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Transcript
The role of aqueous-phase oxidation in the
formation of highly-oxidized organic aerosol A
by
MASSACHUSETT NST TUTE
OF TECHNOLOLGY
Kelly Elizabeth Daumit
MAY 05 2015
LIBRARIES
B.S., Seattle University (2009)
Submitted to the Department of Civil and Environmental Engineering
in partial fulfillment of the requirements for the degree of
Doctor of Philosophy in Environmental Chemistry
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
February 2015
C Massachusetts Institute of Technology 2015. All rights reserved.
Signature redacted
. .. . . ...
.
.
A utho r . . . . .. ........... ........ .............
.... .. . .. . ...
Department of Civil and Environmental Engineering
January 29, 2015
Signature redacted
Certified by .......................
.
. .
.....
Jesse H. Kroll
Associate Professor of Civil, Environmental, and Chemical Engineering
Thesis Supervisor
Signature redacted
Accepted by..........................
/Jfeidi
M N1 f
Donald and Martha Harleman Professor of Civil and Environmental Enginee'ng
Chair, Graduate Program Committee
2
The role of aqueous-phase oxidation in the formation of
highly-oxidized organic aerosol
by
Kelly Elizabeth Daumit
Submitted to the Department of Civil and Environmental Engineering
on January 29, 2015, in partial fulfillment of the
requirements for the degree of
Doctor of Philosophy in Environmental Chemistry
Abstract
Atmospheric particulate matter (or "aerosol") is known to have important implications for cli-
mate change, air quality, and human health. Our ability to predict its formation and fate is hindered by uncertainties associated with one type in particular, organic aerosol (OA). Ambient OA
measurements indicate that it can become highly oxidized in short timescales, but this is generally not reproduced well in laboratory studies or models, suggesting the importance of formation
processes that are not fully understood at present. In this thesis, I focus on the potential for
chemistry within aqueous aerosol to produce highly oxidized OA. I first use a retrosynthetic
modeling approach to constrain the viable precursors and formation pathways of highly oxidized
OA, starting with a target oxidized product and considering possible reverse reactions. Results
suggest three general formation mechanisms are possible: (1) functionalization reactions that add
multiple functional groups per oxidation step, (2) oligomerization of highly oxidized precursors,
or (3) fast aging within the condensed phase, such as oxidation within aqueous particles. The focus of the remainder of the thesis involves experiments designed to study this third pathway. To
examine the importance of the formation of highly oxidized OA in the aqueous phase (wet particles or cloud droplets), I investigate aqueous oxidation of polyols within submicron particles in
an environmental chamber, allowing for significant gas-particle partitioning of reactants,
intermediates, and products. Results are compared to those from analogous oxidation reactions
carried out in bulk solution (the phase in which most previous studies were carried out). Both
sets of experiments result in rapid oxidation, but substantially more carbon is lost from the
submicron particles, likely due to differences in partitioning of early-generation products. Finally, OA is formed from the gas-phase ozonolysis of biogenic precursors in the presence of
reactive aqueous particles, showing that oxidation within the condensed phase can generate
highly oxidized products. The overall results of this thesis demonstrate that aqueous-phase
oxidation can contribute to the rapid formation of highly oxidized OA and therefore its inclusion
in atmospheric models should be considered, but that experiments to constrain such pathways
must be carried out under atmospherically relevant conditions.
Thesis Supervisor: Jesse H. Kroll
Title: Associate Professor of Civil, Environmental, and Chemical Engineering
3
4
Acknowledgements
First, I would like to thank my advisor, Jesse Kroll, for all of his support throughout my
graduate school career. His reputation as a great scientist, and an even better human being, is
what initially caused me to choose MIT, and fortunately he has continued to measure up to this
reputation. Jesse genuinely cares about all of his students and makes each of us feel valued,
excited about research, and proud of our work, improving our confidence as scientists and
making our efforts rewarding and enjoyable. I would also like to thank my committee members,
Phil Gschwend and Colette Heald, for their insightful questions and advice, which have helped
direct and improve the work described in this thesis. Jesse, Phil, and Colette are all excellent role
models, both in their ability to conduct high quality scientific research, and in their decision to
dedicate their careers to studying important environmental problems.
I am also indebted to all my colleagues in the Kroll lab who have helped immensely with
research over the years, helping design and run experiments, giving valuable feedback during
group meetings, and making everything infinitely more fun: Anthony Carrasquillo, James
Hunter, Sean Kessler, Eben Cross, Jon Franklin, Kelsey Boulanger, Ellie Browne, Chris Lim,
David Hagan, and Becca Sugrue. I am especially appreciative of Anthony, James, and Sean who
were here from the beginning and assisted in turning an empty room into a fully functional lab,
and Eben, Chris, and Kelsey who helped with the educational outreach that helped drive my
decision to pursue a teaching career.
I wouldn't have made it through grad school without the kind support of the greater
Parsons community. Thank you especially to Sheila Frankel, Vicky Murphy, Jim Long, and
Jacqui Foster for helping Parsons run smoothly and making everything possible from parties and
posters to faxes and financing (and for all the enjoyable chats that have given me excuses for
breaks from data analysis). I am ever grateful to the many friends I have made here that make it
so much fun to come to work every day. There are far too many to list, but you know who you
are. I would also like to thank all my friends outside of Parsons who keep my life balanced by
filling my free time with social activities and climbing trips. And of course Kyle, for making
every day better, sharing his family with me, and taking me on so many adventures.
Finally, I want to thank my family (my parents, Bryan, Theresa, and all my relatives) for
everything. Thank you for encouraging me to try my best, work hard, think about the future, but
also enjoy life and live in the moment. There is no way I would have gotten here without all of
your support and encouragement. Thank you for always being there, and for always answering
the phone when I needed a break or got homesick. And most importantly, thank you for teaching
me to be kind and loving, and encouraging me to choose a career not based on how much it pays,
but based on what I enjoy and what can help others. I couldn't have asked for a better family and
I dedicate this thesis to them.
Financialsupportfrom the NationalScience Foundation, under grant numbers CHE-1012809
andAGS-1056225 is gratefully acknowledged.
5
6
Contents
1 Introduction
9
1.1
12
2
Research Q uestions ..............................................
1.2 O utline ........................................................
12
1.3
13
R eferences .....................................................
Average chemical properties and potential formation pathways of highly oxidized
organic aerosol
2.1
19
Introduction ....................................................
19
2.2 Methodology .................................................
21
2.3
33
Results ........................................................
2.4 Conclusions ....................................................
49
2.5
A cknow ledgem ents ..............................................
52
2.6
References .....................................................
52
3 Laboratory studies of the aqueous-phase oxidation of polyols: submicron particles
vs. bulk aqueous solution
59
3.1
Introduction ....................................................
59
3.2
Experim ental M ethods ...........................................
62
3.3
Results and Discussion ...........................................
68
3.4
Conclusions and Implications......................................
79
3.5
A cknow ledgem ents ..............................................
81
3.6
R eferences .....................................................
81
3.7 Supplem ent ....................................................
87
4 Role of aqueous-phase oxidants in SOA formation
91
5
4.1
Introduction ....................................................
91
4.2
Experim ental M ethods ...........................................
93
4.3
Results and Discussion ...........................................
97
4.4
Conclusions and Implications .....................................
110
4.5
R eferences ....................................................
112
115
Conclusions
7
8
Chapter 1
Introduction
Aerosols have been shown to play a key role in climate change, air quality, visibility, and
human health (Kanakidou et al., 2005). These fine particles can impact global radiative forcing
directly (by scattering or absorbing solar radiation) or indirectly (by acting as cloud condensation
nuclei and altering cloud formation which in turn affects radiative forcing) (IPCC, 2014).
Numerous studies have found a strong correlation between the presence of aerosol particles and
adverse human health effects, such as increases in lung cancer rates (Pope and Dockery, 2006).
These implications underscore the need for models that can accurately predict the amount and
properties of aerosol. However, our ability to model the formation and fate of particulate matter
is hindered by uncertainties associated with one type in particular, organic aerosol (OA), which
comprises a substantial fraction of fine particulate matter (20-90% of dry mass, Kanakidou et al.,
2005). Although OA has been studied in depth in recent years, there is still much unknown about
the mechanisms by which it is formed. These uncertainties are due in part to the immense
complexity resulting from a large number of reactions and products. Organic particulate matter
can be comprised of primary organic aerosol (POA) that is directly released into the atmosphere,
or secondary organic aerosol (SOA) that is formed from the oxidation and subsequent
condensation of gas-phase organic compounds (Kroll and Seinfeld, 2008). Once formed, SOA
products may continue to evolve or "age" via additional reactions in the gas phase and/or
condensed phase.
9
The largest degree of uncertainty surrounds the most oxidized fraction of OA, whose
sources and chemistry are largely unknown. This highly oxidized material makes up a significant
fraction of OA (~20-70% by mass, Jimenez et al., 2009), and has been shown to form rapidly in
the atmosphere (Volkamer et al., 2006). However, its formation has not been well-reproduced in
the laboratory. Laboratory chamber experiments of SOA formed from various classes of precursors tend to produce aerosol that is relatively unoxidized (average carbon oxidation state (OSc)~
-1.1 to 0.1, Heald et al., 2010; Kroll et al., 2011). Several efforts to age OA in laboratory
chambers have also fallen short of reaching oxidation levels as high as those measured for
ambient aerosol (Donahue et al., 2012; Qi et al., 2010). Those studies that have succeeded in
generating highly oxidized SOA have generally been able to do so only with a select few
precursors (Bahreini et al., 2005; Chhabra et al., 2011) or at OH exposures substantially higher
than those relevant to the formation of oxidized aerosol in the atmosphere (Kessler et al., 2010;
2012; Lambe et al., 2012).
Recent modeling efforts have also attempted to reproduce the amount and degree of
oxidation of OA as measured in field studies, by introducing additional aging chemistry to
simulate oxidation beyond the initial SOA formation (Cappa and Wilson, 2012; Donahue et
al., 2011; Dzepina et al., 2011). However, such models, whether simulating a specific
environment (e.g., Mexico City, Dzepina et al., 2011; 2009; Hodzic et al., 2010), or SOA formation from a specific class of precursors (Aumont et al., 2012; Cappa and Wilson, 2012),
still tend to be unsuccessful in reproducing the formation of highly oxidized aerosol. They
either underpredict OSc at a given loading (Dzepina et al., 2009; 2011; Hodzic et al., 2010)
or predict the formation of highly oxidized OA over longer time scales than what is observed
(Lee-Taylor et al., 2011). These model-measurement discrepancies suggest that traditionally
10
considered pathways may not fully represent the formation of highly oxidized OA.
Aqueous-phase oxidation has received considerable attention as a potential formation
pathway for highly oxidized OA (Ervens et al., 2011; Lim et al., 2010). It is estimated that
particulate liquid water exceeds the total dry aerosol mass by a factor of 2-3, globally
(Carlton & Turpin, 2013; Liao and Seinfeld, 2005). The oxidation of organic species in the
aqueous phase has recently been investigated in the laboratory for a range of water-soluble
species, including small carbonyls (Altieri et al., 2008; Carlton et al., 2007; Kirkland et al.,
2013; Perri et al., 2010; 2009; Tan et al., 2010; 2009), isoprene and its oxidation products
(Altieri et al., 2006; Kameel et al., 2013; Liu et al., 2012; Renard et al., 2013; Zhang et al.,
2010), and phenolic compounds (Smith et al., 2014; Sun et al., 2010). Such bulk-phase
studies have clearly demonstrated that aqueous-phase oxidation, when it occurs, can lead to
the rapid formation of highly oxidized organic species.
However, because a compound can only undergo aqueous-phase oxidation if it is
present in the aqueous phase (cloud droplets or aqueous submicron particles), the
atmospheric relevance of these oxidation processes strongly depends on partitioning
(Donahue et al., 2014). Gas-aqueous partitioning is determined by a compound's effective
Henry's law constant (H*) and the liquid water content (LWC) of the air mass. Because
LWCs are so much lower in the atmosphere (to 106 gg M
g M-3,
3
to 100 pg m 3 for aqueous aerosol, and ~105
for cloud droplets, Ervens et al., 2011), than for bulk aqueous solution (~1 012
the density of water), many compounds that are considered to be "water-soluble" for
bulk solutions will not actually partition significantly into the aqueous-phase in the
atmosphere. This suggests that experiments conducted in a bulk solution may not be fully
representative of aqueous processing in the atmosphere.
11
The overarching goal of this thesis is to determine to what extent aqueous-phase
oxidation can aid in the formation highly-oxidized OA under atmospherically realistic
partitioning conditions. The specific research questions that are addressed as well as an
outline of the thesis are provided in the following sections.
1.1 Research Questions
1) Can highly oxidized organic aerosol be formed from traditional gas-phase oxidation pathways
and precursors? (Chapter 2)
2) How does aqueous oxidation within submicron particles differ from oxidation in a bulk aqueous solution? Is oxidation within submicron particles still an efficient pathway for the generation
of oxidized organic material? (Chapter 3)
3) How does the presence of aqueous-phase oxidants affect SOA formation? Can oxidized OA
be generated from fresh SOA via oxidation within the condensed phase? (Chapter 4)
1.2 Outline
In order to address the first research question (can highly oxidized OA form from
traditional gas-phase oxidation pathways?), we develop a novel retrosynthetic approach for
constraining the viable precursors and formation pathways. This involves starting with the
oxidized product and considering the possible reverse reactions, using a set of simple chemical
rules. We focus on the formation of low-volatility oxidized organic aerosol (LV-OOA),
determined from factor analysis of aerosol mass spectrometer data. The elemental composition
and volatility of the aerosol enable the determination of its position in a three-dimensional
chemical space (defined by the hydrogen-to-carbon ratio, oxygen-to-carbon ratio, and carbon
number) and thus its average chemical formula. Consideration of possible back-reactions then
12
defines the movement taken through this chemical space, constraining potential reaction
pathways and precursors. This work is presented in Chapter 2.
In order to address the second research questions (how does aqueous oxidation differ
within submicron particlesand bulk solution?), we conduct aqueous oxidation experiments on a
set of polyols, which serve as simple surrogates for water-soluble compounds in the atmosphere.
Two analogous sets of experiment are carried out: (1) in a bulk aqueous solution, and (2) within
aqueous submicron particles in an environmental chamber, allowing for significant gas-particle
partitioning of reactants, intermediates, and products. This allows for the first direct comparison
of aqueous oxidation under drastically different partitioning conditions. These experiments are
presented in Chapter 3.
In order to address the final research question (how do aqueous-phase oxidants affect
SOA formation?), we conduct additional chamber experiments with more atmospherically relevant organic precursors and a broader range of condensed phase oxidation conditions. We generate SOA from the gas-phase ozonolysis of biogenic volatile organic compounds with a range of
seed types in order to vary the phase and oxidant levels within the particles. Changes to the
oxidation state of particulate carbon are directly measured using aerosol mass spectrometry.
These experiments are presented in Chapter 4. In Chapter 5, we provide a brief summary of our
overall findings, and discuss implications for future research.
1.3 References
Altieri, K. E., Carlton, A. G., Lim, H.-J., Turpin, B. J. and Seitzinger, S. P.: Evidence for
oligomer formation in clouds: Reactions of isoprene oxidation products, Environ. Sci. Technol.,
40, 4956-4960, doi:10.1021/es052170n, 2006.
Altieri, K. E., Seitzinger, S. P., Carlton, A. G., Turpin, B. J., Klein, G. C. and Marshall, A. G.:
Oligomers formed through in-cloud methylglyoxal reactions: Chemical composition, properties,
and mechanisms investigated by ultra-high resolution FT-ICR mass spectrometry, Atmos.
Environ., 42, 1476-1490, doi:10.1016/j.atmosenv.2007.11.015, 2008.
13
Aumont, B., Valorso, R., Mouchel-Vallon, C., Camredon, M., Lee-Taylor, J. and Madronich, S.:
Modeling SOA formation from the oxidation of intermediate volatility n-alkanes, Atmos. Chem.
Phys., 12, 7577-7589, 2012.
Bahreini, R., Keywood, M. D., Ng, N. L., Varutbangkul, V., Gao, S., Flagan, R. C., Seinfeld, J.
H., Worsnop, D. R. and Jimenez, J. L.: Measurements of secondary organic aerosol from
oxidation of cycloalkenes, terpenes, and m-Xylene using an Aerodyne aerosol mass
spectrometer, Environ. Sci. Technol., 39(15), 5674-5688, doi:10.1021/es048061a, 2005.
Cappa, C. D. and Wilson, K. R.: Multi-generation gas-phase oxidation, equilibrium partitioning,
and the formation and evolution of secondary organic aerosol, Atmos. Chem. Phys., 12, 9505-
9528, 2012.
Carlton, A. G., Turpin, B. J., Altieri, K. E., Seitzinger, S., Reff, A., Lim, H.-J. and Ervens, B.:
Atmospheric oxalic acid and SOA production from glyoxal: Results of aqueous photooxidation
experiments, Atmos. Environ., 41(35), 7588-7602, doi: 10.101 6/j.atmosenv.2007.05.035, 2007.
Carlton, A. G., Turpin, B. J.: Particle partitioning potential of organic compounds is highest in
the Eastern US and driven by anthropogenic water, Atmos. Chem. Phys., 13, 10203-10214,
2013.
Chhabra, P. S., Ng, N. L., Canagaratna, M. R., Corrigan, A. L., Russell, L. M., Worsnop, D. R.,
Flagan, R. C. and Seinfeld, J. H.: Elemental composition and oxidation of chamber organic
aerosol, Atmos. Chem. Phys., 11, 8827-8845, 2011.
Donahue, N. M., Epstein, S. A., Pandis, S. N. and Robinson, A. L.: A two-dimensional volatility
basis set: 1. organic-aerosol mixing thermodynamics, Atmos. Chem. Phys., 11(7), 3303-3318,
2011.
Donahue, N. M., Henry, K. M., Mentel, T. F., Kiendler-Scharr, A., Spindler, C., Bohn, B.,
Brauers, T., Dorn, H. P., Fuchs, H., Tillmann, R., Wahner, A., Saathoff, H., Naumann, K.-H.,
Mohler, 0., Leisner, T., Muller, L., Reinnig, M.-C., Hoffmann, T., Salo, K., Hallquist, M.,
Frosch, M., Bilde, M., Tritscher, T., Barmet, P., Praplan, A. P., DeCarlo, P. F., Dommen, J.,
Prevot, A. S. H. and Baltensperger, U.: Aging of biogenic secondary organic aerosol via gas-
phase OH radical reactions, Proc. Nat]. Acad. Sci. U. S. A., 109(34), 13503-13508,
doi:10.1073/pnas.1 1151 86109/-/DCSupplemental, 2012.
Dzepina, K., Cappa, C. D., Volkamer, R. M., Madronich, S., DeCarlo, P. F., Zaveri, R. A. and
Jimenez, J. L.: Modeling the multiday evolution and aging of secondary organic aerosol during
MILAGRO 2006, Environ. Sci. Technol., 45(8), 3496-3503, 2011.
Dzepina, K., Volkamer, R. M., Madronich, S., Tulet, P., Ulbrich, I. M., Zhang,
Q., Cappa,
C. D.,
Ziemann, P. J. and Jimenez, J. L.: Evaluation of recently-proposed secondary organic aerosol
models for a case study in Mexico City, Atmos. Chem. Phys., 9(15), 5681-5709, 2009.
Ervens, B., Turpin, B. J. and Weber, R. J.: Secondary organic aerosol formation in cloud droplets
and aqueous particles (aqSOA): a review of laboratory, field and model studies, Atmos. Chem.
Phys., 11(21), 11069-11102, doi:10.5194/acp-11-11069-2011, 2011.
14
Heald, C. L., Kroll, J. H., Jimenez, J. L., Docherty, K. S., DeCarlo, P. F., Aiken, A. C., Chen, Q.,
Martin, S. T., Farmer, D. K. and Artaxo, P.: A simplified description of the evolution of organic
aerosol composition in the atmosphere, Geophys. Res. Lett., 37(8), 2010.
Hodzic, A., Jimenez, J. L., Madronich, S., Canagaratna, M. R., DeCarlo, P. F., Kleinman, L. and
Fast, J.: Modeling organic aerosols in a megacity: potential contribution of semi-volatile and
intermediate volatility primary organic compounds to secondary organic aerosol formation,
Atmos. Chem. Phys., 10(12), 5491-5514, 2010.
Intergovernmental Panel on Climate Change: Climate Change 2013 - The Physical Science
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Jimenez, J. L., Canagaratna, M. R., Donahue, N. M., Prevot, A., Zhang, Q., Kroll, J. H.,
DeCarlo, P. F., Allan, J. D., Coe, H. and Ng, N. L.: Evolution of organic aerosols in the
atmosphere, Science, 326(5959), 1525-1529, 2009.
Kameel, F. R., Hoffmann, M. R. and Colussi, A. J.: OH radical-initiated chemistry of isoprene in
aqueous media. Atmospheric implications, J. Phys. Chem. A, 117(24), 5117-5123,
doi:10.1021/jp4026267, 2013.
Kanakidou, M., Seinfeld, J. H., Pandis, S. N., Barnes, I., Dentener, F. J., Facchini, M. C., Van
Dingenen, R., Ervens, B., Nenes, A., Nielsen, C. J., Swietlicki, E., Putaud, J. P., Balkanski, Y.,
Fuzzi, S., Horth, J., Moortgat, G. K., Winterhalter, R., Myhre, C., Tsigaridis, K., Vignati, E.,
Stephanou, E. G. and Wilson, J.: Organic aerosol and global climate modelling: a review, Atmos.
Chem. Phys., 5, 1053-1123, 2005.
Kessler, S. H., Smith, J. D., Che, D. L., Worsnop, D. R., Wilson, K. R. and Kroll, J. H.:
Chemical sinks of organic aerosol: kinetics and products of the heterogeneous oxidation of
erythritol and levoglucosan, Environ. Sci. Technol., 2010.
Kessler, S. H., Nah, T., Daumit, K. E., Smith, J. D., Leone, S. R., Kolb, C. E., Worsnop, D. R.,
Wilson, K. R. and Kroll, J. H.: OH-Initiated Heterogeneous Aging of Highly Oxidized Organic
Aerosol, J. Phys. Chem. A, 116(24), 6358-6365, doi:10.1021/jp212131m, 2012.
Kirkland, J. R., Lim, Y. B., Tan, Y., Altieri, K. E. and Turpin, B. J.: Glyoxal secondary organic
aerosol chemistry: effects of dilute nitrate and ammonium and support for organic radical-radical
oligomer formation, Environ. Chem., 10(3), 158-166, doi: 10.1071/EN 13074, 2013.
Kroll, J. H. and Seinfeld, J. H.: Chemistry of secondary organic aerosol: Formation and evolution
of low-volatility organics in the atmosphere, Atmos. Environ., 42(16), 3593-3624,
doi: 10.101 6/j.atmosenv.2008.01.003, 2008.
Kroll, J. H., Donahue, N. M., Jimenez, J. L., Kessler, S. H., Canagaratna, M. R., Wilson, K. R.,
Altieri, K. E., Mazzoleni, L. R., Wozniak, A. S., Bluhm, H., Mysak, E. R., Smith, J. D., Kolb, C.
E. and Worsnop, D. R.: Carbon oxidation state as a metric for describing the chemistry of
atmospheric organic aerosol, Nat. Chem., 3(2), 133-139, 2011.
15
Lambe, A. T., Onasch, T. B., Croasdale, D. R., Wright, J. P., Martin, A. T., Franklin, J. P.,
Massoli, P., Kroll, J. H., Canagaratna, M. R., Brune, W. H., Worsnop, D. R. and Davidovits, P.:
Transitions from functionalization to fragmentation reactions of laboratory secondary organic
aerosol (SOA) generated from the OH oxidation of alkane precursors, Environ. Sci. Technol.,
46(10), 5430-5437, doi:10.1021/es300274t, 2012.
Lee-Taylor, J., Madronich, S., Aumont, B., Baker, A., Camredon, M., Hodzic, A., Tyndall, G. S.,
Apel, E. and Zaveri, R. A.: Explicit modeling of organic chemistry and secondary organic
aerosol partitioning for Mexico City and its outflow plume, Atmos. Chem. Phys., 11(24), 1321913241, doi:10.5194/acp-l 1-13219-2011, 2011.
Liao. H. and Seinfeld, J. H.: Global impacts of gas-phase chemistry-aerosol interactions on direct
radiative forcing by anthropogenic aerosols and ozone, J. Geophys. Res., 110, D18208,
doi: 10.1 029/2005jd005907, 2005.
Lim, Y. B., Tan, Y., Perri, M. J., Seitzinger, S. P. and Turpin, B. J.: Aqueous chemistry and its
role in secondary organic aerosol (SOA) formation, Atmos. Chem. Phys., 10(21), 10521-10539,
doi:10.5194/acp-10-10521-2010, 2010.
Liu, Y., Monod, A., Tritscher, T., Praplan, A. P., DeCarlo, P. F., Temime-Roussel, B., Quivet,
E., Marchand, N., Dommen, J. and Baltensperger, U.: Aqueous phase processing of secondary
organic aerosol from isoprene photooxidation, Atmos. Chem. Phys., 12(13), 5879-5895,
doi:10.5194/acp-12-5879-2012, 2012.
Nguyen, T. B., Coggon, M. M., Flagan, R. C. and Seinfeld, J. H.: Reactive uptake and photoFenton oxidation of glycolaldehyde in aerosol liquid water, Environ. Sci. Technol., 47(9), 43074316, doi:10.1021/es400538j, 2013.
Perri, M. J., Lim, Y. B., Seitzinger, S. P. and Turpin, B. J.: Organosulfates from glycolaldehyde
in aqueous aerosols and clouds: Laboratory studies, Atmos. Environ., 44(21), 2658-2664,
doi:10.1016/j.atmosenv.2010.03.031, 2010.
Perri, M. J., Seitzinger, S. and Turpin, B. J.: Secondary organic aerosol production from aqueous
photooxidation of glycolaldehyde: Laboratory experiments, Atmos. Environ., 43(8), 1487-1497,
doi: 10.101 6/j.atmosenv.2008.11.037, 2009.
Pope, C. A. and Dockery, D. W.: Health effects of fine particulate air pollution: Lines that
connect, J. Air Waste Manage., 56(6), 709-742, 2006.
Qi, L., Nakao,
S., Malloy, Q., Warren, B. and Cocker, D. R., III: Can secondary organic aerosol
formed in an atmospheric simulation chamber continuously age? Atmos. Environ., 44(25), 29902996, doi: 10.101 6/j.atmosenv.2010.05.020, 2010.
Renard, P., Siekmann, F., Gandolfo, A., Socorro, J., Salque, G., Ravier, S., Quivet, E., Clement,
J. L., Traikia, M., Delort, A. M., Voisin, D., Vuitton, V., Thissen, R. and Monod, A.: Radical
mechanisms of methyl vinyl ketone oligomerization through aqueous phase OH-oxidation: on
the paradoxical role of dissolved molecular oxygen, Atmos. Chem. Phys., 13(13), 6473-6491,
doi:10.5194/acp-13-6473-2013, 2013.
16
Smith, J., Sio, V., Yu, L., Zhang, Q. and Anastasio, C.: Secondary organic aerosol production
from aqueous reactions of atmospheric phenols with an organic triplet excited state, Environ. Sci.
Technol., 48, 1049-1057, 2014.
Sun, Y. L., Zhang, Q., Anastasio, C. and Sun, J.: Insights into secondary organic aerosol formed
via aqueous-phase reactions of phenolic compounds based on high resolution mass spectrometry,
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and wet aerosols: Measurement and prediction of key products, Atmos. Environ., 44(39), 52185226, doi:10.1016/j.atmosenv.2010.08.045, 2010.
Tan, Y., Perri, M. J., Seitzinger, S. P. and Turpin, B. J.: Effects of precursor concentration and
acidic sulfate in aqueous glyoxal - OH radical oxidation and implications for secondary organic
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17
18
Chapter 2
Average chemical properties and potential
formation pathways of highly oxidized
organic aerosol
Adaptedfrom: Daumit, K. E., Kessler, S. H., and Kroll, J. H.: Average chemical properties and
potential formation pathways of highly oxidized organic aerosol, Faraday Discuss., 165, 181-
202, doi:10.1039/c3fdOO045a, 2013. - Reproduced by permission of The Royal Society of
Chemistry http://pubs.rsc.org/en/content/articlepdf/201 3/fd/c3fdOO045a
2.1 Introduction
As described in Chapter 1, organic aerosol (OA) constitutes a substantial fraction of
atmospheric fine particulate matter, and thus a thorough understanding of its formation and
evolution is necessary in order to better evaluate the effects of particulate matter on climate
and human health (Kanakidou et al., 2005). However, despite substantial research efforts,
there remains a great deal unknown about the chemical mechanisms important to OA.
Especially uncertain are the sources and chemistry of the most oxidized fraction of OA,
sometimes termed "humic-like
substances" (HULIS, Graber and Rudich, 2006) (when
measured using offline techniques of fractionated filter samples), or low-volatility oxidized
organic aerosol (LV-OOA) (when determined from factor analysis of online aerosol mass
spectrometer data) (Jimenez et al., 2009). (It is worth noting that as used in the acronym LVOOA, the term volatility is synonymous with saturation vapor concentration.) This highly
oxidized fraction of organic aerosol is ubiquitous in the atmosphere, making up between 20-
19
70% of total organic aerosol mass (Jimenez et al., 2009; Zhang et al., 2007). It is highly
oxidized (with an average carbon oxidation state of approximately 0-1, Kroll et al., 2011),
exceedingly low in saturation vapor concentration (c* -0.1
to <10-7
tg M 3 , Cappa and
Jimenez, 2010), and generated relatively quickly (over time scales of -1-3 days, Jimenez et
al., 2009; Volkamer et al., 2006). While this highly oxidized OA is presumed to be secondary
in nature (formed from chemical transformations of gas-phase organics that subsequently
condense as particulate matter), its detailed formation pathways are poorly understood and
have not been adequately reproduced either in the laboratory or in atmospheric models. This
suggests major gaps in our understanding of the chemistry underlying the formation of
highly oxidized organic aerosol. Unfortunately at present we have very little information as
to whether these gaps relate to uncertainties in reaction mechanisms, SOA precursors, or
both.
Here we present a new approach for constraining the formation of highly oxidized
aerosol, involving the use of chemical properties of the aerosol to better understand its
possible formation pathways and precursors. In contrast to most modeling approaches that
start with potential precursors and simulate their forward evolution in an attempt to generate
products chemically similar to those measured in the atmosphere, we start with the known
products (highly oxidized OA components) and work backwards toward reactants (SOA
precursors). This approach is similar to the retrosynthetic approach of Pun, et al. (2000) for
assessing formation pathways of atmospheric water-soluble organic compounds. That
approach was highly molecule- and reaction- specific, involving the identification of detailed
chemical reactions that link individual precursors to individual aerosol components. Here we
take a more generalized view of the chemistry, assessing classes of reactions that could
20
ultimately lead to the formation of highly oxidized organic products with low saturation
vapor concentrations. Such an approach requires knowledge of the chemical properties of the
"targets" (molecules representing the oxidized fraction of the aerosol); we define these in
terms of the average chemical formulas of their constituent molecules, as determined from
properties measured in field studies as well as structure-activity relationships. From these
targets, a set of simple, general rules governing possible atmospheric reactions lets us work
backwards, allowing for identification of viable reaction pathways and aerosol precursors
(which are also defined in terms of their average chemical properties). By focusing on
generic processes rather than detailed chemical structures and mechanisms, we can draw
general conclusions about the types of processes and precursors that might yield highly
oxidized organic species. These can be used to provide guidance in identifying relevant
reaction conditions and precursors in future studies.
In the following section we describe this methodology in detail, including the
chemical space we use to describe the organics, the characterization of our targets, and the
types and
characteristics
of reverse pathways
considered.
Results -
the chemical
characteristics of our targets (constrained by AMS measurements of LV-OOA components)
and the possible range of aerosol formation pathways - are then described. We conclude with
discussions of implications of this work for our understanding of highly oxidized organic
aerosol, as well as potential areas of future work.
2.2 Methodology
We describe molecules in terms of their position in a three-dimensional chemical
space, defined by their carbon number (nc), oxygen-to-carbon ratio (O/C), and hydrogen-tocarbon ratio (H/C). O/C and H/C have already seen substantial use in describing the average
21
properties of atmospheric organic aerosol using the two-dimensional van Krevelen diagram
(Heald et al., 2010; Ng et al., 2011 ); however this provides no information on the size of the
molecule (a crucial factor governing saturation vapor concentration). By considering nc as a
third dimension to the van Krevelen space, we can represent any molecular formula CxHYOZ
as a single, unique point, and can relate its position in chemical space to c* (Kessler et al.,
2010; 2012). This space is equivalent to the three-dimensional space defined by carbon
number, hydrogen number, and oxygen number (nc vs. nH vs. no); we choose to use O/C and
H/C here since they are also used in the van Krevelen diagram, and are more amenable to
visualization and comparison of elemental composition (hydrogen and oxygen content) of
molecules of different sizes. A previously proposed alternative three-dimensional space uses
molecular weight, heteroatom mass, and double bond equivalents (Wei et al., 2012). While
this has advantages for describing certain atmospheric systems, especially those with
nitrogen or sulfur containing species, the space proposed here is more conducive to the
description of atmospheric reactions in terms of changes to carbon, hydrogen, and oxygen
content of the molecules.
The three-dimensional space we use here is closely related to many of the twodimensional frameworks that have recently been used to describe and/or model organic
aerosol. These include the van Krevelen plot (Heald et al., 2010), polarity vs. nc (Pankow
and Barsanti, 2009), OSc vs. saturation vapor concentration (c*) (Donahue et al., 2011;
Jimenez et al., 2009), the f44 vs. f43 space for plotting AMS data (Ng et al., 2011; 2010),
OSc vs. nc (Kroll et al., 2011), and no vs. nc (Cappa and Wilson, 2012). Although adding a
third dimension introduces complexity to any of these descriptions, it allows for more
chemical information to be represented. It explicitly includes the effects of carbon number,
22
which are missing from functional-group-based frameworks (e.g., van Krevelen and f44-f43),
and it requires fewer assumptions about the identity or distributions of functional groups than
frameworks that reduce the descriptions of functional groups to a single dimension (polaritync, OSc-nc, OSc-c*no-nc).
We note that isomeric species of a given formula CXHyOz are not
distinguished even in this 3-D space, which can lead to errors in the calculation of c* when
the identity of the functional groups changes, as discussed below. Nonetheless, chemical
transformations and properties (e.g., c*) can be represented in this space in a straightforward
manner.
2.2.1 Characterization of the target.
Our general approach is to start with the product (molecules representative of highly
oxidized organic aerosol) and work backwards. We refer to this species as the "target" and
the backwards reactions as "transforms", following the conventions of retrosynthetic analysis
(Corey, 1988). Key to this approach is the accurate determination of the chemical formula
(position in O/C-H/C-nc space) of the target.
Since the target in this case (highly oxidized organic aerosol) is not a single molecule,
but rather a complex mixture, we define it in terms of its average properties and average
chemical formula. In the present study we determine this from field studies using the
Aerodyne High Resolution-Time of Flight-Aerosol Mass Spectrometer (HR-ToF-AMS, or
simply AMS, Canagaratna et al., 2007; DeCarlo et al., 2006) which provides measurements
of the elemental ratios of OA (Aiken et al., 2007; 2008). Positive matrix factorization (PMF)
of AMS data (Ulbrich et al., 2009) typically yields several factors, of which LV-OOA is the
most highly oxidized. We take this to be representative of the most oxidized fraction of
organic aerosol, and use measurements of LV-OOA elemental ratios to define the location of
23
our target in van Krevelen space.
Because the AMS provides no information regarding carbon number (the third
dimension of our chemical space), we determine nc from field measurements of aerosol c*
(Cappa and Jimenez, 2010; Cappa, 2010; Huffman et al., 2009). The carbon number can be
estimated from c* using structure-activity
relationships (SARs) for determining vapor
pressure. In this work we use SIMPOL (Pankow and Asher, 2008), allowing for the direct
determination of c* from nc and the functional group abundance, as described below. We
focus on partitioning only into the condensed organic phase, as described by c*. Partitioning
into liquid water could also be included using this general approach; it would require use of a
SAR for estimating the saturation vapor concentration over water (Henry's law constant, e.g.
Hine and Mookerjee, 1975), but is beyond the scope of this work.
Key inputs for SIMPOL (and most other SARs for estimating vapor pressures) are the
abundances of different functional groups in the molecule (Pankow and Asher, 2008). In
order to determine these from our values for nc, H/C, and O/C, we make two assumptions.
The first is that functional groups in the molecule are limited to carbonyls, hydroxyls, or
some combination of the two (e.g., carboxylic acids), which have been shown to be the major
functional groups in ambient organic aerosol samples (Russell et al., 2011). Since each
functional group contains only one oxygen atom, their abundances can be related to the
oxygen number of the molecule:
no = nc O/C = n-OH+ n 0
(2.1)
where no, nc, and O/C are as defined above, and n-oH and n=o are the number of hydroxyl
groups and carbonyl groups in the molecule, respectively. While other functional groups are
also likely to be present in organic aerosol, several groups can be approximated using this
24
simple treatment, as discussed below.
The second assumption is that all sites of unsaturation (double bond equivalents,
DBE) in the molecule arise from carbonyl groups:
n_
=DBE=1+n((l-+LH/C)
(2.2)
We therefore assume that our target contains no carbon-carbon double bonds or rings. While
this is reasonable for aliphatic C=C bonds, which are highly susceptible to oxidation and
unlikely to survive significant atmospheric processing, cyclic (or aromatic) structures may be
present in highly oxidized OA. The effect of neglecting any rings present is to overestimate
n=o and underestimate n-OH, thereby overestimating c' somewhat.
The two assumptions above (Eqs. 2.1 and 2.2) allow for the straightforward derivation
of expressions relating elemental ratios, functional groups, saturation vapor concentration,
and carbon number. The number of hydroxyl groups
(n-OH)
can be determined by combining
Eqs. 2.1 and 2.2:
=- n -n O
C
O/C -(I+nc (l- i H/C))= -I+n (O/C+-LH/C-1)
The carbon number (nc) and functional group abundances
(n-OH
(2.3)
and n=o) allow for the
calculation of c* (tg m- 3) using SIMPOL:
log
10 c*=log ,(aP)= logO a+bO+bcnc+b n
=
log 0a +bO +bcnc +bO(l +nc(l--jH/C))+b-OH (-
n+b-OH-OH
n (o/C+
H/C -1))
where Psat is the vapor pressure in atm, cc is the conversion from Psat to c
,
and the b terms are
the different group contribution terms for quantifying the contribution of each chemical
moiety to vapor pressure: bo is the zero order term, bc is the carbon number term, b=o is the
carbonyl group term, and b-OH is the hydroxyl group term (for MW = 200 g molEI and T =
293 K, equal to 1.79, -0.438, -0.935, and -2.23, respectively) (Pankow and Asher, 2008). The
25
conversion factor ca is given by a = 10 6(MW)/RT, or 8.314 x 109 at 293 K, where MW is the
average molecular weight (g mol-1) of the molecules making up the absorbing phase, R is the
ideal gas constant (8.21
x
10-5 atm m 3 mol-1 K1), and T is the temperature in K. For these
calculations we use an assumed MW value of 200 g mol-1; the actual value used has little
effect on the results. Rearranging Eq. 2.4 to solve for carbon number, and substituting in
values of n=o and n-OH from Eqs. 2.2 and 2.3, we obtain
logl0 c *-oloa-bo
Cb
+ b= (I - -H/C)+boH
-b 0 +b-OH
(o/C+H/C- 1)
(2.
Equation 2.5 allows for the determination of carbon number from elemental (H/C, O/C) and
c* data. This approach, used in previous descriptions of heterogeneous oxidation systems
(Kessler et al., 2010; 2012), is similar to that of Donahue et al. (2011) for estimating carbon
number from
OSc (or O/C) and
c* alone; however that approach
required
making
assumptions about functional group distribution (specifically that n-OH = n=o). With the
explicit inclusion of H/C, we can directly calculate
n-OH
and n= 0 , which allows for a more
accurate estimation of nc.
Eq. 2.5 relies critically on the assumption that target molecules contain only hydroxyl
and carbonyl groups. While molecules containing other functional groups will be less
accurately represented,
many such functional groups are reasonably approximated
as
acid group (b(=ooH = -3.58) is similar to that of a hydroxyl plus a carbonyl (b-OH
b=o
-
carbonyls, alcohols, or some combination of the two. Because the vapor pressure effect of an
3.165), treating an acid as the sum of the two introduces only a minor error. Likewise,
epoxides are very similar to carbonyls in terms of both bonding and effect on c*: like
carbonyls, they contribute one DBE, and the vapor pressure effect of an ether (b-o- = -0.718)
is similar to that of a carbonyl. Similarly, nitrate and hydroperoxyl groups are connected to
26
the carbon skeleton via a single C-O bond and so have similar bonding as a hydroxyl, and
also have roughly the same vapor pressure effect (b-No3
-2.23,
b-ooH
-2.48). These
functional groups have additional oxygen atoms, but in the AMS these are likely lost as NO 2
or OH fragments (Farmer et al., 2010) and so it is likely that only one oxygen atom is
measured (as is the case for a hydroxyl group). Since these moieties contribute one fewer
hydrogen than a hydroxyl group, the measured H/C may not be exactly the same as for the
corresponding alcohol, but this will have only a minor effect on results. Unfortunately, not
all functional groups are as well-represented in the above treatment. Acyclic ethers have the
same bonding (and contribution to O/C and H/C) as hydroxyl groups, but have a substantially
smaller effect on vapor pressure (b-o- = -0.718 vs. b-OH
-2.23). Similarly, esters, like acids,
will be treated as a carbonyl plus a hydroxyl group, but their actual effect on vapor pressure
is far smaller (b(=o)o_ = -1.20 vs. b-oH
+
b=o
-3.165). While ethers generally make up an
insignificant fraction of functional groups in OA, esters (which are predicted to form from
the condensed-phase reaction of carboxylic and hydroxyl groups) may account for as much
as 20% of functional groups in some organic aerosol (Russell, et al., 2011). There is no good
way to account for these errors using elemental ratios alone; direct measurements of
functional group abundances in OA (e.g., using FTIR, Maria et al., 2002) would thus greatly
aid in this general approach for determining the chemical formula and properties of the
target.
We note there are ways to determine the chemical formula (position in a given
chemical space) other than the elemental- and c*-based approach described above. One
widely-used technique is ultrahigh-resolution electrospray ionization mass spectrometry
(UHR-ESI-MS) of filter samples to directly determine exact chemical formulas (Lin et al.,
27
2012; Wozniak et al., 2008). There are currently far fewer measurements of ambient OA by
UHR-ESI-MS than by AMS, but this is another promising technique for characterizing the
target. While concerns such as variations in sensitivity toward different compounds, and the
possibility of fractionation arising from dissolution in a given solvent, still need to be
addressed, estimates of OSc and nc from AMS and ESI measurements are generally in good
agreement (Kroll et al., 2011).
2.2.2. Transforms
Once the target is well-defined, transforms (the reverse of atmospheric reactions)
allow for the formation chemistry to be traced backwards toward potential precursors. Again,
the aim is not to identify specific formation pathways and precursor molecules, but rather to
examine the key features of viable ones. Here, we consider three reaction classes
functionalization, fragmentation, and oligomerization-each of which is discussed below.
Simple chemical rules for each reaction class help constrain their potential role and possible
precursors.
a) Functionalization
Functionalization reactions are those that add (or interconvert) functional groups
without other changes to the carbon skeleton. They are easily represented in a 2-D van
Krevelen plot since they involve no change in nc. Movement through this space has been
used previously to describe different functional group additions (Heald et al., 2010; Ng et al.,
2011).
Reverse functionalization
reactions connecting target molecules
back to their
precursors involve the loss of carbonyl or hydroxyl groups, as shown in Figure 2-1. The
exact direction in van Krevelen space depends not only on the functional group involved but
also the nature of the precursor, specifically the bond (C-H vs. C=C) being replaced by the
28
(a) C-H bonds
(b) C=C bonds
UA
0
loss of
hydroxyl
O/C
O/C
Figure 2-1. Possible transforms (back-reactions) available to a target molecule, assuming
functionalization reactions only (with no change to carbon number). Loss of functional
groups is illustrated in terms of movement in van Krevelen space, depicting changes in H/C
and O/C. The exact trajectory depends on the identity both of the functional group and the
bond being replaced. Panel a: functionalization of C-H bonds (saturated case), with slopes of
-2 for carbonyl groups and 0 for hydroxyl groups. Panel b: functionalization of C=C bonds or
rings (unsaturated case), with slopes of -I for carbonyl groups and +1 for hydroxyl groups. In
either case, changes to more than one functional group (light grey arrows and points) can be
described by simple vector addition.
functional group. Figure 2-la shows the "saturated" case, in which the functional groups
replace C-H bonds, which has been considered previously (Heald et al., 2010). Removing a
single functional group decreases O/C by i/nc, with the effect on H/C depending on the
identity of the group. Loss of a carbonyl decreases H/C by 2 /nc (following a slope of -2 on
the van Krevelen plot), while removing a hydroxyl has no effect on the H/C ratio (following
a slope of 0). These steps are fully additive, such that the loss of multiple functional groups
can be described simply by the sum of the vectors associated with the individual groups. For
example, removing a hydroxycarbonyl or carboxylic acid (the equivalent of removing one
hydroxyl and one carbonyl) decreases O/C by
29
2 /nc
and increases H/C by
2 /nc
(thereby
following a slope of -1). Figure 2-1b shows the "unsaturated" case, in which the functional
group replaces a carbon-carbon double bond (or C-C single bond within a ring). Loss of a
functional group still decreases O/C by 1/nc, but now removing a carbonyl increases H/C by
only I/nc (slope = -1), whereas removing a hydroxyl actually decreases H/C, by i/nc (slope
= +1). Note that the oxidation of C=C bonds typically involves changes to two functional
groups, so movement through this space can be quite rapid. In either case (saturated or
unsaturated), for a given nc, each point in van Krevelen space corresponds to a value of c*,
allowing the change in c* associated with any transform to be easily computed.
We can define the range of potential precursors of our target by providing two
fundamental limits on what reverse reactions are possible: (1) a molecule cannot lose more of
a given functional group than it actually has; and (2) going backwards, the average carbon
oxidation
state
cannot
increase,
since
reduction
reactions
are
thermodynamically
unfavorable. This can significantly narrow the area in chemical space that defines the
potential precursors, as discussed in the Results section.
b) Fragmentation
Fragmentation reactions decrease the carbon number of the molecule via the cleavage
of carbon-carbon bonds. Since these reactions are oxidative, they will involve the addition of
functional groups at the site of the bond breakage. (The scission of a single bond within a
ring is therefore not considered a fragmentation reaction, and is instead a functionalization
reaction, involving the addition of multiple functional groups.) Fragmentation reactions can
result in rapid oxidation of the carbon, since they can lead to a high abundance of functional
groups on relatively small molecules. However the decrease in carbon number can also lead
to a substantial increase in c*. This tradeoff likely limits the importance of fragmentation
30
pathways for the formation of very oxidized, low-volatility species.
Because nc changes during fragmentation, simple 2-D van Krevelen plots are not
sufficient to accurately capture these transformations. Furthermore, because of the range of
possible precursors (with any number of carbon, oxygen, or hydrogen atoms), there are no
simple generic transforms to describe this change. Nonetheless, we can apply constraints that
considerably narrow the chemical space associated with the possible transforms and
precursors. One approach is to assume that functional groups are evenly distributed across all
the carbon in a molecule, and that this distribution is conserved during the fragmentation
process (with the exception of any functional groups added during fragmentation). This
statistical treatment of fragmentation is similar to that used in the recently-developed
Statistical Oxidation Model (Cappa and Wilson, 2012). For the present study, we assume that
two functional groups (one hydroxyl and one carbonyl) are added to a given fragment, but
this distribution can be altered as necessary. Such assumptions (plus the requirement that nc
of the precursor must exceed that of the target), give the following constraints: nc,p > nc,t , n_
OH,p=
(n-OH
-I
)(nc,p/nc,t) , and n=0 ,p = (n=o,t - 1) (ncp/nct) , where subscript "t" refers to the
target and subscript "p" refers to the precursor. The subtraction of I from n-OH,t and n=o,t
accounts for the addition of the two functional groups upon fragmentation.
An alternative, more exact approach is to allow the functional groups to be localized
somewhere on a molecule, rather than distributed evenly across it. This allows for a broader
range of potential precursors, since fragments can have varying degrees of oxidation (i.e.,
one that is more oxidized than the other). In this case, the number of functional groups in the
target provides constraints on precursors: after taking into account any new functional
groups, the precursor cannot have fewer hydroxyl or carbonyl groups than the target. Again,
31
if we assume that two groups (one hydroxyl and one carbonyl) are added to the fragment,
thereby increasing n=o and n-OH each by one, we obtain the following constraints: nc,p > nc,t,
n-OH,p
(n-OH,t
-
1) , and n=o,p
(n=o,t - 1).
c) Oligomerization
Oligomerization reactions occur when two molecular species combine to form a
larger one. Because of the associated increase in carbon number, oligomerization reactions
are an efficient way to lower vapor pressure. However, this type of reaction is typically nonoxidative (Kroll and Seinfeld, 2008) so is not accompanied by an overall change in OSc.
Therefore the carbon atoms in the precursors must, on average, be as oxidized as those in the
target.
As with fragmentation, oligomerization transforms are more difficult to describe than
those for functionalization, since any number of changes to nc, nH, and no are possible.
However, we can still apply basic constraints related to the allowable number of carbons and
functional groups to narrow the range of potential precursors. The same statistical method
described for fragmentation reactions can be used for oligomerization to constrain the
precursor molecules. Here, we require the nc of the precursor to be smaller than that of the
target, and we assume that the precursor molecules have the same functional group
distribution as the target, giving the following constraints: ncp < nc,t , n-OH,p
nc,p(n-OH,t
/nc,t), and n=o p = nc,p(n=o,t /nc,t).
As with fragmentation, we can expand the range of precursors further by allowing the
functional groups to be unevenly distributed between the two precursors. This allows one of
the oligomerization reactants to be more functionalized that the other. While a large number
of different
oligomerization transformations
32
are possible, we
assume that for any
oligomerization reaction each precursor must have at least one functional group. We also
allow the total oxygen and hydrogen content to change somewhat by the gain or loss of a
water molecule, or by the conversion of a carbonyl to an alcohol (as in a hemiacetal
formation). These assumptions, plus the requirement that the precursor nc be lower than the
target nc, yield the following simple rules for the number of oxygen atoms and carbonyl and
hydroxyl groups: no,p
no,t, n=o,p
(n=o,t + 1) , and n-OH,p
(n-OH,t +
1). It should be noted
that if ester or ether linkages are formed during the oligomerization reaction, they will
introduce an error in the c* calculation. However, these effects are likely to be relatively
small since the change in c* is governed mostly by changes to the carbon number, rather than
by interconversions of functional groups.
2.3 Results
2.3.1 LV-OOA Target.
Table 2-1. HR-ToF-AMS data from measurements at 10 field locations. Elemental ratios are
reported for the average of LV-OOA measurements at each site and are used to calculate
each average LV-OOA OSc (-20/C - H/C).
Measurement Location
O/C
H/C
OSc
Reference
Riverside, CA (2005)
Mexico City Aircraft (2006)
Mexico City Ground (2006)
Kaiping, China (2008)
Barcelona, Spain (2009)
Paris, France (2009)
New York City, NY (2009)
Hong Kong, China (2009)
Shanghai, China (2010)
Sacramento, CA (2010)
Heshan, China (2010)
0.72
1.02
0.84
0.64
0.75
0.73
0.63
0.59
0.65
0.54
0.55
1.27
1.12
1.21
1.30
1.18
1.33
1.29
1.26
1.49
1.32
1.30
0.17
0.92
0.47
-0.02
0.32
0.13
-0.03
-0.08
-0.19
-0.24
-0.20
(Docherty et al., 2011)
(DeCarlo et al., 2010)
(Aiken et al., 2008)
(Huang et al., 2011)
(Mohr et al., 2011)
(Crippa et al., 2013)
(Sun et al., 2011)
(He et al., 2011)
(Huang et al., 2012)
(Setyan et al., 2012)
(Gong et al., 2012)
Properties of the highly oxidized OA target are determined from HR-ToF-AMS field
measurements. Table 2-1 summarizes AMS elemental ratios for studies that report H/C and
33
1.8-
Figure 2-2. Van Krevelen diagram
the reported elemental ra-
v Lshowing
(corrected)
1.6-
tios of LV-OOA from HR-AMS
field campaigns (see Table 2-1) in
the red circles and elemental ratios
o
after
4-
o
1.2
(reported)
1.0-
0.8-
I
I
0.0
0.2
I
0.4
recommended
and H/C from all measurements,
rdiictn
with the red ellipse indicating the
covariance of the data within 1.96
standard deviations of the uncorrected mean value. The blue star and
ellipse are for an average of all the
corrected measurements.
avg LV-OOAwih
2 Mexico City (aircraft)
3 Mexico City (ground)
4 Kaiping, China
the
red star is the average reported O/C
(
1 Riverside, CA
applying
correction of 1.3 and 1.1 to O/C and
H/C, respectively (Canagaratna et
al., 2014) in the blue circles. The
5 Barcelona, Spain
6 Paris, France
7 New York City, NY
8 Hong Kong, China
9 Shanghai, China
10 Sacramento, CA
11 Heshan, China
I
I
I
I
I
0.6
0.8
1.0
1.2
1.4
O/C
O/C for the LV-OOA PMF factor (or, more generally, the most oxidized of multiple OOA
factors, which includes factors such as OOA-I (Aiken et al., 2008) or MO-OOA (Setyan et
al., 2012)). Figure 2-2 shows the location of these as red circles in van Krevelen space. The
degree of oxidation varies widely from study to study, indicating that LV-OOA is not one
class of compounds but rather an aged form of SOA with a composition that varies as a
function of location, emissions profile, and/or oxidant exposure. For example, LV-OOA from
the MILAGRO field campaign in the Mexico City Metropolitan Area (MCMA) (Aiken et al.,
2008; DeCarlo et al., 2010) is substantially more oxidized than that measured at the other
field sites. Nonetheless, we use the average values of LV-OOA H/C and O/C (1.28 and 0.696
respectively) from these studies as our generic highly-oxidized OA target. This is shown as a
red star in Figure 2-2. The ellipse represents the covariance of the data within 1.96 standard
deviations of the uncorrected mean value. AMS elemental ratios for ambient organic aerosol
are typically corrected using empirical factors to account for biases in ion fragmentation
34
(Aiken et al., 2008). Recent calibration studies indicate that the recommended factors result
in an underestimation of O/C and H/C values for a range of multi-functional oxidized organic
standards, indicating that AMS elemental ratios reported for ambient environments will need
to be modified by new correction factors (effectively 1.3 for O/C and 1.1 for H/C,
Canagaratna et al., 2014). The average ratios calculated after using these new proposed
correction factors are shown as blue circles in Figure 2-2.
Saturation vapor concentration of the aerosol, as measured with thermodenuders, has
to our knowledge been reported for only a limited number of field studies (Huffman et al.,
2009), and values for c* have been determined only for the MCMA ground-based data
(Cappa and Jimenez, 2010). The decrease in LV-OOA upon heating in the thermodenuder is
consistent with a range of c* values between 10-7 and 10-1 pg m-3 (Cappa and Jimenez, 2010).
However, since the LV-OOA measured in MCMA appears not to be representative of LVOOA everywhere (Figure 2-2), these c* ranges may not be widely applicable to other
locations. On the other hand, the elemental composition of Riverside LV-OOA is close to
that of the overall LV-OOA average (Figure 2-2), suggesting its c*, which is substantially
higher than that of LV-OOA in MCMA (Huffman et al., 2009), may be more representative
of typical LV-OOA. We choose c* in the range of 10-5 to 10-1 jg m-3 as reasonable values
based on the reported c* distribution (Cappa and Jimenez, 2010). From these c* values,
carbon numbers can be calculated (Eq. 2-5): for loglo(c*) of -1, -3, and -5, the corresponding
chemical formulas are C 9.2H .806.4, C 1 0 5 H 13 4 0 7 3, and C11 8 HI 5s108.2, respectively, for the
reported average, and C 6.8 H 9 .506
1,
C 7.7 Hi 0 9 0 7 , and C8 .7 H 12 .2079, respectively,
for the
corrected average (Canagaratna et al., 2014). These formulas are not meant to define exact
chemical species or specific molecules, but instead represent averages of all the compounds
35
in LV-OOA.
The sources of uncertainty considered in evaluating the average number of carbon
atoms in the LV-OOA mixture arise from estimated errors in the elemental analysis
calculation, the distribution of vapor pressures of the compounds within the mixture, and
variation in the elemental ratios measured in the different field studies. The measurement
error arising from elemental analysis is estimated to be
30% of the reported O/C and +10%
for H/C (Aiken et al., 2007; 2008). Because these errors were reported for individual
compounds, we expect the measurement error of a large mixture of compounds to be
substantially smaller, so that the values given represent the most conservative case. The
uncertainty in c* for the average of all OOA components is taken to be 10-3 2 pLg m 3 , as
approximated from the distribution of values reported (Cappa and Jimenez, 2010). The
variance in elemental ratios for the mixtures is simply the variance of the data presented in
Table 2-1, multiplied by 1.962 in order to reflect the range in which 95% of the data is
expected to fall. These errors are propagated forward using the sensitivity of nc to O/C, H/C,
and loglo(c*) as described by Eq. 2-5, to give three separate values of the uncertainty in the
carbon number, 6nc. The contributions from elemental analysis measurement error, variation
in c*, and variance are 3.3, 1.3, and 3.8, respectively. An overall uncertainty,
6
nc,tot, is
calculated as the square root of sum of squares of each separate uncertainty, to yield a final
estimate of 10.5
5.2 for the LV-OOA carbon number, with the greatest source of
uncertainty being the variance in elemental ratios reported for each field study discussed
here.
Figure 2-3 (OSc vs. nc) shows the location of the different LV-OOA measurements in
carbon oxidation state vs. carbon number space (Kroll et al., 2011). Also shown is the
36
+4-
+3U)
0
CID
+2-
C/)
-
+1
*0
C
0
-eCa
-1
C) -2-
0)
-
-3-
I
I
16
15
I
14
I
13
12
11
I
10
I
9
8
I
I
7
6
I
5
I
4
I
3
I
2
1
Number of carbon atoms (nc)
Figure 2-3. Locations of the different LV-OOA measurements in OSc-nc space. Grey
circles indicate the possible combinations of average carbon oxidation state and carbon
number (Kroll et al., 2011). Numbered points denote LV-OOA from all ten field
campaigns (numbers are the same as in Figure 2-2); OSc is determined from elemental
ratios, and nc from Eq. 2.5, using a c& of 10-3 tg m-3. The red star shows the average
values for all the measurements, with the red ellipse depicting the uncertainty in its nc and
OSc values (see text for details). The blue star shows the position of the average LV-OOA
after applying the recommended correction of 1.3 and 1.1 to O/C and H/C, respectively
(Canagaratna et al., 2014), with the blue ellipse depicting uncertainty in the corrected nc
and OSc values.
average LV-OOA target, with the ellipse depicting the uncertainty associated with its nc and
OSc values. The rotated ellipse arises from the covariance between the outputs and reflects
the fact that more highly oxidized compounds require a smaller carbon number for a given
value of c*. The carbon numbers determined here (nc ~6-15) are slightly larger than those
estimated by Donahue et al. (nc -5-10) using a similar technique and the same general c*
range (Donahue et al., 2012). This difference in part results from the lower value of OSc that
we use, and our direct determination of the abundance of hydroxyl and carbonyl functional
groups individually, rather than assuming the two are equally abundant.
37
Functionalization.
Taking the LV-OOA target determined using c* = 10-3 pg m-3 (CIO. 5 H 1 3 .40 7. 3 ), and
assuming it was formed via functionalization reactions only (with changes to H/C and O/C
but not carbon number), allows us to work backwards toward precursors in van Krevelen
space, using the simple rules described in the Methodology section. Figure 2-4 shows the
potential formation pathways and precursors, but is different than most other van Krevelen
plots used to represent atmospheric OA since it includes gas-phase as well as particulate
organics. The black star denotes the target, and the black circles depict the possible
transforms associated with functionalization (as depicted in Figure 2-1), with each point
corresponding to the loss of an integer number of functional groups. Figure 2-4a shows the
fully saturated case in which functional groups replace C-H bonds only, and Figure 2-4b
shows the unsaturated case in which they replace only C=C bonds. Figures 2-4c and 2-4d
show the corrected values and are discusses at the end of this section.
The parallelograms represent the range of possible transforms and precursors, and are
bounded by the requirement that the target cannot lose more hydroxyl or carbonyl groups
than it has (so that the dimensions of the parallelogram are given by the number of hydroxyl
and carbonyl groups). The left corner of the parallelogram (y-intercept in the van Krevelen
diagram) corresponds to the fully de-functionalized hydrocarbon precursor. For the saturated
case, this gives a precursor with a formula consistent with an acyclic alkane. For the
unsaturated case, it gives an alkene containing no/2 C=C bonds (or 3.7 for our target). This
corresponds to a precursor of formula C10 5 H15 7 , which interestingly is similar to that of
monoterpenes, a well-studied class of SOA precursors. These two limiting cases bound the
H/C of the hydrocarbon precursor to between 1.5 and 2.2.
38
2.2
-
2.2
2.0-
2.0
-1 .8
-
1.8
0
6
1.6
U
0
,O
1 .4
1 .2
1 .0
(a)
08
0.0
(b)
0.2
0.4
O/C
0.6
0.8
1.0
0.0
2.2-
2.2-
2.0-
2.0 -f
1.8-
1.8-
0.2
0.4
0.2
0.4
O/C
0.6
0.8
1.0
0.6
0.8
1.0
0
C-,
1.6-
U
1.6-
T
1.4-
1.4'*~
a
1.2-
1.0-
1.0
0.0
(cV'
'
0,1
0.2
0.4
0.6
'
|
'
0. 8
-
1.2-
0.8
1.0
U.0-t0.0
O/C
O/C
Figure 2-4. Functionalization transforms available to the LV-OOA target (black star). For
the uncorrected O/C and H/C values, these are calculated for nc = 10.5 and c* = 10- 3 Ig m-3,
for the fully saturated case (panel a) and the fully unsaturated case (panel b). For the
corrected O/C and H/C values (multiplied by 1.3 and 1.1, respectively), these are calculated
for nc = 7.7 and c* = 10- pig m-3 , for the fully saturated case (panel c) and the fully
unsaturated case (panel d). Potential oxidative transforms and precursors are bounded by the
parallelograms, whose dimensions are defined by the number of hydroxyl and carbonyl
functional groups in the target. Each black circle depicts an intermediate precursor, formed
from the removal of an integer number of functional groups; the leftmost corner of each
parallelogram (y-intercept) gives the limiting H/C value of the unfunctionalized hydrocarbon
precursor. OSc contours are shown as dashed lines, and shading indicates the predominant
phase of the species: gas phase (white, c& > 10 jig m-3), semi-volatile (light grey, c* 0.1 to 10
pg m-3), or condensed-phase (dark grey, c* < 0.1 pig m-3). Multiple functional groups
(including at least one hydroxyl group) must be lost for the immediate precursor of the target
to be in the gas phase. Nonoxidative processes (e.g. hydrolysis) may also be important in
SOA formation; the open circles indicate immediate hydrolysis precursors.
39
The areas defined by the parallelograms in Figure 2-4, and movement within them,
are not necessarily tied to specific chemistry, but rather describe the range of possible
precursors. The dashed lines are carbon oxidation state contours (OSc z 20/C-H/C). From
these, it can be seen that replacing C-H bonds with functional groups (Figure 2-4a) moves
more directly along the OSc gradient than replacing C=C bonds (Figure 2-4b) does, and
results in a larger increase in OSc per functional group. At the same time, the oxidation of
C=C bonds usually involves the addition of two functional groups per oxidation step. It
should be noted that the areas bounded by the parallelograms are for oxidation reactions
only; non-oxidative transformations (involving movement along lines of constant OSc) could
lead to a wider range of possible precursors. One example is the dehydration of the target
molecule (open circles in Figure 2-4a and 2-4b), whose corresponding forward reaction is the
hydrolysis of a carbonyl or epoxide group to form two hydroxyl groups, believed to be an
important reaction in the formation of isoprene SOA (Paulot et al., 2009).
The shading in Figure 2-4 indicates the saturation vapor concentration (and therefore
predominant phase) as a function of location in van Krevelen space: gas-phase (white, c* >
10 pg m-3), intermediate (light grey, 0.1 < c* < 10 pg m-3), or particle-phase (dark grey, c* <
0.1 ptg m 3 ). This is similar to the work of Kessler et al. in which c* cutoffs were calculated
as a function of carbon number (Kessler et al., 2010; 2012). While the exact cutoffs will vary
with organic aerosol loading, these are reasonable values for most ambient conditions.
Values for c* were calculated using Eq. 2.4, though for Figure 2-4b it was modified to allow
for one DBE to be from a C=C bond:
log,0 c*= log10 a+bo +bcnc +bo(n (l - - HC))+bOH (
OCo+H/C - 1))
(2.6)
While the timescales of formation of highly oxidized OA are highly uncertain, there is
40
evidence (based on Mexico City data) that the process is relatively fast, on the order of a day
(Jimenez et al., 2009; Volkamer et al., 2006). This is faster than the rate of heterogeneous
oxidation (which occurs on the order of several days, George and Abbatt, 2010; Kessler et
al., 2010; 2012; Robinson et al., 2006) suggesting that the immediate precursor to highly
oxidized OA is likely not in the particle phase. The production of products with low c* by
gas-phase oxidation leads to a "trapping" in the particle phase where the rate of oxidation
slows dramatically (Hearn and Smith, 2006; Weitkamp et al., 2008). Thus the rapid
formation of highly oxidized OA via functionalization reactions would seem to require gasphase precursors. Semi-volatile compounds (light grey area in Figure 2-4) may serve as gasphase precursors, but their oxidation reactions tend to be slow, since they spend some
fraction of their time in the particle phase; in addition they are highly susceptible to
depositional losses to other environmental surfaces. Thus, in the absence of fast particle
phase oxidation reactions, the most likely immediate gas-phase precursors to highly oxidized
aerosol are those that are fully in the gas phase; as shown in Figure 2-4, this requires the
precursor to have substantially fewer functional groups than the target (at least two fewer
hydroxyl groups or three fewer mixed hydroxyl/carbonyl groups). Known routes for the
addition of multiple functional groups in a single oxidation step include the oxidation of
carbon-carbon double bonds or oxidative ring-opening reactions, both of which add at least
two (and possibly more) functional groups to the molecule. In addition, some alkoxy
isomerization reactions can add three or more functional groups at a time (Ziemann and
Atkinson, 2012); plus there is evidence for other, poorly-understood mechanisms that involve
the rapid addition of multiple functional groups (Ehn et al., 2012).
The above discussion assumes that the formation of low-volatility organics is rapid
41
compared to the rate of heterogeneous oxidation, and also that gas-particle partitioning is
essentially instantaneous. In general, the formation kinetics of the most oxidized fraction of
OA are not well constrained; if they are substantially slower in most environments than in
MCMA, heterogeneous oxidation could play an important role, allowing the immediate
oxidative precursors to be in the condensed phase. This uncertainty underscores the need for
additional ambient studies of OA at a variety of photochemical ages. Additionally, recent
studies of partitioning kinetics suggest that condensable organics may remain in the gas
phase longer than predicted by assumptions of thermodynamic equilibrium (Shiraiwa and
Seinfeld, 2012). This could potentially allow for additional gas-phase oxidation even when
the precursors have volatilities below the semi-volatile/condensed-phase cutoff. On the other
hand, several experimental studies suggest a mass transfer limitation to the evaporation of
particulate organics (Grieshop et al., 2007; Perraud et al., 2012; Vaden et al., 2011); this
would have the opposite effect, enhancing the "trapping" effect and causing the semi-volatile
precursors to behave more like the least-volatile species.
The potential formation pathways of highly oxidized aerosol also depend on our
knowledge of the physicochemical properties (elemental ratios and c*), which can be
uncertain. While the c* chosen for our target has a relatively minor effect on calculated
carbon number, it strongly influences the potential formation processes, most importantly by
affecting the phase of the precursors. Decreasing the c* of the target (for example, from 1 0-3
to 10-
Ig m-3, increasing the calculated nc from 10.5 to 11.8), increases the number of
functional groups that must be removed in order for the immediate precursor to be in the gas
phase. Therefore targets with low c* are less likely to be formed from functionalization
-
reactions, whereas targets with higher c* can be easily formed from reactions that add only I
42
2 functional groups.
The elemental ratios used for the target in Figure 2-4a and 2-4b represent the average
from a number of studies (Table 2-1), yielding an average chemical formula of C10 .5H 13 4 0 7. 3
(with 2.5 hydroxyl groups and 4.8 carbonyl groups). If we use the corrected values for the
elemental ratios (Canagaratna et al., 2014) (blue star in Figure 2-2), but the same c* as in the
base case (10-3 pg m-3), we obtain a target with a chemical formula of C7 . 7 H 10 .90
7
(3.7
hydroxyl groups and 3.3 carbonyl groups, Figure 2-4c and 2-4d). The higher n-OH/n=O ratio
implies a lower carbon number at a given c*, since hydroxyl groups have a greater effect on
c* than do carbonyl groups. While this has little effect on the number of functional groups
separating the target and a gas-phase precursor (since the separation is governed primarily by
the c* of the target), it can affect the overall pathways connecting the oxidized aerosol to
hydrocarbon precursors; additional oxidation steps will be required to form a target with a
higher number of functional groups.
Fragmentation reactions.
In addition to the chemical constraints listed in the Methodology section, the phase of
the fragmentation precursor also limits the possible transforms associated with this reaction.
As in the functionalization case, we require the precursor to be in the gas phase (c* > 0.1 pig
m- 3) since heterogeneous oxidation is assumed to be too slow to account for the rate of
formation of highly oxidized aerosol.
The overall, average effect of fragmentation reactions on c* is illustrated in Figure 25, which shows the contribution of a single carbon atom (and its associated functional
groups) to the c* of a molecule of nc = 10.5. (Results are only weakly dependent on carbon
number above nc = 7.) This is calculated from Eq. 2.4, assuming that the functional groups
43
-N
6
6A
k5
'
2.2
2.0
1.8
U-
1.6avg LV-OOA
-1.4
1.4-
*
1.2-
MCMA LV-OOA
-1.7
0.0
-
1.00.8 0
X
0
0.2
0.4
0.6
0.8
1.0
O/C
Figure 2-5. Contour plot showing the influence of a single carbon atom in a molecule,
including its associated functional groups, on the molecule's saturation vapor concentration
(D(logio c*)/anc), as calculated from Eq. 2.4. These are calculated for nc = 10.5, but do not
vary significantly with carbon number above nc = 7. Instead H/C and O/C have the strongest
influence on "per-carbon c*", which is greatest when both ratios are high (top right corner,
when hydroxyl groups are abundant) (Kessler et al, 2010; 2012). The discontinuities near the
left of the figure reflect the fact that Eq. 2.4 does not always hold at low 0/C, where
unsaturation arises from C=C double bonds or rings rather than carbonyl groups; thus when
DBE > no, Eq. 2.4 is modified to allow for C=C double bonds. LV-OOA targets used in this
study are denoted by markers.
are evenly distributed throughout the molecule (i.e., the number of groups on the carbon
atom is equal to the number of groups on the entire molecule divided by nc). For our target
(black star in Figure 2-5), each carbon atom decreases the vapor pressure by 1.4 orders of
magnitude. This is a full order of magnitude larger than the effect of a non-functionalized
carbon (e.g., a CH 2 group). This simple view of "per-carbon c*" helps guide assessments of
the viability of fragmentation reactions as steps in the formation of highly oxidized organic
44
aerosol; however, as discussed in the Methodology section, it does not accurately represent
all fragmentation reactions, since functional groups generally are not evenly distributed
throughout a given molecule. Here we consider potential fragmentation precursors both with
and without evenly-distributed functional groups.
Assuming the functional groups are distributed evenly, fragmentation of a gas-phase
molecule cannot form highly oxidized, low-volatility OA. This is because the decrease in c*
from the additional functional groups (in this case, one hydroxyl group and one carbonyl
group, lowering c* by 3.2 orders of magnitude) is largely if not completely offset by the c*
increase from the carbon loss (1.4 orders of magnitude per carbon atom). As a result, the
large decrease in c* needed to form a low-volatility target (2 orders of magnitude) is not
possible, a conclusion broadly consistent with the findings of Cappa and Wilson (2012).
However, when functional groups are not evenly distributed, and reduced carbon can be lost
during fragmentation, the formation of the target via fragmentation is possible in some cases.
The carbon numbers and elemental ratios of viable precursors are depicted in Figure 2-6
(solid-shaded areas). The allowed precursors are only slightly larger than the target, and are
significantly less oxidized, indicating that fragmentation can produce highly oxidized organic
aerosol only when the precursor loses a small (CI or C 2) and essentially unfunctionalized
fragment. (This exact requirement depends on the identity and number of functional groups
added to the fragment). However, molecules that are more volatile than our target are more
likely to form via fragmentation, since they may be more easily formed from gas-phase
precursors. Similarly, less-oxidized species can also be formed from fragmentation reactions,
since the increase in c* from losing carbon atoms is not as great (top left corner of Figure 25). Thus, while fragmentation reactions probably do not lead to the immediate formation of
45
highly oxidized OA, they might occur as part of the overall reaction mechanism, primarily in
the earlier oxidation steps.
2.2-
+1
+2
+3
oligo merization
+4
+5
2.0-
+6
1.8.-
1 .6U~
2
1 .4-
fragmentati on
1.21.0-
0.8 I.I
0.0
0.2
I
0.4
I
0.6
I
0.8
I
1.0
I
1.2
1.4
O/C
Figure 2-6. Ranges of possible precursors of fragmentation (solid-shaded areas) and
oligomerization (lightly-shaded areas) reactions that can form the LV-OOA target (black
star). Three-dimensional chemical space is depicted by showing the range in van Krevelen
space available to precursors of different carbon numbers. The numbers correspond to the
carbon number change of the reaction; for example "-2" refers to a fragmentation reaction in
which the target has two fewer carbon atoms than the precursor. The target can be formed via
fragmentation of a gas-phase precursor only if a small (CI-C 2 ) reduced fragment is lost;
oligomerization is a more viable channel, though it requires the precursors to be low in c
and thus extremely oxidized. In both cases, the range of possible precursors is considerably
narrower if it is assumed that functional groups are evenly distributed on the target and
precursor(s).
Oligomerization reactions.
Because oligomerization reactions decrease c*, they are generally more viable steps in
the formation of oxidized OA components than fragmentation reactions. As with
fragmentation reactions, the phase of the precursor introduces constraints beyond those
described in the Methodology section: since the probability of two gas-phase precursors
reacting is quite low under most conditions, at least one precursor must be present in the
46
particle phase (c* < 10 pg m-3).
Since oligomerization reactions involve changes to nc, their effects can be visualized
similarly to the effects
of fragmentation
reactions, by treating all carbon as evenly
functionalized (Figure 2-5). Assuming that functional groups are evenly-distributed in both
the precursors and the targets, the condensed-phase precursor can be smaller than the target
by one or two carbon atoms. Precursors with even fewer carbon atoms will be so volatile that
they will be present in the gas phase only, and therefore cannot participate in oligomerization
reactions. However, a wider range of precursors is possible if the functional groups are not
evenly distributed, and instead the two precursor molecules have different levels of
functionalization. The range of possible oligomerization precursors are shown as lightlyshaded regions in Figure 2-6; lower carbon numbers are possible when the majority of the
functional groups are on the condensed-phase precursor. Thus oxidized organic aerosol could
form from
the oligomerization of smaller, highly-oxidized
molecular species,
which
themselves may be formed by previous functionalization or fragmentation reactions. This is
broadly consistent with the view of formation of SOA from aqueous chemistry (Ervens et al.,
2011); even though we do not explicitly model partitioning into the aqueous phase, this work
provides some constraints into the chemical nature (H/C and O/C ratios) of these smaller
species. On the other hand, the target itself (of formula C10 .5 H 13.40 7.3 ) is not consistent with
the oligomeric species measured in SOA, such as those from monoterpene oxidation (which
typically have twenty or more carbon atoms and relatively low OSc values, Tolocka et al.,
2004), isoprene photooxidation, or glyoxal oligomerization (which typically have higher H/C
values, Chhabra et al., 2011). Thus, even though these oligomerization reactions can occur
under atmospheric conditions, they do not appear to dominate formation of the most oxidized
47
fraction of the aerosol.
MCMA Case Study.
-
2.2
2.0 -
e
e
'
1.8 -
1.4-
1.2-
0.8
0.0
0.2
0.6
0.4
0.8
1.0
O/C
Figure 2-7. Functionalization transforms available to the LV-OOA target from the
MILAGRO ground site, 3 1 ,3 7 using nc = 10 and c* = 10-5 [g m~ 3 . (These are different than the
values in Figures 2-3 and 2-4 which used c* of 10-3 pg m~ 3 .) This plot is similar to Figure 2-4,
except that the saturated and unsaturated cases are shown together, as black and grey
parallelograms, respectively (the c* cutoffs are given for the saturated case only). Because of
the exceedingly low c* of these organics (Cappa and Jimenez, 2010; Huang et al., 2011),
their formation from gas-phase precursors can occur only if a large number of functional
groups (three or more) are added to the precursor in a single oxidation step.
Although LV-OOA in Mexico City is significantly more oxidized than that measured
in other locations (Table 2-1 and Figure 2-2), it has received substantial study and to our
knowledge is the only time LV-OOA c& and elemental ratios have both been reported. Here,
we examine LV-OOA at the TO ground site at MILAGRO (Aiken et al., 2008; Huffman et
al., 2009) as a case study, in order to gain insight into potential formation pathways for this
exceedingly highly oxidized organic aerosol. Using a c* of 10-5 2 pg m-3 gives a target carbon
48
number estimate of 10.0
3.4 with the chemical formula CoH 12 10 8 .4 , (3.4 hydroxyl groups
and 5 carbonyl groups). The sources of uncertainty studied in this case for the carbon number
are the error in computing elemental ratios and the deviations in c*, which give values of 8nc
of 3.2 and 1.1, respectively. As with the average LV-OOA, the overall error is computed
from the sum of the square of the errors. The potential functionalization precursors are
shown in Figure 2-7 for the saturated and unsaturated cases. Because the target in this case is
so low in c*, the functionalization transforms involve the loss of at least two hydroxyl
groups, or all five carbonyl groups, to form precursors that are semi-volatile, and even more
for the precursor to be present exclusively in the gas phase. This suggests that gas-phase
functionalization reactions would have to be exceedingly rapid to lead to the formation of
organic material that is so high in oxidation state and low in c*. Similarly, this class of
organics cannot be formed directly from fragmentation reactions (even if the loss of small,
reduced fragments are allowed). Again, some fragmentation reactions might occur earlier in
the overall oxidation mechanism, but they do not play a role in the last step that forms this
target. As in our average LV-OOA target (Figure 2-6), oligomerization could be a viable step
in the formation of these compounds, provided highly oxidized organic precursor species are
present in the condensed phase.
2.4 Conclusions
We have presented a new general approach for assessing the potential pathways and
precursors involved in the formation of highly oxidized organic aerosol, which involves
starting from the product or "target" and working backwards. We define this target based on
thermodenuder measurements of c* and average elemental ratios of LV-OOA as measured by
49
the AMS, yielding an average approximate chemical formula of C 10 5 H 13 .4 0 7.3 . The utility of
our retrosynthetic method for constraining its formation chemistry has been demonstrated for
this average LV-OOA target, as well as an exceedingly oxidized case, LV-OOA measured at
the MILAGRO ground site. Our results suggest that the most probable LV-OOA-forming
reactions are (1) functionalization reactions that add multiple functional groups (including at
least one hydroxyl group) at a time, (2) oligomerization reactions of smaller, highly oxidized
precursors, and (3) fast oxidation reactions within the condensed phase, such as aging within
aqueous particles. While this third pathway was not explicitly modeled in the present study,
the first two pathways were determined based on conditions in which fast particle phase
oxidation reactions are not allowed. Therefore, allowing for rapid aging within particles
reduces the constraints on the system, making it a viable alternative pathway. Fragmentation
reactions are only possible as the final step if they involve the loss of a small number of
relatively reduced carbon atoms. While the specific aim of this study was to apply this
technique to LV-OOA, this approach could be useful for understanding a wider range of
targets, including other fractions of the aerosol (e.g., SV-OOA, Jimenez et al., 2009), as well
as SOA generated in laboratory studies. However, this requires some information about c*
and/or carbon number of the aerosol of interest.
In the present work, we limited the partitioning of the organic species to only two
phases, the gas phase and the condensed-organic phase. Recent work has suggested the
importance of the liquid-water phase as a medium for oxidation reactions (Ervens et al.,
2011), which could also be included in this treatment. This could be done using existing
structure activity relationships (e.g. Hine and Mookerjee, 1975) to estimate saturation vapor
concentrations of organics over water (Henry's law constant) from chemical properties
50
(elemental ratios, functional group distributions, etc.). Otherwise the general approach,
including the transforms that describe the chemical reactions, is the same as described in this
work.
This retrosynthetic approach relies critically on the accurate characterization of the
target molecule and key characteristics of its formation. This present work highlights the
need for accurate measurements of the elemental ratios and c* of organic aerosol. Field
measurements of c* are limited, hindering accurate estimates of carbon number. If actual c*
values are higher than those used here, then functionalization pathways would become more
feasible since they would require the addition of a smaller number of functional groups to
move from a gas-phase precursor to the target. Conversely, ambient organic products with
lower c* values will be harder to form via functionalization reactions as they will require a
larger decrease in c*. Therefore, better constraints on c* of the oxidized aerosol could
significantly improve the accuracy of our target carbon number. There is also a need for
better constraints on the kinetics of the formation of highly oxidized aerosol. This has been
measured only in the outflow from MCMA, which might not be widely representative. This
rate is a key constraint on the formation pathway, as it limits the number of oxidation steps
that are possible as well as the phase in which those reactions can happen. Additionally,
characterization of highly-oxidized OA using techniques other than AMS would be useful.
The direct measurement of functional group distributions (e.g., using FTIR, Maria et al.,
2002), can provide an alternative, and potentially more comprehensive, description of the
relationships between chemical structure, saturation vapor concentration, and formation
chemistry. Similarly, direct measurements of nc (e.g., using UHR-ESI-MS) would improve
the accuracy of the target chemical formula and could provide more detailed information on
51
the formation pathways of a wide range of molecular compounds.
We have made an effort to demonstrate the utility of this retrosynthetic approach for
exploring the possible pathways by which highly oxidized aerosol is formed. By starting at
the known endpoint/target, we improve our ability to identify pathways that can actually
result in the formation of low-volatility, highly-oxidized organic compounds. This is in
contrast to standard forward approaches that require some guesswork as to the starting
molecules and pathways that may result in the formation of the target. This general approach
could be particularly powerful when applied to rigorous models of oxidation mechanisms,
gas-particle partitioning, and aerosol chemistry (Aumont et al., 2012; Cappa and Wilson,
2012; Donahue et al., 2012; Zhang and Seinfeld, 2012). Running such models backwards
from well-defined targets may help identify key classes (or even structures) of organic
compounds, as well as reaction conditions, that are most likely to lead to the formation of
this important class of organic aerosol.
2.5 Acknowledgements
This work was supported by the National Science Foundation, under grant numbers
CHE-1012809 and AGS-1056225. We thank Manjula Canagaratna for sharing new AMS
elemental analysis correction factors.
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58
Chapter 3
Laboratory studies of the aqueous-phase
oxidation of polyols: submicron particles vs.
bulk aqueous solution
Adaptedfrom: Daumit, K. E., Carrasquillo, A. J., Hunter, J. F., and Kroll, J. H.: Laboratory
studies of the aqueous-phase oxidation of polyols: submicron particles vs. bulk aqueous solution,
Atmos. Chem. Phys., 14, 10773-10784, doi: 10.5194/acp-14-10773-2014, 2014.
http://www.atmos-chem-phys.net/1 4/10773/2014/acp- 14-10773-2014.pdf
3.1 Introduction
As explained in Chapter 1, the sources and formation mechanisms for the most oxidized
fraction of OA remain highly uncertain. In Chapter 2, we showed that traditional gas-phase
oxidation pathways are not able to fully describe the production of this material and therefore
non-traditional pathways (e.g. oxidation within aqueous particles) may be important. Aqueousphase reactions in atmospheric water droplets have been known to play a role in the fate of
chemicals in the atmosphere for many years (Faust, 1994); more recently, these reactions have
received attention as a possible mechanism for the production and evolution of organic aerosol
(Ervens et al., 2011). As described in Chapter 1, a number of laboratory studies have investigated
oxidation of organic compounds in the aqueous phase and shown that it can lead to the rapid
formation of highly oxidized organic products.
The importance of such oxidation processes relies critically on partitioning (Donahue et
al., 2014): a compound will undergo aqueous-phase oxidation only if it is partitioned to the
59
atmospheric aqueous phase (cloud droplets or aqueous submicron particles), rather than the gas
or condensed-organic phase. The partitioning between the gas and aqueous phases is determined
by a compound's effective Henry's law constant (I)
(or alternatively its saturation vapor
concentration over water (c*aq) (Ervens et al., 2011)), as well the liquid water content (LWC) of
the air mass, as shown in Eq. 3.1:
(LWC)H*RT
1012 +(LWC)H*RT
aq
LWC/cq
l+LWC/c*
()
in which faq is the equilibrium fraction in the aqueous phase, LWC is in units of pg m-3, I
is in
units of M atm-1, and c*aq is in units of pg m-3 and is equal to 1012 H*- R1 T' (where R = 0.0821
L atm mol 1 K' and T is temperature in K). By Eq. 3.1, the partitioning of a single compound
(with a fixed H* or c*aq) can be very different for different values of LWC. Atmospheric LWCs
can span many orders of magnitude, ranging from -1
~I 0 to 106
gg
m-3 for cloud droplets (Ervens et al., 2011), while a bulk aqueous solution has a
C*aq
1013
(jg M-3
103
108
102
1 1
10
-
C.,
-
10
(0
C,.
10 -3
_
-
C,,
10
0
-
-4
10 -4
-
-6
10
I II I I I I I I I I I
10-5
10
10
7 7 7I T
10
H*(M atm 1
)
10
-
U
Figure 3-1. The fraction of a compound
that will partition to the aqueous phase
as a function of its Henry's law constant, H* (or alternatively its saturation
vapor concentration, c*aq), and the liquid
water content (LWC). Compounds with
0.1 M atm < H* < 109 M atm-1 will be
primarily in the aqueous phase at bulk
LWC (blue line), but primarily in the gas
phase at aqueous aerosol LWC (green
line). Cloud water (teal line) represents
the intermediate case, with more
partitioning into the aqueous phase than
for aqueous particles, but still with far
less than for a bulk aqueous solution.
(faq)
0
0~
to 100 pg m 3 for aqueous aerosol, and
60
LWC on the order of 1012 pg m 3 (the density of water). Therefore,
faq
for a given compound will
also vary by many orders of magnitude for these systems, as illustrated in Figure 3-1. Many
compounds that are considered to be "water-soluble" for bulk solutions (H* of 1-1000 M atm-1)
will not actually partition significantly into aqueous submicron particles, or even into cloud
droplets. This could have an important influence on the resulting chemistry, and suggests that a
bulk solution may not always accurately represent aqueous processing under atmospheric
conditions.
Some recent bulk-solution experiments have begun to examine the role of gas-particle
partitioning under lower-LWC conditions, by atomizing or nebulizing the bulk aqueous solution
as it undergoes aqueous oxidation (Lee et al., 201 Ia; 2011 b; 2012; Liu et al., 2012b; OrtizMontalvo et al., 2012; Zhao et al.., 2012). While these studies do indeed show loss of the most
volatile species formed, leaving behind only low-volatility condensed-phase products, the
chemistry that forms these products still takes place in the bulk aqueous solution, with limited
partitioning into the gas phase. To our knowledge, only two studies (Nguyen et al., 2013;
Volkamer et al., 2009) have examined oxidation chemistry within aqueous droplets themselves,
allowing for gas-particle partitioning that mimics what may occur in the atmosphere. These
studies found enhanced uptake and aerosol yield from glyoxal (Volkamer et al., 2009) and
glycolaldehyde (Nguyen et al., 2013) in the presence of aqueous submicron particles; results
from these experiments have not been explicitly compared to those in which oxidation was
carried out in bulk solutions.
Here we describe laboratory studies of the oxidation of water-soluble organic species in
the aqueous phase, with experiments conducted both within bulk aqueous solutions and within
submicron aqueous particles. The goal of these experiments is to compare the oxidation
61
chemistry under very different partitioning conditions. We focus on the oxidation of polyols with
the chemical formula CnH 2n+ 2 On (with one hydroxyl group on each carbon atom). Polyols with
four or more carbon atoms have exceedingly high values of H* (> 1016 M atm-' (Sander, 1999)),
ensuring they will be present in the aqueous phase even at the low LWC in our chamber; thus
any observed partitioning will involve reaction intermediates and products only. To our
knowledge, this work is the first direct comparison of aqueous oxidation in submicron particles
and in bulk aqueous solution.
3.2 Experimental Methods
Two sets of experiments are conducted: (1) bulk oxidation, in which reactions take place
within a bulk aqueous solution of~0.5 L volume, and (2) chamber oxidation, in which reactions
take place within submicron aqueous particles in an environmental chamber. Both sets of
experiments use dark Fenton chemistry (Fe2+ and hydrogen peroxide) as an aqueous-phase
source of oxidants. While there is some uncertainty as to the exact oxidant-forming mechanisms
in Fenton systems, in part due to the complex iron speciation, it is likely that hydroxyl radicals
(OH) are the predominant oxidant formed in our experiments (Ma et al., 2006; Southworth and
Voelker, 2003). Fe2+ reacts with hydrogen peroxide (H 2 0 2 ) to produce OH and Fe3, which is
subsequently converted back to Fe2+ via reaction with other species:
Fe3 + HO- + OH
Fe2++ H 2 0 2
4
Fe 3 + H 2 02
+Fe
Fe 3 ++HO2
2++H0 2
(R3.1)
+H
(R3.2)
Fe2++ 02+ H+
(R3.3)
Alternatively, Fe 3+ can form complexes with water or organic acids; these pathways compete
with the regeneration of Fe2+ and can eventually slow the production of OH:
62
Fe 3 + H20 4 Fe(OH)2+ + H+
(R3.4)
Fe'+ + L 4 Fe(L) 2+ (L = organic ligand)
(R3.5)
A more detailed treatment of this chemistry is described elsewhere (Arakaki and Faust, 1998;
Faust and Hoigne, 1990; Hoffmann et al., 1996; Ma et al., 2006; Nguyen et al., 2013; Ou et al.,
2008). In both sets of experiments, we start with a mixture of a polyol and Fe2+, and initiate the
oxidation chemistry with the addition of H202. The use of Fenton chemistry to initiate oxidation
within aqueous submicron particles was first demonstrated in a recent glycolaldehyde uptake
study by Nguyen et al. (2013); the present experiments differ from that work in that we focus on
dark Fenton chemistry, using the addition of H202 rather than exposure to UV lights to initiate
the reaction.
3.2.1 Bulk aqueous oxidation
clean
40 W blacklights (350 nm)
-F
air in
air in
id
water
40 W blacklights (350 nm)
settling
3
7.5 m Teflon
Environmental
Chamber
-
clean_
[1
clean
settling
chamber
(a)
Ll chamber
II
-
polyol
FeSO 4
j(H 202)
H
40 W blacklights (350 nm)
-03
-- HR-AMS
FeSO,
HR-AMS
SM PS
Monitor
Monitor
- NOX
SMPS
clean
air in
40 W blacklights (350 nm)
H202
(b)
Figure 3-2. Experimental setup for (a) bulk experiments and (b) chamber experiments. The
atomizer serves as a reactor for bulk oxidation but only as a source of aqueous particles for
chamber oxidation. In the bulk experiments, the solution is continuously atomized, with H 2 0 2
added after the polyol and FeSO 4 have been sampled for 1 hour. In the chamber experiments, a
solution containing the polyol and FeSO 4 is atomized into the chamber for 1 hour, with gasphase H 20 2 introduced later via the two-neck flask. Humidified make-up air is continuously
supplied to the chamber via the bubbler. While the focus of this work is dark Fenton chemistry,
UV lights are turned on following oxidation in both setups in order to assess the photo-reactivity
of the reaction products.
63
For bulk oxidation experiments, the general technique of Lee et al. (201 la) is used, in
which an atomizer serves both as the reactor containing the aqueous solution and as the method
for aerosolizing the solution, enabling online analysis by aerosol instrumentation. Figure 3-2a
shows a simple schematic of the experimental setup. Reactions are carried out in a I L
borosilicate reaction bottle, and the solution is continuously atomized using a constant output
atomizer (TSI, Model 3076) with a backing pressure of ~30 psi of pure compressed air (Aadco
737-13A/C with methane reactor), giving 2.5-3 LPM aerosol output flow. The atomizer output is
passed through an empty I L bottle to remove excess liquid water, then through a diffusion dryer,
and finally into the aerosol instruments, described in Section 3.2.3.
A major difference between the present experiments and previous bulk oxidation
experiments (Lee et al., 2011 a; 2011 b; 2012; Liu et al., 2012b; Ortiz-Montalvo et al., 2012; Zhao
et al., 2012) is the oxidation scheme: here OH radicals are generated via dark Fenton chemistry
rather than photolysis of hydrogen peroxide, avoiding the use of 254 nm lights. The initial
atomization solution contains iron(II) sulfate heptahydrate (99+%) and a polyol (glycerol,
99.5+%, erythritol, 99+%, adonitol 99+%, mannitol 98+%, or volemitol, all Sigma-Aldrich) fully
dissolved in Milli-Q water; concentrations are given in Table 3-1. Prior to H 20 2 addition, this
solution is atomized into the AMS (see below) to characterize the reactants and ensure they are
inert with respect to each other. To initiate Fenton chemistry, hydrogen peroxide (30% w/w in
water, Alfa Aesar) is added and the solution is shaken to facilitate mixing. After ~3 hours, once
the reaction appears to have gone to completion, the solution is exposed to UV light for
additional oxidation (via photo-Fenton chemistry) and/or direct photolysis, using four external
blacklights (Sylvania BL-350 ECO, output 300-400 nm). Photo-Fenton reactions regenerate Fe2
and can produce additional OH:
64
+
Fe3(OH-) + hv
Fe2+ + OH
(R3.6)
Fe3+(L-) + hv 4 Fe2++ L
(R3.7)
Table 3-1. Initial concentrations of reactants in the atomizer solutions.
Polyol Precursor
Bulk Experiments
Chamber Experiments
Name &
Chemical
[Polyol]
[FeSO 4]
[H 2 0 2]
[Polyol]a
[FeSO 4]a
[H 2 0 2 ]b
Formula
Structure
(mM)
(mM)
(mM)
(mM)
(mM)
(mM)
0.20
0.45
2.0
2.0
0.40
27
0.20
0.42
2.0
2.0
0.39
27
0.20
0.41
2.0
2.0
0.40
27
0.22
0.42
2.0
2.0
0.38
27
0.19
0.40
2.0
2.0
0.40
27
Glycerol
OH
(C 3 H 8 0 3 )
OH
HO
Erythritol
OH
OH
HO
(C 4 H 100 4 )
Adonitol
OH
OH
OH
(C 5H 1205 )
Mannitol
OH
OH
OH
HO
(C 6H 1406 )
Volemitol
OH
HO
OH
OH OH
OH OH
OH
HO
OH
(C 7HI 607 )
OH OH
a Initial concentrations in the atomizer solution; the aqueous
particles themselves will be
substantially more concentrated (by a factor of 1000).
b Estimated concentrations within the particles, calculated by assuming
full evaporation of H 20 2
into the chamber, with Henry's law partitioning into the aqueous particles; likely represent an
upper limit.
3.2.2 Chamber oxidation
Chamber experiments are conducted to allow for aqueous oxidation in submicron
particles, providing partitioning conditions representative of deliquesced atmospheric aerosol.
The chamber oxidation setup is shown in Figure 3-2b. The MIT chamber is a 7.5 m 3 Teflon (5
mil PFA) bag within a temperature-controlled environmental room. Two banks of 24 blacklights
(Sylvania BL-350 ECO) on opposite sides of the chamber provide UV irradiation when needed.
The chamber is run as a semi-batch reactor with 5 LPM pure air added to balance instrument
65
sample flows. A fraction of this air is sent through a bubbler containing Milli-Q water in order to
maintain a relative humidity of between 67% and 79%, ensuring that all sulfate-containing
particles are aqueous. Prior to reaction, seed particles composed of iron(II) sulfate (FeSO 4) and a
polyol are introduced into the chamber via atomization. The liquid water content in the chamber
is not directly measured, but is estimated to be in the range of ~20 to -150 pig m-3 prior to
oxidation, based on concentrations of particulate water measured by the AMS. These values are
broadly consistent with liquid water contents estimated by assuming the particles have a similar
hygroscopic growth factor as particles of ammonium sulfate (Seinfeld & Pandis, 2006).
Concentrations of the atomizer solution are given in Table 3-1; it should be noted that within the
aqueous particles themselves, concentrations will be substantially higher than these values, since
the solution is concentrated upon atomization by a factor of -1000 (based on measured mass
loadings and estimated LWC).
,
Seed particles are allowed to mix in the chamber for one hour prior to addition of H 2 0 2
which initiates the reaction. H 20 2 is not added directly to the atomizer solution since this would
initiate oxidation in the bulk solution prior to atomization, and so is instead introduced in the gas
phase, from which it subsequently partitions into the aqueous phase. Gas-phase H 20 2 is
introduced by sending 1-1.5 LPM air through a two-neck flask containing 6.0 piL of 30%
aqueous H 2 0 2 solution and into the chamber via a Teflon line; full evaporation of the H 2 0 2 takes
approximately 30 min. Assuming all the H2 02 enters the chamber, and partitions into the
aqueous particles according to Henry's law (with H* = 105 M atm- 1 (Sander, 1999)), this gives
270 ppb H2 0 2 in the gas phase, and 27 mM in the aqueous particles. This aqueous concentration
is higher than in the bulk experiments, but the concentrations of Fe2 and polyol are also higher
in the particles than in the bulk solution. However, because some H 2 0 2 is expected to be lost to
66
surfaces, such as the Teflon inlet tubing and chamber walls, these H 2 0 2 concentrations likely
represent upper limits. After completion of the dark Fenton chemistry, the blacklights are turned
on for additional photolytic and/or oxidative chemistry, as in the bulk experiments. Under dark
conditions, gas-phase oxidation is unlikely; however, during irradiation the photolysis of H 2 0 2
will lead to the formation of some gas-phase OH; as discussed below, this is unlikely to affect
the observed chemistry.
3.2.3 Aerosol analysis
Aerosol size distribution, volume, and number density are monitored using a scanning
mobility particle sizer (SMPS, TSI, Inc.). Chemical composition of non-refractory particulate
matter (here, operationally defined as all material that is instantly vaporized on the 600 *C
tungsten heater) is measured using a high-resolution time-of-flight aerosol mass spectrometer
(AMS, Aerodyne Research, Inc.). Although the AMS uses electron impact ionization, the
resulting mass spectra are also influenced by the heater, and should be compared to standard
spectra in the AMS spectral database rather than the NIST database. The AMS allows for the
measurement of chemical families, such as total organic, sulfate, and ammonium (Canagaratna et
al., 2007; DeCarlo et al., 2006), as well as elemental ratios of the organic species, most
importantly oxygen-to-carbon (0/C) and hydrogen-to-carbon (H/C) (Aiken et al., 2007; 2008).
All data reported here was taken with the instrument operated in V mode. Because the amount of
sulfate per particle is constant, abundances of all reported ions and families are normalized to
sulfate, accounting for possible changes in collection efficiency (CE), variations in atomizer
output, dilution in the chamber or atomizer, and loss of particles to surfaces. Some sulfate signal
(10-25%) is measured in the "closed" mass spectra, suggesting that FeSO 4 does not flashvaporize at the temperature of the AMS vaporizer (-550-600'C). However, this fraction does not
67
vary significantly over any experiment, suggesting the AMS "diff' signal is sufficient for this
normalization. In most chamber experiments the aerosol is not sent through a dryer prior to
analysis, though drying appears to have no effect on measured elemental ratios or sulfatenormalized concentrations. For chamber oxidation experiments, we also monitor temperature and
relative humidity (Vaisala), NOx (Horiba, Inc.), and ozone (2B Tech).
3.3 Results and Discussion
A series of control experiments were carried out to verify that aqueous-phase, dark
Fenton chemistry was indeed responsible for any chemistry observed. No reaction was observed
between the polyols and FeSO 4 in the absence of H 2 0 2 , nor between the polyols and H 20 2 in the
absence of FeSO 4, confirming that both Fe2 and H 20 2 are necessary for oxidation to occur.
Furthermore, no chemical changes were observed upon addition of H 2 0 2 or exposure to UV
when a chamber experiment (using erythritol-FeSO
4
particles) was conducted under dry
conditions (RH < 4%). The sulfate-normalized organic signal gradually decayed over the course
of this experiment, but this is likely a result of gradual evaporation of organic material as the
chamber air is diluted. The lack of a reaction under such dry conditions confirms that the
chemistry described below indeed takes place only in the aqueous phase.
3.3.1 Bulk oxidation of erythritol
Results for the bulk oxidation of erythritol (C 4 H 100 4 ) are shown in Fig. 3-3. Figure 3-3a
shows the aerosol mass spectrum of the aqueous erythritol solution, taken over a 30-minute
period immediately prior to the addition of H 2 0 2 . Figure 3-3b shows the time traces of key AMS
ions and species (all normalized to sulfate). Once H 2 0 2 is added, oxidation occurs immediately.
Key ions associated with erythritol (m/z 29, 61, 73, 91) begin to decay rapidly (initial lifetime
68
I
0.20
I
*-
00
Uh
U,
0.15
-I-
mz 73
mz 91
0
0.05
0
0
1
-3
(b)
-4_
10-
0.00
E
0.4
0.3
C02
10-2
4-J
Cu
E
0
Org
N
(~)
~i-
1.5-
4-J
1.0
H/C
-
0.2
2.0-
0
C
H202 added
-
0.10
E 1 0-1
E
0.1
-
0.5
O/c
H 20 2
0.0
0.0
20
40
60
80
10(
m/z
(d)
I
-50
added
I
0
50
I
I
100
150
reaction time (min)
Figure 3-3. Results for the oxidation of erythritol within the bulk aqueous phase: (a) AMS
spectra of unreacted erythritol; (b) sulfate-normalized mass concentrations of total organic
(Org), ammonium (NH 4 +), CO 2+, and key ions associated with erythritol (m/z 29, 61, 73, 91)
as a function of reaction time; (c) AMS spectra of oxidation products; and (d) oxygen-tocarbon and hydrogen-to-carbon ratios as a function of reaction time. Results in panels a, c,
and d are from high-resolution mass spectrometric analysis. All traces in panel b are from unit
mass resolution, except NH 4+ and CO 2+, which are high-resolution traces.
-1.8 min), presumably by reaction with OH. This compares to a lifetime of -14 hr in the gas
x
106 molec cm-3
,
phase (assuming a kOH of 2 x 1011 cm 3 molec~ 1 s-1 and OH concentration of I
Seinfeld & Pandis, 2006). Using a kOH of 1.9 x 10 9 M' s~' (Herrmann et al., 2010), this decay is
consistent with an initial aqueous OH concentration of 4.8 x 10-1 M, which is within an order of
magnitude of the average OH concentrations estimated for ambient deliquesced particles
69
(Herrmann et al., 2010). The OH concentration, estimated from the time dependence of the
polyol concentration (see Supporting Information), drops over the course of the experiment,
reaching a final OH exposure of 2.7 x 10-9 M s. Sulfate-normalized organic signal initially
decreases, likely due to the formation and evaporation of compounds that are more volatile than
erythritol, but then gradually rises, presumably from the addition of oxygen-containing
functional groups that increase the molecular weight and reduce the volatility of the organic
species. Ammonium (NH 4+) also rises, likely due to uptake of ammonia from the laboratory air;
this is consistent with acidification of the solution, which is known to occur upon initiation of
dark Fenton chemistry (Nguyen et al., 2013). While aqueous ammonia can undergo oxidation by
OH to form nitrite and nitrate (which are photosensitizers), this process is expected to be too
slow to be important here (with
kOH+NH3 =
1 x 108 M' s,
-20 times slower than
kOH+erythritol,
Huang et al., 2008). The uptake of NH 4 + is accompanied by an instantaneous change in the
appearance of the atomizer solution from colorless to yellow-orange, consistent with the
formation of Fe(OH)2+ (Zuo and Hoigne, 1992).
Rapid growth of the C02+ ion in the AMS is observed upon oxidation (Fig. 3-3b). This
highly oxidized ion fragment, which typically indicates the presence of carboxylic acids in the
AMS, is the most abundant ion in the product mass spectra, shown in Fig. 3-3c. Figure 3-3d
shows the elemental ratios of the particulate organic species as a function of time; the rapid rise
in O/C and drop in H/C upon addition of H 2 02 is also consistent with rapid oxidation of the
organic species. It should be noted that the initial H/C and O/C measured by the AMS do not
match the true values for erythritol. This is a result of using the default elemental analysis
correction factors, which have been determined empirically for an ensemble of species rather
than individual compounds (Aiken et al., 2008); however, errors in the absolute elemental ratios
70
are not expected to affect the observed trends in H/C and O/C, nor the overall conclusions of this
work. The rapid oxidation of dissolved organic species observed here is broadly consistent with
the findings of other bulk oxidation studies (Altieri et al., 2006; 2008; Carlton et al., 2007;
Kirkland et al., 2013; Lee et al., 201 Ia; 201 ib; 2012; Liu et al., 2012a; Perri et al., 2009; Sun et
al., 2010; Tan et al., 2009; 2010; Zhang et al., 2010; Zhao et al., 2012).
The AMS mass spectrum of the oxidation products closely resembles that of oxalate
(C 2 0 4 ) (Mensah et al., 2011; Takegawa et al., 2007), which has been shown to be a major
product in other aqueous-phase oxidation systems (Carlton et al., 2007; Perri et al., 2009; Tan et
al., 2010). A pathway for the formation of oxalic acid from the aqueous oxidation of ethylene
glycol (a C2 diol) has been shown by Tilgner & Herrmann (2010) and mechanisms for the
formation of oxalic acid from larger diacids have been described by Ervens et al. (2004). We did
not observe the formation of organonitrogen compounds, which were major reaction products in
the uptake of glyoxal (Galloway et al., 2009; Nozibre et al., 2009) and glycolaldehyde (Nguyen
et al., 2013). In those studies, imidazoles and other N-containing products were formed via
nucleophilic attack of ketones and aldehydes by ammonia (Galloway et al., 2009; Nozibre et al.,
2009); such reactions are unlikely in the present system given the lack of carbonyl moieties in
the polyols studied here.
The formation of oxalate (a highly-oxidized C2 compound) from erythritol (a lessoxidized C4 compound) likely occurs via multiple generations of oxidation. Individual product
ions (shown in Fig. 3-4) have significantly different temporal behavior, with some growing in
and decaying before others, suggesting complex chemistry with intermediate products formed at
different stages of oxidation. The varying rates of decay of erythritol ions in Fig. 3-3b (e.g., m/z
61 vs. 91) provide further evidence that multigenerational oxidation is occurring in this system.
71
.
3-
l
.
I
I
-
2n
c00
I C)
m/z 76m/z 77
-- M/z 87-
-3
Figure 3-4. Sulfate-normalized concentrations of key ions associated
intermediate products in the bulk
0 )with
s-
oxidation of erythrito0l.
Cc0
3-
E2--
-40
40
80
0
reaction time (min)
Upon exposure to UV, the organic ions decay still further, and the total organic signal
decreases dramatically, in a short period of time (lifetime ~36 min; see Fig. Sla, Supporting
Information). This is likely a result of either further oxidation by OH generated from photoFenton chemistry (R3.6) or direct photolysis (R3.7) to form small volatile species that cannot be
detected by the AMS. This provides additional evidence for oxalate, since Fe(III) oxalato
complexes are known to rapidly photolyze, a process that has been suggested as an important
sink of atmospheric oxalate (Sorooshian et al., 2013). Given the abundance of iron in the system,
our results suggest that the main condensed-phase oxidation product is oxalate, present as an iron
oxalato complex.
3.3.2 Chamber oxidation of erythritol
Results for the oxidation of erythritol in aqueous particles in the chamber are shown in
Fig. 3-5. As in the bulk experiment, dark Fenton chemistry within submicron particles in the
chamber was found to lead to the rapid decay of erythritol and formation of oxidized products. A
maximum OH concentration of 3.5 x 1012 M is reached between 30 and 40 min (where erythritol
72
0.25
0.20
100
-
0
0.15
E
~0
0.10
10
c
~r
10
0
4-.1
0.05
C,)
- ~
0.00
0.4
10-
-4m/z
j - - a)
73
(b)
:added
2.0-
0
a
0.3
0
0.2
1.5H/C
1.0E
0.1
0.5 E
H202
(C)
0.0
20
40
60
m/z
80
0.0
(d)
i
-50
10(
added
1
0
50
100
reaction time (min)
Figure 3-5. Same as Figure 3-3, but for the oxidation of erythritol within submicron particles
(chamber experiments): (a) AMS spectra of unreacted erythritol; (b) sulfate-normalized mass
concentrations of total organic (Org), ammonium (NH 4 +), CO2, and key ions associated with
erythritol (m/z 29, 61, 73, 91) as a function of reaction time; (c) AMS spectra of oxidation
products; and (d) oxygen-to-carbon and hydrogen-to-carbon ratios as a function of reaction
time.
lifetime is ~2.5 min), and subsequently decreases to give a final OH exposure of 4 x 10 9 M s;
these values are similar (within a factor of two) to those of the bulk experiments. The initial
organic mass spectrum of unreacted erythritol in the chamber (Fig. 3-5a) closely matches that of
erythritol in the bulk (Fig. 3-3a). The observed chemical changes in the two oxidation systems,
as described by the time traces of key AMS ions from the chamber experiment (Fig. 3-5b), the
73
product mass spectrum (Fig. 3-5c), and changes to O/C and H/C (Fig. 3-5d) are also similar to
the bulk oxidation results. The primary difference is that the total organic signal decreases
substantially more than in the bulk, with no subsequent increase, discussed in detail below. As in
the bulk experiment, a rapid loss of organic mass is observed upon UV irradiation (Fig. Slb).
This loss is unlikely to arise from oxidation by gas-phase OH (formed from H 20 2 photolysis):
the rapid loss of the organic species (lifetime -7 min) would require an OH mixing ratio of > 6 x
10 7 molec cm- 3 (assuming a koH of < 3 x 1011 cm 3 molec- s-1), far greater than can be produced
by H 20 2 photolysis. Instead, the rapid loss of organic mass probably arises from the direct
photolysis of Fe(III) oxalato complexes, as in the bulk oxidation experiment.
Our finding of rapid oxidation by Fenton chemistry in the absence of UV light differs
from the results of Nguyen et al. (2013), who found oxidation to occur only upon UV irradiation
(photo-Fenton chemistry). In that work, the H 202 was added directly to the atomizer solution
(along with FeSO4 ), and so dark Fenton chemistry may have gone to completion prior to the
introduction of the organic species (glycolaldehyde) in the gas phase. By contrast, in the present
experiments, H 2 0 2 is added last, so that all OH is produced in the presence of the aerosol-phase
organic compound. Because of such differences, and the differences in the organic species
studied, results from the two chemical systems are difficult to compare directly. Nonetheless, our
results are broadly consistent with those of Nguyen et al., in that they indicate that aqueousphase oxidation in submicron particles can lead to the rapid formation of highly oxidized organic
aerosol.
3.3.3 Oxidation of other polyols
Similar results are seen for the oxidation of most of the other polyols, in both the bulk
phase and the chamber. The one exception is the oxidation of glycerol (a C3 polyol), which
74
behaved differently in both the bulk and chamber experiments, presumably due to its high
volatility. In the glycerol bulk experiments, the measured organic mass actually increased after
oxidation, likely because the less volatile oxidation products evaporated less than glycerol upon
atomization; and in the chamber experiments, the LWC was too low for any measureable
particulate glycerol. However, the C5-C7 polyols (adonitol, mannitol, and volemitol) all behaved
similarly to erythritol upon oxidation, with decreases in the intensity of the polyol ions and H/C,
increases in the CO 2+ ion intensity and O/C, and product mass spectra resembling that of oxalate
(Figs. S2-S4 in the Supplement). Estimated OH concentrations and exposures were also similar
between the bulk and chamber cases (see Fig. S5 in the Supplement). As in the erythritol case,
exposure to UV led to a rapid decrease in all organic ion signal intensity, as well as a decrease in
total organic mass. In each case, the fraction of organic material remaining after oxidation was
smaller for the chamber experiment than for the bulk experiment.
Shown in Fig. 3-6 is the fraction of carbon remaining in the condensed phase (fc) after
reaction in the bulk solution or deliquesced particles for each polyol (except glycerol, for which
meaningful values of fc could not be determined). This is calculated from the changes in organic
mass concentration and elemental ratios:
[IOrgI 1 (160/C, + H/C, +12)
COrgI,
(60/Cf
(3.2)
+ H/Cf +12)
in which [Org] is the average high-resolution sulfate-normalized mass concentration of total
organic, O/C and H/C are the oxygen-to-carbon and hydrogen-to-carbon ratios, respectively, of
the organic species, with the subscripts i and f denoting initial and final conditions (before and
after H 20 2 addition), respectively. For example, in the case .of erythritol oxidation, 51% of the
initial carbon remained after bulk oxidation, but only 16% remained after oxidation in aqueous
75
aerosol in the chamber. For all bulk experiments, between 48% and 58% of the original carbon
remained in the condensed phase after oxidation, with no obvious dependence on carbon number.
However, a clear trend is observed for the chamber experiments, with fc increasing with the size
of the polyol.
0.7-
Chamber
Bulk
Figure 3-6. The fraction of carbon re-
0.6-
maining in the condensed phase after
_C
S
E
a
0.5-
oxidation (fc) in the bulk solution (blue
bars) or in submicron particles (red
bars), as calculated from Eq. 3.2, using
high-resolution V mode data. Meaningful values could not be determined for
the oxidation of glycerol (C3) due to
from the particle phase
prior to oxidation.
0.40.3
0.2 -evaporation
U 0 0.1-1
-
0.0
4
5
6
7
(erythritol) (adonitol) (mannitol) (volemitol)
carbon number
3.3.4 Differences between chamber and bulk experiments
The above observations - that a larger fraction of carbon remains in the condensed phase
for the bulk experiments than for the chamber experiments, and that this difference gets smaller
with increasing carbon number - can be explained either by differences in reactivity or
differences in partitioning between the two systems. The submicron particles in the chamber
have higher reactant concentrations and a greater ionic strength than the dilute bulk solution, and
also require dissolution of gas-phase H 2 0 2 prior to oxidation; these differences may cause the
observed differences in oxidation kinetics. Furthermore, the availability of gas-phase H 20 2 in the
chamber that can partition into the submicron particles, could allow for a longer production of
76
OH. However, the ratio of polyol to Fe2 is also greater in the chamber, so the OH may react
away more quickly. These complex effects are difficult to quantify, but could result in different
chemical reactions and degrees of oxidation between the two systems.
Nonetheless, it is unlikely that differences in reactivity are driving the differences in fc
between the bulk and chamber experiments, given the similarities in mass spectra, changes to
individual ions, and elemental ratios in the two cases (as shown in Figures 3-3, 3-5, and S1-4).
The final OH exposures for the oxidation of erythritol in the bulk solution (4.7 x 10-" M s) and
in the submicron particles (2.7 x 10-9 M s) agree to within a factor of two. This is true for the
larger polyols (C5-C7) as well (Fig. S5), suggesting that differences in oxidant availability are
not likely to account for the observed differences between the bulk and chamber results.
Furthermore, the variation in fc for different polyols oxidized in the chamber (Figure 3-6) is also
unlikely to result from differences in chemistry, given that the oxidation conditions were the
same.
Rather, the difference in total organic mass remaining in the condensed phase appears to
be a result of differences in gas-aqueous partitioning of intermediate species (early-generation
oxidation products). A general mechanism is illustrated in Fig. 3-7. The polyol is oxidized to
form multiple generations of products, ultimately forming oxalate (in the form of an iron (III)
oxalato complex). In the bulk aqueous solution, where the liquid water content is very high, the
reactants, intermediates, and products remain in the condensed phase. (Some of these species
may evaporate upon atomization and not be detected by the AMS, but they still would have been
present in the condensed phase during oxidation.) However, in the chamber experiments, in
which LWC is low, relatively volatile compounds can partition out of the condensed phase. The
polyols and oxalate are sufficiently water-soluble to remain in the particle phase at this low LWC,
77
-+
-+
bulk & chamber
gas phase
intermediates
(g)
chamber only
gas phase
aqueous phase
poIyoI(aq)
products
hv or OH
t
O
OHintermediates(q)
~
oxalate(aq)
Figure 3-7. Simplified mechanism to explain observed differences between oxidation in the
bulk and in submicron particles. The polyol reacts to form intermediates with lower H*
(higher volatility over water), which remain fully in the aqueous phase during bulk
experiments, but partition to the gas phase (red arrows) during chamber experiments. This
partitioning, which occurs to a greater extent for smaller polyol precursors, competes with
further aqueous oxidation, and has the effect of lowering the product (oxalate) yield. Upon
exposure to ultraviolet light, the Fe(III) oxalato complex photolyzes, forming small gas-phase
products.
suggesting that the partitioning of intermediate compounds from the aqueous phase to the gas
phase (red arrow in Fig. 3-7) accounts for the increased loss of organic material in the chamber.
These early-generation oxidation products are likely to be more volatile (lower H*) than the
polyol precursor, due to the oxidative conversion of hydroxyl groups to carbonyl groups (Bethel
et al., 2003) and/or the formation of smaller molecular species via fragmentation reactions (Kroll
et al., 2009). Evaporation of these species may also be promoted by a decrease in particulate
water, which is observed upon H 20 2 addition and likely occurs due to a change in the
hygroscopicity of iron salts upon initiation of Fenton chemistry. Regardless of the exact
mechanism, partitioning of intermediates into the gas phase will compete with further oxidation,
as shown in Fig. 3-7, resulting in lower yields of oxalate than were measured in the bulk
experiments. This general mechanism also explains the correlation between fc and carbon
number (Fig. 3-6), since the size, and thus the volatility, of the intermediate compounds should
be related to the size of the polyol precursor.
78
3.4 Conclusions and Implications
To our knowledge, this work presents the first direct comparison of aqueous-phase
oxidation within submicron particles to that within the bulk phase. In both systems, dark Fenton
chemistry leads to the rapid conversion of polyol precursors to highly-oxidized organic material,
a result that is broadly consistent with previous aqueous-phase oxidation studies (Altieri et al.,
2006; 2008; Carlton et al., 2007; Kirkland et al., 2013; Lee et al., 201 Ia; 2011 b; 2012; Liu et al.,
2012a; Nguyen et al., 2013; Perri et al., 2009; Sun et al., 2010; Tan et al., 2009; 2010; Zhang et
al., 2010; Zhao et al., 2012). The primary difference between the two systems is that less carbon
remains in the condensed phase in the chamber experiments, with the smallest precursors having
the greatest differences in fc. As shown in Fig. 3-1, under bulk conditions, any compound with
H* 2 0.1 M atm- 1 should be present primarily in the aqueous phase. However, at the lower LWC
of the chamber, compounds with H* as high as 109 M atm- 1 will instead be present predominantly
in the gas phase. Thus, species with H* between these limits will remain dissolved in bulk-phase
water, but will partition out of submicron particles into the gas phase. Our results suggest that the
reaction intermediates formed from the oxidation of polyols appear to have H* in this range.
Therefore, the partitioning of these early-generation products from aqueous aerosol to the gas
phase can explain the dramatic differences in fc observed in the bulk and chamber experiments.
The present results confirm those of previous studies showing that aqueous-phase
oxidation is an efficient pathway for the rapid formation of highly oxidized material. However,
when oxidation occurs within submicron particles, the fraction of carbon remaining in the
condensed phase is substantially smaller than in the bulk oxidation experiments, implying that
the formation of highly oxidized OA by aqueous chemistry may be somewhat less important than
bulk-phase experiments suggest. Bulk oxidation experiments may not accurately simulate the
79
chemistry that takes place in the atmospheric aqueous phase, due to large differences in LWC
(and therefore partitioning) between the bulk solution and atmospheric droplets or particles. This
difference points to the importance of running aqueous-phase oxidation experiments under
atmospherically-relevant partitioning conditions. This is analogous to chamber studies of
secondary organic aerosol (SOA) formation via partitioning into the condensed-organic phase,
which to be representative of atmospheric conditions must be run at low total organic mass
concentrations
(COA)
(Presto and Donahue, 2006). A major difference between such "traditional"
SOA chamber experiments and aqueous-phase oxidation, however, is that LWC can vary by a
great deal more than
COA
between the laboratory and atmosphere. Our results show that even
bulk oxidation experiments that allow for evaporation of oxidation products (Lee et al., 2011 a;
2011 b; 2012; Liu et al., 2012b; Ortiz-Montalvo et al., 2012; Zhao et al., 2012), may not fully
simulate atmospheric processing, since they do not include the effects of partitioning of reaction
intermediates. Further, the experiments with glycerol oxidation demonstrate that in bulk-phase
experiments it is possible to oxidize compounds that would not actually be present in the
atmospheric aqueous phase under most conditions. Results from the oxidation of larger polyols
indicate that differences in LWC between bulk and chamber oxidation result in significant
differences in partitioning, which in turn can affect the chemistry. For experiments like ours with
a single product, this affects only the final yield. However, when multiple oxidation products are
formed, with intermediates of varying volatility, changing the LWC may also alter the product
distributions.
This
study
underscores
atmospherically-relevant
the
need
for
conducting
oxidation
experiments
at
liquid water contents, and extending the existing suite of bulk
experiments to additional partitioning conditions. Oxidizing previously-studied compounds
80
(small carbonyls, acids, isoprene oxidation products, etc.) within aqueous submicron particles
would provide valuable information on the role of partitioning and LWC on the formation of
highly-oxidized OA for a wider range of water-soluble organic compounds. Further, the large
differences in partitioning between deliquesced particles and clouds (Fig. 3-1) suggest that a
similar set of experiments is a useful next step for the accurate study of atmospheric cloud
processing. Oxidation experiments involving actual cloud droplets, with LWCs much higher than
those of aqueous particles but still far lower than that of the bulk aqueous phase, would improve
our understanding of this potentially important source of oxidized organic aerosol.
3.5 Acknowledgements
This work was supported by the National Science Foundation, under grant number AGS1056225.
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86
..
...
...
....
3.7 Supplement
add H2)2
UV
on
Org
(OU)
NH,4+
1
C0 2
+
4
CO
UV on
add H202
10
10
rn/z 29
m/iz 61
C
0
a)N 10
10
102
m/z 73
m/z 91
-3
20 E
E c 10
10
10
-4
(a)
1
300
100 200
reaction time (min)
0
(b)
10
1
-50
400
0
50 100 150
reaction time (min)
Figure S1. Results (including photolysis) for the oxidation of erythritol (C 4 H1 00 4 ), showing
sulfate-normalized mass concentrations of total organic (Org), ammonium (NH 4+), CO 2+, and
key ions associated with erythritol (m/z 29, 61, 73, 91) as a function of reaction time for (a) bulk
+
oxidation and (b) chamber oxidation. All data is shown in unit mass resolution, except for NH 4
and CO2, which are high-resolution traces. Dark Fenton chemistry is indicated by grey shading,
and exposure to UV by yellow shading.
add H202
UV
on
add H202
10o0
Org
-1
CO0
4Q)
CUa)
O N
102
0
10
NH,
C0
-
CU U
2
m/z 29
10 -2
imz 61
M/Z 91
Ec
-4
-5
10
10
m/z 73
o 0
10
on
+
C
UV
10
10-2
10
(a)
-50
10
0
50 100 150 200
reaction time (min)
-3
-4
(b)
-50
0
50 100 150 200
reaction time (min)
Figure S2. Same as Figure S1, but for the oxidation of adonitol (C 5H 12 0 5 ).
87
add H202
UV
add
on
C --
Org
1U
0
10
c0 2
+
CO
UV
10
10
4
H202
10
C a)
0ON
10
-2
rn/z 29
rnz 61
m/z 73
m/z 91
-3
10
10
-1
-2
c
E c0
10
10
-4
-5
10
(a)
10
0
-3
-4
(b)
-50
200
100
reaction time (min)
0
50
100 150
reaction time (min)
Figure S3. Same as Figure S1, but for the oxidation of mannitol (C 6H 140 6).
add
H202
UV
add H202
on
10
10
UV
on
-T
Org
NHI.
c0 2
+
C Cn
10
rn/z 29
10
m/z 61
m/z 73
m/z
-2
91
10
10-2
-2
10
105
10
10,
(a)
0
100 200 300 400
reaction time (min)
(b)
0
200
100
reaction time (min)
Figure S4. Same as Figure S1, but for the oxidation of volemitol (C 7 H 160 7 ).
88
on
-9
5x10 -9
(a)
-
5x10
4-
4-
3
3-
(b)
0
2-
20
0
1
-
1Bulk (m/z 61)
Chamber (m/z 61),
00
50
100
150
reaction time (min)
0
-9
-
5x10
Bulk (m/z 61)
Chamber (m/z 61)
0-
5x10
(c)
4-
40
80
reaction time (min)
120
-9
(d)
4-
U)
a)
30
Q.
X
30
Q.
2-
a)
2
0
1
a)
IZ
1
0-
-
-
0
X
Bulk (m/z 73)
Chamber (m/z 73)
I
0
Bulk (m/z 73)
Chamber (m/z 73)
0-
I
I
100
50
150
reaction time (min)
0
100
200
reaction time (min)
Figure S5. Estimated OH exposures for all experiments. These are determined from OH
exposure = ln([polyol]o/[polyol]t)/koH, where [polyol]o and [polyol]t are the sulfate-normalized
mass concentrations of the fastest decaying tracer ion at times 0 and t, respectively, and koH is
the rate constant for aqueous reaction with OH (Herrmann et al., 2010). OH exposures for both
bulk and chamber oxidation are shown as a function of time from 30 min prior to adding H 202
until exposure to UV lights for (a) erythritol using m/z 61 as a tracer and koH = 1.9 x 109 M~'s-Q,
(b) adonitol using m/z 61 as a tracer and kOH
tracer and kOH
1.6 x 10 9 M's 1 , (c) mannitol using m/z 73 as a
1.6 x 109 M's , and (d) volemitol using m/z 73 as a tracer and koH
=
1.6 x 10 9
M' s -. (Because no kOH has been reported for volemitol, it was assumed to be equal to the koH
for mannitol.)
89
90
Chapter 4
Role of aqueous-phase oxidants in SOA
formation
4.1 Introduction
As shown in Chapter 2, there are only a few potential pathways for the formation of
oxidized organic aerosol (OA), one of which is rapid oxidation within the condensed phase.
Further oxidation of SOA within particles is often neglected since heterogeneous oxidation (in
which gas-phase oxidants react with condensed-phase molecules at the particle surface) is known
to be slow, with lifetimes on the order of several days (George and Abbatt, 2010; Kessler et al.,
2010; 2012; Robinson et al., 2006). However, if oxidants are also present within the condensed
phase, then oxidation within SOA particles could be an efficient pathway for the production of
highly oxidized OA. In Chapter 3, we showed that rapid oxidation of water-soluble organic
compounds is possible within aqueous systems.
In this chapter, we investigate whether the presence of condensed-phase oxidants can
lead to the production of highly oxidized OA from the oxidation of gas-phase precursors. As
described in Chapter 1, SOA is formed from the oxidation and subsequent condensation of gasphase organic precursors. This can be an efficient pathway for formation of OA, but the organic
material is generally not very oxidized. A number of studies have focused on oxidation within
the condensed phase as a means for aging this aerosol. As described in Chapter 3, numerous
oxidation studies using bulk aqueous solutions have shown aqueous oxidation to be an efficient
91
pathway for the formation of oxidized organic material (Daumit et al., 2014). Some work has
also been done to examine oxidation within the submicron particles. A couple studies have
examined the role of photoenhanced chemical reactions within aqueous particles containing apinene ozonolysis SOA (Lignell and Hinks, 2014; Wong et al., 2014). Volkamer et al. studied
the formation of SOA from acetylene and glyoxal for a range of seed compositions and RH, and
found that organic photochemistry within aqueous aerosol can accelerate SOA formation
(Volkamer et al., 2009). Several studies have used iron sulfate (FS) seed particles to generate
aqueous oxidants via Fenton-like chemistry in submicron aerosol. Chu et al. observed reduced
yields in photooxidation experiments with FS seed particles (Chu et al., 2012; 2014), and
Nguyen et al. showed that FS seed enhanced the condensed-phase oxidation of glycolaldehdye
(Nguyen et al., 2013). In Chapter 3, we focused on the oxidation of simple single component
systems in deliquesced FS particles, using polyols as surrogates for water-soluble atmospheric
oxidation products. We saw evidence for multiple generations of aqueous oxidation, as well as
partitioning of oxidation intermediates out of the condensed phase, resulting in decreased yields.
In this chapter, we expand our focus to more atmospherically relevant organic
compounds and a broader range of condensed phase oxidation conditions. Rather than starting
with a single organic molecule in the condensed phase, we generate a mixture of condensable
products from gas-phase ozonolysis of biogenic VOCs. We still use deliquesced submicron
particles as the aqueous reactors, which we have shown to be more atmospherically
representative than bulk aqueous solutions. However, we now use a range of seed types
including dry and wet ammonium sulfate and iron sulfate in order to vary the phase and oxidant
levels within the particles, while keeping the gas phase oxidants constant. While we discuss the
effects of this oxidation on aerosol yield (the fraction of carbon that ends up in the condensed
92
phase) the main focus of this work is the effect of condensed-phase oxidation on chemistry in the
aerosol, specifically changes to the oxidation state of particulate carbon. This is the first study, to
our knowledge, to directly examine how varying oxidant levels and photochemical processes
within aerosol can affect the oxidation state of SOA.
4.2 Experimental Methods
SOA is formed from both ozonolysis of a-pinene and ozonolysis of isoprene under a
number of different seed and humidity conditions, in order to systematically vary the oxidant
levels within the particles: (1) phase of particles (aqueous vs. dry), (2) added oxidant source
(none vs. H 2 0 2 vs. Fenton chemistry), (3) irradiation (dark vs. UV lights). As described
previously (Daumit et al., 2014), Fenton-type chemistry can produce aqueous-phase hydroxyl
radicals (OH) via the cycling of Fe2+ and Fe3 in the presence of H2 0 2 ; in this work we find that
other ROS species, such as those formed from alkene ozonolysis, can drive this chemistry as
well. The conditions for each experiment are provided in Table 4-1 in the Results and
Discussion.
4.2.1 Experimental setup
Much of the experimental setup (shown in Fig. 4-1) was previously described in Chapter
3 (Daumit et al., 2014). Here, we will briefly highlight key features and important changes for
this present study. The MIT chamber is a 7.5 m 3 Teflon bag run in semi-batch mode at 20'C,
with relative humidity controlled by a bubbler containing Milli-Q water. Deliquesced seed
particles are introduced by directly atomizing a I g L-i solution of either ammonium sulfate (AS,
Sigma-Aldrich, > 99.0%) or iron(II) sulfate heptahydrate (FS, Alfa Aesear, > 99.999% metals
basis) into the chamber for 0.5-1 h. This generates a seed volume of between ~55 and 170 nL
m-3, depending on the RH and seed type. During atomization, 20 ppb of (-)-a-pinene (1.0 ptL,
93
clean
air in
Hi-i
bubbler
settling
40 W black
lights (350 nm
chamber
4
44
7.5 m 3 Teflon
settling
chamber
F
-GC
.
clean
air in
Environmental
Chamber
-NOn Monitor
_03 Monitor
HR-AMS
FeS04
a-pinene,
HFB
clean
clean
air in
clean
air in
40 W black lights (350 nm)
--
Ozone
SMPS
-
air in
Generator
Figure 4-1. Experimental setup.
Sigma-Aldrich, 98%) or 400 ppb of isoprene (13.5 pl, Sigma-Aldrich, 99%) is injected through
a septum into a 1/4 inch OD stainless steel line with -1
Lpm of clean air flow. A non-reactive
dilution tracer (2.0 ptL of hexafluorobenzene, Sigma-Aldrich, > 99%) is also injected. After at
least 20 minutes to allow for mixing, SOA formation is initiated by introducing an excess of
ozone (80-100 ppb for a-pinene, -900 ppb for isoprene), generated by sending 1.5 Lpm of clean
air through a corona ozone generator (Enaly). This also generates a small amount of NO, (< 5
ppb for a-pinene, < 25 ppb for isoprene). An OH scavenger was not used in these experiments.
As in the previous polyol study, 6.0 ptL of 30% w/w H2 0 2 in water (Alfa Aesar) is added to the
gas-phase, giving -27 mM H 20 2 in the aqueous particles, in order to initiate the Fenton-like
chemistry once the SOA precursor has reacted away (-3 h for a-pinene, -5 h for isoprene).
However, as described in the Results and Discussion, it was found that H 20 2 is not necessary in
order to initiate the Fenton-like chemistry, and therefore it was only added for some experiments.
94
Once the reactions appear to have gone to completion (-6 h for a-pinene, -10 h for isoprene),
two banks of 24 black lights are turned on to assess the photo-reactivity of the reaction products
and/or initiate any further condensed-phase photochemistry.
4.2.2 Instrumentation
As described in our previous work, a high-resolution time-of-flight aerosol mass
spectrometer (AMS, Aerodyne Research Inc.) and a scanning mobility particle sizer (SMPS, TSI
Inc.) are used to measure the aerosol chemical composition and the aerosol size distribution,
volume, and number concentration, respectively (Daumit et al., 2014). All reported AMS data
was collected in V-mode. Concentrations of a-pinene, isoprene, and hexafluorobenzene are
monitored using gas chromatography-flame ionization detection (GC-FID, SRI). We also
monitor NOx (Horiba Inc.), ozone (2B Tech), and temperature and relative humidity (Vaisala).
4.2.3 Yield calculation
SOA mass yields (Y) are given by eq. 4.1
-
(4.1)
AcA
AHC
in which AcOA is the wall-loss and dilution-corrected increase in organic aerosol mass loading
and AHC is the dilution-corrected decrease in mass concentration of hydrocarbon (a-pinene or
isoprene) lost to reaction with ozone. The hydrocarbon concentration is corrected for dilution
using the measured hexafluorobenzene decay rate. The GC-FID calibration factors were
determined by averaging the peak area across experiments conducted within a given time period
and assuming it to be equal to 114 ptg m 3 for a-pinene or 1135 ptg m 3 for isoprene.
For standard chamber experiments that use dry AS seed,
COA
can be estimated from the
AMS organic-to-sulfate ratio (org/S04) and the initial SO 4 mass (which is calculated from the
SMPS volume and AS density). However, in the present set of experiments, uncertainties in seed
95
hygroscopicity, density, and AMS collection efficiency prevent the accurate determination of the
initial S04 mass, and necessitate the use of a more complex approach.
Here, the absolute organic mass loading is determined from SMPS data at a single point
in time (2.5-3 h after ozonolysis of a-pinene, 4.5-5 h after ozonolysis of isoprene). The SMPS
volume concentration is first corrected for dilution and deposition to the walls using a decay rate
fitted over a time period where the AMS data indicate that chemical composition (and density) is
not changing. The organic volume concentration is then estimated by subtracting the seed
volume from the total wall-loss corrected volume. For most experiments, the seed volume is
measured just prior to the addition of ozone. However, for the wet FS experiments, the addition
of ozone appears to change the density and/or hygroscopicity of the seed particles, making the
initial seed volume a poor measure of the seed volume throughout the experiment. For such
experiments, the seed volume is instead estimated from the total volume at the end of the
experiment after most of the organic material is lost to the gas phase (as described in the
Results). The organic volume is converted to coA using a density of 1.4 g Cm 3 (the implications
of this assumption are discussed in the Results and Discussion). The time dependence of COA is
then determined from changes to the AMS org/SO 4 ratio.
Carbon yield (Yc) is given by eq. 4.2
(4.2)
YC = Ac
AHCC
where Acoc is the wall-loss and dilution corrected mass of carbon in organic aerosol and AHCc
is the mass of a-pinene carbon that has reacted. coc is calculated from eq. 4.3
COC =
1oc
(4.3)
12+16 O/C+ H/C
96
where O/C and H/C are the oxygen-to-carbon and hydrogen-to-carbon ratios of the organic
aerosol.
4.3 Results and Discussion
Table 4-1: Reaction conditions and results for a-pinene (Expt. 1-7) and isoprene (Expt. 8-9).
Note that because the data reported here have been rounded to the nearest hundredth place, very
small changes are clearer in the figures than in the table (e.g. see Fig. 4-3 for comparison of
cha ges in O/C after UV for Ex eriments 1-4b).
Seed
I
2
3
4a
4b
5
6
7
8
9
Dry AS
Dry AS
Aq AS
Aq AS
A AS
Dry FS
Aq FS
Aq FS
Dry AS
Ag FS
O/C
initial
0.55
0.47
0.47
0.51
0.45
0.62
0.95
1.06
0.59
1.45
AO/C
after
AO/C
after
H 20 2
UV
(0.02)
0
(0.02)
0
0
0.07
(0.08)
0.20
0.02
0.01
0.03
0.08
0.06
0.06
0.04
-0.09
0.02
-0.81
(--)
0.48
initial
0.27
0.26
0.19
0.18
0.16
0.13
0.12
0.12
0.038
0.020
Yield
after
after
Carbon Yield
initial after after
H 20 2
UV
(0.26)
0.26
(0.16)
0.15
0.13
0.13
(0.12)
0.11
(--)
0.023
0.25
0.25
0.16
0.14
0.12
0.08
0.06
0.02
0.16
0.17
0.12
0.11
0.10
0.08
0.06
0.05
0.036
0
H 20 2
UV
(0.16)
0.17
(0.10)
0.10
0.08
0.07
(0.05)
0.05
0.15
0.16
0.10
0.08
0.08
0.04
0.02
0.01
0.022
(--)
0.018
0.007
0.007
0
Experimental conditions and key results for each experiment are provided in Table 4-1.
Experiments 1 through 7 used a-pinene; Experiments 8 and 9 used isoprene. The "Seed" column
indicates the seed type and humidity conditions (dry experiments were run at RH < 12%, below
the efflorescence RH of ammonium sulfate ensuring the seed particles were dry whereas aqueous
experiments were run at 70% < RH < 87%, above the deliquescence RH ensuring that seed
particles contained liquid water (Seinfeld and Pandis, 2006)). The initial O/C, yield, and carbon
yield were measured just prior to the addition of H 2 0 2 (t = 3 h for a-pinene, t = 5 h for isoprene).
The yield, carbon yield, and change in O/C are also reported after the addition of H 2 0 2 (t = 6 h
for a-pinene, t = 10 h for isoprene), and after UV irradiation (t = 9 h for a-pinene, t = 15 h for
isoprene). Experiments in which H 20 2 was not added are indicated by parenthesis.
97
4.3.1 "Traditional"a-pinene SOA (dry AS seed)
-n2.0
UV
(a)
(b)
30
25
1.5
E 20-
2
0
1.0
-
U-pnene/1
15-
100.5
o-oi
5
1"
0
200
100
300
reaction time (min)
400
500
.
0.0
0-
10
20
30
40
50
60
70
80
90
100
110
120
m/z
0
_o
2-
0.*
mC 4
-5
4--
0.0*
0
Figure 4-2. Results for formation of SOA
from ozonolysis of a-pinene in the presence of dry AS seed (Expt. 1): (a) mass
concentrations of organic aerosol and apinene, and O/C and H/C of organic aerosol, (b) average organic mass spectrum,
(c) sulfate-normalized total organic mass
and fragment ions.
3-
6'
0
100
200
300
reaction time (min)
400
500
Results for Experiment I (ozonolysis of a-pinene with dry AS seed) are shown in Figure
4-2. SOA formation in this experiment occurs in the absence of any condensed-phase oxidants
and therefore represents the base degree of oxidation against which other experiments are
compared. Figure 4-2a shows the mass concentration of organic aerosol and a-pinene, and the
O/C and H/C of the organic aerosol, Figure 4-2b shows the organic mass spectrum of the SOA,
and Figure 4-2c shows the sulfate-normalized mass concentrations of total organic signal and
individual ion fragments. Oxidation begins immediately following the addition of ozone,
producing SOA with an O/C between 0.5 and 0.6, and H/C between 1.3 and 1.4. Most of the a-
98
pinene reaction and SOA formation is completed within -3 hours. Exposure to UV light does not
appear to have any effect, causing no appreciable change to the elemental ratios or organic mass
loading. The individual ion fragments all follow the same trend, implying that the composition of
the organic material does not change following condensation.
In a similar experiment in which 270 ppb H 20 2 was added prior to UV irradiation (Expt.
2), a similar degree of oxidation was observed (Table 4-1), again with no change to O/C upon
UV exposure. This indicates that photolysis of gas-phase H 2 0 2 has no measureable effect on
SOA.
4.3.2 a-pinene SOA formed on aqueous AS seed
o
0.65 -u
-
0.60
0.50
*
0.40
-
*
0
0.35
,
200
250
300
350
400
Dry AS
Dry AS, H 20 2
Wet AS
Wet AS, H 2 0 #1
2
Wet AS, H 2 0 2 #2
450
500
reaction time (min)
Figure 4-3. Effect of UV irradiation on O/C for the different a-pinene ozonolysis experiments
with AS seed (Expt. 1-3, 4a, 4b).
SOA formed on aqueous AS seed (Expt. 3, 4a, 4b) had a similar degree of oxidation
(Table 4-1). However, unlike with dry AS, there was an increase in O/C upon exposure to UV,
suggesting that UV irradiation leads to oxidant formation within the aqueous particles. This can
be seen in Fig. 4-3, which shows O/C before and after UV irradiation for all the AS seed
experiments. The variation in absolute O/C values gives a sense of the reproducibility. Despite
99
this variability, it is clear that UV causes O/C to increase in the aqueous AS experiments, but has
no effect in the dry AS experiments. It is hypothesized that this occurs via photochemical
excitation of aqueous-phase SOA molecules, resulting in the production of aqueous oxidants.
The increase in O/C was greater when H 20 2 had been added, likely due to additional oxidant
formation from the photolysis of aqueous H 20 2 to OH. Because H 20 2 did not enhance oxidation
in the dry AS experiment, it is not likely that gas-phase oxidants are responsible for the increase
in oxidation upon UV irradiation. This enhancement in the degree of oxidation for irradiated
aqueous AS seed but not dry AS seed is consistent with the findings of Chu et al. (2014);
however, the results of their FS seed experiments differ from ours as is discussed below.
4.3.3 a-pinene SOA formed on aqueous FS seed
14-
2.0(b)
[
pinene/10
12
o20 /1C
O
H/C
15
10
2
0
-1.0
_ 6
0
00
4
-
0
U
-
-0.0
400
reaction time (min)
200
0
0.5
600
80
0
0.01-
--
0.001
0
30
40
50
60
70
mlz
80
90
100
110
120
Figure 4-4. Results for formation of SOA
from ozonolysis of a-pinene in the presence of aqueous FS seed (Expt. 6): (a)
mass concentrations of organic aerosol
and a-pinene, and O/C and H/C of organic
aerosol, (b) average organic mass spectrum prior to UV irradiation, (c) sulfatenormalized total organic mass and fragment ions.
0.17
(n
20
UV on
(C)
0
10
200
org
m'z4
m/z 44
m/z 45
mlz 55
400
reaction time (min)
600
800
100
SOA formed with aqueous FS seed (Expt. 6 and 7) is fundamentally different from the
SOA formed with AS seed (Table 4-1), with the condensed-phase organics having undergone
substantially more oxidation. Figure 4-4a shows the mass concentration of OA and a-pinene, and
the OA elemental ratios for Experiment 6. Prior to UV irradiation, the organic material reaches
an O/C > I and H/C < 1, with a mass spectrum dominated by m/z 44 (C0 2+), a highly oxidized
ion fragment which typically indicates the presence of carboxylic acids (Fig. 4-4b). The less
oxidized ion fragments (m/z 43 and m/z 55) grow in earlier and gradually decrease as the more
oxidized fragments (m/z 44 and m/z 45) gradually increase (Fig 4-4c). UV irradiation leads to a
rapid loss of organic material from the condensed phase, likely due to the photolysis of oxidation
products to smaller volatile species which partition out of the condensed phase. The initial sharp
decline in O/C following UV suggests that the products undergoing photolysis are the most
highly oxidized species. Oxalate is one potential product that is both highly oxidized (O/C = 2)
and known to undergo rapid photolysis when present as an iron complex (Sorooshian et al.,
2013). The organic mass spectrum (Fig. 4-4b) is consistent with the presence of some oxalate
(Mensah et al., 2011; Takegawa et al., 2007). Following its initial decline upon UV exposure,
O/C beings to rise indicating that UV also leads to increased concentrations of aqueous oxidants,
possibly from photo-Fenton chemistry.
For the first 3 h of oxidation, Expt. 6 and 7 had identical conditions. Therefore the
differences in initial O/C (0.95 vs. 1.06) are due to random variability between experiments and
give insight into the reproducibility. Regardless, the addition of H 2 0 2 in Experiment 7 appeared
to accelerate the Fenton-like chemistry, clearly increasing the rate at which O/C rises, leading to
a higher degree of oxidation and a greater loss of organic material upon photolysis. Figure 4-5
shows O/C for all the aqueous experiments, highlighting the dramatic enhancement in the degree
101
of oxidation when SOA is formed in the presence of aqueous oxidants. To our knowledge, no
other ozonolysis experiment reported in the literature has ever generated SOA that is as highly
oxidized as that generated in Expt. 6 and 7.
*
b
1.2-
*
1.02
*
-
Wet AS
Wet AS, H,2 0#1
Wet AS, H 0#2
added
Wet FS
Wet FS, H 2 0
2
O~
1.0
1
H 20 2
0.8-
0.6
0.40
50
100
150
200
250
reaction time (min)
300
350
Figure 4-5. Differences in O/C for a-pinene ozonolysis with aqueous FS (red/pink traces) and
aqueous AS seed (green/blue traces).
The immediate production of highly oxidized organic material indicates that aqueous
oxidants are generated as soon as SOA formation begins, and that Fenton-like chemistry is
initiated without the addition of H 20 2. To investigate whether this chemistry is initiated directly
by ozone, we re-ran the previously described chamber oxidation of erythritol in aqueous FS
particles (Daumit et al., 2014), but with ozone added to the chamber prior to the addition of
H 20 2 . Although ozone dissolved in water can generate OH (von Gunten, 2003), in this study
ozone did not lead to any aqueous oxidation of erythritol, indicating that only an insignificant
amount of OH was generated from the reaction of ozone with water or ozone with iron. This
suggests that the Fenton-like chemistry in the present study is not initiated by ozone itself, but
rather by a product of ozonolysis such as HO 2, R0 2 , or ROOH. A proposed general reaction
scheme for ozonolysis of a-pinene in the presence of FS seed is shown in Figure 4-6.
102
+
03 -
SOA products(g)
ROSg
O(g)S
OA p ro duc ts( a)
Fe 2++ ROS + Fe 3++ OH
Fe3+ + ROS + Fe 2
+
ROSaq)
SOA products
jOH
OOA products
gas phase
products
hv or OH
Figure 4-6. Proposed reaction scheme for ozonolysis of a-pinene in the presence of aqueous FS
seed. Following the gas-phase reaction of ozone and a-pinene, water-soluble organic products
and reactive oxygen species (ROS) partition into the condensed phase. ROS react with iron to
initiate Fenton-like chemistry and produce aqueous hydroxyl radicals (OH). The organic
products then undergo further oxidation by OH within the particles. Some of this oxidation leads
to smaller volatile products which partition out of the particles. Upon exposure to UV, the more
oxidized organic molecules undergo photolysis to gas-phase products. The addition of H 20 2
(which is a ROS) accelerates the formation of OOA by increasing [ROS] and therefore the rate
of OH(aq) production via reactions with iron.
An additional experiment was conducted with FS seed and H2 0 2 at low RH (Expt. 5).
This represents an intermediate case, with SOA that is somewhat more oxidized than for dry AS
seed, but still substantially less oxidized than for aqueous FS seed (Table 4-1). Similarly to the
aqueous FS experiment, UV irradiation led to some loss of organic compounds from the
condensed phase, suggesting that some oxidized photolabile products had been formed.
However, the lower degree of oxidation relative to the aqueous FS experiments points to the
importance of the aqueous phase for the generation of oxidants. The reason for higher oxidation
than in the AS experiments is not clear, but could be due to some chemistry at the particle
interface and/or the presence of a small amount of water within the particles. These results differ
from those of Chu et al. (2014) who observed higher levels of oxidation with aqueous AS than
103
dry FS; this difference may be due to the continuous exposure to UV irradiation in those
experiments and/or differences in gas-phase oxidants (OH vs. ozone).
4.3.4 Additional a-pinene SOA experiments
In order to investigate the ability for SOA generated in the absence of aqueous oxidants
to be later oxidized within separate seed particles, a-pinene SOA was initially formed on
aqueous AS seed, and FS seed was later introduced. Figure 4-7 shows the time evolution of O/C,
H/C, and ammonium-normalized total organic mass and individual fragment ions. (Because the
sulfate concentration increases when FS is added, ions are normalized to ammonium instead of
sulfate.) The initial SOA resembled that produced in the other aqueous AS experiments (O/C
~0.5, H/C ~1.3). However, as soon as FS seed particles are added, the SOA undergoes further
oxidation with m/z 44 growing in as less oxidized ion fragments decay away, generating oxidized
OA with a final O/C of ~0.9 and H/C of ~1. As with the other aqueous FS experiments, UV
irradiation leads to photolysis of oxidized products and loss of organic material to the gas phase.
Fo o/C
2.0 - (a)
start adding FS
1
(b)
H/C
start adding FS
stop adding FS
stop adding FS
1.5
-3.U)
1.0-
0..01
0
200
400
reaction
600
time (min)
800
0
1000
200
600
reaction time (min)
400
800
1000
Figure 4-7. Results for SOA from a-pinene ozonolysis initially formed on aqueous AS seed,
with the later addition of ES seed: (a) 0/C and H/C of the organic aerosol, (b) ammoniumnormalized total organic mass and fragment ions.
These results suggest that SOA products are able to partition out of unreactive aqueous
particles and into reactive aqueous particles where they can undergo further oxidation. While it is
104
possible that some coagulation of unreactive and reactive particles occurs, this is expected to be
slow relative to the rates of partitioning. Partitioning timescales for a-pinene SOA products have
been estimated to be < 5 min for 80% RH and ammonium sulfate particles with a diameter _ 200
nm (Shiraiwa et al., 2013); because our experiments were run under the same conditions, we
expect our partitioning timescales to be similar. The theoretical coagulation timescale for the
particles in this experiment is -1 h; the coagulation timescale estimated from the rates of decay
of number and volume concentration over the first 20 min after the addition of FS seed stopped
is -2 h (assuming the rates of wall loss for number concentration and volume concentration are
the same). Therefore, it appears that partitioning plays a key role.
The primary oxidants in the aqueous FS experiments are likely to be hydroxyl radicals.
Therefore, the results should not be unique to iron-containing particles, but instead applicable to
deliquesced particles with any OH source. In order to estimate aqueous hydroxyl radical
concentrations, a repeat of the aqueous FS + H 2 0 2 experiment was run with 2 mM of adonitol (a
C5 polyol) added to the seed particles as an aqueous OH tracer. Based on the rate of decay of
adonitol (from tracer ions m/z 61 and 73) maximum [OH]aq was measured to be 3.3 x 10-3 M
during ozonolysis and 2.5 x 10-12 M after the addition of H2 0 2 , using kOH Of 1.6 x 109 M- s(Herrmann et al., 2010). However, because adonitol may act as a quencher of OH, the maximum
[OH]aq during the original experiment is likely greater than this. The mass concentration of
adonitol was -3 times greater than the mass concentration of SOA; if we assume that adonitol
and a-pinene ozonolysis products have similar rate constants for aqueous reaction with OH, then
[OH]aq during the original experiment could be -4 times as high. This is still within an order of
magnitude of the average OH concentrations estimated for ambient deliquesced particles
(Hern-mann et al., 2010); however, the estimated ambient concentrations are highly uncertain.
105
In order to determine the contribution from background organic material, a blank was run
under the same conditions as the aqueous FS + H202 experiment, but without the addition of any
a-pinene. This resulted in a COA of 2.6 [tg m-3, or -18% of the SOA in the version containing apinene. While this background may seem somewhat high, it likely undergoes the same types of
condensed-phase oxidation as the SOA, and therefore does not change our conclusions about the
ability of condensed-phase oxidants to produce more highly oxidized OA.
4.3.5 Isoprene SOA experiments
The isoprene experiments gave similar results to the a-pinene experiments. Ozonolysis of
isoprene with dry AS seed produced SOA with an O/C of 0.6 and H/C of 1.4 (Expt. 8 in Table 41). As with a-pinene ozonolysis, the SOA composition did not change over the course of the
experiments and was not affected by exposure to UV. Isoprene SOA formed with aqueous FS
seed and H 2 0 2 (Expt. 9) was extremely oxidized reaching an reaching an O/C of 1.9 and H/C of
0.6, and with a mass spectra that closely resembles that of oxalic acid (not shown, Mensah et al.,
2011; Takegawa et al., 2007). This establishes that the enhancement in the degree of oxidation
when SOA is formed in the presence of aqueous-phase oxidants is not unique to a-pinene. The
time evolution of sulfate-normalized mass concentrations of total organic signal and key ion
fragments are shown in Figure 4-8. The rise and decay of different ions at different times (e.g.
m/z 29 vs. 45 vs. 55) suggests that multiple generations of oxidation occur. Here, the addition of
H20 2 has a more substantial influence on the degree of oxidation than it did for a-pinene SOA,
and appears to initiate new chemistry (e.g. the divergence of m/z 29 and 43). As with a-pinene
SOA, exposure to UV results in the rapid photolysis of organic matter, except here nearly all of
the SOA is lost to the gas-phase. This is consistent with all the material having been converted to
oxalic acid as implied by the mass spectrum.
106
add H2 0 2
1
UV on
n~l
---- m/z
-m/z
-- m/z
29
mz 43
0.1g
44
45
69
m/z 551
0.1---
0
E
0
0.01-
0
0.001
0
200
800
600
400
reaction time (min)
Figure 4-8. Sulfate-normalized total organic and fragment ion time traces for ozonolysis of
isoprene with aqueous FS seed (Expt. 9).
4.3.6 Yields
As described in the experimental section, an accurate determination of yields is made
difficult by uncertainties in particle seed density, hygroscopicity, AMS collection efficiency, and
the relative detection of water in the AMS and SMPS. Therefore, the mass yields and carbon
yields presented in Table 4-1 are only approximate and should be treated as such. Regardless, we
will discuss possible explanations for trends in the reported values.
The dry AS experiments (I & 2) have the highest reported yields. These are somewhat
higher than previously reported yields for a-pinene ozonolysis, but this could be a result of
different experimental conditions (e.g. higher seed concentration, no use of an OH scavenger)
(Griffin et al., 1999; Hallquist et al., 2009). Based on the similarity in the composition of SOA in
the dry and aqueous AS experiments, it would be expected for aqueous AS yields to be similar,
but instead they are -35-50% lower. One possible explanation is if the condensation of organic
material decreases the hygroscopicity of the seed particles. This would drive some water out of
107
the condensed phase, causing an underestimate of yield for the aqueous AS experiments. Cocker
et al. (2001) also measured lower yields for a-pinene ozonolysis with aqueous AS than with dry
AS. However, they reported total mass yields (mass organic + mass water), so it is also possible
their yields were lower due to loss of water rather than less condensed organic material.
On average, the FS experiments have lower yields than the AS experiments, with a
greater difference between carbon yields than mass yields (due to the high oxygen content in the
organic material formed with FS seed). This suggests that aqueous oxidation results in some
fragmentation to smaller products that partition out of condensed phase. These yield comparisons
assume that the fresh SOA formed on dry AS has the same density as the oxidized SOA formed
on aqueous FS. However, it is likely that the oxidized SOA has a higher density; if we instead
use a density of 1.9 g cm- 3 (that of solid oxalic acid, so an upper limit for oxalate in solution) for
the FS experiments, their yields increase by a factor of 1.4 and are no longer significantly lower
than for the aqueous AS experiments. While these uncertainties make it difficult to say
conclusively, it is likely that the difference in densities is smaller than this and that the presence
of aqueous oxidants does in fact result in the loss of carbon to the gas phase.
Isoprene SOA formed in the presences of aqueous oxidants has a 48% lower yield and
68% lower carbon yield than isoprene SOA formed in the absence of condensed-phase oxidants.
However, because the product mass spectrum in the aqueous FS experiment resembles oxalic
acid, the SOA may have a density closer to 1.9 g cm~ 3 than 1.4 g cm- 3. Using this higher density
gives an initial yield of 0.027 and carbon yield of 0.010. These are both still lower than for the
dry AS experiment, further suggesting that aqueous oxidation leads to the loss of carbon from the
condensed phase.
108
4.3.7 Kinetics
10
-
0
U
.
aqueous phase
gas phase t
10
10-610
-
10-
-
0)gm
1
6(3
0
a
10*-6 - 0.a
10
100
*
-
00
.3
-6
X~
L)
i
1o
10
U
i
CO
Em
0)0
.0u
10,
1
>
WC
18c)
-2
d
= 10 pg/m)
6>
S
-1
0Ug
d
(liquid water content
14
o
-10
0
c
1
3
=
-16>
3
aqueous fine particles
(liquid water content
C
10
(a)
aqueous phase
gas phase
M10,80
0-1
I 14
-
10
1
)
0
00)
i 01
ioi
i
O-x "LL
10/
0
8
3
1
0
1012
10
10 18
Mb
/iMatnmy
u
V
1o
10
10
1
10
0
3
6
09
L-RM/atml)
10
10
10
1
Figure 4-9. Comparison of effective oxidation rates in the aqueous phase (blue circles) and gas
phase (red squares) in aqueous fine particles (a) and cloud droplets (b) for compounds with
varying Henry's law constants.
The atmospheric importance of these results depends in part on how the rates of
transformation within aqueous aerosol compare to gas phase transformation rates. Figure 4-9
compares the effective rates of oxidation in the gas phase to those in the atmospheric liquid water
phase (aqueous aerosol in panel a, or cloud droplets in panel b) for compounds with varying
Henry's law constants. These are calculated for reaction with hydroxyl radicals using an average
[OH] of 106 molec cm- 3 in the gas phase, 108 molec cm- 3 in aqueous aerosol, and 107 molec cm~ 3
in cloud droplets (Seinfeld & Pandis, 2006, Herrmann et al., 2010). The Henry's law constants
and rate constants (koH,aq and koH,gas) are compiled or estimated from the literature for each
compound (Sander, 1999; Haag & Yao, 1992; Kwok & Atkinson, 1995; NIST Webbook). These
estimates suggest that aqueous oxidation can outcompete gas-phase oxidation for compounds
with Henry's law constants > 108 M atm 1 for aqueous aerosol and > 100 M atm1 for cloud
droplets. The oxidation rates within aqueous particles in the experiments discussed here are on
the order of 10-4 s-' (e.g. rapid decay of m/z 43 in Fig. 4-7b following addition of FS seed),
109
suggesting that this condensed-phase chemistry could compete with gas-phase oxidation under
typical atmospheric conditions.
4.4 Conclusions and Implications
Our previous work showed that aqueous oxidants within submicron particles can lead to
the efficient oxidation of simple water-soluble compounds that start out in the condensed phase
(Daumit et al., 2014). In the present study, we have shown that this holds true for a more
complex, atmospherically relevant SOA system. By directly measuring the degree of oxidation,
we found that the presence of aqueous oxidants can have a significant effect on the formation of
SOA, and that chemistry within the condensed-phase can lead to the formation of highly
oxidized OA. This was observed for SOA formed from the ozonolysis of both a-pinene and
isoprene, suggesting that the effects are not unique to a single type of SOA. Furthermore, the
partitioning experiment suggests that even SOA initially formed on unreactive seed particles can
later undergo further oxidation if it encounters reactive aerosol. Our overall findings are
consistent with previous studies that have shown aqueous oxidation to be an efficient pathway
for the formation of oxidized organic material (Chu et al., 2014; Daumit et al., 2014; Lee et al.,
2011; Nguyen et al., 2013).
We also found that exposure to UV irradiation caused additional significant changes to
OA mass loading and composition. For experiments with aqueous AS seed, UV resulted in
higher degrees of oxidation, likely via the formation of aqueous oxidants from photolysis of
SOA products and/or H 2 0 2 . For example, organic peroxides have been shown to make up -2050% by mass of a-pinene ozonolysis SOA (Docherty et al., 2005; Epstein et al., 2014), and are
known to generate OH radicals upon photolysis (Lim et al., 2010; Ng et al., 2011). For the
experiments with FS seed, exposure to UV again led to an increased production of aqueous
110
oxidants (here, presumably via photo-Fenton-like chemistry), but also resulted in photolysis of
the most oxidized products and a loss of organic material from the condensed phase.
A number of other studies have also shown UV irradiation to cause important changes to
SOA. Wong et al. (2014) found that photolysis of SOA from a-pinene ozonolysis resulted in loss
of organic material from the particles and produced more oxidized SOA, both effects that were
greater at higher RH. They attributed the higher degree of oxidation to both the loss of less
oxidized products such as carbonyls, and the formation of additional oxidants. The latter
mechanism is consistent with all of our aqueous experiments. However, in our FS experiments,
we saw a dramatic decrease in OA mass, likely due to the photolysis of the more oxidized
products (presumably photolabile iron-carboxylate complexes) (Sorooshian et al., 2013). Unlike
Wong et al. (2014), in our experiments UV irradiation had no effect at low RH and did not affect
mass yields for any AS experiments; this could be due to experimental differences (e.g. SOA
from filter extracts vs. formed in situ). In another study, SOA from a-pinene ozonolysis was
found to increase the photolysis rate of 2,3-dinitrophenol, suggesting it can enhance indirect
photodegradation (Lignell and Hinks, 2014). A number of low RH studies have found UV
irradiation to result in loss of material from the condensed phase for SOA generated from
ozonolysis of a-pinene (Epstein et al., 2014; Presto et al., 2005), ozonolysis of d-limonene
(Mang et al., 2008), and low-NOx photooxidation of isoprene (Surratt et al., 2006). It is unclear
why we do not observe this loss of organic material in the low RH AS experiments, but it is
likely to be a result of differences between experiments (e.g. Presto et al. irradiated the particles
during SOA formation; Epstein et al. used denuders to remove VOCs and ozone; and the other
studies used different SOA precursors or oxidants). Regardless, our finding that UV effects are
111
enhanced at high RH suggests that the previously documented UV effects may be even greater
for humid systems.
These results demonstrate that the presence of aqueous oxidants can have a dramatic
effect on SOA formation. It has been estimated that on a global scale, aerosol water exceeds dry
aerosol mass, making it a potentially important phase (Ervens et al., 2011). However, because
there is currently great uncertainty regarding the ambient concentrations of condensed-phase
oxidants, it is difficult to quantify what effect condensed-phase chemistry may have on overall
SOA production. Currently, the only estimates of oxidant concentrations within deliquesced
particles come from models (Herrmann et al., 2010). Therefore, direct measurements of
concentrations of condensed-phase oxidants in the atmosphere would be extremely valuable for
future studies and could help determine the overall importance of oxidation within the condensed
phase.
4.5 References
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seed aerosols on secondary organic aerosol formation from photooxidation of a- pinene/NOx and
toluene/NOx, Atmos. Environ., 55, 26-34, doi:10.101 6/j.atmosenv.2012.03.006, 2012.
Chu, B., Wang, K., Takekawa, H., Li, J., Zhou, W., Jiang, J., Ma, Q., He, H. and Hao, J.:
Hygroscopicity of particles generated from photooxidation of a-pinene under different oxidation
conditions in the presence of sulfate seed aerosols, J. Environ. Sci., 26(1), 129-139,
doi:10.1016/S1001-0742(13)60402-7, 2014.
Cocker III, D. R., Clegg, S. L., Flagan, R. C., and Seinfeld, J. H.: The effect of water on gasparticle partitioning of secondary organic aerosol. Part I: a-pinene/ozone system, Atmos.
Environ., 35, 6049-6072.
Daumit, K. E., Carrasquillo, A. J., Hunter, J. F. and Kroll, J. H.: Laboratory studies of the
aqueous-phase oxidation of polyols: Submicron particles vs. bulk aqueous solution, Atmos.
Chem. Phys., 14, 10773-10784, doi:10.5194/acp-14-10773-2014, 2014.
Epstein, S. A., Blair, S. L. and Nizkorodov, S. A.: Direct photolysis of a-pinene ozonolysis
secondary organic aerosol: Effect on particle mass and peroxide content, Environ. Sci. Technol.,
48, 11251-11258, doi:10.1021/es502350u, 2014.
112
Ervens, B., Turpin, B. J. and Weber, R. J.: Secondary organic aerosol formation in cloud droplets
and aqueous particles (aqSOA): a review of laboratory, field and model studies, Atmos. Chem.
Phys., 11(21), 11069-11102, doi:10.5194/acp-11-11069-2011, 2011.
George, I. J. and Abbatt, J. P. D.: Heterogeneous oxidation of atmospheric aerosol particles by
gas-phase radicals, Nat. Chem., 2(9), 713-722, doi:10.1038/nchem.806, 2010.
Gomez Alvarez, E., Wortham, H., Strekowski, R., Zetzsch, C., Gligorovski, S.: Atmospheric
photosensitized heterogeneous and multiphase reactions: From outdoors to indoors, Environ. Sci.
Technol., 46(4), 1955-1963, 2012.
Griffin, R. J., Cocker 111, D. R., Flagan, R. C., Seinfeld, J. H.: Organic aerosol formation from
the oxidation of biogenic hydrocarbons, J. Geophys. Res., 104(D3), 3555-3567, 1999.
Hallquist, M., Wenger, J. C., Baltensperger, U., Rudich, Y., Simpson, D., Claeys, M., Dommen,
J., Donahue, N. M., George, C., Goldstein, A. H., Hamilton, J. F., Herrmann, H., Hoffmann, T.,
linuma, Y., Jang, M., Jenkin, M. E., Jimenez, J. L., Kiendler-Scharr, A., Maenhaut, W.,
McFiggans, G., Mentel, Th. F., Monod, A., Prdvot, A. S. H., Seinfeld, J. H., Surratt, J. D.,
Szmigielski, R., and Wildt, J.: The formation, properties and impact of secondary organic
aerosol: Current and emerging issues, Atmos. Chem. Phys., 9, 5155-5236, 2009.
Herrmann, H., Hoffmann, D., Schaefer, T., Brduer, P. and Tilgner, A.: Tropospheric
aqueous-phase free-radical chemistry: Radical sources, spectra, reaction kinetics and prediction
tools, Chem. Phys. Chem., 11(18), 3796-3822, doi:10.1002/cphc.201000533, 2010.
Kessler, S. H., Smith, J. D., Che, D. L., Worsnop, D. R., Wilson, K. R. and Kroll, J. H.:
Chemical sinks of organic aerosol: kinetics and products of the heterogeneous oxidation of
erythritol and levoglucosan, Environ. Sci. Technol., 44, 7005-7010, 2010.
Kessler, S. H., Nah, T., Daumit, K. E., Smith, J. D., Leone, S. R., Kolb, C. E., Worsnop, D. R.,
Wilson, K. R. and Kroll, J. H.: OH-initiated heterogeneous aging of highly oxidized organic
aerosol, J. Phys. Chem. A, 116(24), 6358-6365, doi:10.1021/jp212131m, 2012.
Lee, A. K., Herckes, P., Leaitch, W. R., Macdonald, A. M. and Abbatt, J.: Aqueous OH
oxidation of ambient organic aerosol and cloud water organics: Formation of highly oxidized
products, Geophys. Res. Lett., 38(11), doi:10.1029/201 1GL047439, 2011.
Lignell, H. and Hinks, M. L.: Exploring matrix effects on photochemistry of organic aerosols,
Proc. Natl. Acad. Sci. U. S. A., 111(38), 13780-13785, 2014.
Mang, S. A., Henricksen, D. K., Bateman, A. P., Andersen, M. P. S., Blake, D. R. and
Nizkorodov, S. A.: Contribution of carbonyl photochemistry to aging of atmospheric secondary
organic aerosol, J. Phys. Chem. A, 112(36), 8337-8344, doi:10.1021/jp804376c, 2008.
Mensah, A. A., Buchholz, A., Mentel, T. F., Tillmann, R. and Kiendler-Scharr, A.: Aerosol mass
spectrometric measurements of stable crystal hydrates of oxalates and inferred relative ionization
efficiency of water, J. Aerosol Sci., 42(1), 11-19, doi:10.1016/j.jaerosci.2010.10.003, 2011.
113
Nguyen, T. B., Coggon, M. M., Flagan, R. C. and Seinfeld, J. H.: Reactive uptake and photoFenton oxidation of glycolaldehyde in aerosol liquid water, Environ. Sci. Technol., 47(9), 4307-
4316, doi:10.1021/es400538j, 2013.
Presto, A. A., Hartz, K. and Donahue, N. M.: Secondary organic aerosol production from terpene
ozonolysis. 1. Effect of UV radiation, Environ. Sci. Technol, 39(18), 7036-7045, 2005.
Robinson, A. L., Donahue, N. M. and Rogge, W. F.: Photochemical oxidation and changes in
molecular composition of organic aerosol in the regional context, J. Geophys. Res., 111 (D3),
D03302, doi: 10.1 029/2005JD006265, 2006.
Seinfeld, J. H. and Pandis, S. N.: Atmospheric chemistry and physics: From air pollution to
climate change, 2nd Edition, John Wiley & Sons, Inc., Hoboken, New Jersey, 2006.
Shiraiwa, M., Zuend, A., Bertram, A. K. and Seinfeld, J. H.: Gas-particle partitioning of
atmospheric aerosols: interplay of physical state, non-ideal mixing and morphology, Phys.
Chem. Chem. Phys., 15(27), 11441-11453, doi:10.1039/c3cp51595h, 2013.
Sorooshian, A., Wang, Z., Coggon, M. M., Jonsson, H. H. and Ervens, B.: Observations of sharp
oxalate reductions in stratocumulus clouds at variable altitudes: Organic acid and metal
measurements during the 2011 E-PEACE campaign, Environ. Sci. Technol, 47(14), 7747-7756,
doi:10.1021/es4012383, 2013.
Surratt, J. D., Murphy, S. M., Kroll, J. H., Ng, N. L., Hildebrandt, L., Sorooshian, A.,
Szmigielski, R., Vermeylen, R., Maenhaut, W., Claeys, M., Flagan, R. C. and Seinfeld, J. H.:
Chemical composition of secondary organic aerosol formed from the photooxidation of isoprene,
J. Phys. Chem. A, 110(31), 9665-9690, doi:10.1021/jp061734m, 2006.
Takegawa, N., Miyakawa, T., Kawamura, K. and Kondo, Y.: Contribution of selected
dicarboxylic and wo-oxocarboxylic acids in ambient aerosol to the m/z 44 signal of an Aerodyne
aerosol mass spectrometer, Aerosol Sci. Technol., 41(4), 418-437,
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Volkamer, R., Ziemann, P. J. and Molina, M. J.: Secondary organic aerosol formation from
acetylene (C 2H 2): Seed effect on SOA yields due to organic photochemistry in the aerosol
aqueous phase, Atmos. Chem. Phys., 9(6), 1907-1928, 2009.
von Gunten, U.: Ozonation of drinking water: Part 1. Oxidation kinetics and product formation,
Water Res., 37(7), 1443-1467, 2003.
Wong, J., Zhou, S. and Abbatt, J.: Changes in secondary organic aerosol composition and mass
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114
Chapter 5
Conclusions
A thorough understanding of the mechanisms governing the formation and evolution of
organic aerosol (OA) is necessary in order to predict the impact of particulate matter on climate
change, air quality, and human health. This thesis has aimed to improve our understanding of the
formation of highly oxidized OA, with specific attention given to the role of aqueous-phase oxidation. Here, we summarize our key findings, discuss their broader implications, and suggest
possible areas for future research.
In Chapter 2, we developed a novel retrosynthetic approach for constraining the potential
formation pathways and precursors of highly oxidized OA. This involved starting with the
known products (oxidized OA components), and moving backwards toward possible reactants
(SOA precursors). The "target" product was defined in terms of an average chemical formula,
determined from properties measured in field studies as well as structure-activity relationships. A
set of simple, general rules were developed to constrain possible atmospheric reactions and allow
for the identification of viable reaction pathways and aerosol precursors. Three general formation
mechanisms were found to be possible: (1) functionalization reactions that add multiple function
groups per oxidation step, (2) oligomerization of highly oxidized precursors, or (3) fast aging
within the condensed phase, such as oxidation within aqueous particles. The accuracy of this approach was somewhat hindered by the limited number of field measurements of OA volatility
(which allows for the estimation of carbon number) and formation kinetics (which constrain the
115
number of oxidation steps). Therefore, additional measurements of these parameters in future
field studies could improve this method by allowing for further characterization of the target
molecule and better constraints on possible formation mechanisms. While the partitioning of organic species in this study was limited to either the gas phase or condensed-organic phase, this
approach could be extended to explicitly include partitioning to and reactions within the liquidwater phase. This could be done using existing structure-activity relationships to determine Henry's law constants from chemical properties (elemental ratios, functional group distributions,
etc.), and could be valuable given that aqueous-phase aging was suggested as a possible pathways for oxidized OA formation.
Chapters 3 and 4 focused on experiments designed to study the role of chemistry within
the aqueous-phase (wet particles or cloud droplets) in the formation of highly oxidized OA. In
Chapter 3, we described experiments designed to directly compare aqueous oxidation chemistry
under vastly different partitioning conditions. Polyols (which served as simple surrogates for water-soluble compounds in the atmosphere) were oxidized both in aqueous submicron particles
and in bulk aqueous solution. Both sets of experiments resulted in rapid oxidation, but substantially more carbon was lost from the submicron particles, likely due to differences in partitioning
of early-generation products. These results confirmed that aqueous-phase oxidation is an efficient pathway for the rapid formation of highly oxidized material. However, they also implied
that bulk oxidation experiments may not accurately simulate the chemistry that occurs in the atmospheric aqueous phase, due to large differences in liquid water content (LWC) and therefore
partitioning. This underscores the need for future experiments to be carried out within aqueous
submicron particles where partitioning conditions are more representative of the atmosphere.
116
In Chapter 4, we examined the role of aqueous oxidants on the formation of SOA from
the gas-phase ozonolysis of atmospherically important biogenic precursors. Shifting the focus to
a more complex and realistic SOA system allowed for better insight into the potential effects of
aqueous oxidation in the atmosphere. In these experiments the presence of aqueous oxidants was
found to dramatically influence SOA formation, leading to substantially higher degrees of oxidation. This demonstrated that chemistry within aerosol particles can result in the formation of
highly oxidized material. In some cases, exposure to UV irradiation was shown to enhance the
degree of oxidation presumably via increased production of condensed-phase oxidants. These
experiments represent the first time organic aerosol with this high a degree of oxidation has been
generated from ozonolysis of gas-phase precursors.
This thesis has provided compelling evidence that oxidation within aqueous particles
could play an important role in the formation of highly oxidized OA. We have shown that aqueous-phase chemistry, when it occurs, can result in efficient oxidation of organic material. We
also demonstrated the need for future studies to be conducted under atmospherically relevant
partitioning conditions, since oxidation within bulk aqueous solutions was found to differ from
oxidation in aqueous aerosol. Our overall findings suggest that the inclusion of aqueous-phase
chemistry in atmospheric models needs to be considered. However, the atmospheric importance
of these processes, as well as their integration into models, are both strongly dependent on the
concentrations of oxidants within atmospheric particles. These concentrations, which currently
only exist as estimations from models, are highly uncertain, largely due to the absence of direct
measurements of aqueous oxidants in the atmosphere. Therefore, future measurements of ambient aqueous oxidant concentrations could prove extremely valuable. These could be used along
with detailed information regarding liquid water contents to better estimate the relative im-
117
portance of aqueous- and gas-phase chemistry under different conditions. These measurements
together with the results of this thesis would prove a valuable addition to current models.
118