Download Evidence for High Molecular Weight Nitrogen

Environ. Sci. Technol. 2010, 44, 4441–4446
Evidence for High Molecular Weight
Nitrogen-Containing Organic Salts in
Urban Aerosols
XIAOFEI WANG,† SONG GAO,‡
X I N Y A N G , * ,† H O N G C H E N , †
JIANMIN CHEN,† GUOSHUN ZHUANG,†
JASON D. SURRATT,§ MAN NIN CHAN,|
A N D J O H N H . S E I N F E L D * ,§,|
Department of Environmental Science and Engineering, Fudan
University, 220 Handan Road, Shanghai, 200433, China,
Division of Math, Science and Technology, Nova Southeastern
University, Fort Lauderdale, Florida 33314, and Division of
Chemistry and Chemical Engineering and Division of
Engineering and Applied Science, California Institute of
Technology, Pasadena, California 91125
Received January 12, 2010. Revised manuscript received
April 27, 2010. Accepted May 4, 2010.
High molecular weight (Mw) species were observed at
substantial intensities in the positive-ion mass spectra in
urban Shanghai aerosols collected from a single-particle timeof-flight mass spectrometer (in the m/z range 250-500)
during three separate periods over 2007-2009. These species
correlate well with the CN- mass signal, suggesting that
C-N bonds are prevalent and that the observed high-Mw species
are potentially nitrogen-containing organic salts. Anticorrelation with the ambient O3 concentration suggests that
photochemical oxidants are not involved directly in the formation
of these species. The Mannich reaction, among amines (or
ammonia), formaldehyde, and carbonyls with an adjacent, acidic
proton, is proposed as a plausible pathway leading to these
organic salts. Although the high-Mw species observed in the singleparticle mass spectra appear to be nitrogen-containing
organics, further chemical confirmation is desired to verify if
the proposed Mannich reaction can explain the formation of
these high-Mw species in regions where ammonia, amines, and
carbonyls are prevalent.
Introduction
Atmospheric fine particulate matter comprises a large fraction
of organic material (1). It is well established that oxidation
of volatile organic compounds (VOCs) produces low-volatility
compounds that partition into the particle phase to yield
secondary organic aerosol (SOA) (2). Particle-phase accretion
reactions, which are neither fully understood nor currently
incorporated in atmospheric models, lead to low-volatility
products and subsequently increase the particulate mass
fraction of SOA (2, 3). High-Mw species identified in SOA by
a variety of mass spectrometric techniques have suggested
* Address correspondence to either author. E-mail: yangxin@
fudan.edu.cn (X.Y.); [email protected] (J.H.S.).
†
Fudan University.
‡
Nova Southeastern University.
§
Division of Chemistry and Chemical Engineering, California
Institute of Technology.
|
Division of Engineering and Applied Science, California Institute
of Technology.
10.1021/es1001117
 2010 American Chemical Society
Published on Web 05/17/2010
the occurrence of particle-phase accretion reactions (4-8).
Mass spectra of SOA generated in laboratory studies frequently show a series of high-Mw ion peaks separated by
∆m/z 12, 14, and 16, indicating the likely presence of
oligomeric species (9, 10). In addition, gas-phase glyoxal and
methyl glyoxal can be absorbed into the aqueous particle
phase and undergo accretion reactions (11-14), which may
be acid-catalyzed (15). While an increase of the seed particle
acidity is found to enhance oligomer formation (7), oligomeric
species are present even when particles are not acidic
(4, 6, 7, 16). Although high-Mw species have been clearly
observed in laboratory-generated organic aerosols, the
ambient significance of particle-phase reactions responsible
for these species is not well established. High-Mw organosulfates (Mw up to 374) have been identified in ambient
aerosol from the Southeastern U.S. and forested regions of
Europe as well as in laboratory-generated SOA produced
from the photooxidation of biogenic VOCs (such as isoprene
and monoterpenes) in the presence of acidified sulfate seed
aerosol (17-19). This is currently one of the only laboratoryobserved accretion reactions to be found atmospherically
significant.
Despite recent observations of high-Mw organosulfates,
information on the exact nature and formation mechanisms
of other possible classes of high-Mw species in ambient
aerosols is lacking. By deploying an aerosol single-particle
time-of-flight mass spectrometer (ATOFMS), we report for
the first time high-Mw nitrogen-containing organic salts that
are widely present in aerosols in an urban atmosphere. Our
results also demonstrate a relationship with particle acidity
in the formation of this class of aerosol components.
Experimental Section
1. Real-Time Single Particle Analysis by ATOFMS. The size
and chemical composition of individual particles were
characterized in real time by an ATOFMS (TSI-3800), the
design and operation of which have been described previously (20). Briefly, aerosols are introduced into a vacuum
region through an aerodynamic lens that is designed to
effectively transmit particles in the size range of 100 nm to
3 µm. Each particle is accelerated to a size-dependent
terminal velocity. The velocity and thus the aerodynamic
diameter of each particle are determined by the time-offlight between two orthogonal, continuous diode-pumped
green lasers. The firing of the desorption/ionization laser
(Nd:YAG laser, 266 nm) is triggered based on the velocity
measurement when the particle enters the center of the
source region of the mass spectrometer. Both positive and
negative ions generated from laser ablation are analyzed
simultaneously. The power density of the desorption/
ionization laser was kept at about 1.0 × 108 W cm-2 in the
measurements reported here. Monodisperse polystyrene
latex spheres (Nanosphere Size Standards, Duke Scientific
Corp., Palo Alto) of 0.22-2.00 µm diameter were suspended
using an atomizer (TSI-3076) for size calibration.
The ATOFMS was located on the campus of Fudan
University in Shanghai at an urban site close to residential,
traffic, and construction emissions. Ambient aerosols were
sampled into the ATOFMS through a 4 m copper tube of 1.27
cm diameter. A cyclone was used on the inlet and pulled at
10 L min-1 to minimize particle residence time in the
sampling line. The inlet of the sampling tube was about 5 m
above the ground and 0.5 m above the roof of the building
on which the instrument was located. The ATOFMS was
operated around the clock in separate campaigns during
August 2007, December 2008, and March, May, and June
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FIGURE 1. Average mass spectrum of particles containing high-Mw species identified by ATOFMS.
2009. Local meteorological data, including temperature,
relative humidity (RH), atmospheric pressure, wind speed,
and direction, and hourly average O3 concentration were
provided by the Shanghai Meteorological Bureau.
2. Bulk Aerosol Sampling and LC-MS Analysis. PM2.5
aerosol samples were collected daily in 24 h intervals by a
medium-volume sampler (Model: (TSP/PM10/PM2.5)-2, flow
rate: 76.67 L min-1) in August 2007. An 8-stage impactor
(Anderson 8-Stage Non-Viable Cascade Impactor, Series
20-800, Thermo Fisher Scientific) was used to collect sizesegregated aerosol samples during May and June in 2009. All
the samples were collected on Whatman 41 polycarbonate
filters (Whatman Inc., Maidstone, U.K., diameter: 90 mm).
Samples were placed in polyethylene plastic bags immediately after sampling and stored in a freezer (-18 °C). All
the filters were weighed before and after sampling with an
analytical balance (Sartorius 2004 MP, precision 10 µg) after
stabilizing under constant temperature (20 ( 1 °C) and
humidity (40 ( 2%). All procedures were strictly qualitycontrolled to avoid possible contamination of the samples.
PM2.5 samples collected on August 1 and 2, 2007 and fine
particles collected in stages 5 (3.3-2.1 µm) to 8 (0.7-0.4 µm)
from the 8-stage impactor during May 24-25, June 11-12,
and June 13-15, 2009 were chemically characterized offline by ultra performance liquid chromatography (UPLC)
interfaced to an electrospray ionization time-of-flight mass
spectrometer (ESI-TOFMS). Coarse-mode aerosols (i.e.,
stages 1 to 4) were not chemically characterized in the present
study, since they were not measured by the ATOFMS. To
provide sufficient mass for detection, each film was extracted
individually in 5 mL of high-purity methanol (LC-MS
CHROMASOLV-Grade, Sigma-Aldrich) via 45 min of sonication, combined into a single 20 mL methanol extract, and
then dried under a gentle ultrahigh purity N2 stream at room
temperature. The dried sample was then reconstituted with
100 µL of 1:1 (v/v) solvent mixture of 0.1% acetic acid in
water and 0.1% acetic acid in methanol. Five microliters of
each reconstituted sample extract was then injected into a
Waters ACQUITY UPLC system, coupled with a Waters LCT
Premier TOFMS equipped with an ESI source, allowing for
accurate mass measurement for each observed ion. Operation
protocols, including column information and chromatographic method, for the UPLC/ESI-TOFMS technique have
been previously described (17). All sample extracts were
analyzed in both the positive and negative ion modes; for
the present study only the positive ion mode data are
considered. Blank films were also extracted and prepared in
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the same manner as the aerosol samples; the chemical
analyses of these blank films, as well as the solvent mixture
used to reconstitute the dried samples, by the UPLC/ESITOFMS technique served as the controls and were carefully
compared to the ambient samples to disregard any background signals. Using the MassLynx Version 4.1 software,
molecular formula searches for each measured ion employed
the following elemental limits: C (up to 1000 atoms), H (up
to 1000 atoms), N (up to 10 atoms), O (up to 1000 atoms),
S (up to 5 atoms), Na (up to 1 atom), K (up to 1 atom), Ca
(up to 1 atom), and Fe (up to 1 atom). In addition to using
these elemental limits, we considered only those molecular
formulas that were within 5 mDa and had an i-Fit (i.e., a
measure of how well the isotopic distributions match, closer
to zero is best) within 100.
Results and Discussion
Ambient ATOFMS measurements were carried out over 40
days in separate campaigns during August 2007, December
2008, and March, May, and June 2009. Here we focus on the
measurements during August 1-5 and 8-9, 2007, which are
characteristic of all samples (Supporting Information, Figure
S1). During the 7 days of sampling in August 2007, a total of
97,716 particles were identified as containing ion peaks in
the m/z range 250 to 500 (high-Mw species), which account
for 25.7% of the total collected particles for which both
positive and negative ion mode mass spectra were available.
Figure 1 shows the average mass spectra of particles
containing high-Mw species. These spectra are unique in that
most of the high-Mw ion peaks appear in the positive ion
mode, whereas relatively few appear in the negative ion
spectra. In a study of SOA generated in a laboratory chamber,
Gross et al. (21) found that high-Mw ion peaks appear in both
positive and negative ion spectra, but predominantly in the
negative ion spectra. Ambient ATOFMS measurements
identified oxygenated high-Mw ion peaks, also present mostly
in the negative ion spectra (10), attributed largely to the
negative charge centers on oxygen. Ion peaks of neutral
organic compounds would be present in both positive and
negative spectra; organic ion peaks that appear mostly in the
positive spectrum suggest strongly the presence of other
classes of compounds, such as organic salts (22, 23).
Cations of organic salts are likely responsible for the
numerous high-Mw ion peaks found in the positive ion
spectrum in Figure 1; some of the respective counterions,
inorganic in nature, can be seen in the negative ion mass
FIGURE 2. Comparison of hourly averaged relative peak areas
of high-Mw signals with (a) those of the CN- signal in the mass
spectra and (b) ambient O3.
spectrum. An organic salt cation requires a stable positive
charge center other than carbon, such as nitrogen. Indeed,
the strong signal at m/z 26 in the negative ion mode data can
be attributed to CN-, which is a definitive marker for species
with carbon-nitrogen (C-N) bonds. The CN- m/z of 26 can
also be assigned to C2H2-; however, the ion at m/z 26
correlates well with that of C3N- at m/z 50 (as shown in
Supporting Information, Figure S2), which confirms the CNassignment. Figure 2a shows the comparison of hourly
averaged relative peak areas of CN- and high-Mw signals.
The degree of correlation strongly suggests that C-N bonds
are prevalent in these high-Mw species. The possibility that
some of the signals represent fragments without N, generated
in laser ablation, cannot be totally ruled out, but such signals
would tend to be equally distributed in both positive and
negative ion spectra, not exclusively in the positive spectrum
(considering that C and H have lower electronegativity than
N). The mass signal for (C2H5)2NCH2+ (m/z 86) in Figure 1,
one of the marker ions for alkylamines (24), indicates that
amines were probably involved in the formation of at least
some of the high-Mw species observed. The m/z 86 ion could
be an aerosol component or a fragment from larger compounds. In either case, amines were likely involved in its
formation.
Amines are major nitrogen-containing atmospheric organic compounds, which are emitted by a variety of sources
including industry, motor vehicles, animal husbandry, sewage
treatment plants, and waste incinerators. Amines can easily
form amine salts with acids (25) or react with O3, OH, or NO3
and partition to the particle phase (26-28). Amine salts
produced by the direct reaction with inorganic acids usually
are of relatively low molecular weight (Mw < 150 Da), while
products from ozonolysis or other oxidation reactions would
not give the mass spectral patterns in Figure 1 (predominantly
present in the positive ion mode).
Particulate C-N compounds have been recently identified
in laboratory studies via reactions of glyoxal with ammonium
sulfate, amino acids, and amines (26-32). For example,
imidazoles have been suggested as a major form of C-N
compounds, which can be protonated to form organic salts.
With sufficient ambient abundance of glyoxal, ammonia,
sulfate, and amines, imidazole salts could be the source of
some of the ion peaks in Figure 1. However, the elevated
baseline with no simple, repetitive mass patterns as shown
in Figure 1 suggests that the high-Mw species result from a
variety of precursors. Even though ATOFMS data (with
relatively low mass resolution at around 700) do not allow
one to pinpoint the exact structures of these compounds, it
is unlikely that imidazole salts alone can explain all of the
complex mass spectral patterns observed. Taking into
consideration the elevated concentrations of formaldehyde
in Shanghai (33), a general mechanism for the formation of
C-N containing products based on the Mannich reaction
(34) can be proposed. As shown in Figure 3, in this mechanism
primary or secondary amines react with formaldehyde (or
other aldehydes) to form iminium cations. Note that tertiary
amines lack an N-H proton to form the intermediate
iminium cations. Under acidic conditions, compounds with
a carbonyl functional group can tautomerize to the enol form,
which can attack the iminium ion via nucleophilic addition
to form Mannich bases. The bases can be converted to organic
cations in the presence of H+. In the particle phase, aldehydes
or ketones can undergo aldol condensation and/or carboxyl
chemistry forming high-Mw species. Mannich bases can
condense further with formaldehyde and amines through
similar pathways, giving rise to a network of condensation
reactions and eventually forming a number of high-Mw
species. Ammonia, which is more abundant than amines in
the atmosphere, can also undergo Mannich reactions.
The overall Mannich reaction among formaldehyde, a
primary or secondary amine (or ammonia), and a carbonyl
that has an adjacent, acidic proton, can be expressed as
follows:
Altogether, this proposed class of organic salts is relatively
non-volatile and chemically stable (as opposed to accretion
reaction products that are reversible in formation). Like
organosulfates, they may represent a substantial fraction of
previously unidentified species in ambient organic aerosols,
especially in the presence of abundant formaldehyde and
amines (or ammonia). The extent to which these species
contribute to the total organic aerosol mass cannot be
accurately known until some of these compounds can be
quantified, or at least reasonable surrogate standard compounds can be identified for quantification.
For this work, we used soft-ionization MS techniques to
further examine the presence of these species and corresponding structures. The control filter blanks show multiple
peaks in the positive ion mode mass spectra, indicating the
presence of background species either in the filter media, in
the solvents, or because of the relatively long storage duration.
Nevertheless, after disregarding ion signals observed in the
base peak ion chromatograms (BPCs) of both the blank films
(or filters) and solvent only samples, there are noticeable
peaks in the BPCs of the Shanghai aerosol filter samples (see
Supporting Information, Figure S3). Peaks with very similar
retention times are seen in samples from different sampling
periods, indicating the presence of common species in these
samples. By comparing each chromatographic peak in the
controls versus the samples (see Supporting Information,
Figure S3 as an example), a number of molecular ions (i.e.,
[M+H]+ under the positive ion mode) are identified, as listed
VOL. 44, NO. 12, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Proposed formation mechanism of organic salts: Amines react with aldehydes and/or ketones in the presence of an acid.
TABLE 1. Nitrogen-Containing Organic Constituents Observed by UPLC/(+)ESI-TOFMS
TOFMS suggested ion formula [M+H]+
retention
time (RT) (min)
measured mass
actual mass of the TOFMS
suggested ion formula
C
H
N
0.95
1.57
1.81
3.91
4.66
5.66
6.96
7.09
8.33
8.73
9.58
102.1279
102.1276
102.1279
163.1240
128.1432
130.1592
182.1903
226.1417
240.1587
244.2086
282.2048
102.1283
102.1283
102.1283
163.1235
128.1439
130.1596
182.1909
226.1443
240.1600
244.2065
282.2045
6
6
6
10
8
8
12
12
13
17
12
16
16
16
15
18
20
24
20
22
26
24
1
1
1
2
1
1
1
1
1
1
1
in Table 1. The accurate mass measurements for these species
are also listed, along with the elemental composition
suggested by the ESI-TOFMS software (with a maximum
tolerance of (5 mDa error between the measured and
theoretical mass). The m/z 102 [M+H]+ ion corresponds most
likely to triethylamine or its isomeric alkylamines (26). While
it is difficult to pinpoint the structures of other species from
these data alone, it is likely that many of these species are
nitrogen-containing (e.g., amines) owing to odd-number
molecular masses. For those species with nitrogen but no
oxygen, amines are likely structures, analogous to triethylamine. They can exist either in the original aerosol samples
as alkylaminium salts formed from acid-base reactions (35)
or be derived from the Mannich-type products through
degradation during analysis. On the other hand, species that
contain both nitrogen and oxygen are also present. Based on
the C:H ratio, it can be seen that some of these species (e.g.,
C13H22NO3 and C12H20NO3) have unsaturated bonds such as
CdO. Since the observed high-Mw species are anticorrelated
with O3 (see below), direct photooxidation is unlikely the
dominant mechanism leading to formation of these positiveion mode species. Overall, the ESI-TOFMS data provide some
further, albeit indirect, evidence that the Mannich reaction
is a plausible pathway to form some of the observed highMw species in the positive ion mode ATOFMS spectra.
The relationship between particle acidity and the abundance of high-Mw species can be explored. The relative acidity
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O
S
Na
error (mDa)
i-Fit
1
-0.4
-0.7
-0.4
-0.5
-0.7
-0.4
-0.6
-2.6
-1.3
2.1
0.3
1.5
25.9
36.1
4.1
9.3
0.9
23.8
21.8
12.0
53.1
2.3
3
3
3
FIGURE 4. Relative peak area of high-Mw species versus hourly
averaged relative acidity.
of particles is defined as the ratio of the sum of the absolute
average peak areas of NO3- (m/z 62) and HSO4- (m/z 97) to
that of NH4+ (m/z 18) (10). Figure 4 compares hourly averaged
relative peak area of high-Mw compounds and the relative
acidity. The data suggest that formation of organic salts is
associated with elevated particle acidity, which is consistent
with the fact that the proposed reactions are generally acidcatalyzed. The leveling-off for high-Mw species exceeding
about 1500 in the relative acidity is consistent with proton
saturation of amines, which react with carbonyl groups less
readily at high acidity (36).
To rule out the possibility of SOA formation by ozonolysis
or other oxidation reactions as the main source of the mass
signals, the correlation of the high-Mw species with O3 levels
is examined. Figure 2b shows the temporal profiles for the
relative peak areas of high-Mw signals in the mass spectra
and for ambient O3. The anti-correlation between high-Mw
species and O3 suggests a pathway of SOA formation involving
nitrogen-containing species that occurs under relatively low
oxidant levels. High concentrations of oxidants may either
oxidize the carbonyls, ammonia, and amines or degrade the
Mannich-type products. Considering the presence of ammonia, amines, and carbonyls in the urban atmosphere, this
pathway could account for a substantial fraction of organic
aerosol mass.
Acknowledgments
The ATOFMS work here was supported by The National
Natural Science Foundation of China (20937001, 40875074
and 40875073). Support of J.D.S. and J.H.S. by the Electric
Power Research Institute is acknowledged. The authors thank
Douglas Worsnop for helpful discussions.
Supporting Information Available
Mass spectra of particles containing high-Mw species obtained
in other sampling periods and the correlation between the
signal intensities of CN- and C3N-. This material is available
free of charge via the Internet at http://pubs.acs.org.
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