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SECONDARY AEROSOL FORMATION MECHANISMS IN POLAR AREAS BY DIRECT
MEASUREMENT OF CLUSTER CHEMICAL COMPOSITION AND CONDENSING VAPOURS
M. SIPILÄ1, T. JOKINEN1, N. SARNELA1, H. JUNNINEN1, J. KONTKANEN1, O. PERÄKYLÄ1, D.
WIMMER1, O. KAUSIALA1,2, L. AHONEN1, J. MIKKILÄ1, T. PETÄJÄ1, D.R. WORSNOP1,3, I. E.
NIELSEN4, H. SKOV4, A. MASSLING4, A. VIRKKULA5, M. KULMALA1 and V.-M. KERMINEN1
1
Department of Physics, University of Helsinki, Helsinki, Finland
2
Tampere University of Technology, Tampere, Finland
3
Aerodyne Research Inc. Billerica, Massachusetts, USA
4
Arctic Research Centre, Department of Environmental Science, Aarhus University, Denmark
5
Finnish Meteorological Institute, Helsinki, Finland
Keywords: CLUSTER, NUCLEATION, MASS SPECTROMETRY.
INTRODUCTION
Formation of new aerosol particles from trace gases is a major source of cloud condensation nuclei (CCN)
in the global atmosphere (Dunne et al., 2016). Controlled laboratory experiments have recently provided
detailed, molecular level insight into different nucleation pathways (e.g. Kirkby et al., 2011 and subsequent
work performed in CERN CLOUD experiment) but molecular steps and compounds involved in real
atmospheric nucleation have been resolved only in very rare locations (Sipilä et al., 2016; Bianchi et al.,
2016). Secondary aerosol formation is particularly important in the Arctic and Antarctic atmospheres with
low anthropogenic aerosol emissions. Here, we summarize our recent observation of molecular steps of new
particle formation from Antarctica and from Arctic polar areas.
METHODS
Field experiments were performed in Queen Maud Land, Antarctica (Nov 2014 – Feb 2015) and at Villum
Resarch Station, Station Nord (Feb - Aug 2015), northern Greenland. Both campaigns utilized an
Atmospheric Pressure Interface – mass spectrometer (APi-TOF) for measuring the chemical composition of
ion clusters and a nitrate ion based Chemical Ionization APi-TOF mass spectrometer for measuring the
chemical composition of neutral clusters as well as concentrations of low volatility particle precursor
vapours, such as sulphuric acid, methane sulphonic acid, iodic acid (Sipilä et al., 2016) and highly oxidized
extremely low vapour pressure organic compounds, ELVOC (Ehn et al., 2014). Particle Size Magnifier
(PSM), Neutral cluster and Air Ion Spectrometer and a Differential Mobility Particle Sizer (DMPS) were
used for measuring the particle size distributions between ca. 1.3 and 800 nm in diameter.
RESULTS AND CONCLUSIONS
Several NPF events, showing the formation of nucleation mode particles and their further growth, were
during the Antarctic campaign (Fig. 1). Air mass trajectory analysis revealed that the NPF events occurred
only under marine air masses. During the NPF events, relatively high concentrations of sulphuric acid, likely
originating from the oxidation of dimethyl sulphide, were observed. Methane sulphonic acid and iodic acid
(HIO3) were also detected but with lower concentrations. Measurements revealed the also the chemical
composition of freshly nucleated clusters and their further growth mechanism to climatically relevant sizes.
Several NPF events were recorded also in Northern Greenland. There, at least two different type of cluster
formation mechanism were observed. Fist mechanism was likely related to iodine emissions from sea ice,
subsequent gas phase conversion of molecular iodine to iodic acid and iodic acid homogeneous nucleation
and possibly subsequent restructuring to I2O5 (Figure 2) (Sipilä et al., 2016). The other mechanism was
likely connected to emissions from the open, ice free, ocean. Results from both sites will be presented in
detail during the conference.
Figure 1. New particle formation event recorded by a DMPS in Antarctica in early January 2015.
Figure 2. Example mass spectrum of iodic acid clusters recorded at Villum Research station, Station Nord
in spring 2015. Mass defect describes the difference between the measured mass (in Da) and the sum of
nucleons in the atomic nuclei of the clusters and, together with the cluster mass, unambiguously reveals
the atomic composition of the clusters. Area of the dot is related to the observed signal strength. For
clarity, only the iodic acid and iodine oxide clusters are depicted.
ACKNOWLEDGEMENTS
This work was supported by Academy of Finland, European Research Council and Nordic Centre of
Excellence CRAICC. Villum Research Station, Station Nord and Finnish Antarctic Research Program are
acknowledged for logistics support.
REFERENCES
Bianchi et al. (2016). New particle formation in the free troposphere: A question of chemistry and timing.
Science, DOI: 10.1126/science.aad5456.
Dunne et al. (2016). Global atmospheric particle formation from CERN CLOUD measurements. Science,
DOI: 10.1126/science.aaf2649.
Ehn et al. (2014). A large source of low-volatility secondary organic aerosol. Nature, 506(7489):476–479.
Kirkby J. et al. (2011). Role of sulphuric acid, ammonia and galactic cosmic rays in atmospheric aerosol
nucleation. Nature, 476:429–U77.
Sipilä M. et al. (2016). Molecular-scale evidence of aerosol particle formation via sequential addition of
HIO3. Nature, 537, 532–534.