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Volume 17, issue 10 | Copyright
Atmos. Chem. Phys., 17, 6373-6391, 2017
https://doi.org/10.5194/acp-17-6373-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 29 May 2017

Research article | 29 May 2017

Chemical and isotopic composition of secondary organic aerosol generated by α-pinene ozonolysis

Carl Meusinger1,a, Ulrike Dusek2,3, Stephanie M. King1,4, Rupert Holzinger2, Thomas Rosenørn1,5, Peter Sperlich6,b, Maxime Julien7, Gerald S. Remaud7, Merete Bilde1,c, Thomas Röckmann2, and Matthew S. Johnson1 Carl Meusinger et al.
  • 1Department of Chemistry, University of Copenhagen, 2100, Copenhagen Ø, Denmark
  • 2Institute for Marine and Atmospheric research Utrecht (IMAU), Utrecht University, 3584 CC, Utrecht, the Netherlands
  • 3Centre for Isotope Research, Energy and Sustainability Research Institute Groningen, 9747 AG Groningen, the Netherlands
  • 4now at: Haldor Topsøe A/S, 2800, Kgs. Lyngby, Denmark
  • 5Infuser ApS, 2200, Copenhagen N, Denmark
  • 6Max-Planck Institute for Biogeochemistry, 07745 Jena, Germany
  • 7CEISAM, UMR CNRS6230, BP 92208, Nantes 44322 cedex 3, France
  • anow at: Airlabs, 2200, Copenhagen N, Denmark
  • bnow at: National Institute of Water and Atmospheric Research (NIWA), Wellington 6021, New Zealand
  • cnow at: Aarhus University, Department of Chemistry, 8000 Aarhus C, Denmark

Abstract. Secondary organic aerosol (SOA) plays a central role in air pollution and climate. However, the description of the sources and mechanisms leading to SOA is elusive despite decades of research. While stable isotope analysis is increasingly used to constrain sources of ambient aerosol, in many cases it is difficult to apply because neither the isotopic composition of aerosol precursors nor the fractionation of aerosol forming processes is well characterised. In this paper, SOA formation from ozonolysis of α-pinene – an important precursor and perhaps the best-known model system used in laboratory studies – was investigated using position-dependent and average determinations of 13C in α-pinene and advanced analysis of reaction products using thermal-desorption proton-transfer-reaction mass spectrometry (PTR-MS). The total carbon (TC) isotopic composition δ13C of the initial α-pinene was measured, and the δ13C of the specific carbon atom sites was determined using position-specific isotope analysis (PSIA). The PSIA analysis showed variations at individual positions from −6.9 to +10. 5 ‰ relative to the bulk composition. SOA was formed from α-pinene and ozone in a constant-flow chamber under dark, dry, and low-NOx conditions, with OH scavengers and in the absence of seed particles. The excess of ozone and long residence time in the flow chamber ensured that virtually all α-pinene had reacted. Product SOA was collected on two sequential quartz filters. The filters were analysed offline by heating them stepwise from 100 to 400°C to desorb organic compounds that were (i) detected using PTR-MS for chemical analysis and to determine the O:C ratio, and (ii) converted to CO2 for 13C analysis.

More than 400 ions in the mass range 39–800Da were detected from the desorbed material and quantified using a PTR-MS. The largest amount desorbed at 150°C. The O:C ratio of material from the front filter increased from 0.18 to 0.25 as the desorption temperature was raised from 100 to 250°C. At temperatures above 250°C, the O:C ratio of thermally desorbed material, presumably from oligomeric precursors, was constant. The observation of a number of components that occurred across the full range of desorption temperatures suggests that they are generated by thermal decomposition of oligomers.

The isotopic composition of SOA was more or less independent of desorption temperature above 100°C. TC analysis showed that SOA was enriched in 13C by 0.6–1.2‰ relative to the initial α-pinene. According to mass balance, gas-phase products will be depleted relative to the initial α-pinene. Accordingly, organic material on the back filters, which contain adsorbed gas-phase compounds, is depleted in 13C in TC by 0.7‰ relative to the initial α-pinene, and by 1.3‰ compared to SOA collected on the front filter. The observed difference in 13C between the gas and particle phases may arise from isotope-dependent changes in the branching ratios in the α-pinene + O3 reaction. Alternatively, some gas-phase products involve carbon atoms from highly enriched and depleted sites, as shown in the PSIA analysis, giving a non-kinetic origin to the observed fractionations. In either case, the present study suggests that the site-specific distribution of 13C in the source material itself governs the abundance of 13C in SOA.

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Isotope studies can constrain budgets of secondary organic aerosol (SOA) that is pivotal to air pollution and climate. SOA from α-pinene ozonolysis was found to be enriched in 13C relative to the precursor. The observed difference in 13C between the gas and particle phases may arise from isotope-dependent changes in branching ratios. Alternatively, some gas-phase products involve carbon atoms from highly enriched and depleted sites, giving a non-kinetic origin to the observed fractionations.
Isotope studies can constrain budgets of secondary organic aerosol (SOA) that is pivotal to air...
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