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Volume 18, issue 10
Atmos. Chem. Phys., 18, 7041–7056, 2018
https://doi.org/10.5194/acp-18-7041-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue: CHemistry and AeRosols Mediterranean EXperiments (ChArMEx)...

Atmos. Chem. Phys., 18, 7041–7056, 2018
https://doi.org/10.5194/acp-18-7041-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 23 May 2018

Research article | 23 May 2018

Aerosol composition and the contribution of SOA formation over Mediterranean forests

Evelyn Freney1, Karine Sellegri1, Mounir Chrit2, Kouji Adachi3, Joel Brito1, Antoine Waked1,a, Agnès Borbon1, Aurélie Colomb1, Régis Dupuy1, Jean-Marc Pichon1, Laetitia Bouvier1, Claire Delon4, Corinne Jambert4, Pierre Durand4, Thierry Bourianne5, Cécile Gaimoz6, Sylvain Triquet6, Anaïs Féron6, Matthias Beekmann6, François Dulac7, and Karine Sartelet2 Evelyn Freney et al.
  • 1Laboratoire de Météorologie Physique, CNRS-Université Clermont Auvergne, UMR6016, 63117, Clermont Ferrand, France
  • 2CEREA, Joint Laboratoire École des Ponts ParisTech – EDF R & D, Université Paris-Est, 77455 Marne-la-Vallée, France
  • 3Meteorological research institute, Atmospheric Environment and Applied Meteorology Research Department, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan
  • 4Laboratoire d'Aérologie, CNRS-Université de Toulouse, CNRS, UPS, Toulouse, France
  • 5Centre National de Recherches Météorologiques, Météo-France-CNRS, Toulouse, URA1357, France
  • 6Laboratoire Interuniversitaire des Systèmes Atmosphériques, LISA/IPSL, UMR CNRS 7583, Université Paris Est Créteil (UPEC), France
  • 7Laboratoire des Sciences du Climat et de l'Environnement, LSCE/IPSL, UMR 8212 CEA-CNRS-UVSQ, Université Paris-Saclay, Gif-sur-Yvette, France
  • anow at: IMT Lille Douai, Sciences de l'Atmosphère et Génie de l'Environnement (SAGE), 59508 Douai CEDEX, France

Abstract. As part of the Chemistry-Aerosol Mediterranean Experiment (ChArMEx), a series of aerosol and gas-phase measurements were deployed aboard the SAFIRE ATR42 research aircraft in summer 2014. The present study focuses on the four flights performed in late June early July over two forested regions in the south of France. We combine in situ observations and model simulations to aid in the understanding of secondary organic aerosol (SOA) formation over these forested areas in the Mediterranean and to highlight the role of different gas-phase precursors. The non-refractory particulate species measured by a compact aerosol time-of-flight mass spectrometer (cToF-AMS) were dominated by organics (60 to 72 %) followed by a combined contribution of 25 % by ammonia and sulfate aerosols. The contribution from nitrate and black carbon (BC) particles was less than 5 % of the total PM1 mass concentration. Measurements of non-refractory species from off-line transmission electron microscopy (TEM) showed that particles have different mixing states and that large fractions (35 %) of the measured particles were organic aerosol containing C, O, and S but without inclusions of crystalline sulfate particles. The organic aerosol measured using the cToF-AMS contained only evidence of oxidized organic aerosol (OOA), without a contribution of fresh primary organic aerosol. Positive matrix factorization (PMF) on the combined organic–inorganic matrices separated the oxidized organic aerosol into a more-oxidized organic aerosol (MOOA), and a less-oxidized organic aerosol (LOOA). The MOOA component is associated with inorganic species and had higher contributions of mz 44 than the LOOA factor. The LOOA factor is not associated with inorganic species and correlates well with biogenic volatile organic species measured with a proton-transfer-reaction mass spectrometer, such as isoprene and its oxidation products (methyl vinyl ketone, MVK; methacroleine, MACR; and isoprene hydroxyhydroperoxides, ISOPOOH). Despite a significantly high mixing ratio of isoprene (0.4 to 1.2 ppbV) and its oxidation products (0.2 and 0.8 ppbV), the contribution of specific signatures for isoprene epoxydiols SOA (IEPOX-SOA) within the aerosol organic mass spectrum (mz 53 and mz 82) were very weak, suggesting that the presence of isoprene-derived SOA was either too low to be detected by the cToF-AMS, or that SOA was not formed through IEPOX. This was corroborated through simulations performed with the Polyphemus model showing that although 60 to 80 % of SOA originated from biogenic precursors, only about 15 to 32 % was related to isoprene (non-IEPOX) SOA; the remainder was 10 % sesquiterpene SOA and 35 to 40 % monoterpene SOA. The model results show that despite the zone of sampling being far from industrial or urban sources, a total contribution of 20 to 34 % of the SOA was attributed to purely anthropogenic precursors (aromatics and intermediate or semi-volatile compounds).

The measurements obtained during this study allow us to evaluate how biogenic emissions contribute to increasing SOA concentrations over Mediterranean forested areas. Directly comparing these measurements with the Polyphemus model provides insight into the SOA formation pathways that are prevailing in these forested areas as well as processes that need to be implemented in future simulations.

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The focus of these experiments, within the ChArMEx project, were to better understand the chemical properties of ambient aerosols over the Mediterranean region. A series of airborne measurements were performed aboard the French research aircraft, the ATR42, during the summer period. Aerosol and gas-phase chemical mass spectrometry allowed us to understand the sources and formation of organic aerosols. Numerical models were incorporated into this study to help interpret our observations.
The focus of these experiments, within the ChArMEx project, were to better understand the...
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