Aerosol composition and the contribution of SOA formation over Mediterranean forests
Authors: Freney Evelyn , Sellegri Karine , Chrit Mounir , Adachi Kouji , Brito Joel , Waked Antoine , Borbon Agnès , Colomb Aurélie , Dupuy Régis , Pichon Jean-Marc , Bouvier Laetitia , Delon Claire , Jambert Corinne , Durand Pierre , Bourianne Thierry , Gaimoz Cécile , Triquet Sylvain , Féron Anaïs , Beekmann Matthias , Dulac François , Sartelet Karine 
Published: July 2017

Abstract. As part of the Chemistry-Aerosol Mediterranean Experiment (ChArMEx), a series of aerosol and gas phase measurements were deployed aboard the SAFIRE ATR-42 research aircraft in summer 2014. The present study focuses on the 4 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 C-ToF-AMS instrument were dominated by organic species (60 to 72 %) followed by a combined contribution of 25 % by ammonia and sulphate aerosols. The contribution from the anthropogenic nitrate and black carbon (BC) concentrations, measured by an SP2, never contributed to more than 5 % each to the total PM1 mass concentration. Measurements of non-refractory species from off-line transmission electron microscopy (TEM) were coherent with the C-ToF-AMS instrument, showing a large contribution of externally mixed organic aerosol and externally mixed sulphate particles. Externally mixed organic aerosols, were equally identified with S signals, which may suggest the presence of organo-sulphates. Measurements of refractory species from TEM analysis showed a significant contribution of both sea salt and dust particles depending on the air mass trajectory. The organic aerosol measured by the C-ToF-AMS contained only evidence of oxidised organic aerosol (OOA), without a contribution of fresh primary organic aerosol. Positive matrix factorization (PMF) on the combined organic/inorganic matrices separated the oxidised organic aerosol into a more oxidised organic aerosol (MOOA), and a less oxidised organic aerosol (LOOA). The MOOA component is associated with inorganics species and had higher O : C ratios than the LOOA factor. The LOOA factor is not associated with inorganic species and correlates well with biogenic volatile organic species measured with a PTR-MS, such as isoprene and its oxidation products (methylvinylketone (MVK), methacroleine (MACR), and isoprene hydroxyhydroperoxides (ISOPOOH)). Despite a significantly high mixing ratio of isoprene (2– ppbV) and oxidation products (0.6 and 1.2 ppbV), the contribution of specific signatures for isoprene epoxydiols SOA (IEPOX) within the aerosol organic mass spectrum (m / z 53 and m / z 82) were very weak, suggesting that isoprene SOA may be formed through a non-IEPOX route here, or with different precursors without clear mass spectral signatures in the C-ToF-AMS. This was corroborated through simulations performed with the Polyphemus model showing that 60 to 80 % of SOA originated from biogenic precursors: about 15 to 32 % isoprene (non-IEPOX) SOA, 10 % sesquiterpenes SOA and 35 to 40 % monoterpenes SOA). A total of 20 to 34 % was attributed to purely anthropogenic precursors (aromatics and intermediate/semi volatile compounds).