Journal of Atmospheric Chemistry 35: 59–75, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 59 Contribution of Secondary VOC to the Composition of Aqueous Atmospheric Particles: A Modeling Approach BERNARD AUMONT 1,2 , SASHA MADRONICH 2 , ISABELLE BEY 1 and GEOFFREY S. TYNDALL 2 1 Laboratoire Interuniversitaire des Systèmes Atmosphériques, UMR CNRS 7583, Universités Paris 7 et Paris 12, 94010 Créteil Cedex, France 2 National Center for Atmospheric Research, Atmospheric Chemistry Division, PO Box 3000, Boulder, Colorado 80307, U.S.A. (Received: 8 July 1998; accepted: 15 April 1999) Abstract. Measurements show that 20–60% of the carbon mass present in fine atmospheric par- ticulate matter consists of water soluble organic compounds (WSOC). However, only 5–20% of this WSOC has been identified, mainly as dicarboxylic acids. Because of their high solubility in water, multifunctional secondary compounds derived from the gas-phase oxidation of volatile organic compounds (VOC) are suspected to be key contributors to the WSOC. To test this assumption, an estimate of aqueous uptake of secondary VOC was included in a highly detailed gas-phase mechan- ism which treats explicitly the formation of the secondary VOC from a set of representative primary species. Simulations were conducted for 2 scenarios, representing typical rural and urban areas. It was observed that the uptake of secondary VOC can lead to WSOC mass concentrations in the range of a few μgC m −3 , in fairly good agreement with typical WSOC mass concentrations measured. Speciation of WSOC was found to be mainly as tri- or higher multifunctional hydroxy-carbonyl species and hydroxy-hydroperoxide-carbonyl species, in urban and rural environments, respectively. However, it was also found that taking into account only the absorption of secondary VOC does not bring the carboxylic acids mass concentration in agreement with measurements. An attempt was made to explain this discrepancy by introducing chemistry occurring within deliquescent aerosols. Key words: organic particle composition, organic aerosol, water-soluble organics, multifunctional compounds. 1. Introduction Measurements performed in the lower continental troposphere show that 20–60% of the carbon mass present in fine atmospheric Particulate Matter (PM) can be attributed to Water Soluble Organic Compounds (WSOC) (Mueller et al., 1982; Cadle and Groblicki, 1982; Sempere and Kawamura, 1994). Particulate organic matter is expected to affect significantly the chemical and physical properties of aerosols including total mass concentration, hygroscopic behavior (hence the abil- ity of particles to act as CCN), acidity, and radiative properties. However, the way