Atmospheric Environment 40 (2006) 2105–2116 Assessment of vapor pressure estimation methods for secondary organic aerosol modeling Marie Camredon à , Bernard Aumont Laboratoire Interuniversitaire des Syste`mes Atmosphe´riques, UMR-CNRS-7583, Universite´s Paris 12 et Paris 7, Avenue du Ge´ne´ral de Gaulle, 94000 Cre´teil, France Received 30 April 2005; received in revised form 1 September 2005; accepted 14 November 2005 Abstract Vapor pressure (P vap ) is a fundamental property controlling the gas–particle partitioning of organic species. Therefore this pure substance property is a critical parameter for modeling the formation of secondary organic aerosols (SOA). Structure–property relationships are needed to estimate P vap because (i) very few experimental data for P vap are available for semi-volatile organics and (ii) the number of contributors to SOA is extremely large. The Lee and Kesler method, a modified form of the Mackay equation, the Myrdal and Yalkowsky method and the UNIFAC-p o L method are commonly used to estimate P vap in gas–particle partitioning models. The objectives of this study are (i) to assess the accuracy of these four methods on a large experimental database selected to be representative of SOA contributors and (ii) to compare the estimates provided by the various methods for compounds detected in the aerosol phase. r 2005 Elsevier Ltd. All rights reserved. Keywords: Vapor pressure; Secondary organic aerosols; Gas–particle partitioning modeling; Structure–property relationship 1. Introduction Volatile organic compounds (VOC) emitted into the atmosphere are progressively oxidized, leading to the formation of a myriad of intermediates, more functionalized and thus more polar and less volatile than their parent compounds (e.g., Aumont et al., 2005). These semi-volatile species form secondary organic aerosol (SOA) by nucleation and/or con- densation on pre-existing aerosols (i.e., absorption and/or adsorption). The dominant process control- ling this gas–particle partitioning is expected to be an absorption mechanism (Pankow, 1994a,b; Odum et al., 1996; Hoffmann et al., 1997; Griffin et al., 1999; Kalberer et al., 2000). The equilibrium of organic species can then be described on the basis of Raoult’s law P i ¼ g i x i P vap i , (1) where P i is the partial pressure of a species i in the gas phase, g i and x i are its activity coefficient and its mole fraction in the aerosol phase, respectively, and P vap i is its vapor pressure as a pure liquid, subcooled if necessary (Seinfeld and Pankow, 2003), at the temperature of interest. Note that whether absorp- tion or adsorption is controlling the partitioning, both need vapor pressure data (Pankow, 1994a,b). Vapor pressure is, therefore, a fundamental ARTICLE IN PRESS www.elsevier.com/locate/atmosenv 1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2005.11.051 à Corresponding author. Fax: +33 1 45 17 15 64. E-mail address: camredon@lisa.univ-paris12.fr (M. Camredon).