Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 7 4 2007 10.5194/acpd-7-11049-2007 http://www.atmos-chem-phys-discuss.net/7/11049/2007/ http://www.atmos-chem-phys-discuss.net/7/11049/2007/acpd-7-11049-2007.html http://www.atmos-chem-phys-discuss.net/7/11049/2007/acpd-7-11049-2007.pdf 11049 11089 2007-07-26 Effects of uncertainties in the thermodynamic properties of aerosol components in an air quality model – Part II: Predictions of the vapour pressures of organic compounds S. L. Clegg s.clegg@uea.ac.uk M. J. Kleeman R. J. Griffin J. H. Seinfeld School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK Department of Civil and Environmental Engineering, University of California, Davis CA 95616, USA Institute for the Study of Earth, Oceans and Space, and Department of Earth Sciences, University of New Hampshire, Durham, NH 03824, USA Department of Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA Air quality models that generate the concentrations of semi-volatile and other condensable organic compounds using an explicit reaction mechanism require estimates of the vapour pressures of the organic compounds that partition between the aerosol and gas phases. The model of Griffin, Kleeman and co-workers (e.g., Griffin et al., 2005) assumes that aerosol particles consist of an aqueous phase, containing inorganic electrolytes and soluble organic compounds, and a hydrophobic phase containing mainly primary hydrocarbon material. Thirty eight semi-volatile reaction products are grouped into ten surrogate species. In Part I of this work (Clegg et al., 2007) the thermodynamic elements of the gas/aerosol partitioning calculation are examined, and the effects of uncertainties and approximations assessed, using a simulation for the South Coast Air Basin around Los Angeles as an example. Here we compare several different methods of predicting vapour pressures of organic compounds, and use the results to determine the likely uncertainties in the vapour pressures of the semi-volatile surrogate species in the model. These are typically an order of magnitude or greater, and are further increased when the fact that each compound represents a range of reaction products (for which vapour pressures can be independently estimated) is taken into account. 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