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Air quality models that generate the concentrations of semi-volatile andother condensable organic compounds using an explicit reaction mechanismrequire estimates of the physical and thermodynamic properties of thecompounds that affect gas/aerosol partitioning: vapour pressure (as asubcooled liquid), and activity coefficients in the aerosol phase. The modelof Griffin, Kleeman and co-workers (e.g., Griffin et al., 2003; Kleeman et al., 1999) assumes that aerosol particles consist of an aqueous phase,containing inorganic electrolytes and soluble organic compounds, and ahydrophobic phase containing mainly primary hydrocarbon material. Thirtyeight semi-volatile reaction products are grouped into ten surrogate specieswhich partition between the gas phase and both phases in the aerosol.Activity coefficients of the organic compounds are calculated using UNIFAC.In a companion paper (Clegg et al., 2008) we examine the likelyuncertainties in the vapour pressures of the semi-volatile compounds andtheir effects on partitioning over a range of atmospheric relativehumidities. In this work a simulation for the South Coast Air Basinsurrounding Los Angeles, using lower vapour pressures of the semi-volatilesurrogate compounds consistent with estimated uncertainties in the boilingpoints on which they are based, yields a doubling of the predicted 24-haverage secondary organic aerosol concentrations. The dependency of organiccompound partitioning on the treatment of inorganic electrolytes in the airquality model, and the performance of this component of the model, aredetermined by analysing the results of a trajectory calculation using anextended version of the <i>Aerosol Inorganics Model</i> of Wexler and Clegg (2002). Simplifications areidentified where substantial efficiency gains can be made, principally: theomission of dissociation of the organic acid surrogates; restriction ofaerosol organic compounds to one of the two phases (aqueous or hydrophobic)where equilibrium calculations suggest partitioning strongly in either direction; a single calculation of activity coefficients of the organiccompounds for simulations where they are determined by the presence of one component at high concentration in either phase (i.e., water in theaqueous phase, or a hydrocarbon surrogate compound P8 in the hydrophobicphase) and are therefore almost invariant. The implications of the resultsfor the development of aerosol models are discussed

atmospheric chemistry and physics

Effects of uncertainties in the thermodynamic properties of aerosol components in an air quality model – Part 1: Treatment of inorganic electrolytes and organic compounds in the condensed phase