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 S. L. Clegg1, M. J. Kleeman2, R. J. Griffin3, and J. H. Seinfeld4 1School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK 2Department of Civil and Environmental Engineering, University of California, Davis CA 95616, USA 3Institute for the Study of Earth, Oceans and Space, and Department of Earth Sciences, University of New Hampshire, Durham, NH 03824, USA 4Department of Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA
Abstract. Air quality models that generate the concentrations of semi-volatile and
other condensable organic compounds using an explicit reaction mechanism
require estimates of the physical and thermodynamic properties of the
compounds that affect gas/aerosol partitioning: vapour pressure (as a
subcooled liquid), and activity coefficients in the aerosol phase. The model
of 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 a
hydrophobic phase containing mainly primary hydrocarbon material. Thirty
eight semi-volatile reaction products are grouped into ten surrogate species
which 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 likely
uncertainties in the vapour pressures of the semi-volatile compounds and
their effects on partitioning over a range of atmospheric relative
humidities. In this work a simulation for the South Coast Air Basin
surrounding Los Angeles, using lower vapour pressures of the semi-volatile
surrogate compounds consistent with estimated uncertainties in the boiling
points on which they are based, yields a doubling of the predicted 24-h
average secondary organic aerosol concentrations. The dependency of organic
compound partitioning on the treatment of inorganic electrolytes in the air
quality model, and the performance of this component of the model, are
determined by analysing the results of a trajectory calculation using an
extended version of the Aerosol Inorganics Model of Wexler and Clegg (2002). Simplifications are
identified where substantial efficiency gains can be made, principally: the
omission of dissociation of the organic acid surrogates; restriction of
aerosol 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 organic
compounds for simulations where they are determined by the presence of one component at high concentration in either phase (i.e., water in the
aqueous phase, or a hydrocarbon surrogate compound P8 in the hydrophobic
phase) and are therefore almost invariant. The implications of the results
for the development of aerosol models are discussed.
Citation: Clegg, S. L., Kleeman, M. J., Griffin, R. J., and Seinfeld, J. H.: 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, Atmos. Chem. Phys., 8, 1057-1085, doi:10.5194/acp-8-1057-2008, 2008.