Modeling urban and regional aerosols—condensation and evaporation near acid neutrality

Mathematically predicting the size and composition distribution of atmospheric aerosols can help to elucidate the complex link between emissions and particulate air quality. Wexler et al. (1994) identified and analyzed the atmospheric aerosol processes that govern particulate mass concentrations and estimated the relative importance of each term using parameters typical in South Coast Air Basin (SoCAB). The result was a general dynamic equation including only the relevant terms. In this paper we describe practical difficulties integrating these equations under the acid-neutral conditions common in the SoCAB. By introducing an acid equilibrium assumption, that is, the aerosol hydrogen ion concentration can be assumed to be in equilibrium with the gas-phase acidity, the mass distribution can be predicted using modest computer resources.