Atmospheric aerosol models for systems including the ions H+, NH4+, Na+, SO42−, NO3−, Cl−, Br−, and H2O

Abstract

Mole fraction based equations for aqueous phase activities, together with equilibrium constants for the formation of gases and solids, have been combined with a Gibbs free energy minimization algorithm to create equilibrium phase partitioning models of inorganic atmospheric aerosols. The water content, phase state (solid or liquid), and gas/aerosol partitioning are predicted for known ionic composition, relative humidity, and temperature. The models are valid from <200 to 328 K for the subsystems (H+‐SO 42−‐NO 3−‐Cl−‐Br−‐H 2 O) and (H+‐NH 4+‐SO 42−‐NO 3−‐H 2 O), and 298.15 K only for (H+‐NH 4+‐Na+‐SO 42−‐NO 3−‐Cl−‐H 2 O). The models involve no simplifying assumptions and include all solid phases identified in bulk experiments, including hydrated and double salt forms not treated in most other studies. The Henry's law constant of H 2 SO 4 is derived as a function of temperature, based upon available data, and the model treatment of the solubility of HBr in aqueous H 2 SO 4 is revised. Phase diagrams are calculated for the (NH 4)2 SO 4/H 2 SO 4/H 2 O system to low temperature. The models are also used to explore the importance of the double salts in urban inorganic aerosols. These Aerosol Inorganics Model (AIM) models can be run on the Web for a variety of problem types at http://mae.ucdavis.edu/wexler/aim.html and http://www.uea.ac.uk/∼e770/aim.html, and their use is summarized here.