A comparative review of inorganic aerosol thermodynamic equilibrium modules: similarities, differences, and their likely causes

A comprehensive comparison of five inorganic aerosol thermodynamic equilibr ium modules, MARS-A, SEQUILIB, SCAPE2, EQUISOLV II, and AIM2 was conducted for a variety of atmospheric concentrations of particulate matter (PM) cons tituents, relative humidities (RHs), and temperatures. Our results show tha t although the PM compositions and concentrations predicted by these module s are generally comparable under most conditions, significant discrepancies exist under some conditions, especially at high nitrate/chloride concentra tions and low/medium RHs. As a consequence, the absolute differences in tot al PM concentrations predicted by these modules under all simulation condit ions are 7.7-12.3% on average and as much as 68% for specific cases. The PM predictions are highly sensitive to changes in the molar ratios of ammoniu m to sulfate, nitrate to sulfate, and sodium chloride to sulfate, relative humidity, and temperature. The similarities and differences in simulation r esults predicted by the five modules are analyzed and the likely causes for these differences are discussed in detail. Recommendations are provided re garding the relative advantages of these modules, possible improvements of their performance, and applications in three-dimensional PM modeling studie s. (C) 1999 Elsevier Science Ltd. All rights reserved.