Prediction of multicomponent inorganic atmospheric aerosol behavior

Many existing models calculate the composition of the atmospheric aerosol s ystem by solving a set of algebraic equations based on reversible reactions derived from thermodynamic equilibrium. Some models rely on an a priori kn owledge of the presence of components in certain relative humidity regimes, and often fail to accurately predict deliquescence point depression and mu ltistage aerosol growth. The present approach, relying on adjusted thermody namic parameters of solid salts and a state of the art activity coefficient model, directly minimizes the Gibbs free energy (according to thermodynami c equilibrium principles) given temperature, relative humidity and the tota l (gas plus aerosol) ammonia, nitric acid, sulfate, sodium, and hydrochlori c acid concentrations. A direct minimization, while requiring nb additional assumptions in its algorithm, allows the elimination of many of the assump tions used in previous models such as divided relative humidity (rh) and co mposition domains where only certain reactions are assumed to occur and con stant DRH values despite varying temperature and composition. Moreover, the current approach predicts aerosol deliquescence and efflorescence behavior explaining the existence of supersaturated aerosol solutions. A comparison is conducted between our approach and available experimental results under several conditions. The current model agrees with experimental results for single salt systems although it shows sensitivity to thermodynamic paramet ers used in the minimization algorithm. A set of Delta G(f)(0) for solid sa lts is estimated that is consistent with available laboratory measurements and significantly improves model performance. I;or multicomponent systems, the current approach with adjusted Delta G(f)(0) accurately reproduces obse rved multistage growth patterns and deliquescence point depression over a b road temperature range. Finally, the direct Gibbs free energy minimization accurately reproduces aerosol efflorescence behavior. (C) 1999 Elsevier Sci ence Ltd. All rights reserved.