Prediction of multicomponent inorganic atmospheric aerosol behavior

Abstract

Many existing models calculate the composition of the atmospheric aerosol system by solving a set of algebraic equations based on reversible reactions derived from thermodynamic equilibrium. Some models rely on an a priori knowledge of the presence of components in certain relative humidity regimes, and often fail to accurately predict deliquescence point depression and multistage aerosol growth. The present approach, relying on adjusted thermodynamic parameters of solid salts and a state of the art activity coefficient model, directly minimizes the Gibbs free energy (according to thermodynamic equilibrium principles) given temperature, relative humidity and the total (gas plus aerosol) ammonia, nitric acid, sulfate, sodium, and hydrochloric acid concentrations. A direct minimization, while requiring no additional assumptions in its algorithm, allows the elimination of many of the assumptions used in previous models such as divided relative humidity (rh) and composition domains where only certain reactions are assumed to occur and constant 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 parameters used in the minimization algorithm. A set of ΔG(f)/0 for solid salts is estimated that is consistent with available laboratory measurements and significantly improves model performance. For multicomponent systems, the current approach with adjusted ΔG(f)/0 accurately reproduces observed multistage growth patterns and deliquescence point depression over a broad temperature range. Finally, the direct Gibbs free energy minimization accurately reproduces aerosol efflorescence behavior.