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A multicomponent mole-fraction-based thermodynamic model is used to represent aqueous phase activities, equilibrium partial pressures (of H2O, HNO3, and NH3), and saturation with respect to solid phases (H2SO4 and HNO3 hydrates, (NH4)2SO4(cr), (NH4)3H(SO4)2(cr), NH4HSO4(cr), (NH4)2SO4·2NH4NO3(cr), (NH4)2SO4·3NH4NO3(cr), and NH4HSO4·NH4NO3 (cr)) in the system H+−NH4 +−SO4 2-−NO3 -−H2O. The model is valid from 328 to <200 K, dependent upon liquid-phase composition. Parameters for H2SO4−H2O, HNO3−H2O, and (NH4)2SO4−H2O interactions were adopted from previous studies, and values for NH4NO3−H2O obtained from vapor pressures (including data for supersaturated solutions), enthalpies, and heat capacities. Parameters for ternary interactions were determined from extensive literature data for salt solubilities, electromotive forces (emfs), and vapor pressures with an emphasis upon measurements of supersaturated H2SO4−(NH4)2SO4−H2O solutions. Comparisons suggest that the model satisfactorily represents partial pressures of both NH3 and H2SO4 above acidic sulfate mixtures in addition to that of HNO3, and salt solubilities and water activities.

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Abstract. Acidity, defined as pH, is a central component of aqueous chemistry. In the atmosphere, the acidity of condensed phases (aerosol particles, cloud water, and fog droplets) governs the phase partitioning of semivolatile gases such as HNO3, NH3, HCl, and organic acids and bases as well as chemical reaction rates. It has implications for the atmospheric lifetime of pollutants, deposition, and human health. Despite its fundamental role in atmospheric processes, only recently has this field seen a growth in the number of studies on particle acidity. Even with this growth, many fine-particle pH estimates must be based on thermodynamic model calculations since no operational techniques exist for direct measurements. Current information indicates acidic fine particles are ubiquitous, but observationally constrained pH estimates are limited in spatial and temporal coverage. Clouds and fogs are also generally acidic, but to a lesser degree than particles, and have a range of pH that is quite sensitive to anthropogenic emissions of sulfur and nitrogen oxides, as well as ambient ammonia. Historical measurements indicate that cloud and fog droplet pH has changed in recent decades in response to controls on anthropogenic emissions, while the limited trend data for aerosol particles indicate acidity may be relatively constant due to the semivolatile nature of the key acids and bases and buffering in particles. This paper reviews and synthesizes the current state of knowledge on the acidity of atmospheric condensed phases, specifically particles and cloud droplets. It includes recommendations for estimating acidity and pH, standard nomenclature, a synthesis of current pH estimates based on observations, and new model calculations on the local and global scale.

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 fro