Identifying solid-aqueous-phase transitions in atmospheric aerosols. II. Acidic solutions

Atmospheric aerosols are often found to be acidic. Predicting the composition of these multi-component aerosols as a function of relative humidity is important in estimating their water content, phase state and acidity. In this work, a computational method that was previously developed and applied to neutral acidity solutions to determine deliquescence relative humidities is extended to acidic solutions. Three sets of acid aerosol compositions are analyzed: H+---NH4+---HSO4−---SO42−---NO3−, H+---NH4+---HSO4−---SO42−---Cl− and H+---NH4---NO3−---Cl−. Solubility products are used to determine phase state and water activity at saturation. Activity coefficients are predicted using the recently developed mole-fraction-based thermodynamic model of Clegg and coworkers, along with Pitzerʹs-molality-based method. Partial dissociation of bisulfate ion, HSO4− = H+ + SO42−, is considered explicitly. For each set of components, water activities at deliquescence are plotted as a function of composition. Trajectories are included that show variation in the composition of the aqueous phase in a multiphase aerosol as a function of relative humidity.