IDENTIFYING SOLID-AQUEOUS-PHASE TRANSITIONS IN ATMOSPHERIC AEROSOLS .2. ACIDIC SOLUTIONS

Atmospheric aerosols are often found to be acidic. Predicting the comp osition of these multicomponent aerosols as a function of relative hum idity is important in estimating their water content, phase state and acidity. In this work, a computational method that was previously deve loped and applied to neutral acidity solutions to determine deliquesce nce 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 use d 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 Pitzer's- molality-based method. Partial dissociation of bisulfate ion, HSO4- = H+ + SO42-, is considered explicitly. For each set of components, wate r 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 hu midity.