Studying the effects of calcium and magnesium on size-distributed nitrate and ammonium with EQUISOLV II

A chemical equilibrium code was improved and used to show that calcium and magnesium have a large yet different effect on the aerosol size distributio n in different regions of Los Angeles. In the code, a new technique of solv ing individual equilibrium equations was developed. The technique, the anal ytical equilibrium iteration (AEI) method, gives the same solutions (to at least 7 decimal places) as the previous technique used, the mass-flux itera tion (MFI) method, but consumes 13-48 times less computer time. The model w as also updated to include treatment of potassium, calcium, magnesium, and carbonate. Previously, it treated only nitrate, ammonium, chloride, sulfate , and sodium. Predictions from the updated code, EQUISOLV II, were compared with data from an eight-stage Berner impactor at Long Beach, Claremont, an d Riverside during the Southern California Air Quality Study. When any equi librium salver is applied between the gas phase and multiple aerosol size b ins, unique solutions are possible only when solids (e.g., NH4NO3) that for m from two gas-phase species are absent. For this study, unique solutions w ere possible only when the relative humidity exceeded 62%, and only cases i n this regime are discussed. Base-case predictions of nitrate and ammonium matched observations well in most size bins of every case. When Ca and Mg w ere removed from calculations, coarse-mode nitrate decreased at Long Beach, as expected, to maintain charge balance. At Riverside, removing Ca and Mg had the opposite effect, increasing coarse-mode nitrate, shifting it from t he accumulation made. The reason is explained in terms of mean mixed activi ty coefficients. At Claremont, the charge-balance and activity-coefficient effects nearly canceled each other. (C) 1999 Elsevier Science Ltd. All righ ts reserved.