Atmospheric Gas-Aerosol Equilibrium II. Analysis of Common Approximations and Activity Coefficient Calculation Methods

The gas-aerosol equilibrium model, SCAPE, presented in Part I is used to evaluate the sensitivity of thermodynamic calculations of aerosol composition to common approximations and the choice of activity coefficient estimation method. The treatment of weak electrolytes, associated ammonia, NH_3(aq), and bisulfate ion, HSO_4^−, is analyzed. Comparisons of the three multi-component activity coefficient estimation methods are carried out with a variety of data. On the basis of the sensitivity analysis results, recommendations are provided for treating these electrolytes and for selecting an activity coefficient estimation method in atmospheric gas-aerosol equilibrium calculations. The two gas-aerosol equilibrium models, SCAPE and AIM, are compared. Remaining questions in gas-aerosol equilibrium are highlighted.