The H_2SO_4-HNO_3-NH_3 System at High Humidities and in Fogs: 2. Comparison of Field Data With Thermodynamic Calculations

Concentrations of HNO_3(g) and NH_3(g) determined in the field were compared to predictions from aerosol equilibrium models. The products of HNO_3(g) and NH_3(g) concentrations measured under cool and humid nonfoggy conditions agreed in magnitude with predictions from a comprehensive thermodynamic model for the atmospheric H_2SO_4-HNO_3-NH_3-H_2O system. Observed concentrations of NH_3(g) in fogs were generally consistent with those predicted at equilibrium with fog water, but important discrepancies were noted in some cases. These discrepancies may be due to fluctuations in fog water composition over the course of sample collection or to the sampling of nonfoggy pockets of air present within the fog. Detectable concentrations of HNO_3(g) (up to 23 neq m^(−3)) were often found in fogs with pH < 5 and were attributed to the sampling of nonfoggy air or to the slow rate of HNO_3(g) diffusion to fog droplets. Concentrations of HNO_3(g) in fogs with pH > 5 were below the detection limit of 4–8 neq m^(−3).