Predictions of size-resolved aerosol concentrations of ammonium, chloride and nitrate at a bayside site using EQUISOLV II

Measured ambient air concentrations of ammonium, chloride and nitrate were compared with concentrations produced by EQUISOLV II, an aerosol thermodynamic equilibrium model. The monitoring equipment was located 50 m from Old Tampa Bay at the eastern end of the Gandy Bridge in Tampa, FL. Size-segregated ion concentrations of ammonium, sodium, potassium, calcium, magnesium, fluoride, chloride, nitrite, nitrate, sulfate and phosphate were determined from 6- and 1-day integrated cascade impactor samples obtained in May and August 2001, respectively. EQUISOLV II was initialized with these ion concentrations by size bin, and by gas phase concentrations of the volatile species, acquired with a collocated annular denuder system. The model redistributed the ions between the size bins until gas and particle concentrations reached equilibrium. The model calculated predominantly fine particle ammonium and coarse particle chloride and nitrate. For the May sampling period, when the average relative humidity was below 65%, the model predicted the formation of seven solids KNO3, K2SO4, (NH4)2SO4, Na2SO4, NaCl, NaNO3, and CaSO4•2H2O. The amounts of ammonium in the fine fraction and of nitrate and chloride in the coarse fraction were predicted within the combined measurement and modeling uncertainty in the majority of cases.