Simple expressions are fitted to the results obtained from ion interaction thermodynamic models for calculating HNO3 and H2O vapor pressures over the NH3/H2SO4/HNO3/H2O system at cold temperatures. The vapor pressure expressions are incorporated into a mass conserving equilibrium solver for computing aerosol compositions in the lower stratosphere and upper troposphere. The compositions calculated from the aerosol physical chemistry model (APCM) are compared against previous parameterizations. The APCM compositions are in better agreement with the compositions obtained from ion interaction models than from other previous formulations of the NH3/H2SO4/HNO3/H2O system. The only advantage of the APCM over the ion interaction approach is that the numerical scheme used in the model is fast and efficient for incorporation into large-scale models. The APCM is used to calculate HNO3 solubility in ammoniated aerosols as a function of HNO3, H2SO4 and NH3 mass loadings in the lower stratosphere and upper troposphere. While the uptake of HNO3 by ammoniated aerosols is strongly dependent upon the solution neutrality (or pH), we find that in both the lower stratosphere and upper troposphere a significant fraction of HNO3 will exist in aerosol solutions near and below the ice frost point irrespective of solution neutrality. ¿ 2001 American Geophysical Union