Abstract: The atmospheric NH3-HNO3-NO3--SO42- system has been studied. A theoretical model and an available computer program were established for calculating NH4NO3 dissociation constant under different atmospheric conditions and concentrations of gaseous NH3, HNO3 and particulates of NH4NO3 according to basic thermodynamic theory. This model was applied to the atmospheric system in southwestern (acidic) and northern (non-acidic) areas in China. The theoretical results from the model calculating were compatible with those observed from monitoring in both areas.The concentrations of ammonium in air,and those of am monium salts in particles and precipitations are different in southwestern and northern areas. It is more possible to have ammonium nitrate formed in the southwestern aerosols than that in the northern areas.