Inorganic thermodynamic equilibrium model (ISORROPIA) coupled with the Models-3 Community Multi-scale Air Quality modeling system (CMAQ) was used to understand the role of ammonia (NH 3) in sulfur dioxide (SO 2) transformation and sulfur (S) deposition in China. The simulated S and nitrogen (N) compounds in gas (SO 2, NH 3) and aerosol (sulfate (SO 4 2−), ammonium (NH 4 +)) phases and wet deposition were compared against observations at two non-urban sites: Tie Shan Ping (TSP) and Cai Jia Tang (CJT). Observations showed that SO 4 2− was the predominant anion in aerosol and precipitation at the sites, and there were not enough NH 3 and alkaline dusts (i.e. mineral oxides and carbonates) to neutralize all acidity. Most of the NH 4 + was therefore probably bound to SO 4 2−. The presence of NH 3 significantly altered the regional distributions of S compounds in gas and aerosol phases. Model simulations showed SO 4 2− concentration increased by 10–15% while SO 2 concentration decreased by 10–60% over polluted regions due to NH 3. Furthermore, the presence of NH 3 enhanced long-range S transportation for the longer lifetime of sulfate than of SO 2 in the atmosphere. S deposition thereby decreased by 10–30% over polluted regions.