Sulfate aerosol (SO 4 2- ) is a major component of particulate matter in Japan. The Japanese model intercomparison study, J-STREAM, found that although SO 4 2- is well captured by models, it is underestimated during winter. In the first phase of J-STREAM, we refined the Fe- and Mn-catalyzed oxidation and partly improved the underestimation. The winter haze in December 2016 was a target period in the second phase. The results from the Community Multiscale Air Quality (CMAQ) and Comprehensive Air quality Model with eXtentions (CAMx) regional chemical transport models were compared with observations from the network over Japan and intensive observations at Nagoya and Tokyo. Statistical analysis showed both models satisfied the suggested model performance criteria. CMAQ sensitivity simulations explained the improvements in model performance. CMAQ modeled lower SO 4 2- concentrations than CAMx, despite increased aqueous oxidation via the metal catalysis pathway and NO 2 reaction in CMAQ. Deposition explained this difference. A scatter plot demonstrated that the lower SO 4 2- concentration in CMAQ than in CAMx arose from the lower SO 2 concentration and higher SO 4 2- wet deposition in CMAQ. The dry deposition velocity caused the difference in SO 2 concentration. These results suggest the importance of deposition in improving our understanding of ambient concentration behavior.
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