Model performance differences in fine-mode nitrate aerosol during wintertime over Japan in the J-STREAM model inter-comparison study

In this study, the results for nitrate (NO3-) aerosol during winter from the first-phase model inter-comparison study of Japan's Study for Reference Air Quality Modeling (J-STREAM) were analyzed. To investigate the models' external and internal settings, the results were limited to Community Multiscale Air Quality (CMAQ) models. All submitted models generally underestimated NO₃-over the urban areas in Japan (e.g., Osaka, Nagoya, and Tokyo); however, some model settings showed distinct behavior. The differences due to the model external settings were larger than the model internal settings. Emissions were an important factor, and emissions configured with lower NO_x emissions and higher NH₃ emissions led to a higher NO₃-concentration as the NH₃ was consumed under NH₃-rich conditions. The model internal settings of the chemical mechanisms caused differences over China, and this could affect western Japan; however, the difference over Tokyo was lower. To obtain a higher NO₃-concentration over the urban areas in Japan, the selection of the HONO option for the heterogenous reaction and the inline calculation of photolysis was desired. For future studies, the external settings of the boundary condition and the meteorological field require further investigation.