We refined the aqueous-phase sulfate (SO 4 2− ) production in the state-of-the-art Community Multiscale Air Quality (CMAQ) model during the Japanese model inter-comparison project, known as Japan's Study for Reference Air Quality Modeling (J-STREAM). In Japan, SO 4 2− is the major component of PM 2.5 , and CMAQ reproduces the observed seasonal variation of SO 4 2− with the summer maxima and winter minima. However, CMAQ underestimates the concentration during winter over Japan. Based on a review of the current modeling system, we identified a possible reason as being the inadequate aqueous-phase SO 4 2− production by Fe-and Mn-catalyzed O 2 oxidation. This is because these trace metals are not properly included in the Asian emission inventories. Fe and Mn observations over Japan showed that the model concentrations based on the latest Japanese emission inventory were substantially underestimated. Thus, we conducted sensitivity simulations where the modeled Fe and Mn concentrations were adjusted to the observed levels, the Fe and Mn solubilities were increased, and the oxidation rate constant was revised. Adjusting the concentration increased the SO 4 2− concentration during winter, as did increasing the solubilities and revising the rate constant to consider pH dependencies. Statistical analysis showed that these sensitivity simulations improved model performance. The approach adopted in this study can partly improve model performance in terms of the underestimation of SO 4 2− concentration during winter. From our findings, we demonstrated the importance of developing and evaluating trace metal emission inventories in Asia.Atmosphere 2018, 9, 132 Fund (5-1601) of the Environmental Restoration and Conservation Agency. The authors appreciate the helpful comments on the coarse-mode speciation of Fe and Mn in CMAQ from Golam Sarwar, Sergey Napelenok, and Kathleen Fahey at the US Environmental Protection Agency.Author Contributions: Syuichi Itahashi developed the sub-model for the aqueous-phase reaction and wrote this paper; Kazuyo Yamaji was the sub-leader of the model inter-comparison, and prepared the meteorological inputs and initial and boundary conditions; Hiroshi Hayami was the sub-leader of the inorganic aerosol measurements and contributed to the sub-model development; Satoru Chatani was the leader of the J-STREAM project, and prepared the emission inputs and discussed the sub-model development.
Conflicts of Interest:The authors declare no conflict of interest.