Refinement of Modeled Aqueous-Phase Sulfate Production via the Fe- and Mn-Catalyzed Oxidation Pathway

We refined the aqueous-phase sulfate (SO42-) 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, SO42- is the major component of PM2.5, and CMAQ reproduces the observed seasonal variation of SO42- 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 SO42- 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 SO42- concentration during winter. From our findings, we demonstrated the importance of developing and evaluating trace metal emission inventories in Asia.