LAUSR

Numerical models have been developed to elucidate air pollution caused by sulfate aerosols (SO42–). However, typical models generally underestimate SO42–, and oxidation processes have not been validated. This study improves the modeling of SO42– formation processes using the mass-independent oxygen isotopic composition [17O-excess; Δ17O(SO42–)], which reflects pathways from sulfur dioxide (SO2) to SO42–, at the background site in Japan throughout 2015. The standard setting in the Community Multiscale Air Quality (CMAQ) model captured SO42– concentration, whereas Δ17O(SO42–) was underestimated, suggesting that oxidation processes were not correctly represented. The dust inline calculation improved Δ17O(SO42–) because dust-derived increases in cloud-water pH promoted acidity-driven SO42– production, but Δ17O(SO42–) was still overestimated during winter as a result. Increasing solubilities of the transition-metal ions, such as iron, which are a highly uncertain modeling parameter, decreased the overestimated Δ17O(SO42–) in winter. Thus, dust and high metal solubility are essential factors for SO42– formation in the region downstream of China. It was estimated that the remaining mismatch of Δ17O(SO42–) between the observation and model can be explained by the proposed SO42– formation mechanisms in Chinese pollution. These accurately modeled SO42– formation mechanisms validated by Δ17O(SO42–) will contribute to emission regulation strategies required for better air quality and precise climate change predictions over East Asia.