Role of Dust and Iron Solubility in Sulfate Formation during the Long-Range Transport in East Asia Evidenced by (17)O-Excess Signatures

[Image: see text] Numerical models have been developed to elucidate air pollution caused by sulfate aerosols (SO(4)(2–)). However, typical models generally underestimate SO(4)(2–), and oxidation processes have not been validated. This study improves the modeling of SO(4)(2–) formation processes using the mass-independent oxygen isotopic composition [(17)O-excess; Δ(17)O(SO(4)(2–))], which reflects pathways from sulfur dioxide (SO(2)) to SO(4)(2–), at the background site in Japan throughout 2015. The standard setting in the Community Multiscale Air Quality (CMAQ) model captured SO(4)(2–) concentration, whereas Δ(17)O(SO(4)(2–)) was underestimated, suggesting that oxidation processes were not correctly represented. The dust inline calculation improved Δ(17)O(SO(4)(2–)) because dust-derived increases in cloud-water pH promoted acidity-driven SO(4)(2–) production, but Δ(17)O(SO(4)(2–)) 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 Δ(17)O(SO(4)(2–)) in winter. Thus, dust and high metal solubility are essential factors for SO(4)(2–) formation in the region downstream of China. It was estimated that the remaining mismatch of Δ(17)O(SO(4)(2–)) between the observation and model can be explained by the proposed SO(4)(2–) formation mechanisms in Chinese pollution. These accurately modeled SO(4)(2–) formation mechanisms validated by Δ(17)O(SO(4)(2–)) will contribute to emission regulation strategies required for better air quality and precise climate change predictions over East Asia.