Quantitative analysis of sulfate formation from crop burning in Northeast China: Unveiling the primary processes and transboundary transport to South Korea

The substantial amount of pollutants produced by crop burning leads to particulate matter (PM2.5) formation through chemical reactions in the atmosphere. In this study, we quantitatively examined the characteristics of PM2.5 component concentration changes and the production mechanism of sulfate (SO42-) due to crop burning in China, during a period of high PM2.5 concentrations in South Korea. Crop burning emissions were obtained from the Fire INventory from NCAR (FINN), and simulations were conducted using the Community Multi-scale Air Quality (CMAQ) model. The concentrations of PM2.5, nitrate, ammonium, organic carbon, and elemental carbon caused by crop burning were the highest in Northeast China, while the SO42- concentration was higher in the Shandong Peninsula and the Yellow Sea. SO42- was mainly produced through the oxidation reaction by hydrogen peroxide (H2O2) (H2O2 + S(IV) = S(VI) + H2O) (1.44 mu g/m(3), 80.49%). H2O2 that contributed to SO42- production was primarily produced through the self-reaction of hydroperoxy radical (HO2). Moreover, HO2, which contributed to the production of H2O2, was mainly formed through the decomposition of peroxynitric acid (PNA), which is produced by nitric oxide (NOx) emissions from crop burning. SO42- in the Yellow Sea was reported to be predominantly produced by the oxidation of sulfur dioxide (SO2) by hydroxyl radical (OH) (OH + SO2 = SULF + HO2); however, our study suggests that the PNA formed by NOx emissions from crop burning could contribute to an increase in H2O2, which subsequently promotes SO42- production. The produced SO42- was then transported across the Yellow Sea, impacting the elevated PM2.5 concentrations in Jeju Island, a suburban area of South Korea. These findings demonstrate that the mechanisms of SO42- production by crop burning emissions differ from those of anthropogenic emissions.