Pathways of sulfate enhancement by natural and anthropogenic mineral aerosols in China

China, the world's largest consumer of coal, emits approximately 30 million tons of sulfur dioxide (SO2) per year. SO2 is subsequently oxidized to sulfate in the atmosphere. However, large gaps exist between model-predicted and measured sulfate levels in China. Long-term field observations and numerical simulations were integrated to investigate the effect of mineral aerosols on sulfate formation. We found that mineral aerosols contributed a nationwide average of approximately 22% to sulfate production in 2006. The increased sulfate concentration was approximately 2 mu gm(-3) in the entire China. In East China and the Sichuan Basin, the increments reached 6.3 mu gm(-3) and 7.3 mu gm(-3), respectively. Mineral aerosols led to faster SO2 oxidation through three pathways. First, more SO2 was dissolved as cloud water alkalinity increased due to water-soluble mineral cations. Sulfate production was then enhanced through the aqueous-phase oxidation of S(IV) (dissolved sulfur in oxidation state + 4). The contribution to the national sulfate production was 5%. Second, sulfate was enhanced through S(IV) catalyzed oxidation by transition metals. The contribution to the annual sulfate production was 8%, with 19% during the winter that decreased to 2% during the summer. Third, SO2 reacts on the surface of mineral aerosols to produce sulfate. The contribution to the national average sulfate concentration was 9% with 16% during the winter and a negligible effect during the summer. The inclusion of mineral aerosols does resolve model discrepancies with sulfate observations in China, especially during the winter. These three pathways, which are not fully considered in most current chemistry-climate models, will significantly impact assessments regarding the effects of aerosol on climate change in China.