Sulfate formation in haze pollution using multiple sulfur isotopes

Air pollution has become a serious problem in some parts of the world. The mechanism of sulfate formation during haze events is still not clear. This research looks at the different sulfur isotope compositions of sulfate in PM2.5 (from 2015 to 2016) in Beijing and in seasonal samples of PM2.5, PM1.0, and TSP from rural, suburban, urban, industrial, and coastal areas of North China (in 2017). The goal is to figure out the mechanism by which SO2 oxidizes at different levels of air pollution. An obvious seasonal variation (with positive values in spring, summer, and autumn and negative values in winter) is shown by the Δ33S values of sulfate in aerosols, except for those samples collected in rural areas. The Δ33S value (S-MIF) of sulfate in PM2.5 shows a pronounced seasonality, with positive values in spring, summer, and autumn and negative values in winter. The negative Δ33S changes that happen during winter haze events are mostly caused by SO2 being oxidized by H2O2 and transition metal ion catalysis (TMI) in the troposphere, which is most likely caused by coal burning. The positive Δ33S results observed on clean days are mainly attributed to tropospheric SO2 oxidation and stratospheric SO2 photolysis. These results provide important information on sulfate formation during haze events and clean days.