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 PM_2.5 (from 2015 to 2016) in Beijing and in seasonal samples of PM_2.5, PM_1.0, 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 PM_2.5 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.