Multiple Sulfur Isotopic Evidence for Sulfate Formation in Haze Pollution

The mechanism of sulfate formation during winter haze events in North China remains largely elusive. In this study, the multiple sulfur isotopic composition of sulfate in different grain-size aerosol fractions collected seasonally from sampling sites in rural, suburban, urban, industrial, and coastal areas of North China are used to constrain the mechanism of SO2 oxidation at different levels of air pollution. The Delta S-33 values of sulfate in aerosols show an obvious seasonal variation, except for those samples collected in the rural area. The positive Delta S-33 signatures (0 parts per thousand < Delta S-33 < 0.439 parts per thousand) observed on clean days are mainly influenced by tropospheric SO2 oxidation and stratospheric SO2 photolysis. The negative Delta S-33 signatures (-0.236 parts per thousand < Delta S-33 < similar to 0 parts per thousand) observed during winter haze events (PM2.5 > 200 mu g/m(3)) are mainly attributed to SO2 oxidation by H2O2 and transition metal ion catalysis (TMI) in the troposphere. These results reveal that both the H2O2 and TMI pathways play critical roles in sulfate formation during haze events in North China. Additionally, these new data provide evidence that the tropospheric oxidation of SO2 can produce significant negative Delta S-33 values in sulfate aerosols.

Automated summary

This study uses multiple sulfur isotopic composition of sulfate in different aerosol fractions to uncover the mechanism of SO2 oxidation during winter haze events in North China. The results highlight the importance of H2O2 and transition metal ion catalysis in sulfate formation.