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 SO 2 oxidation at different levels of air pollution. The Δ 33 S values of sulfate in aerosols show an obvious seasonal variation, except for those samples collected in the rural area. The positive Δ 33 S signatures (0‰ < Δ 33 S < 0.439‰) observed on clean days are mainly influenced by tropospheric SO 2 oxidation and stratospheric SO 2 photolysis. The negative Δ 33 S signatures (-0.236‰ < Δ 33 S < ∼0‰) observed during winter haze events (PM 2.5 > 200 μg/m 3 ) are mainly attributed to SO 2 oxidation by H 2 O 2 and transition metal ion catalysis (TMI) in the troposphere. These results reveal that both the H 2 O 2 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 SO 2 can produce significant negative Δ 33 S values in sulfate aerosols.