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 Δ³³S values of sulfate in aerosols show an obvious seasonal variation, except for those samples collected in the rural area. The positive Δ³³S signatures (0‰ < Δ³³S < 0.439‰) observed on clean days are mainly influenced by tropospheric SO₂ oxidation and stratospheric SO₂ photolysis. The negative Δ³³S signatures (-0.236‰ < Δ³³S < ∼0‰) observed during winter haze events (PM_2.5 > 200 μg/m³) 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 Δ³³S values in sulfate aerosols.