Heterogeneous SO₂ Oxidation in Sulfate Formation by Photolysis of Particulate Nitrate

Heterogeneous oxidation of sulfur dioxide (SO2) is suggested to be one of the most important pathways for sulfate formation during extreme haze events in China, yet the exact mechanism remains highly uncertain. We propose a much less explored pathway for aqueous-phase SO2 oxidation for the formation of particulate sulfate by NO2 and OH radicals produced from photolysis of particulate nitrate. Reactive uptake experiments with SO2 and ammonium nitrate particles under ultraviolet irradiation show measured SO2 uptake coefficients of ∼10–5. Model calculations of sulfate production rates, comparing known mechanisms of oxidation by O3, NO2, H2O2, and transition metal ions, and the nitrate photolysis mechanism suggest that the nitrate photolysis pathway could contribute significantly to the overall sulfate production at pH 4–6. This study provides new insight into the current debate about sulfate production pathways under typical haze conditions in China.