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.