Heterogeneous oxidation of sulfur dioxide (SO 2 ) 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 SO 2 oxidation for the formation of particulate sulfate by NO 2 and OH radicals produced from photolysis of particulate nitrate. Reactive uptake experiments with SO 2 and ammonium nitrate particles under ultraviolet irradiation show measured SO 2 uptake coefficients of 10 -5 . Model calculations of sulfate production rates, comparing known mechanisms of oxidation by O 3 , NO 2 , H 2 O 2 , 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.