Effects of SO 2 on NH 4 NO 3 Photolysis: The Role of Reducibility and Acidic Products.

Nitrate photolysis is a vital process in secondary NOx release into the atmosphere. The heterogeneous oxidation of SO 2 due to nitrate photolysis has been widely reported, while the influence of SO 2 on nitrate photolysis has rarely been investigated. In this study, the photolysis of nitrate on different substrates was investigated in the absence and presence of SO 2 . In the photolysis of NH 4 NO 3 on the membrane without mineral oxides, NO, NO 2 , HONO, and NH 3 decreased by 17.1, 6.0, 12.6, and 57.1% due to the presence of SO 2 , respectively. In the photolysis of NH 4 NO 3 on the surface of mineral oxides, SO 2 also exhibited an inhibitory effect on the production of NOx, HONO, and NH 3 due to its reducibility and acidic products, while the increase in surface acidity due to the accumulation of abundant sulfate on TiO 2 and MgO promoted the release of HONO. On the photoactive oxide TiO 2 , HSO 3 - , generated by the uptake of SO 2 , could compete for holes with nitrate to block nitrate photolysis. This study highlights the interaction between the heterogeneous oxidation of SO 2 and nitrate photolysis and provides a new perspective on how SO 2 affects the photolysis of nitrate absorbed on the photoactive oxides.