SO2-enhanced nitrate photolysis on TiO2 minerals: A vital role of photochemically reactive holes

In this study, we investigated the nitrate photolysis on photoactive TiO2 particles in the presence of SO2 through theoretic DFT calculation and in situ DRIFTS analysis, and found that in a clean atmosphere nitrate was oxidized to & nbsp;center dot NO3 radicals by the holes generated on the surface of TiO2 under solar irradiation, followed reactive nitrogen species generation via center dot NO3 & nbsp;radicals reduction by photoinduced electrons. After shifting to the SO2-polluted environment, center dot NO3 radicals preferentially reacted with abundant sulfites to significantly increase nitrogen species formation, the online measured concentrations of NOx and HONO increased by approximately 2-fold and 6-fold respectively in comparison with clean atmosphere. These results demonstrate that photogenerated holes play a key role in nitrate photolysis on photoactive mineral dusts with or without the coexistence of SO2, providing new insights into the source of NOx and HONO in complex air polluted areas during the daytime.