Comprehensive Study about the Photolysis of Nitrates on Mineral Oxides

Nitrates formed on mineral dust through heterogeneous reactions in high NO areas can undergo photolysis to regenerate NO and potentially interfere in the photochemistry in the downwind low NO areas. However, little is known about such renoxification processes. In this study, photolysis of various nitrates on different mineral oxides was comprehensively investigated in a flow reactor and in situ diffuse reflectance Fourier-transform infrared spectroscopy (in situ DRIFTS). TiO₂ was found much more reactive than Al₂O₃ and SiO₂ with both NO₂ and HONO as the two major photolysis products. The yields of NO₂ and HONO depend on the cation basicity of the nitrate salts or the acidity of particles. As such, NH₄NO₃ is much more productive than other nitrates like Fe(NO₃)₃, Ca(NO₃)₂, and KNO₃. SO₂ and water vapor promote the photodegradation by increasing the surface acidity due to the photoinduced formation of H₂SO₄/sulfate and H⁺, respectively. O₂ enables the photo-oxidation of NO to regenerate nitrate and thus inhibits the NO yield. Overall, our results demonstrated that the photolysis of nitrate can be accelerated under complex air pollution conditions, which are helpful for understanding the transformation of nitrate and the nitrogen cycle in the atmosphere.