Combined effects of high relative humidity and ultraviolet irradiation: Enhancing the production of gaseous NO2 from the photolysis of NH4NO3

Abstract

Free radicals and nitrogen-containing species produced by nitrate photolysis can affect various atmospheric chemical processes, and thereby the photochemical behavior of atmospheric nitrate aerosols has been attracting much attention. However, the photolysis mechanism of NH₄NO₃ and its products under different atmospheric conditions remain unclear. In this study, the effects of relative humidity (RH), pH, NH₃, ultraviolet (UV) light intensity and halogen ions (Cl^-, Br^- and I^-) on the photolysis of particulate NH₄NO₃ have been investigated through a flow tube reactor. The results show that RH can significantly enhance the production of gaseous NO₂ from the photolysis of NH₄NO₃ when RH is higher than its deliquescence RH, but almost no NO₂ is generated under dry conditions. Under high RH and UV light, the main product of NH₄NO₃ photolysis is NO₂, rather than NO and HONO, and another main species HNO₃ which mainly comes from the hydrolysis of product NO₂ in the gas path was detected. Almost no NO₂ and HNO₃ are produced under high RH without UV light or low RH with UV light, showing the combined effect of high RH and UV irradiation on the photolysis of NH₄NO₃. In addition, under high RH, the lower the pH and the stronger the light intensity, the higher the NO₂ production. Furthermore, surprising yields of NO and HONO are detected in the presence of halogen ions, especially in the presence of I^-, indicating the important role of halogen ion in the nitrate photolysis. These results provide new insights into the photolysis of atmospheric nitrate aerosols, and may contribute to elucidating the formation and migration of atmospheric nitrate aerosols and the potential mechanisms of the occurrence and evolution of atmospheric pollution and ozone pollution.

Keywords: Halogen ions; Light intensity; NH(3); Nitrate photolysis; RH; pH.