Fast near-surface ClNO₂ production and its impact on O₃ formation during a heavy pollution event in South China

Atmospheric nitryl chloride (ClNO₂) acts as a reservoir of both NO_x and reactive chlorine radicals and therefore affects atmospheric oxidation chemistry and the production of secondary pollutants such as ozone (O₃). However, the factors contributing to ClNO₂ production and its impact on O₃ formation in the polluted atmosphere are not fully understood. Here, we investigated the causes and impacts of extremely high ClNO₂ levels (up to 8.3 ppbv, 1 min average) in a winter pollution episode at a semi-rural surface site in South China. Anthropogenic activities (mainly coal burning) produced an abundant amount of chloride, and high NO₃ production rates and efficient N₂O₅ uptake by aerosols facilitated ClNO₂ production at night. We used a chemical box model to assess the ClNO₂ impact on next-day O₃ production both at the site and in downwind areas. The model results showed that ClNO₂ chemistry led to 6.6 % enhancement of net O_x (=NO₂ + O₃) production at the site, while the enhancement was increased to 11.2 % in the air mass transporting downwind, which resulted in 20 ppbv (38.5 % max) increment of peak O₃ concentration. ClNO₂ also changed the response of O₃ to reduction in the concentration of O₃ precursors (NO_x and anthropogenic volatile organic compounds (VOCs)), thereby affecting the design of NO_x and VOC reduction strategies for O₃ pollution mitigation. Reducing chloride emissions can help alleviate the emission reduction burden for NO_x and anthropogenic VOCs.