Fast near-surface ClNO production and its impact on O formation during a heavy pollution event in South China.

Atmospheric nitryl chloride (ClNO) acts as a reservoir of both NO and reactive chlorine radicals and therefore affects atmospheric oxidation chemistry and the production of secondary pollutants such as ozone (O). However, the factors contributing to ClNO production and its impact on O formation in the polluted atmosphere are not fully understood. Here, we investigated the causes and impacts of extremely high ClNO 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 NO production rates and efficient NO uptake by aerosols facilitated ClNO production at night. We used a chemical box model to assess the ClNO impact on next-day O production both at the site and in downwind areas. The model results showed that ClNO chemistry led to 6.6 % enhancement of net O (=NO + O) 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 O concentration. ClNO also changed the response of O to reduction in the concentration of O precursors (NO and anthropogenic volatile organic compounds (VOCs)), thereby affecting the design of NO and VOC reduction strategies for O pollution mitigation. Reducing chloride emissions can help alleviate the emission reduction burden for NO and anthropogenic VOCs.