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

Abstract

Atmospheric nitryl chloride (ClNO2) acts as a reservoir of both NOx 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 Ox (=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 (NOx and anthropogenic volatile organic compounds (VOCs)), thereby affecting the design of NOx and VOC reduction strategies for O3 pollution mitigation. Reducing chloride emissions can help alleviate the emission reduction burden for NOx and anthropogenic VOCs.