Comment on acp-2022-408

Abstract

Here we present a field measurement of ClNO2 (nitryl chloride) and N2O5 (dinitrogen pentoxide) by a state-of-the-art instrument at a regional site in Pearl River Delta during a photochemical pollution season from Sept. 26th to Nov. 17th, 2019. Three patterns of air masses are sampled during this campaign, including the dominating air masses from north and northeast urban regions (Type A), the southeast coast (Type B) and the South China Sea (Type C). The concentration of ClNO2 and N2O5 were observed much higher in Type A and B than those in Type C, indicated the urban nighttime chemistry is more active than the background marine regions. Two key parameters that regulating ClNO2 formations, N2O5 uptake coefficient and ClNO2 production yield, were estimated by measured parameters, and the performance of the previously derived parameterizations were assessed. We find the ClNO2 formation was limited by the N2O5 uptake rather than N2O5 source at this site. By examining the relationship of particulate chloride and other species, we implied that anthropogenic emissions (e.g., biomass burning) rather than sea salt particles dominate the origin of particulate chloride, despite the site is only about 100 km away from the ocean. Model simulations showed the chloride radical liberated by ClNO2 photolysis during the next day had a small increase in concentrations of OH, HO2 and RO2 radicals, as well as minor contributions to RO2 radical and O3 formation (<5 %, on daytime average) in all the three types of air masses. Relative higher contributions were observed in Type A and B. The overall low contributions of ClNO2 to ozone pollution are consistent with those reported recently from wintertime observations in China (included Shanghai, Beijing, Wangdu and Mt. Tai). This may be attributed to: (1) Relative low particle mass concentration limited ClNO2 formation; (2) Other reactions channels had larger radical formation rate during the ozone pollution episodes and weakened the ClNO2 contribution indirectly. The results provided scientific insights into the role of nighttime chemistry in photochemical pollution under various scenarios in coastal areas.