High levels of ClNO2 enhance wintertime atmospheric oxidation capacity at a coastal site in North China.

Nitryl chloride (ClNO2) is a reservoir for chlorine radicals and NOx, and thereby plays a potentially important role in atmospheric oxidation chemistry and the formation of secondary air pollutants. Despite a growing number of observations and modeling studies of ClNO2, knowledge on its atmospheric abundance and impact under diverse environmental conditions remains incomplete. In this study, we investigated the sources and impacts of ClNO2 during winter 2022 at a suburban-coastal site in North China. The 1-minute average mixing ratio in the field campaign period was 404.5 ± 502.9 pptv, which is higher than those previously reported winter values in polluted regions of North China. Anthropogenic activities, such as biomass burning, are significant contributors to atmospheric chlorine. The high ClNO2 levels were attributed to efficient N2O5 uptake by aerosols and low NO titration of NO3. Calculations with a chemical box model show that ClNO2 contributed ROx (OH+HO2+RO2) production by 10 %, which is higher than the contribution of ozone photolysis (< 1 %), but lower than that of HONO (20 %) and HCHO (13 %). The net Ox (O3 + NO2) production is also promoted by 19 % from ClNO2 photolysis at this site. These results highlight the considerable contribution of ClNO2 to winter atmospheric oxidation capacity and the need to consider it in evaluating winter air quality.