Formation drivers and photochemical effects of ClNO ₂ in a coastal city of Southeast China

Abstract. Nitryl chloride (ClNO2) is an important precursor of chlorine (Cl) radical, significantly affecting ozone (O3) formation and photochemical oxidation. However, the key drivers of ClNO2 production are not fully understood. In this study, the field observations of ClNO2 and related parameters were conducted in a coastal city of Southeast China during the autumn of 2022, combining with machine learning and model simulations to elucidate its key influencing factors and atmospheric impacts. Elevated concentrations of ClNO2 (>500 ppt) were notably observed during nighttime in late autumn, accompanied by increased levels of dinitrogen pentoxide (N2O5) and nitrate (NO3-). Nighttime concentrations of ClNO2 peaked at 3.4 ppb, while its daytime levels remained significant, reaching up to 100 ppt and sustaining at approximately 40 ppt at noon. Machine learning and field observations identified nighttime N2O5 heterogeneous uptake as the predominant pathway for ClNO2 production, whereas NO3- photolysis may contribute to its daytime generation. Additionally, ambient temperature (T) and relative humidity (RH) emerged as primary meteorological factors affecting ClNO2 formation, mainly through their effects on thermal equilibrium and N2O5 hydrolysis processes, respectively. Ultraviolet (UV) radiation was found to play a dual role in ClNO2 concentrations around noon. Box model simulations showed that, under high-ClNO2 conditions, the rates of alkane oxidation by Cl radical in the early morning exceeded those by OH radical. Consequently, volatile organic compound (VOC) oxidation by Cl radical contributed ∼19 % to ROx production rates, thereby significantly impacting O3 formation and atmospheric oxidation capacity. This research enriched the understanding of ClNO2 generation and loss pathways, providing valuable insights for the regulation of photochemical pollution in coastal regions.