Prolonged daytime presence and oxidative impact of nitryl chloride, ClNO2, in winter urban environment☆

Abstract

Nitryl chloride (ClNO2) is widely recognized as a nocturnally formed species that influences next-day air quality through early-morning photolysis. However, its formation mechanisms and diurnal behavior remain poorly constrained, particularly its persistence beyond the morning. Here, we present the first wintertime observations of ClNO2 in South Korea, revealing its sustained presence and photochemical impacts under urban conditions. Observed ClNO2 concentration averaged 208 pptv in the morning and 27 pptv in the afternoon, with a campaign maximum of 2.25 ppbv (1-min resolution). These patterns, along with supporting chemical and meteorological parameters, suggest that elevated morning ClNO2 resulted from reduced photolytic loss under weak solar radiation and continued N2O5 uptake. In contrast, observational evidence indicates a possible linkage between particulate NO3- photolysis and additional afternoon formation, particularly in aerosols enriched with anthropogenic chloride. This persistent ClNO2 shifted the diurnal peak of Cl radical production to late morning, with similar to 33 % of daily production occurring in the afternoon-surpassing the morning contribution (22 %). Observation-constrained box modeling further showed that ClNO2-driven ozone (O-3) production was comparable in both morning and afternoon periods, each contributing similar to 38 % to the total ClNO2-related O-3 enhancement. This demonstrates that substantial Cl-initiated oxidation can persist well into the afternoon, even under moderate NO3- levels (6.4 +/- 6.9 mu g/m(3), average +/- 1 sigma). These findings underscore the importance of considering ClNO2-driven oxidation throughout the day and highlight the need for further observations across seasons and urban environments to better constrain its atmospheric role.