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

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 ∼33 % of daily production occurring in the afternoon-surpassing the morning contribution (22 %). Observation-constrained box modeling further showed that ClNO₂-driven ozone (O₃) production was comparable in both morning and afternoon periods, each contributing ∼38 % to the total ClNO₂-related O₃ enhancement. This demonstrates that substantial Cl-initiated oxidation can persist well into the afternoon, even under moderate NO₃^- levels (6.4 ± 6.9 μg/m³, average ± 1σ). 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.