Influence of Nighttime Chemical Processes on Daytime Oxidation through ClNO2

Abstract

Nitryl chloride (ClNO2) can play a significant role in local daytime atmospheric oxidation as a major source of chlorine (Cl) radicals. Previous field studies have reported a wide range of ClNO2 concentrations across various regions, suggesting that Cl radical production may also vary substantially. However, our understanding of the underlying causes of this variability in ClNO2 concentrations and the corresponding impacts on atmospheric oxidation remain poorly constrained. This dissertation aims to investigate the chemical processes governing ClNO2 concentrations and its oxidative effects, based on intensive field observations conducted in Gwangju, South Korea, during the winter of 2022-2023. Specifically, the role of ClNO2 in enhancing daytime ozone (O3) production during the campaign was evaluated and found to be effective even through the afternoon. Additionally, the contributions of relevant chemical and meteorological parameters controlling ClNO2 abundances were assessed. This analysis revealed that nocturnal O3 availability served as a limiting factor for ClNO2 concentration in this campaign. Moreover, a simplified model capable of estimating ClNO2 production rates even without direct ClNO2 measurement was developed and validated in not only this campaign but also under various environmental conditions. Finally, to clearly establish the applicable conditions of the developed model, a broadband cavity-enhanced absorption spectrometer for the precise measurement of nitrate (NO3) radicals was built and deployed in the field. This research provides a scientific basis for understanding nighttime and daytime chemistry as an integrated process mediated by ClNO2, and for interpreting the variability of ClNO2 concentrations and the resulting impacts on atmospheric oxidation.