Investigating the causes and reduction approaches of nocturnal ozone increase events over Tai'an in the North China Plain

Abstract

China's O3 pollution events continue to be frequent, and accurately understanding the sources and sinks of O3 pollution is crucial. Previous research predominantly focused on controlling O3 pollution through daytime photochemical production, overlooking the significant impact of nocturnal high O3 values on the subsequent days' O3 concentrations. Understanding the causes of nocturnal O3 pollution and implementing effective control measures are essential for O3 pollution alleviation. In light of this, we utilized clustering analysis, combined with meteorological and chemical factors to comprehend the nocturnal variation characteristics and mechanisms of nocturnal O3 increase (NOI) events in Tai'an from July 2021 to June 2022. The results qualitatively identified that three clusters have distinct influencing factors: horizontal transport predominantly drives NOIflucts, meanwhile, NOIdecline events, mainly occurring in summer, experienced more intense photochemical reactions. For NOIstable events, primarily observed in spring, there was greater downward transport of high-altitude O3 and more intense titration reactions. Further, using interpretable machine learning models, we quantitatively assessed the contributions of several factors to nocturnal O3. The results reveal that nocturnal titration is the predominant factor, followed by the accumulation of daytime photochemical generation. Additionally, we explored approaches for reducing daytime nitrogen dioxide (NO2) and formaldehyde (HCHO), quantifying their impact on nocturnal O3 concentrations. The results indicate that reducing emissions of HCHO and NO2 can effectively lower O3 levels in NOIdecline and focusing on reducing HCHO emissions proves to be more beneficial in NOIstable events, with NOIflucts events falling in between these two clusters. Targeted implementation of diverse reduction approaches and proportions in different NOI clusters can more effectively mitigate the occurrence of nocturnal O3 pollution, thereby influencing the O3 levels of the following day.