Heat-Promoted Oxidative Capacity and Ozone Accumulation: Episode-Based Evidence from China’s Three Urban Agglomerations

Comprehending the long-term distribution patterns of surface ozone is essential, yet research in this area continues to encounter substantial obstacles due to the scarcity of continuous monitoring data. This study examines the interannual and monthly fluctuations in ozone levels across three major urban agglomerations in eastern China-the Pearl River Delta (PRD), Yangtze River Delta (YRD), and Beijing–Tianjin–Hebei (BTH)-over the period 2000–2023. Linear regression analysis reveals observed growth rates are 0.25, 0.48, and 0.45 ppb yr−1, respectively, corresponding to cumulative increases of roughly 6, 12, and 11 ppb. To further elucidate the interplay among meteorological factors, atmospheric oxidizing capacity, and ozone contamination in these areas, the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) was employed to simulate an unprecedented severe ozone episode that affected all three megacity clusters in late September 2019. The results demonstrate that photochemical production constituted the predominant mechanism driving daytime ozone enhancement across these regions. Furthermore, atmospheric dynamic processes facilitate the nocturnal storage of ozone and its precursor compounds in the upper atmospheric layers, followed by vertical transport to the surface during the subsequent morning hours, thereby contributing to elevated daytime ground-level ozone concentrations. Our analysis also reveals that elevated temperatures accelerate the chain reactions involving atmospheric hydroxyl radicals and peroxy radicals, consequently strengthening the atmospheric oxidizing potential and promoting ozone formation. These findings establish that high-temperature conditions create a favorable environment for the development of ozone pollution episodes.These mechanistic findings are diagnosed from a representative severe episode and are not intended to represent the entire 2000–2023 period.