Ozone variability and the impacts of associated synoptic patterns over China during summer 2016–2020 based on a regional atmospheric composition reanalysis dataset

Ozone (O₃) pollution, which not only depends on emissions but is also closely related to prevailing meteorological conditions, is a major concern in China. In the context of China's emission controls imposed during the 13th Five-Year Plan, the variations of synoptic circulations and associated summertime O₃ variations were investigated during 2016–2020. Different from the common use in previous studies of observations and numerical models, a regional atmospheric composition reanalysis dataset at a refined spatial (45 km) and temporal (1 h) resolution was applied here, in which O₃ and its major precursors, emissions, and meteorology were jointly assimilated to reduce the impacts of uncertainty. With this continuous and optimal dataset, the impacts of regional synoptic variations on O₃ interannual variability were explored through an objective circulation classification approach during 2016–2020 over China. On the one hand, from the perspective of O₃ variability, increasing trends of O₃ levels were detected. Compared to the Yangtze River Delta (YRD), Pearl River Delta (PRD), and Sichuan Basin (SCB) with less summer pollution, the Beijing–Tianjin–Hebei (BTH) and Fenwei Plain (FWP) regions had more severe summer O₃ pollution with the frequency of days exceeding Grade 3 tends to be around 50%. On the other hand, from the perspective of O₃ variability driven by meteorological conditions, obvious interannual variations of synoptic circulation patterns occurred, and about half of type C occurrences were accompanied by O₃ pollution episodes in BTH and FWP (i.e., 52.73% and 45.65%), while far fewer pollution episodes occurred with type C in YRD and PRD (i.e., 2.78% and 0.59%). In addition, according to the quantitative assessment of the meteorological contribution, the contribution of interannual variations of synoptic circulations to changing the O₃ variability amounted to 13%–31% in BTH, YRD, PRD, FWP, and SCB. Therefore, the interannual variability of O₃ from 2016 to 2020 over China was closely linked with the regulations of O₃ precursors. This work provides an understanding of O₃ variation under the impacts of emission regulations and meteorological conditions over China.