Impact of regional transport on high ozone episodes in southeast coastal regions of China

Severe ozone (O3) pollution can be harmful to human health and ecosystems. The O3 level in Fujian, a coast province in southeast China, is generally 30???60 ??g m- 3 lower than in highly polluted regions in China, including North China Plain (NCP), Yangtze River Delta (YRD), and Pearl River Delta (PRD). Yet, Fujian still experiences high O3 episodes that exceed the national standard. Here we investigate whether and how these high O3 episodes are impacted by regional O3 transport from polluted regions. A backward trajectory model, HYSPLIT, is combined with observational data to identify transport pathways and O3 source regions. The results show that the exceedance days (8 h O3 concentrations above 160 ??g m??? 3), averaged over 2015???2020, range 12???34 days for coastal cities and 3???9 days for inland cities, with two peaks in late-spring and early-autumn. On average, O3 transport from non-Fujian regions can impact 29???38% of exceedance days in coastal cities, and 44???50% of exceedance days in inland cities. The dominant transport pathways are from YRD and NCP to the East China Sea near Fujian in the lower troposphere, and then to Fujian near the ground level. Regional transport from PRD, mainly in spring, also happens. Using synoptic weather pattern (SWP) classification, we identify three SWPs in spring and two SWPs in autumn that are favorable to regional O3 transport. In summer, such transport is mainly associated with typhoon activities. This study underscores the importance of regional O3 transport to a clean coastal region and helps predict regional air pollution.