What is the cause(s) of ozone trends in three megacity clusters in eastern China during 2015–2020?

Abstract. Thanks to a strong emission control policy, major air pollutants in China, including PM2.5, SO2, NO2 and CO had shown remarkable reductions in 2015–2020. However, ozone (O3) had increased significantly and emerged as a major air pollutant in eastern China at the same time. The annual mean concentration of maximum daily 8-hour average O3 (MDA8) in three megacity clusters in eastern China, namely Beijing-Tianjin-Hebei (BTH), Yangtze River Delta (YRD) and Pearl River Delta (PRD), showed alarming large upward linear trends of 2.4, 1.1 and 2.0 ppb yr-1, respectively during the period 2015–2020. Furthermore, drastic trends of approximately three-fold increase in the number of O3-exceeding days (defined as MDA8 O3 >75 ppb) were observed during the same period. Our analysis of the upward trends of the annual mean concentration of MDA8 found that the trends were almost entirely attributable to the increase in the number of consecutive O3-exceeding days. In addition, a widespread expansion of high O3 from urban centers to surrounding rural regions was found in 2015–2017, which had made the O3 spatial distribution becoming more uniform after 2017. Finally, we found that the O3 episodes with four or more consecutive O3-exceeding days in the three megacity clusters were closely associated with the position and strength of the West Pacific subtropical high (WPSH), which contributed to the meteorological conditions characterized by clear sky, sinking motion and high vertical stability in the lower troposphere, and high solar radiation and positive temperature anomaly at the surface. These meteorological conditions were highly conducive to O3 formation. Hence we hypothesize that the cause of the worsening O3 trends in BTH, YRD and PRD from 2015 to 2020 is attributable to the increased occurrence of meteorological conditions of high solar radiation and positive temperature anomaly under the influence of WPSH, tropical cyclones as well as mid-high latitude wave activities.