Processes conducive to high ozone formation in Pearl River Delta in the presence of Pacific tropical cyclones

Surface ozone (O3) pollution in the warm season (June–November), especially persistent O3 exceedance (O3 exceedances events lasting for two days or more), has been the primary air pollution problem of Pearl River Delta (PRD), China since 2015. Most of persistent O3 exceedances in PRD were related to the presence of tropical cyclones (TCs) in the northwestern Pacific Ocean. However, the understanding of the relationship between persistent O3 exceedances and TCs remains limited. In this study, we divided TCs in the northwestern Pacific Ocean in 2015–2020 into four categories according to their presence and distances from PRD. We found that TCs with midrange distances of 700 km–4500 km from PRD (MTCs) occurred in 50% of total days in the warm season but disproportionally accounted for 80.7% of persistent O3 exceedances during the same period. In addition, analysis of the vertical velocity revealed that extensive downdrafts prevailed over PRD during both persistent O3 exceedances and MTCs. Downdrafts were characterized by meteorological conditions of clear sky, low winds, high boundary layer and low relative humidity, which were highly conducive to the photochemical formation and accumulation of O3, and hence the genesis of persistent O3 exceedances. Moreover, the presence of downdraft conditions on 1-day-before O3 exceedance days was found to contribute to the genesis of all O3 exceedance events. In contrast, during periods of other three TC categories, there were prevailing updrafts characterized by unstable atmosphere, overcast sky, high humidity and high winds which were unfavorable for O3 formation. We therefore hypothesize that the downdraft and stable atmospheric conditions which frequently occurred during the MTCs were the primary process conducive to high O3 formation of persistent O3 exceedances and thus controlled the interannual variability of O3 in PRD in 2006–2020. In addition, we propose that the downdraft brought forth by MTCs is highly likely the primary cause of the extreme high O3 in 2019 and thus the rapid increasing trend of O3 in 2015–2020. The implication to the formulation of O3 control strategy in PRD is that more stringent emission controls should be implemented during the presence of MTCs.