An important mechanism of regional O ₃ transport for summer smog over the Yangtze River Delta in East China

Abstract. Severe ozone (O3) pollution episodes plague a few regions in Eastern China at times, e.g., the Yangtze River Delta (YRD). The formation mechanisms including contributing meteorological factors of these severe pollution events remain elusive. A severe summer smog stretched over the YRD region from August 22 to 25, 2016 with hourly surface O3 concentrations exceeding 300 µg m−3 on August 25 in Nanjing, located in the western YRD. The weather pattern of this episode was characterized by near-surface prevailing easterly wind and continuous high air temperature. The formation mechanism of this O3 episode over the YRD area, particularly the extreme values over western YRD, was investigated using observation data and simulation with the Weather Research and Forecasting model with Chemistry (WRF-Chem). The O3 pollution episode was generally well simulated by the WRF-Chem air quality model. On August 24, the high O3 levels with the peak values of 250 µg m−3 occurred in the daytime mixing layer over the eastern YRD area. During nighttime, a shallow stable boundary layer formed near the surface, which decoupled the residual layer (RL) above it from the surface. O3 in the decoupled RL remained nearly constant, resulting an O3-rich reservoir, due to lack of NO titration and absence of dry deposition. The prevailing easterly wind in the lower troposphere transported the O3-rich air mass in the nocturnal RL from the eastern to western YRD. Consequently, the O3 concentrations in the RL over the western YRD area increased to 170 µg m−3 in the wee hours of August 25, 2016. Due to the growth of the convective boundary layer after the sunrise, entrainment of O3-rich RL air and boundary layer mixing contributed considerably to the rapid increase of surface O3. Process analysis indicated vertical mixing contributed ~ 40 µg m−3 h−1 of O3 accumulation over Nanjing in the morning of August 25, 2016, which played an important role in contributing to the severe daytime O3 pollution in the western YRD area. The mechanism of regional O3 transport through the nocturnal RL has a great implication for understanding O3 pollution in air quality change.

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