An important mechanism of regional O3 transport for summer smog over the Yangtze River Delta in eastern China

Severe ozone (O-3) pollution episodes plague a few regions in eastern China at certain times of the year, e.g., the Yangtze River Delta (YRD). However, the formation mechanisms, including meteorological factors, contributing to these severe pollution events remain elusive. A severe summer smog stretched over the YRD region from 22 to 25 August 2016. This event displayed hourly surface O-3 concentrations that exceeded 300 mu g m(-3) on 25 August in Nanjing, an urban area in the western YRD. The weather pattern during this period was characterized by near-surface prevailing easterly winds and continuous high air temperatures. The formation mechanism responsible for this O-3 pollution episode over the YRD region, particularly the extreme values over the western YRD, was investigated using observation data and by running simulations with the Weather Research and Forecasting model with Chemistry (WRF-Chem). The results showed that the extremely high surface O-3 concentration in the western YRD area on 25 August was largely due to regional O-3 transport in the nocturnal residual layer (RL) and the diurnal change in the atmospheric boundary layer. On 24 August, high O-3 levels, with peak values of 220 mu g m(-3), occurred in the daytime mixing layer over the eastern YRD region. During nighttime from 24 to 25 August, a shallow stable boundary layer formed near the surface which decoupled the RL above it from the surface. Ozone in the decoupled RL remained quite constant, which resulted in an O-3 -rich "reservoir" forming in this layer. This reservoir persisted due to the absence of O-3 consumption from nitrogen oxide (NO) titration or dry deposition during nighttime. The prevailing easterly winds in the lower troposphere governed the regional transport of this O-3-rich air mass in the nocturnal RL from the eastern to the western YRD. As the regional O3 transport reached the RL over the western YRD, O-3 concentrations in the RL accumulated and rose to 200 mu g m(-3) over the western Nanjing site during the sunrise hours on 25 August. The development of the daytime convective boundary layer after sunrise resulted in the disappearance of the RL, as the vertical mixing in the convective boundary layer uniformly redistributed O-3 from the upper levels via the entrainment of O-3-rich RL air down to the O-3-poor air at the ground. This net downward transport flux reached up to 35 mu g m(-3) h(-1) , and contributed a considerable surface O-3 accumulation, resulting in severe daytime O-3 pollution during the summer smog event on 25 August in the western YRD region. the mechanism of regional O-3 transport through the nocturnal RL revealed in this study has great implications regrading O-3 pollution and air quality change.