Impact of a subtropical high and a typhoon on a severe ozone pollution episode in the Pearl River Delta, China

A record-breaking severe ozone (O-3) pollution episode occurred in the Pearl River Delta (PRD) in early autumn 2019 when the PRD was under the influence of a Pacific subtropical high followed by Typhoon Mitag. In this study, we analyzed the effects of meteorological and photochemical processes on the O-3 concentration in the PRD during this episode by carrying out the Weather Research Forecast-Community Multiscale Air Quality (WRF-CMAQ) model simulations. Results showed that low relative humidity, high boundary layer height, weak northerly surface wind, and strong downdrafts were the main meteorological factors contributing to O-3 pollution. Moreover, delayed sea breezes that lasted into the night would transport O-3 from the sea back to the land and resulted in secondary O-3 maxima at night. In addition, O-3 and its precursors stored in the residual layer above the surface layer at night can be mixed down to the surface in the next morning, further enhancing the daytime ground-level O-3 concentration on the following day. Photochemical production of O-3, with a daytime average production rate of about 7.2 ppb h(-1) (parts per billion), is found to be the predominate positive contributor to the O-3 budget of the boundary layer (0-1260 m) during the entire O-3 episode, while the horizontal and vertical transport fluxes are the dominant negative contributors. This O-3 episode accounted for 10 out of the yearly total of 51 d when the maximum daily 8 h average (MDA8) O-3 concentration exceeded the national standard of 75 ppb in the PRD in 2019. Based on these results, we propose that the enhanced photochemical production of O-3 during the episode is a major cause of the most severe O-3 pollution year since the official O-3 observation started in the PRD in 2006. Moreover, since this O-3 episode is a synoptic-scale phenomenon covering the entire eastern China, we also suggest that the enhanced photochemical production of O-3 in this O-3 episode is a major cause of the extraordinarily high O-3 concentrations observed in eastern China in 2019.