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

Abstract

Abstract. A record-breaking severe ozone (O3) pollution episode occurred in the Pearl River Delta (PRD) in early Autumn 2019 when PRD was under the influence of a Pacific subtropical high followed by Typhoon Mina. In this study, we analyzed the effects of meteorological and photochemical processes on the O3 concentration in 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, northerly surface winds and strong downdrafts were the main meteorological factors contributing to O3 pollution. Moreover, delayed sea breezes that lasted into the night would transport O3 from the sea back to land and resulted in secondary O3 maxima at night. In addition, O3 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 O3 concentration the following day. Photochemical production of O3, with daytime average production rate of about 7.2 ppb/h, is found to be the predominate positive contributor to the O3 budget of the boundary layer (0–1260 m) during the entire O3 episode; while the horizontal and vertical transport fluxes are the dominant negative contributors. This O3 episode accounted for 10 out of the yearly total of 51 days when the maximum daily 8-h average (MDA8) O3 concentrations exceeded the national standard of 75 ppb in PRD in 2019. Based on these results, we propose that the enhanced photochemical production of O3 during the episode is a major cause of the most severe O3 pollution year since the official O3 observation started in PRD in 2006. Moreover, since this O3 episode is a synoptic scale phenomenon covering the entire eastern China, we also suggest that the enhanced photochemical production of O3 in this O3 episode is a major cause of the extraordinary high O3 concentrations observed in eastern China in 2019.

Impacts of Meteorological Conditions on Autumn Surface Ozone During 2014–2020 in the Pearl River Delta, China

“…To understand possible impacts of AS on O 3 concentrations, we investigated the spatial correlation patterns between meteorological conditions and the number of AS days in the PRD (Figure 5). During AS days, the eastern PRD was dominated by anomalous cyclones and the PRD was located on the periphery of the cyclone (Figure 5a), which was controlled by downdraft airflow in the middle troposphere (Figure 5b) and was closely associated with O 3 pollution in the PRD (Jiang et al., 2015; Ouyang et al., 2022; Zhao et al., 2022). These conditions were always accompanied by increases in DSWRF and daily maximum temperature and decreases in TCC and RH.…”