Measurement and analysis of a multiday photochemical smog episode in the Pearl River Delta of China

Recent measurements of a photochemical episode in September of 2001 in the Pearl River delta (PRD) were analyzed to gain insights into the meteorological and chemical processes affecting ozone (O3) concentrations in the subtropical southern China coast. High concentrations (>120 ppbv) of O3were observed at a rural coastal site in western Hong Kong for six consecutive days, with maximum 1-h O3concentration reaching 191 ppbv and visibility decreasing to 1.8 km. Comparison with O3data obtained from six other sites in the region indicated the regional nature of the O3pollution. Examination of synoptic charts showed that this unusually severe and prolonged pollution episode was induced by a quasi-stationary tropical cyclone in the East China Sea that caused air subsidence and stagnation over the PRD. Weak northerly winds were observed from radiosonde and at a mountaintop site, but surface winds showed a complex pattern owing to land-sea breezes and the topography effects. The measurements of O3, carbon monoxide (CO), sulfur dioxide (SO2), nitric oxide (NO), and total reactive nitrogen (NOy) at the western Hong Kong site were analyzed to show the possible sources and emission characteristics of O3-laden plumes. The daytime high concentrations of O3and other pollutants were caused by the diffusion/advection of urban plumes under light north-northeast winds; and their reduced concentrations in the late afternoon were due to the stronger sea breezes. The large values of CO/NOyand SO2/NOyon some days implied the contribution of regional emissions to the high O3in western Hong Kong. The data from the western site were compared with those from an eastern site to illustrate the spatial variability of air pollutants in the coastal environment of the study region.