Study on an ozone episode in hot season in Hong Kong and transboundary air pollution over Pearl River Delta region of China

An ozone (O3) episode that occurred in Hong Kong during 18–22 August 1999 was investigated. This episode lasted nearly 2 days with Typhoon Sam located at 500 km away from Hong Kong in the southeast. The maximum ground-level concentration of O3 reached 334 μg m−3, which violated the Hong Kong Air Quality Objective for O3 (240 μg m−3, 1 h). The visibility was reduced to 3.2 km and UV radiation was reduced by 50%. Investigations of local meteorological factors revealed that cloudless sky with high temperature and weak wind was responsible for the formation of O3, which was trapped by sea-breeze circulations developed under stable atmospheric conditions. Backward trajectories indicated that transboundary transport induced by synoptic northwest wind due to the cyclone system played a critical role in this episode. It carried aged air mass of O3 as well as its precursors from areas with heavy emissions. The PATH (Pollutants in the Atmosphere and their Transport over Hong Kong) model system was used to simulate the flow field and O3 concentration. The computed results were basically consistent with field observations. Comparison with measurements showed that the model generally performed well in simulating flow structure over coastal complex terrain. The computed O3 peaks at most stations were underestimated by 10%, which might be due to the uncertainty in NOx and VOC emission inventory in the study region. The H2O2/HNO3 ratio of 0.8–1.2 was identified to be the transition value for separating NOx- and VOC-sensitive regimes of O3 chemistry. A process analysis further revealed the importance of horizontal transport in all physical and chemical evolution phases. Sensitivity study suggested that regional transport contributed to 60–90% of O3 level at urban and rural areas of Hong Kong during the study period.