Effects of synoptic weather systems upon the air quality in an Asian megacity

The Hong Kong Environmental Protection Department (HK EPD) monitoring data for SO2, NO, NO2 and O-3 from 1995 and 1996 at three stations spanning the length of Hong Kong have been analyzed in the contexts of selected meteorological parameters, and the types of synoptic weather system. High level and episode days were defined according to two criteria. Higher pollutant gas concentrations were found for systems involving the transport of continental air masses to Hong Kong (cold front (type CF), north/northeast monsoon (type NE)), or when tropical cyclones approached (type AC) Hong Kong. Lower concentrations were observed when the cleaner maritime southerly (type S) airstreams were incident at Hong Kong. Most notably, tropical cyclones (type C) ventilated and diluted local pollution. Days with high levels of SO2, NO and NO2 were generally characterized by low windspeed and lower relative humidity conditions. By contrast, high concentrations of O-3 occurred both under low windspeed conditions (type AC) and under the advection of a continental air mass, generally at high windspeeds. The diurnal variation of O-3 levels is discussed: destruction is more important than production in determining local levels.