Cool season pollution episodes in Hong Kong, 1996–2002

Seven serious pollution episodes occurred in Hong Kong during the winter and spring seasons of 1996–2002 with NO2 and/or PM10 concentrations exceeding the Hong Kong Air Quality Objectives. Analyses were made with respect to general meteorological and synoptic conditions, air mass back trajectories, satellite images and PM10 chemical species characterization. Six of the episodes are related to continental anticyclonic systems in the cool season, with one believed to be caused by cyclogenesis over southwestern China. Episodic conditions were usually associated with protracted weak northeast monsoon conditions, but one of the episodes was caused by a surge of easterly monsoon. On seven out of nine of the episode days, interactions are observed between the anticyclonic system and/or dust events with coastal sea breezes. Coupled with intense low level inversions, pollutants accumulated to episodic levels. Long range transport of pollutants from mainland China by the northeast monsoon is evident from the analyses. It is remarkable that all the three episodes in the spring season are also traceable to dust storm events originating in deserts in north or northwestern China. High concentrations of As, Mn, V, non-sea-salt sulphate and ammonium_N on the episode days indicate the significant impact of fossil fuel burning, while enhanced Al, Ca, Fe and Mn concentrations show the effect of mineral dusts and crustal materials.