Characteristics of ozone episodes and relations to synoptic patterns in Hong Kong

In this paper, seasonal variation, diurnal and duration characteristics of ozone episodes in the urban and rural areas of Hong Kong are presented. The seasonal behavior of ozone episodes in this subtropical and coastal city is observed to be different from those measured in other parts of the world. Ozone episode spikes occur throughout the year with distinct and higher frequencies found in late summer and early autumn months (Aug-Oct). There are also frequent episodes in spring and winter months. Elevated ozone concentrations occur throughout the diurnal cycle and persist for only a short period. Higher frequencies are observed from early afternoon to early evening (>60 ppb) and high ozone concentrations (>80 and 100 ppb) are found in the afternoon. The synoptic meteorological and weather conditions associated with these episode days can be classified into six main categories. They are cyclonic, northerly (N), north-easterly (NE), easterly (E), high (H) and trough (T). The seasonal variation of ozone episodes is high related to the seasonal alternation of these synoptic patterns. The diurnal characteristics and duration of occurrence of these episodes have variations for different synoptic patterns. Type C is associated with higher ozone concentration episodes. Type N, NE and E are associated with longer duration and overnight episodes. While all these synoptic situations are conducive to local photochemical ozone formation, type N, NE and E are highly related to transport of ozone and its precursors from continental sources from the industrial and urban centers of the blooming Pearl River Delta north of Hong Kong.