The outflow patterns for BC events in northerly, northwesterly, and westerly pathways were comparable to those of Asian dust storms that usually occurred in spring and winter, and associated with cold air outbreaks that often caused the Mongolian cyclonic depression and frontal systems [Husar et al., 2001; Sun et al., 2001]. Cheng et al. [2006] reported that the outflow pattern for BC events observed at Hok Tsui was obviously associated with the transport of air masses in the boundary layer behind the typical cold fronts over the Asian continent during winter and triggered by the extension of the Siberian anticyclone southeastward over China.