The unique physicochemical properties of total gaseous mercury (TGM: primarily in its elemental form, Hg^o), characterized by strong stability, and long atmospheric residence time of approximately 1 yr, permit analysis of its temporal distribution patterns to be a potential indicator of air-quality change and associated source processes (Schroeder and Munthe 1998). In fact, large differences in concentration levels of Hg exist in many countries depending on their level of development and approaches to air quality management. In many Western countries, where significant efforts have been made in air quality control, the presence of moderately low Hg concentration values around 2.0 ng m^-3 can readily be observed, even in locations adjacent to major metropolitan areas, Perch River area in New York, USA (Ames et al. 1998) or at certain source processes; for example, a coal-fired power station in Harwell, UK (Lee et al. 1998). This trend, however, contrasts sharply with what is observed in some Asian countries.