A dual site study of the rainwater chemistry within the Western Pacific region

The two-year (1999-2000) rainwater chemistry at two monitoring sites in nearby coastal areas [Taiwan (TW) and Hong Kong (HK)] within the Western Pacific region has been studied. The volume weighted average pH values for the entire sampling period in TW and HK were 4.6 and 4.2, respectively. Sea salt Na ⁺ and Cl^- were the most abundant species in the TW samples but SO₄^2- and H⁺ were the most abundant in the HK samples. The sea salt and NH₄⁺ concentrations at TW were higher than those at HK both in the cold and warm seasons. Chloride depletion was minimal in the rainwater samples at both sites. Non seasalt-SO₄^2- was associated with NH₄ ⁺. Under the influence of the East Asian Winter Monsoon, the back-trajectory studies revealed that elevated anthropogenic species concentrations were associated with trajectories (1) very near to the continental boundary layer of Mainland China; or (2) along the coastline of Eastern China where large cities/industrial areas are located or (3) passing through the region of stagnant air over Northern/Eastern China. The lowest anthropogenic and crustal species concentrations measured in HK are associated with the summer monsoon and are attributed not only to the clean marine air masses but also to the relatively low SO₂, NO _x and NH₃ emissions from the South/ South East Asian countries, as well as infrequent biomass burning activities and wet scavenging at sources during the summer months. Approaching tropical cyclones led to the lowest pH values (4.2 in TW and 3.8 in HK) amongst the other weather categories. The findings here have been compared with other studies within East Asia and elsewhere.