Four-year Measurements of Wet Mercury Deposition at a Tropical Mountain Site in Central Taiwan

Rainwater samples were collected between 2010 and 2013 at Lulin Atmospheric Background Station (LABS) to study the distribution and characteristics of wet mercury (Hg) deposition, and possible driving mechanisms. Sample Hg concentrations ranged from 0.8 to 35.1 ng L^(-1) with an overall volume-weighted mean (VWM) concentration of 9.2 ng L^(-1). Annual wet Hg deposition fluxes ranged between 24.4 and 48.9 μg m^(-2) with a mean value of 32.3 μg m^(-2). This mean annual wet flux was about 1.5-6.0 times the values measured at 15 sites in the U.S. and 4-16 times the values reported from mountain and high-elevation sites in China. Both rainwater Hg concentration and rainfall amount contributed to this geographical difference, but rainfall amount played a more important role. This indicated that tropical mountains in East and Southeast Asia, especially the windward maritime slopes with abundant rainfall, could be hot spots of wet Hg deposition. Wet Hg deposition flux was high in summer because of elevated rainwater Hg concentrations and high rainfall. The seasonal pattern of rainwater Hg concentrations was different from that of the East Asian air pollutant export, indicating other factors, e.g., rainfall type, were also influencing rainwater Hg concentrations. A clear difference in seasonal frequency distribution of rainfall types was observed, with rain events associated with the Pacific high pressure type (PH) occurring more frequently in summer months. PH rainfall type had the highest VWM Hg concentration (13.5 ng L^(-1)), 2.3- 6.2 ng L^(-1) greater than those of the other rainfall types. Because of intense surface heating under summer PH conditions, precipitation systems usually form locally due to strong convection, resulting in afternoon shower. Therefore, the elevated rainwater Hg levels in summer at LABS were likely due to the scavenging of free tropospheric gaseous oxidized Hg (GOM) by deep convection.