Wet deposition of mercury at a remote site in the Tibetan Plateau: Concentrations, speciation, and fluxes

Precipitation samples collected at a remote high elevation site (i.e., Nam Co Station, 4730 m a.s.l.) in the southern Tibetan Plateau were analyzed for total mercury (HgT) between July 2009 and 2011, particulate-bound mercury (HgP) between July 2010 and 2011 and methylmercury (MeHg) from July through August of 2009. The volume-weighted mean (VWM) concentrations and wet deposition fluxes of HgT and MeHg in precipitation were 4.8 ng L−1 and 1.75 μg m−2 yr−1, 0.031 ng L−1 and 0.01 μg m−2 yr−1, respectively. VWM HgT concentration was approximately two times higher during the non-monsoon season than during the monsoon season, while 83% of the HgT wet deposition fluxes occurred during the monsoon season. The HgT and MeHg concentrations are comparable to the reported data for some of the most remote alpine and polar regions worldwide (e.g., Churchill), but the wet deposition fluxes of HgT and MeHg were among the lowest in the world. Analysis of Hg speciation has presented that HgP and MeHg concentrations are high, making up 71.2% and 1.82% of the HgT on average (VWM), respectively. The high HgP%, as well as a significantly positive between HgT and HgP (R2 = 0.91; n = 44; p < 0.001), confirmed that atmospheric deposition of Hg in the Tibetan Plateau was occurring in the form of HgP. A decreasing trend in HgT concentrations with increasing amount of precipitation (R2 = 0.08; N = 101; p < 0.005) was found at Nam Co Station, indicative that scavenging of HgP from the atmosphere was an important mechanism contributing Hg to precipitation. The precipitation amount, rather than HgT concentration, was found to be the governing factor affecting HgT wet deposition flux. Moreover, a comparison between measured wet deposition flux of Hg at Nam Co Station and the estimates from environmental records indicated that both snowpits and lake sediments appear to be reliable archives for estimating historical Hg accumulation rates over the Tibetan Plateau.