Transport of Exogenous Anthropogenic Atmospheric Mercury to the Tibetan Plateau Identified Using Mercury Stable Isotopes

Transport of exogenous anthropogenic mercury (Hg) is an important source of Hg pollution in the Tibetan Plateau (TP) and its downstream water ecosystems, but the origins and contributions of Hg sources remain uncertain. Here, we investigate the concentrations and isotopic compositions of gaseous elemental mercury (GEM) at four rural sites in the TP and three urban sites surrounding the TP to quantify the sources of GEM in the TP. GEM concentrations in the surrounding cities (site-specific means: 2.36–9.12 ng m⁻³) were highly elevated mainly due to strong local anthropogenic emissions as indicated by their negative δ²⁰²Hg and near zero Δ¹⁹⁹Hg and Δ²⁰⁰Hg signatures. GEM isotopes indicate that GEM pollution in the TP, typically observed during the summer monsoon and the pre-monsoon, were mainly caused by trans-boundary transport of anthropogenic Hg from surroundings. Using an Hg isotope mixing model, we estimate that exogenous anthropogenic emissions on average contributed 26 ± 5% (1sd) to the GEM in the TP. Further analysis of the transport of anthropogenic Hg emissions based on the backward trajectory and gridded anthropogenic Hg emissions suggests that 16 ± 9% and 6 ± 13% of the GEM in the TP were derived from anthropogenic sources in South Asia and China, respectively. Our study suggests that anthropogenic Hg emissions in South Asia could be effectively transported to the TP across the Himalayan range. Future studies are needed to better assess the role of rapidly increasing anthropogenic Hg emissions in South Asia on the regional to global scale atmospheric Hg cycling.