Quantifying Altitudinal\nMercury Accumulation in Biomonitors\nalong Himalayan Valleys Using Mercury Isotopes

The Himalayan valleys are important transport channels\nof atmospheric\npollutants from South Asia to the Tibetan Plateau. This study aims\nto demonstrate the use of biomonitors (i.e., tree foliage, bark, mosses,\nand lichens) in the Himalayas to understand the sources and accumulation\nof mercury (Hg), including the transboundary atmospheric Hg transport\nacross the Himalayas. Results showed that the significant variability\nin the physiological characteristics and nutrient uptake pathways,\ncoupled with rapid changes in topography and climate-forced precipitation,\nled to significant differences in concentrations and isotopic compositions\namong biomonitor species. Δ<sup>199</sup>Hg values (−0.32\nto −0.10‰) at the lower altitudes were slightly more\npositive than values at upper altitudes, likely reflecting signals\nof transboundary transport of anthropogenic Hg from South Asia. The\nisotope mixing model determined atmospheric Hg<sup>0</sup> as the\nmain source of Hg in most biomonitors (67 ± 13% to 88 ±\n13%), except for Usnea longissimas (i.e.,\na unique type of lichen) with 61 ± 16% contribution of atmospheric\nHg<sup>2+</sup>. Additionally, the morphological structure and epiphytic\nenvironment of U. longissimas facilitate\naqueous Hg secondary reactions. Our results suggest that the Hg cycling\nin the Himalayan valleys could mix multiple impacts from montane environments\nand signals of transboundary transport of anthropogenic Hg from South\nAsia.