Westerlies-driven transboundary transport of atmospheric mercury to the north-central Tibetan Plateau

The transboundary mercury (Hg) pollution has caused adverse effects on fragile ecosystems of the Tibetan Plateau (TP). Yet, knowledge of transport paths and source regions of atmospheric Hg on the inland TP remains poor. Continuous measurements of atmospheric total gaseous mercury (TGM) were conducted in the central TP (Tanggula station, 5100 m a.s.l., June -October). Atmospheric TGM level at Tanggula station (1.90 +/- 0.30 ng m -3 ) was higher than the background level in the Northern Hemisphere. The identified high -potential source regions of atmospheric TGM were primarily located in the northern South Asia region. TGM concentrations were lower during the Indian summer monsoon (ISM) -dominant period (1.81 +/- 0.25 ng m -3 ) than those of the westerly -receding period (2.18 +/- 0.40 ng m -3 ) and westerly -intensifying period (1.91 +/- 0.26 ng m -3 ), contrary to the seasonal pattern in southern TP. The distinct TGM minima during the ISM -dominant period indicated lesser importance of ISM -transported Hg to Tanggula station located in the northern boundary of ISM intrusion, compared to stations in proximity to South and Southeast Asia source regions. Instead, from the ISM -dominant period to the westerly -intensifying period, TGM concentrations showed an increasing trend as westerlies intensified, indicating the key role of westerlies in transboundary transport of atmospheric Hg to the inland TP.