Isotopic composition of total gaseous mercury at a high-altitude tropical forest site influenced by air masses from the East Asia continent and the Pacific Ocean

Gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), particulate-bound mercury (PBM), and the isotopic composition of total gaseous mercury (TGM = GEM + GOM) were measured at the Lulin Atmospheric Background Station (LABS), a mountain background site downwind of the East Asia continent, from February 2015 to January 2016. Mean (+/- S.D.) concentrations of GEM, GOM, and PBM were 1.54 +/- 0.34 ng m(-3), 14.0 +/- 27.8 pg m(-3), and 2.6 +/- 3.7 pg m(-3), respectively. Concentration-Weighted Trajectory (CWT) analysis indicated that East and coastal China were the main source regions of elevated GEM observed at LABS. Weekly integrated TGM showed significant variations in delta Hg-202(TGM) (-0.42 to 0.41 parts per thousand) and Delta Hg-199(TGM) (-0.31 to -0.13 parts per thousand). Both delta Hg-202(TGM) and Delta Hg-199(TGM) showed clear seasonal variations, but the seasonal pattern of delta Hg-202(TGM) was different from that of Delta Hg-199(TGM). The lowest mean delta Hg-202(TGM) was observed in spring (-0.15 parts per thousand), while the lowest mean Delta Hg-199(TGM) was observed in summer (-0.24 parts per thousand). A significant positive correlation (R-2 = 0.38, p < 0.01) existed between delta Hg-202(TGM) and normalized difference vegetation index (NDVI), suggesting vegetation activity was an important factor controlling the seasonal variation of delta Hg-202(TGM). In contrast, air mass transport path, which influences the source region of TGM to LABS, was likely the driver for the seasonal variations of Delta Hg-199(TGM). Furthermore, results from the air mass classification showed distinguishable Delta Hg-199(TGM) signature for different air mass source regions, with air masses from South China Sea and western Pacific Ocean being characterized by more negative Delta Hg-199(TGM) (-0.25 +/- 0.04 parts per thousand). Results of this research demonstrated a clear difference in isotopic compositions of atmospheric Hg between continental and marine air masses.