Significant Seasonal Variations in Isotopic Composition of Atmospheric Total Gaseous Mercury at Forest Sites in China Caused by Vegetation and Mercury Sources

In this study, isotopic compositions of atmospheric total gaseous mercury (TGM) were measured in the Mt. Changbai (MCB) temperate deciduous forest and the Mt. Ailao (MAL) subtropical evergreen forest over a 1-year period. Higher δ²⁰²Hg_TGM values were observed under the forest canopy than above the forest canopy in the MCB forest. The vertical gradients in δ²⁰²Hg_TGM and Δ¹⁹⁹Hg_TGM are positively correlated with the satellite-based normalized difference vegetation index (NDVI, representing the vegetation photosynthetic activity), suggesting that a strong vegetation activity (high NDVI) induces both mass-dependent and mass-independent fractionation of TGM isotopes. The observed δ²⁰²Hg_TGM and Δ¹⁹⁹Hg_TGM showed seasonal variations. Mean δ²⁰²Hg_TGM and Δ¹⁹⁹Hg_TGM in summer were 0.35–0.99‰ and 0.06–0.09‰ higher than those in other seasons in the MCB forest. In contrast, the highest seasonal δ²⁰²Hg_TGM in the MAL forest was observed in winter at 0.07–0.40‰ higher than the values found in other seasons. The variability of δ²⁰²Hg_TGM and Δ¹⁹⁹Hg_TGM in MCB was attributed to vegetation activities, whereas the seasonal δ²⁰²Hg_TGM in the MAL forest was driven by the exposure of air masses to anthropogenic emissions. Using the data in this study and in the literature, we concluded that vegetation activity and anthropogenic Hg release are the main drivers for the spatial variations in TGM isotopic compositions in the northern hemisphere.