Quantification of Atmospheric Mercury Deposition to and Legacy Re-emission from a Subtropical Forest Floor by Mercury Isotopes

Air–soil exchange of elemental mercury vapor (Hg0) is an important component in the budget of the global mercury cycle. However, its mechanistic detail is poorly understood. In this study, stable Hg isotopes in air, soil, and pore gases are characterized in a subtropical evergreen forest to understand the mechanical features of the air–soil Hg0 exchange. Strong HgII reduction in soil releases Hg0 to pore gas during spring–autumn but diminishes in winter, limiting the evasion in cold seasons. Δ199Hg in air modified by the Hg0 efflux during flux chamber measurement exhibit seasonality, from −0.33 ± 0.05‰ in summer to −0.08 ± 0.05‰ in winter. The observed seasonal variation is caused by a strong pore-gas driven soil efflux caused by photoreduction in summer, which weakens significantly in winter. The annual Hg0 gross deposition is 42 ± 33 μg m–2 yr–1, and the corresponding Hg0 evasion from the forest floor is 50 ± 41 μg m–2 yr–1. The results of this study, although still with uncertainty, offer new insights into the complexity of the air-surface exchange of Hg0 over the forest land for model implementation in future global assessments.