Using Mercury Stable Isotopes to Quantify Bidirectional Water-Atmosphere Hg(0) Exchange Fluxes and Explore Controlling Factors.

In this study, exchange fluxes and Hg isotope fractionation during water-atmosphere Hg(0) exchange were investigated at three lakes in China. Water-atmosphere exchange was overall characterized by net Hg(0) emissions, with lake-specific mean exchange fluxes ranging from 0.9 to 1.8 ng m h, which produced negative δHg (mean: -1.61 to -0.03‰) and ΔHg (-0.34 to -0.16‰) values. Emission-controlled experiments conducted using Hg-free air over the water surface at Hongfeng lake (HFL) showed negative δHg and ΔHg in Hg(0) emitted from water, and similar values were observed between daytime (mean δHg: -0.95‰, ΔHg: -0.25‰) and nighttime (δHg: -1.00‰, ΔHg: -0.26‰). Results of the Hg isotope suggest that Hg(0) emission from water is mainly controlled by photochemical Hg(0) production in water. Deposition-controlled experiments at HFL showed that heavier Hg(0) isotopes (mean εHg: -0.38‰) preferentially deposited to water, likely indicating an important role of aqueous Hg(0) oxidation played during the deposition process. A ΔHg mixing model showed that lake-specific mean emission fluxes from water surfaces were 2.1-4.1 ng m h and deposition fluxes to water surfaces were 1.2-2.3 ng m h at the three lakes. Results from the this study indicate that atmospheric Hg(0) deposition to water surfaces indeed plays an important role in Hg cycling between atmosphere and water bodies.