Using mercury stable isotopes to quantify bidirectional water-atmosphere Hg(0) exchange fluxes and explore controlling factors

Abstract

Water-atmosphere exchange of gaseous elemental mercury (Hg(0)) plays a critical role in global Hg cycling. However, the controlling factors and Hg isotope fractionation during the bidirectional water-atmosphere Hg(0) exchange are not well understood. 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, e.g., lake-specific mean exchange fluxes ranged from 0.9 to 1.8 ng m^-2 h^-1, which produced negative δ²⁰²Hg (means: -1.61 to -0.03‰) and Δ¹⁹⁹Hg (means: -0.34 to -0.16‰) values in net Hg(0) emissions. Emission-controlled experiments conducted in a chamber using Hg-free air over water surface at Hongfeng lake (HFL) showed negative δ²⁰²Hg and Δ¹⁹⁹Hg in Hg(0) emitted from water similar between daytime (mean δ²⁰²Hg: -0.95‰, mean Δ¹⁹⁹Hg: -0.25‰) and nighttime (mean δ²⁰²Hg: -1.00‰, mean Δ¹⁹⁹Hg: -0.26‰). Based on the diagnostic isotopic ratio Δ¹⁹⁹Hg/Δ²⁰¹Hg of unity, we propose that Hg(0) emission from water is mainly controlled by photochemical Hg(0) production in water, which subsequently emits into the atmosphere via gas transfer. Deposition-controlled experiments at HFL showed that heavier Hg(0) isotopes (mean ε²⁰²Hg: -0.38‰) preferentially deposited to water, likely indicating an important role of aqueous Hg(0) oxidation played during deposition process. A Δ²⁰⁰Hg mixing model was used to quantify the bidirectional water-atmosphere Hg(0) exchange fluxes, which yielded lake-specific mean emission fluxes from waters surfaces of 1.9 to 6.8 and deposition fluxes to water surfaces of 0.9 to 5.0 ng m^-2 h^-1 at the three lakes. Results from the present study are in agreement with the contribution of Hg(0) deposition generated from sediments and seawater Hg using isotope approaches, indicating that atmospheric Hg(0) deposition to water surfaces indeed play an important role in Hg cycling between atmosphere and large water bodies.