No abstract

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 −2 h −1 , which produced negative δ 202 Hg (mean: −1.61 to −0.03‰) and Δ 199 Hg (−0.34 to −0.16‰) values. Emission-controlled experiments conducted using Hg−free air over the water surface at Hongfeng lake (HFL) showed negative δ 202 Hg and Δ 199 Hg in Hg(0) emitted from water, and similar values were observed between daytime (mean δ 202 Hg: −0.95‰, Δ 199 Hg: −0.25‰) and nighttime (δ 202 Hg: −1.00‰, Δ 199 Hg: −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 ε 202 Hg: −0.38‰) preferentially deposited to water, likely indicating an important role of aqueous Hg(0) oxidation played during the deposition process. A Δ 200 Hg mixing model showed that lake-specific mean emission fluxes from water surfaces were 2.1−4.1 ng m −2 h −1 and deposition fluxes to water surfaces were 1.2−2.3 ng m −2 h −1 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.

China is the largest contributor to the global total anthropogenic mercury (Hg) emissions. However, the trend in anthropogenic Hg emissions in recent years in China has not been effectively evaluated due to the lack of long‐term atmospheric Hg observations. This study documents the changes in atmospheric gaseous elemental mercury (GEM) concentrations and isotopic compositions at Mt. Changbai (MCB) in northeastern China during 2015–2021 and Mt. Ailao (MAL) in southwestern China during 2017–2021, and explores the potential factors controlling these changes. GEM concentrations showed continuous declines from 2015 to 2021 (−2.1 ± 0.6% yr−1) at MCB and from 2017 to 2021 (−4.0 ± 1.4% yr−1) at MAL. Accompanied with these GEM declines are positive shifts in δ202Hg (medians: from 0.42 to 0.46‰ at MCB and from 0.17 to 0.57‰ at MAL), and negative shifts in Δ199Hg (medians: from −0.17‰ to −0.21‰ at MCB and from −0.10‰ to −0.17‰ at MAL) and Δ200Hg values (medians: from −0.07‰ to −0.08‰ at MCB and from −0.03‰ to −0.05‰ at MAL) (at significant levels for Δ199Hg at MCB and δ202Hg and Δ199Hg at MAL). These changes were mainly caused by the decreases in regional anthropogenic emissions in the study areas. Based on a ternary mixing model with Δ199Hg and Δ200Hg as input, we estimate decline rates of 5.8 ± 2.8 and 4.8 ± 3.0% yr−1 for the regional anthropogenic GEM emissions in northeastern and southwestern China, respectively.