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

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.

Using Mercury Stable Isotopes to Quantify Directional Soil–Atmosphere Hg(0) Exchanges in Rice Paddy Ecosystems: Implications for Hg(0) Emissions to the Atmosphere from Land Surfaces

“…However, the average isotopic compositions of the Hg(0) collected from the DFC outlet at night were similar to those collected during the day, as seen from the signature values presented above. This phenomenon may be because very little abiotic dark reduction occurs at night, and a delayed release of Hg(0) was previously produced during the day . Therefore, Hg(0) collected at night cannot be used as an endmember for abiotic dark reduction in the mixing model.…”

“…In 2021, Hg(0) exchange flux between the paddy soil and the atmosphere was measured at Gouxi using the DFC method (Figure S4). , Sampling was performed on the 50, 90, and 110th day after transplanting and first flooding, with each sampling period lasting 24 h. Given the significant impacts of flooding and solar radiation on Hg(0) emissions from paddy soils, , day 50 and 90th were selected to assess the variations in Hg(0) emissions under different solar radiation intensities. Additionally, day 110th was chosen for sampling to examine the changes in soil Hg(0) emissions between flooded and dry conditions.…”

“…This phenomenon may be because very little abiotic dark reduction occurs at night, and a delayed release of Hg(0) was previously produced during the day. 49 Therefore, Hg(0) collected at night cannot be used as an endmember for abiotic dark reduction in the mixing model.…”

“…Moreover, the average Hg(0) emission fluxes were significantly higher during day-than nighttime on both the 50 and 90th day (as presented in Section 3.3), possibly because the rate of photoreduction was significantly higher than that of nonphotoreduction. 49 However, on the 110th day, the contribution of photoreduction decreased due to the low penetration of solar radiation in dry paddy soils, 58 while the relative contributions of microbial reduction and abiotic dark reduction increased correspondingly.…”

Isotope Data Constrains Redox Chemistry of Atmospheric Mercury

“…We speculate that 60% of deposited snowfall-Hg can be re-emitted to the air and estimated that the ε 202 Hg of snow-released Hg of −0.53‰, and assumed the E 199 Hg and E 201 Hg of 1.40‰ representing the level of annual re-emission. g Mercury isotope fractionation in the oceanic environment remains unexplored. Model studies have indicated that Hg 0 released into the ocean is subject to isotopic alteration due to reduction processes occurring within the ocean. , Drawing from investigations into Hg II reduction in NOM-rich waters and freshwater–air Hg 0 exchanges, we hypothesize that oceanic reduction follows a similar fractionation pattern and assume the enrichment factors of ε 202 Hg = −0.13‰, E 199 Hg = E 201 Hg = −0.15‰. …”