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Photochemistry of oxidized Hg(I) and Hg(II) species suggests missing mercury oxidation in the troposphere.

ABSTRACT: Mercury (Hg), a global contaminant, is emitted mainly in its elemental form Hg⁰ to the atmosphere where it is oxidized to reactive Hg^II compounds, which efficiently deposit to surface ecosystems. Therefore, the chemical cycling between the elemental and oxidized Hg forms in the atmosphere determines the scale and geographical pattern of global Hg deposition. Recent advances in the photochemistry of gas-phase oxidized Hg^I and Hg^II species postulate their photodissociation back to Hg⁰ as a crucial step in the atmospheric Hg redox cycle. However, the significance of these photodissociation mechanisms on atmospheric Hg chemistry, lifetime, and surface deposition remains uncertain. Here we implement a comprehensive and quantitative mechanism of the photochemical and thermal atmospheric reactions between Hg⁰, Hg^I, and Hg^II species in a global model and evaluate the results against atmospheric Hg observations. We find that the photochemistry of Hg^I and Hg^II leads to insufficient Hg oxidation globally. The combined efficient photoreduction of Hg^I and Hg^II to Hg⁰ competes with thermal oxidation of Hg⁰, resulting in a large model overestimation of 99% of measured Hg⁰ and underestimation of 51% of oxidized Hg and ∼66% of Hg^II wet deposition. This in turn leads to a significant increase in the calculated global atmospheric Hg lifetime of 20 mo, which is unrealistically longer than the 3-6-mo range based on observed atmospheric Hg variability. These results show that the Hg^I and Hg^II photoreduction processes largely offset the efficiency of bromine-initiated Hg⁰ oxidation and reveal missing Hg oxidation processes in the troposphere.

SUBMITTER: Saiz-Lopez A

PROVIDER: S-EPMC7733835 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Understanding atmospheric mercury chemistry is the key for explaining the biogeochemical cycle of mercury and for improving the predictive capability of computational models. Increased efforts are being made to ensure comparable Hg speciation measurements in the air through establishing metrological traceability. While traceability for elemental mercury has been recently set, this is by no means the case for gaseous oxidized mercury (GOM). Since a calibration unit suitable for traceable GOM calibrations based on evaporation of HgCl2 solution was recently developed, the purpose of our work was to extensively evaluate its performance. A highly specific and sensitive 197Hg radiotracer was used for validation over a wide range of concentrations. By comparing experimental and calculated values, we obtained recoveries for the calibration unit. The average recoveries ranged from 88.5% for 1178 ng m-3 HgCl2 gas concentration to 39.4% for 5.90 ng m-3 HgCl2 gas concentration. The losses were due to the adsorption of oxidized Hg on the inner walls of the calibrator and tubing. An adsorption isotherm was applied to estimate adsorption enthalpy (ΔHads); a ΔHads value of -12.33 kJ mol-1 was obtained, suggesting exothermal adsorption. The results of the calibrator performance evaluation suggest that a newly developed calibration unit is only suitable for concentrations of HgCl2 higher than 1 µg m-3. The concentration dependence of recoveries prevents the system from being used for calibration of instruments for ambient GOM measurements. Moreover, the previously assessed uncertainty of this unit at µg m-3 level (2.0%, k = 2) was re-evaluated by including uncertainty related to recovery and was found to be 4.1%, k = 2. Calibrator performance was also evaluated for HgBr2 gas calibration; the recoveries were much lower for HgBr2 gas than for HgCl2 gas even at a high HgBr2 gas concentration (>1 µg m-3). As HgBr2 is often used as a proxy for various atmospheric HgBr species, the suitability of the unit for such calibration must be further developed.

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Photochemistry of oxidized Hg(I) and Hg(II) species suggests missing mercury oxidation in the troposphere.

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