Photochemistry of oxidized Hg(I) and Hg(II) species suggests missing mercury oxidation in the troposphere
8 pags., 5 figs., 2 tabs. ; Mercury (Hg), a global contaminant, is emitted mainly in its elemental form Hgto the atmosphere where it is oxidized to reactive Hgcompounds, 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 Hgand Hgspecies postulate their photodissociation back to Hgas 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 Hg, Hg, and Hgspecies in a global model and evaluate the results against atmospheric Hg observations. We find that the photochemistry of Hgand Hgleads to insufficient Hg oxidation globally. The combined efficient photoreduction of Hgand Hgto Hgcompetes with thermal oxidation of Hg, resulting in a large model overestimation of 99% of measured Hgand underestimation of 51% of oxidized Hg and ∼66% of Hgwet 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 Hgand Hgphotoreduction processes largely offset the efficiency of bromine-initiated Hgoxidation and reveal missing Hg oxidation processes in the troposphere. ; This study has received funding from the European Research Council Executive Agency under the European Union's Horizon 2020 Research and Innovation programme (Project ERC-2016- COG 726349 CLIMAHAL) and the Spanish Ministerio de Economía y Competitividad (MINECO) /Fondo Europeo de Desarrollo Regional (FEDER) (Projects CTQ2017-87054-C2-2-P, RYC-2015-19234, and CEX2019-000919-M). This work was supported by the Consejo Superior de Investigaciones Científicas (CSIC) Spain. A.F.-M. acknowledges the Generalitat Valenciana and the European Social Fund (Contract APOSTD/2019/149 and Project GV/2020/ 226) for the financial support. J.C.-G. acknowledges the Universitat de València for his Masters Scholarship. M.J. acknowledges funding by the Swiss National Science Foundation (Grant PZ00P2_174101). The ETMEP measurements as well as ground-based measurements of the GMOS network were funded by the EU FP7-ENV-2010 project (GMOS, Grant Agreement 265113). J.S.F. acknowledges the H2020 ERA-PLANET (689443) Integrated Global Observing Systems for Persistent Pollutants (iGOSP) and Integrative and Comprehensive Understanding on Polar Environments (iCUPE) programs
