The atmospheric chemistry of mercury, a global priority pollutant, is key to its transport and deposition to the surface environment. Assessments of its risks to humans and ecosystems rely on an accurate understanding of global mercury cycling.

This work shows that the set of chemical reactions and rates currently employed to interpret the delicate balance between Hg oxidation and reduction in the atmosphere fails to explain the observed atmospheric mercury concentrations and deposition. We report that model simulations incorporating recent theoretical developments in the photoreduction mechanisms of the oxidized forms of mercury (HgI and HgII) lead to a significant model underestimation of global observations of these oxidized species in the troposphere and their surface wet deposition. This implies that there must be currently unidentified mercury oxidation processes in the troposphere.

Alfonso Saiz-Lopez, Oleg Travnikov, Jeroen E. Sonke, Colin P. Thackray, Daniel J. Jacob, Javier Carmona-García, Antonio Francés-Monerris, Daniel Roca-Sanjuán, A. Ulises Acuña, Juan Z. Dávalos, Carlos A. Cuevas, Martin Jiskra, Feiyue Wang, Johannes Bieser, John M. C. Plane, and Joseph S. Francisco. Photochemistry of oxidized Hg(I) and Hg(II) species suggests missing mercury oxidation in the troposphere. PNAS. DOI: 10.1073/pnas.1922486117