Shifts in Atmospheric Composition Since the Preindustrial Era Modified the Transport and Deposition of Mercury

Abstract The atmospheric lifetime of mercury (Hg) determines its global spread and the delivery of this toxic pollutant to remote ecosystems. Previous studies have generally assumed that the chemical lifetime of elemental mercury (Hg 0 ) has remained constant across historical time periods, mirroring present‐day (PD, 2010–2019) conditions. However, since preindustrial times (PI, 1850) anthropogenic emissions have altered the concentrations of key oxidants that affect the Hg 0 lifetime, including bromine radicals (Br), hydroxyl radicals (OH), and ozone (O 3 ). Here, we use chemistry‐climate modeling to analyze the changes in Hg redox chemistry between PI and PD and consequent impacts on Hg transport and deposition. While increasing concentrations of OH and O 3 lead to 16% faster Hg 0 oxidation in the PD Northern Hemisphere, the increased partitioning of Br to reservoir species slows Hg 0 oxidation by 20% in the Southern Hemisphere compared to PI. On the global scale, these competing mechanisms lead to an overall buffering of the tropospheric chemical lifetime of Hg 0 . The shift from PI to PD atmospheric composition drives 15% more Hg deposition to tropical and subtropical fisheries, which are the major global source of toxic methylmercury for human exposure. The PI atmosphere was more conducive to the spread of Hg to the remote Southern Hemisphere extratropics, impacting the interpretation of historical records of Hg deposition from natural archives and the supply of Hg to the Southern Ocean marine sediment sink. This study reveals the previously overlooked role of changing atmospheric composition in aggravating human Hg exposure risk via altered deposition patterns.