Aryeh Feinberg , Noelle Eckley Selin , Christine F Braban, Kai-Lan Chang, Danilo Custódio, Daniel A. Jaffe, Katriina Kyllönen, Matthew S. Landis, Sarah R. Leeson, Koketso M. Molepo, Marijana Murovec, Michelle G. Nerentorp Mastromonaco, Katrine Aspmo Pfaffhuber, Julian Rüdiger, Guey-Rong Sheu, Vincent L. St.Louis

Anthropogenic activities emit ~2000 Mg yr-1 of the toxic heavy metal mercury (Hg) into the atmosphere, where it can be transported long distances and deposited to remote ecosystems. Existing global anthropogenic emissions inventories report increases in Northern Hemispheric (NH) Hg emissions during the last three decades, in contradiction with the observed decline in atmospheric Hg concentrations at NH measurement stations. Many factors can obscure the link between anthropogenic emissions and atmospheric Hg concentrations, including trends in the re-emissions of previously released anthropogenic (“legacy”) Hg, atmospheric sink variability, and spatial heterogeneity of monitoring data. Here we assess the observed trends in total gaseous mercury (TGM) in the NH and apply biogeochemical box modeling and chemical transport modeling to understand the trend drivers. Using linear mixed effects modeling of observational data from 51 stations, we find negative TGM trends in most NH regions, with an overall trend for 2005–2020 of -0.011 ± 0.006 ng m-3 yr-1 (±2 SD). We attribute this trend to a decline in NH anthropogenic emissions of at least 165 Mg yr-1 between the years 2005 and 2020, using an ensemble of simulations in a biogeochemical box model. Faster declines in 95th percentile TGM values than median values in Europe, North America, and East Asian measurement stations corroborate that the likely cause is a decline in nearby anthropogenic emissions rather than background legacy re-emissions. Our results are relevant for evaluating the effectiveness of the Minamata Convention on Mercury and demonstrate that existing emissions inventories are incompatible with the observed TGM declines.