Aryeh Feinberg , Noelle Eckley Selin , Christine F Braban, Kai-Lan Chang, Danilo Custódio, Daniel A. Jaffe, Katriina Kyllönen, Matthew S. Landis, Sarah R. Leeson, Winston Luke, 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 pollutant mercury (Hg) into the atmosphere, leading to long-range transport and deposition to remote ecosystems. 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 gaseous elemental mercury (Hg⁰) 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 Hg⁰ trends in most NH regions, with an overall trend for 2005–2020 of ‑0.011 ± 0.006 ng m^-3 yr^-1 (±2 SD). In contrast to existing emission inventories, our modelling analysis suggests that NH anthropogenic emissions must have declined by at least 140 Mg yr^-1 between the years 2005 and 2020 to be consistent with observed trends. Faster declines in 95th percentile Hg⁰ 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, demonstrating that existing emissions inventories are incompatible with the observed Hg⁰ declines.