Large-scale impact of anthropogenic mercury during the 1970s revealed by polar firn air

Mercury (Hg) is an extremely toxic pollutant, and its biogeochemical cycle has been perturbed by anthropogenic emissions during recent centuries. In the atmosphere, gaseous elemental mercury (GEM; Hg°) is the predominant form of mercury (up to 95%). GEM concentrations measured in the interstitial air of the firn (unconsolidated snow) at Summit (central Greenland) enabled reconstruction of the atmospheric history of this species in mid- to high-northern latitudes back to the twentieth century. GEM concentrations increased rapidly after World War II from ~1.5 ng m-3 reaching a maximum of ~3 ng m-3 around 1970 and decreased until stabilizing at ~1.7 ng m-3 around 1995. This reconstruction reproduces real-time measurements available from the Arctic since 1995 and exhibits the same general trend observed in Europe since 1990. Anthropogenic emissions caused a two-fold rise in boreal atmospheric GEM concentrations before the 1970s, which likely contributed to higher deposition of mercury in both industrialized and remotes areas. Once deposited, this toxin becomes available for methylation and, subsequently, the contamination of ecosystems. Implementation of air pollution regulations, however, enabled a large-scale decline in atmospheric mercury levels during the 1980s. Our results suggest that potential increases in emissions in the coming decades could have a similar large-scale impact on atmospheric Hg levels.