Mercury Emissions, Atmospheric Concentrations, and Wet Deposition across the Conterminous United States: Changes over 20 Years of Monitoring

Mercury is a global pollutant whose supply to the environment has greatly increased due to human activities resulting in adverse effects on human and wildlife health. In response, the United States has substantially reduced mercury emissions over the last two decades, largely through air quality management; however, questions remain on the effectiveness of these emission controls in reducing mercury inputs to local ecosystems, due to a disproportional influence of global pools over local sources. Here, we show that trends in mercury concentrations in air and precipitation are largely decreasing across monitoring sites in the United States, coincident with decreases in stationary mercury emissions. Significant decreasing trends primarily occurred in the eastern United States, while trends in the western United States were more often insignificant or increasing, despite no clear increases in local, stationary emissions. For the eastern United States, local emissions appear to have had larger impacts on atmospheric and precipitation mercury concentrations than has been suggested from simulations using atmospheric models, suggesting that reductions in local mercury emissions can confer meaningful benefits to mercury-contaminated areas. These findings highlight the success of air management policies in the United States and are especially timely, as the United States Environmental Protection Agency has decided to roll back the Mercury and Air Toxics Standards.