Atmospheric mercury sources in a coastal-urban environment: a case study in Boston, Massachusetts, USA
Résumé
Mercury (Hg) is an environmental toxicant dangerous to human health and the environment. Its anthropogenic emissions are regulated by global, regional, and local policies. Here, we investigate Hg sources in the coastal city of Boston, the third largest metropolitan area in the Northeastern United States. With a median of 1.37 ng m À3 , atmospheric Hg concentrations measured from August 2017 to April 2019 were at the low end of the range reported in the Northern Hemisphere and in the range reported at North American rural sites. Despite relatively low ambient Hg concentrations, we estimate anthropogenic emissions to be 3-7 times higher than in current emission inventories using a measurement-model framework, suggesting an underestimation of small point and/or nonpoint emissions. We also test the hypothesis that a legacy Hg source from the ocean contributes to atmospheric Hg concentrations in the study area; legacy emissions (recycling of previously deposited Hg) account for $60% of Hg emitted annually worldwide (and much of this recycling takes place through the oceans). We find that elevated concentrations observed during easterly oceanic winds can be fully explained by low wind speeds and recirculating air allowing for accumulation of land-based emissions. This study suggests that the influence of nonpoint land-based emissions may be comparable in size to point sources in some regions and highlights the benefits of further top-down studies in other areas. Environmental signicance This work investigates the sources of mercury, an environmental toxicant dangerous to human health and the environment, in the coastal city of Boston, United States. Using a measurement-model framework, we show that current inventories underestimate mercury emissions in the study area by a factor of 3-7. While emission inventories are thought to be relatively accurate for large point sources, this study adds to the growing body of literature suggesting that fugitive emissions from unknown point and nonpoint sources must be better constrained to improve emission inventories. These emissions are likely to become relatively more important as major point sources are controlled under domestic and global mitigation policies. This study therefore highlights the need of further top-down studies in other regions.