Elevated methylmercury in Arctic rain and aerosol linked to air-sea exchange of dimethylmercury.

Mercury (Hg) is a global pollutant with substantial human health impacts. While most studies focus on atmospheric total Hg (THg) deposition, contributions of methylated Hg (MeHg), including monomethylmercury (MMHg) and dimethylmercury (DMHg), remain poorly understood. To examine this, we use rain and aerosol Hg speciation data and high-resolution surface DMHg measurements, collected on a transect from Alaskan coastal waters to the Bering and Chukchi Seas. We observed a significant fivefold increase in the MeHg:THg fraction in rain and a 10-fold increase for aerosols, closely linked to elevated surface DMHg and the highest DMHg evasion (9.4 picomoles per square meter per hour) found in upwelling waters near the Aleutian Islands. These data highlight a previously underexplored aspect of MeHg air-sea exchange and its importance to Hg cycling and human health concerns. Our findings emphasize the importance of DMHg evasion by demonstrating that atmospheric MeHg can be transported long distances (1700 kilometers) in the Arctic, posing risks to human health and ecosystems.