Dr. Noelle Eckley Selin of the MIT Center for Global Change Science. Abstract: Concern about the toxicity of mercury depositing to ecosystems has prompted efforts to regulate its emissions in both national and international policy arenas. Current concentrations of atmospheric mercury have increased three-fold since pre-industrial times, and the main source of anthropogenic mercury emissions is coal-fired power generation. Despite increasing attention to mercury as an environmental problem, the global budget of mercury and its chemistry and cycling in the atmosphere are not well understood. This research uses a global, 3D chemical transport model (GEOS-Chem) in conjunction with worldwide atmospheric observations to constrain the redox chemistry, biogeochemical cycling and deposition of mercury. Global budget analysis of mercury shows that anthropogenic emissions will continue to affect the atmosphere-ocean-biosphere system for centuries to millennia. Over the U.S., the Midwest is affected by deposition of regional emissions, but areas such as the Southeast experience high levels of wet deposition resulting from convective scavenging of reactive mercury from higher altitudes. We then conduct a policy-focused analysis combining atmospheric, ocean and ecosystem models to explore the influence of domestic and international mercury controls on exposure to fish methylmercury. Results show that regional and ecosystem variation has a strong effect on the ability of domestic controls to address excess exposures. Research results will be discussed in the context of existing and proposed national and international policy actions on mercury. This lecture is a part of the MIT Chemical Oceanography and Biogeochemistry Seminar Series.