The severity and spatial extent of the health impact of anthropogenic mercury (Hg) emission to the atmosphere depend on the emission rate and chemical form of the emitted species. The few measurements of combustion flue gas give highly variable results about how the emission is divided between the elemental (Hg¿) and reactive forms and are difficult to extrapolate to a regional scale. Here we combine measurement of total gaseous mercury (TGM) and carbon dioxide (CO2) concentrations at a background site in the winter with carbon (C) emission inventory to show that at a regional (500 km) scale, the effective anthropogenic Hg¿ flux is 41(¿2) g km-2 yr-1 in the north-east United States. This regional-scale flux was higher under clear skies than under cloudy skies, suggesting some removal of Hg¿ by cloud water, but the physical mechanisms of the removal are yet to be identified. ¿ 2001 American Geophysical Union