There are few reliable data on the speciation of Hg in ambient air, although this information is critical to understanding the fate of Hg once released from point sources. The water soluble species of Hg that are thought to exist in flue gases would be subject to far greater local removal rates than is elemental Hg vapor, but methods are lacking to quantify this species. We developed a method using refluxing mist chambers (MC) to measure the airborne concentrations of reactive gaseous mercury (RGM) in short-term samples under ambient conditions, and have simulated the atmospheric transport and fate of anthropogenic mercury emissions over the contiguous United States. The MC method exhibits an effective detection limit of 0.02 ng/m(3) and a precision for ambient concentration levels of +/-20-30%. The average RGM concentrations measured with our method at sites in Tennessee (TN) and Indiana (IN) were similar to 0.06 and similar to 0.10 ng/m(3), respectively, These averages represent about 3% of total gaseous mercury (TGM), and RGM generally exceeds regional particulate Hg. The 24-h simulated RGM concentration averages in the modeling grid cells representing TN and IN are 0.05 and 0.098 ng/m(3), respectively, in good agreement with the data. These concentrations are high enough to suggest that RGM can play an important role in both wet and dry deposition on a regional scale. (C) 2000 Elsevier Science B.V. All rights reserved.