Speciation and distribution of atmospheric mercury: Significance of reactive gaseous mercury in the the global mercury cycle

The importance of reactive gaseous mercury (RGHg) to Hg biogeochemistry has been neglected until recently, mainly due to the lack of proper sampling techniques. However, although low ambient RGHg concentrations are generally assumed, the dry depositional flux of RGHg can still be a significant source of Hg to many environmental systems because of its high dry deposition velocity. Recent advancement in RGHg sampling techniques has improved the situation and enabled researchers to explore the significance of RGHg. In this study, three commonly used RGHg sampling techniques, including the filter pack method, the refluxing mist chamber method, and the KCl-coated denuder method, have been tested in field side-by-side with comparable results being obtained. These results demonstrate that the use of the filter pack method for the rest of the study was appropriate. The speciation and distribution of atmospheric Hg around and over the Chesapeake Bay was studied. Elevated concentrations of total atmospheric Hg (THg), RGHg, and particulate Hg (Hg-P) were detected in downtown Baltimore, as compared to the rural sites. Therefore, Baltimore's urban air is an important source of Hg to the northern Chesapeake Bay, and the estimated dry depositional flux of Hg was the same magnitude as that of the wet flux measured. Even at rural sites, the dry depositional flux was a significant portion of the total depositional flux. It is generally assumed that the anthropogenic emission is the major source of RGHg to the atmosphere. Since both wet and dry depositional processes efficiently remove RGHg, its concentration should be low in marine air if the assumption was valid. However, substantial amounts of RGHg were often detected in marine air over the Atlantic Ocean, and sometimes they reached the level measured in polluted urban air. A laboratory-based oxidation study was conducted at the Chesapeake Biological Laboratory. It was demonstrated that in the presence of light and deliquescent NaCl, gaseous Hg0 was quickly oxidized. The results of this study support the contention that halogen radical-mediated Hg0 oxidation could be an important RGHg source in the marine boundary layer; that Hg may be quickly recycled at the air-ocean interface; and that the dry deposition of RGHg is an important flux of Hg to the open ocean.