precise measurement of concentration gradients of mercury in air over soils: a review of past and recent measurements

Measurements of the atmospheric concentration gradients of mercury (Hg) vapor over soils can be used to determine the direction and magnitude of exchange rates of Hg if certain assumptions are met. However, these gradients are quite small and require highly precise sampling to achieve accurate data. We have developed a sampling and analysis procedure which allows quantification of gradients over background soils. With this procedure we can now measure atmospheric Hg at ambient levels with a precision of similar to 0.5 to 2% (expressed as relative standard error). This level of precision is well above those published in earlier gradient studies. In our recent studies, gradients measured between 25 and 165 cm above background forest soils at Walker Branch Watershed, Tennessee were quite small, ranging from 0.02 to 0.39 ng/m super(3) (expressed as concentration differences). These gradients indicated that Hg emission was about 3 times more frequent than dry deposition. Gradients measured over soils at Lake Gaardsjoen, Sweden were generally smaller but also indicated bidirectional fluxes. By comparison, gradients above Hg-contaminated soils in Tennessee were far larger as expected, ranging from 0.12 to 5.60 ng/m super(3). These gradients consistently indicated emission of Hg. A number of tests were performed to validate that these gradients were true indications of Hg exchange rates.