MICROMETEOROLOGICAL GRADIENT APPROACH FOR QUANTIFYING AIR-SURFACE EXCHANGE OF MERCURY-VAPOR - TESTS OVER CONTAMINATED SOILS

The micrometeorological modified Bowen ratio method was used for the first time to quantify fluxes of elemental mercury Vapor (Hg-0) over contaminated soils during the spring and fall of 1993. We determined fluxes using relationships among the concentration gradients and fluxes of water vapor (measured with fast-response sensors and eddy correlation) and measured gradients in Hg-0 (sampled with a multi-replicate, high-precision device). The contaminated soils consistently exhibited significant emission of Hg-0 to the atmosphere. Fluxes associated with winds from the primary source area ranged from similar to 10 to similar to 200 ng m(-2) h(-1), up to several orders of magnitude above background. Fluxes increased exponentially with soil temperature, yielding a temperature coefficient similar to nitrogen oxide (exp[(0.065 +/- 0.011)T-soil], for T-soil = 7-29 degrees C). These results provided confidence in the method for Hg-0 because the gradients and fluxes behaved predictably. Fluxes appear to be controlled by volatilization of existing contaminant elemental Hg plus reduction of Hg2+ to Hg-0 in soil solution. We modeled the emission from an area of contaminated soils within similar to 20-50 m of our site using the flux/soil temperature relationship and annual temperature data. The predicted hourly fluxes were within 30-40% of our measurements and scaled up to an annual Hg emission rate of similar to 800 mu g m(-2) y(-1) We estimate the magnitude of the Hg-0 emission from the 100-ha contaminated area to be on the order of 1-10 kg/y.