Application of a rule-based model to estimate mercury exchange for three background biomes in the continental United States.

Ecosystems that have low mercury (Hg) concentrations (i.e., not enriched or impacted by geologic or anthropogenic processes) cover most of the terrestrial surface area of the earth yet their role as a net source or sink for atmospheric Hg is uncertain. Here we use empirical data to develop a rule-based model implemented within a geographic information system framework to estimate the spatial and temporal patterns of Hg flux for semiarid deserts, grasslands, and deciduous forests representing 45% of the continental United States. This exercise provides an indication of whether these ecosystems are a net source or sink for atmospheric Hg as well as a basis for recommendation of data to collect in future field sampling campaigns. Results indicated that soil alone was a small net source of atmospheric Hg and that emitted Hg could be accounted for based on Hg input by wet deposition. When foliar assimilation and wet deposition are added to the area estimate of soil Hg flux these biomes are a sink for atmospheric Hg.

Assessing the potential for re-emission of mercury deposited in precipitation from arid soils using a stable isotope.

A solution containing 198Hg in the form of HgCl2 was added to a 4 m2 area of desert soils in Nevada, and soil Hg fluxes were measured using three dynamic flux chambers. There was an immediate release of 198Hg after it was applied, and then emissions decreased exponentially. Within the first 6 h after the isotope was added to the soil, approximately 12 ng m(-2) of 198Hg was emitted to the atmosphere, followed by a relatively steady flux of the isotope at 0.2 +/- 0.2 ng m(-2) h(-1) for the remainder of the experiment (62 days). Over this time, approximately 200 ng m(-2) or 2% of the 198Hg isotope was emitted from the soil, and we estimate that approximately 6% of the isotope would be re-emitted in a year's time. During the experiment, dry deposition of elemental Hg from the atmosphere was measured with an average deposition rate of 0.2 +/- 0.1 ng m(-2) h(-1). Emission of ambient Hg from the soil was observed after soil wetting with the isotope solution and after a storm event. However, the added moisture from the storm event did not affect 198Hg flux. Results suggest that in this desert environment, where there is limited precipitation, Hg deposited by wet processes is not readily re-emitted and that dry deposition of elemental Hg may be an important process.