Measurements of gaseous elemental mercury fluxes over intact tallgrass prairie monoliths during one full year

The atmosphere is an important pathway by which mercury is transported and distributed to pristine ecosystems. The significance of anthropogenic versus natural mercury contributions to the atmosphere is controversial, and the importance of re-emission of deposited mercury from ecosystems is not known. Here we present a continuous year-long data set of gaseous elemental mercury exchange between intact soil-plant monoliths of tallgrass prairie and the atmosphere. Mercury fluxes were measured using large open-flow gas exchange chambers (7.3×5.5×4.5 m3, L×W×D). Approximately 60 μg m−2 of elemental gaseous mercury was lost from four replicate grassland ecosystems (9 m2 surface area each) to the atmosphere over the course of 1 yr. Deposition was an important flux in the winter and emissions were dominant in spring, summer, and fall. Solar radiation and air temperature were most strongly correlated with mercury emissions. Gaseous elemental mercury losses to the atmosphere exceeded other measured fluxes of mercury in and out of the grassland ecosystems. These results indicate that mercury emissions from uncontaminated terrestrial ecosystems to the atmosphere may be a significant source of atmospheric mercury. We hypothesize that most of the mercury being emitted is previously deposited mercury and that re-emissions of mercury from terrestrial ecosystems is an important process whereby mercury is continually cycled between the air and terrestrial ecosystems.