Air-surface exchange of gaseous mercury over a mixed sawgrass-cattail stand within the Florida Everglades

The Florida Everglades Dry Deposition Study (FEDDS) was conducted to test newly developed measurement techniques used to quantify the dry-deposition of speciated mercury to the Florida Everglades ecosystem. One component of FEDDS employed micrometeorological and dynamic flux chamber techniques to study the air-surface exchange of gaseous elemental mercury over a mixed sawgrass/cattail marsh within Water Conservation Area 3. These measurements provided information related to the magnitudes of, and processes associated with, the bidirectional flux of gaseous elemental mercury within this ecosystem for use in the development of a canopy-scale mercury dry deposition model. During the 2000 FEDDS study period, on average elemental gaseous mercury was emitted from the ecosystem during the daytime (16 30 ng m(-2) h(-1)), with deposition observed at night (-1 +/- 4 ng m(-2) h(-1)). The measured fluxes followed a diurnal pattern and were positively correlated with ambient temperature, solar radiation, and water vapor fluxes and negatively correlated with carbon dioxide fluxes. Our study results suggest that while flux-gradient techniques currently used in many mercury air-surface exchange studies can provide useful information on the general nature of mercury fluxes and the environmental parameters that influence them, there is sufficient uncertainty in the measurement of vertical gradients of gaseous elemental mercury to warrant caution in the interpretation of such measurements, especially during periods typically characterized as having small gradients (day/night transition periods and well-mixed conditions).