Development of an automated micrometeorological method for measuring the emission of mercury vapor from wetland vegetation

The ability of green plants to act as conduits to enhance the transport of Hg from soils to the atmosphere is now established, but the data base is severely limited. The potential role of this process in mobilizing Hg in global and regional cycles makes it imperative that automated methods be developed to increase our capability to measure and understand the process in a variety of ecosystems. We previously published a tower-based micrometeorological gradient method for measuring gas-phase Hg° fluxes in terrestrial systems based on the Modified Bowen ratio (MBR) approach. The method relied on demanding and time-consuming manual sampling of Hg gradients. Automated Hg sampling methods now exist, and we describe here applications of the Tekran Hg analyzer to automated near-real-time measurements of Hg gradients over wetland vegetation. We use these data with MBR to compute fluxes of Hg from those of other trace gases. From 1996 to 1998 we sampled Hg fluxes over emergent macrophytes in the Florida Everglades, beginning with manual methods, but later deploying automated methods for most of the study to collect more than 500 30-min fluxes over 2 y. The limitations of the manual method resulted in considerable uncertainly in our earlier observations, even to the extent that we initially doubted that vegetation emissions were real. However, the automated method allowed us to quantify the actual development of Hg emission gradients over wetland vegetation. Following sunrise Hg fluxes show diel patterns similar to those of CO2 and H2O, providing information on the possible mechanisms of Hg emission. Our data suggest mean daytime emission rates of Hg over these wetlands on the order of 30 ng m-2 h-1. Fluxes were influenced by temperature, solar radiation, and atmospheric turbulence. There exists a significant biotic re-emission of Hg° from the oceans, and our data provide the first direct evidence of a similar process in subtropical wetlands.