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

Abstract

The ability of green plants to act as conduits to enhance the transport ofHg from soils to the atmosphere is now established, but the data base isseverely limited. The potential role of this process in mobilizing Hg inglobal and regional cycles makes it imperative that automated methods bedeveloped to increase our capability to measure and understand the processin a variety of ecosystems. We previously published a tower-basedmicrometeorological 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 manualsampling of Hg gradients. Automated Hg sampling methods now exist, andwe describe here applications of the Tekran Hg analyzer to automatednear-real-time measurements of Hg gradients over wetland vegetation. Weuse these data with MBR to compute fluxes of Hg from those of othertrace gases. From 1996 to 1998 we sampled Hg fluxes over emergentmacrophytes in the Florida Everglades, beginning with manual methods, butlater deploying automated methods for most of the study to collect morethan 500 30-min fluxes over 2 y. The limitations of the manual methodresulted in considerable uncertainly in our earlier observations, even to theextent that we initially doubted that vegetation emissions were real. However, the automated method allowed us to quantify the actualdevelopment of Hg emission gradients over wetland vegetation. Followingsunrise Hg fluxes show diel patterns similar to those of CO2 andH2O, providing information on the possible mechanisms of Hgemission. Our data suggest mean daytime emission rates of Hg over thesewetlands on the order of 30 ng m-2 h-1. Fluxes wereinfluenced 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 insubtropical wetlands.