Explorative and innovative dynamic flux bag method development and testing for mercury air–vegetation gas exchange fluxes

An intensive field study quantifying total gaseous mercury (TGM) and mercury speciation fluxes in a wetland ecosystem (Bay St. François wetlands, Québec, Canada) was conducted in summer 2003. This study is one of the first attempts to design and develop an innovative approach—dynamic flux bag (DFB) technique to measure in situ mercury air–vegetation exchange with a monoculture of river bulrush (Scirpus fluviatilis). Air–vegetation flux measurements were conducted under dry condition at site 1 and flood condition at site 2. TGM fluxes fluctuated from −0.91 to 0.64 ng/m2 (leaf area)/h with an average value of –0.26±0.28 ng/m2 (leaf area)/h at site 1 and ranged from −0.98 to 0.08 ng/m2 (leaf area)/h with a mean flux of −0.33±0.24 ng/m2 (leaf area)/h at site 2 (positive sign means volatilization, and negative sign indicates deposition). The data indicated that TGM air–vegetation exchange is bidirectional. However, the net flux is primarily featured by dry deposition of TGM from atmosphere to the vegetation. In mercury speciation study using the DFB approach, particulate mercury (PM) and reactive gaseous mercury (RGM) represented less than 1% of total mercury. Ambient ozone concentrations had significant influences on RGM concentrations (r=0.54, p<0.05), implicating oxidation of gaseous elemental mercury (GEM) by ozone to form RGM. A discussion about the similarities and discrepancies between the DFB and other approaches (dynamic flux chamber and modified Bowen ratio) is presented. During the course of this study, some operational effects associated with the bag design, mainly the emergence of condensation within the bag, were encountered. Several improvements relating to the DFB design were recommended. Upon improvement, the DFB method could be one of the most promising techniques to study the role of a single plant in air–vegetation exchange of mercury.