Evaluation of use of EcoCELL technology for quantifying total gaseous mercury fluxes over background substrates

Total gaseous mercury (Hg) fluxes from large (7.3×5.5×4.5 m, L×W×D) climate-controlled gas exchange mesocosms (Ecologically Controlled Enclosed Lysimeter Laboratories or EcoCELLs) containing tallgrass prairie soil–plant monoliths were measured from 2002 to 2005. EcoCELL Hg fluxes (calculated based on the difference in air Hg concentrations inside mesocosms and in incoming air, soil area of the monoliths, and airflow through the system) indicated a net annual emission of 102 μg m−2, while soil Hg fluxes measured simultaneously using a dynamic flux chamber were an order of magnitude lower. Since Hg fluxes measured from empty EcoCELLs in winter and when housing the soil–plant monoliths at the same time of year were similar, we hypothesized that the Hg signal generated by the tallgrass prairie soil–plant monoliths was too low to be detected using the EcoCELL technology. Because mesocosm Hg exchange was correlated with solar radiation and temperature, with the largest emissions occurring at midday and in the summer, we also hypothesized that the flux from mesocosm infrastructure would change over time. Limited by the ongoing experiment, the EcoCELLs were manipulated to test the above hypotheses. When monoliths were completely covered and excluded from the exchange with the surrounding air, mesocosm Hg exchange was unaffected. Furthermore, removal of vegetation at the end of each growing season did not affect mesocosm Hg fluxes. Tests with changing mesocosm airflow also indicated that the signal from the tallgrass prairie monoliths was not being measured. These results suggest that, although EcoCELLs performed well in a study using Hg contaminated soils and have been successfully applied to understand processes controlling Hg fluxes, there are limitations of this technology for quantifying Hg exchange from background substrates. Prior to the use of similar systems the detection limit and Hg exchange from an empty system need to be carefully quantified.