A comprehensive database of global soil-atmosphere flux studies of mercury

Wednesday, 17 December 2014: 5:45 PM
Yannick Agnan1, Daniel Obrist1, Chris Moore1 and Théo Le Dantec2, (1)Desert Research Institute, Reno, NV, United States, (2)Institut National Polytechnique de Toulouse, EcoLab- Laboratoire Ecologie Fonctionnelle et Environnement, Toulouse, France
The surface-atmosphere exchange of mercury (Hg) is temporally and spatially dynamic and soils can serve as large reservoirs for atmospheric deposition of Hg. However, due to semi-volatile behavior of elemental Hg (Hg0) and reduction processes, Hg0 can re-emit to the atmosphere, undergoing a bi-directional flux. These processes, particularly re-emissions of previously deposited Hg, are estimated to globally account for up to 60% of current atmospheric Hg loads. Due to the variation in Hg0 fluxes among areas, the purpose of this study was to build a global database of surface-atmosphere Hg0 flux measurements and use that database to evaluate our current level of understanding.

Hg0 fluxes have been measured over a wide range of areas from typical background sites to locations naturally and anthropogenically enhanced in Hg. Our database analysis showed that most flux measurements of Hg0 have been focused in the United States, Europe, and East Asia. Large areas of the globe have poorly documented surface-atmosphere Hg0 exchange. Measured fluxes span a very large range from -5,500 ng m-2 hr-1 to +110,000 ng m-2 hr-1, with fluxes measured before the year 2005 being 3 times higher than fluxes measured after 2005. This corresponds to a shift from measuring over enriched sites towards measurement over background areas and improvements in measurement methods.

Measurements to date have experienced a strong bias; for instance, daytime measurements (85%) prevail over nighttime ones, as summer and spring measurements (65%) dominate over those taken in winter and fall. Bare soils account for over 40% of all terrestrial data, and 57% of studies were conducted over substrate concentrations that are considered “background”. At the same time, only 40% of measurements were performed under air Hg concentrations typical of background air (<2 ng m-3). 85% of all flux studies were conducted with chamber methods using a variety of different designs, materials, and measurement protocols, with the rest based on micrometeorological flux techniques. Our findings indicate that to effectively estimate global and regional budgets of surface-atmosphere Hg0 exchange and their dependence on environmental variables, careful consideration should be given to the potential biases introduced by selectively sampling areas and methods used.