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

Abstract:

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.