A Comprehensive Database of Global Soil-Atmosphere Flux Studies of Mercury.

In this study, we built a global database to comprehensively constrain and summarize flux measurements of gaseous elemental mercury (Hg⁰) between soils and terrestrial environments and the atmosphere. Soils serve as large reservoirs for atmospheric deposition of Hg, but due to the semi-volatile nature of Hg⁰ and reduction processes, Hg⁰ can re-emit from terrestrial environment to the atmosphere. Such Hg⁰ re-emissions, or legacy emissions, are estimated to globally account for up to 60% of current atmospheric Hg loads.

Our analysis of information contained in the database shows that global flux measurements of Hg⁰ have mainly been conducted in the United States, Europe, and East Asia, with a few additional measurements from South America and Australia. Large areas of the globe are missing information on surface-atmosphere exchange of Hg⁰, in particular in Africa, Eurasia, and large areas in the northern latitudes. Measurements are strongly biased towards daytime measurements (85%) as compared to nighttime, as well as towards spring and summer measurements (65% of all measurements). We found that bare soils account for over 40% of all terrestrial Hg⁰ flux measurements, and 57% of all studies were conducted over sites considered to have background soil Hg concentrations. On the other hand, only 40% of all flux measurements were conducted under air Hg concentrations at and near background levels (i.e., <2 ng m^-3) showing that a majority of measurements were conducted over sites affected by high Hg levels either due to natural enrichment, mining, or atmospheric contamination. Additionally, there was a large difference in the number of studies using specific techniques for measuring Hg⁰ flux, with 85% of all studies being conducted with chamber methods with a variety of different designs, materials, and measurement protocols. In contrast, only 15% of fluxes were based on other methods such as micrometeorological techniques.

Fluxes of Hg⁰ span a wide range of -5,500 ng m^-2 hr^-1 (i.e., deposition) to + 110,000 ng m^-2 hr^-1, and fluxes measured before the year 2005 were on average over three times higher than fluxes measured after 2005. This difference is likely due to improvement of measurement techniques and a shift towards more measurements over background areas. Our analysis indicates that to estimate global and regional budgets of surface-atmosphere exchange and their dependence on environmental variables, careful consideration should be given to measurement methods and biases of available data.