New constraints on terrestrial surface-atmosphere fluxes of gaseous elemental mercury using a global database

Despite 30 years of study, gaseous elemental mercury (Hg⁰) exchange magnitude and controls between terrestrial surfaces and the atmosphere still remain uncertain. We compiled data from 132 studies, including 1290 reported fluxes from more than 200,000 individual measurements, into a database to statistically examine flux magnitudes and controls. We found that fluxes were unevenly distributed, both spatially and temporally, with strong biases toward Hg-enriched sites, daytime and summertime measurements. Fluxes at Hg-enriched sites were positively correlated with substrate concentrations, but this was absent at background sites. Median fluxes over litter- and snow-covered soils were lower than over bare soils, and chamber measurements showed higher emission compared to micrometeorological measurements. Due to low spatial extent, estimated emissions from Hg-enriched areas (217 Mg·a⁻¹) were lower than previous estimates. Globally, areas with enhanced atmospheric Hg⁰ levels (particularly East Asia) showed an emerging importance of Hg⁰ emissions accounting for half of the total global emissions estimated at 607 Mg·a⁻¹, although with a large uncertainty range (-513 to 1353 Mg·a⁻¹ [range of 37.5th and 62.5th percentiles]). The largest uncertainties in Hg⁰ fluxes stem from forests (-513 to 1353 Mg·a⁻¹ [range of 37.5th and 62.5th percentiles]), largely driven by a shortage of whole-ecosystem fluxes and uncertain contributions of leaf-atmosphere exchanges, questioning to what degree ecosystems are net sinks or sources of atmospheric Hg⁰.18 page(s