Atmospheric Hg, emitted from anthropogenic sources as well as natural sources, deposits via various wet and dry deposition processes. We now understand that the dominant source of Hg in many terrestrial ecosystems is from atmospheric elemental Hg(0) deposition taken up by vegetation and transferred to soils when plants die off or shed leaves (also termed “litterfall”). Here, we discuss recent evidence that Hg(0) dominates as a source to ecosystems, including stable Hg isotopes measurements, direct flux characterization of Hg(0) in the field, proxy measurements such as litterfall deposition and observational and modeling evidence of atmospheric Hg(0) dynamics.

First, we show evidence based on litterfall deposition that has been used to estimate dry Hg deposition in forests. We also discuss limitations of using litterfall as a proxy to estimate Hg(0) deposition, including that it fails to capture deposition of woody tissues, contains both Hg(0) and oxidized Hg(II), does not consider re-emissions of Hg(0) via photochemical processes, and does not account for additional Hg(0) deposition such as direct uptake in soils and under snow. Second, we provide evidence based on stable Hg isotope studies that now show that Hg(0) accounts for 57-94% of Hg in soils of central North American forests, 71% in Alaskan tundra soils, 79% in central European peat soils, and 90% in boreal forest soils in Northern Sweden. Third, we provide evidence that a strong global vegetation Hg(0) sink induces pronounced seasonal variability and diurnal variation of atmospheric Hg(0) concentrations, in particular summertime atmospheric Hg(0) minima at remote sites. Fourth, we provide evidence that provides based on annual measurement series of direct Hg(0) deposition measurements from three ecosystems using micrometeorological measurements and provide preliminary data from a temperate forest in the Eastern United States using flux-gradient measurements. Finally, we provide an overview on how to best integrate Hg(0) deposition into global and regional chemical transport models and show how currently employed resistance-in-series approaches perform against field measurements. For the future, we suggest to deploy direct micrometeorological flux methods across a series of ecosystems to address the critical lack of temporal and spatial data on Hg(0) exchange across ecosystems and to provide direct Hg(0) deposition datasets from remote ecosystems to allow parameterizations of Hg(0) deposition in models against field observations.