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Mercury (Hg), a neurotoxic heavy metal, is transferred to marine and terrestrial ecosystems through atmospheric transport. Recent studies have highlighted the role of vegetation uptake as a sink for atmospheric elemental mercury (Hg0) and a source of Hg to soils. However, the global magnitude of the Hg0 vegetation uptake flux is highly uncertain, with estimates ranging 1000–4000 Mg per year. To constrain this sink, we compare simulations in the chemical transport model GEOS-Chem with a compiled database of litterfall, throughfall, and flux tower measurements from 93 forested sites. The prior version of GEOS-Chem predicts median Hg0 dry deposition velocities similar to litterfall measurements from Northern hemisphere temperate and boreal forests (~0.03 cm s-1 yet it underestimates measurements from a flux tower study (0.04 cm s-1 vs. 0.07 cm s-1and Amazon litterfall (0.05 cm s-1 vs. 0.17 cm s-1). After revising the Hg0 reactivity within the dry deposition parametrization to match flux tower and Amazon measurements, GEOS-Chem displays improved agreement with the seasonality of atmospheric Hg0 observations in the Northern midlatitudes. Additionally, the modelled bias in Hg0 concentrations in South America decreases from +0.21 ng m-3 +0.05 ng m-3. We calculate a global flux of Hg0 dry deposition to land of 2276 Mg per year, approximately double previous model estimates. The Amazon rainforest contributes 29% of the total Hg0 land sink, yet continued deforestation and climate change threatens the rainforest's stability and thus its role as an important Hg sink. In an illustrative worst-case scenario where the Amazon is completely converted to savannah, GEOS-Chem predicts that an additional 283 Mg Hg per year would deposit to the ocean, where it can bioaccumulate in the marine food chain. Biosphere–atmosphere interactions thus play a crucial role in global Hg cycling and should be considered in assessments of future Hg pollution. 

Abstract. The tundra plays a pivotal role in the Arctic mercury(Hg) cycle by storing atmospheric Hg deposition and shuttling it to theArctic Ocean. A recent study revealed that 70 % of the atmospheric Hgdeposition to the tundra occurs through gaseous elemental mercury (GEM or Hg(0))uptake by vegetation and soils. Processes controlling land–atmosphereexchange of Hg(0) in the Arctic tundra are central, but remainunderstudied. Here, we combine Hg stable isotope analysis of Hg(0) in theatmosphere, interstitial snow air, and soil pore air, with Hg(0) fluxmeasurements in a tundra ecosystem at Toolik Field Station in northernAlaska (USA). In the dark winter months, planetary boundary layer (PBL)conditions and Hg(0) concentrations were generally stable throughout the dayand small Hg(0) net deposition occurred. In spring, halogen-inducedatmospheric mercury depletion events (AMDEs) occurred, with the fastre-emission of Hg(0) after AMDEs resulting in net emission fluxes of Hg(0).During the short snow-free growing season in summer, vegetation uptake ofatmospheric Hg(0) enhanced atmospheric Hg(0) net deposition to the Arctictundra. At night, when PBL conditions were stable, ecosystem uptake ofatmospheric Hg(0) led to a depletion of atmospheric Hg(0). The night-timedecline of atmospheric Hg(0) was concomitant with a depletion of lighterHg(0) isotopes in the atmospheric Hg pool. The enrichment factor,ε202Hgvegetationuptake=-4.2 ‰ (±1.0 ‰) was consistentwith the preferential uptake of light Hg(0) isotopes by vegetation. Hg(0)flux measurements indicated a partial re-emission of Hg(0) during daytime,when solar radiation was strongest. Hg(0) concentrations in soil pore airwere depleted relative to atmospheric Hg(0) concentrations, concomitant withan enrichment of lighter Hg(0) isotopes in the soil pore air, ε202Hgsoilair-atmosphere=-1.00 ‰(±0.25 ‰) and E199Hgsoilair-atmosphere=0.07 ‰ (±0.04 ‰). Thesefirst Hg stable isotope measurements of Hg(0) in soil pore air areconsistent with the fractionation previously observed during Hg(0) oxidationby natural humic acids, suggesting abiotic oxidation as a cause for observedsoil Hg(0) uptake. The combination of Hg stable isotope fingerprints withHg(0) flux measurements and PBL stability assessment confirmed a dominantrole of Hg(0) uptake by vegetation in the terrestrial–atmosphere exchange ofHg(0) in the Arctic tundra.