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Sources of neurotoxic mercury in forests are dominated by atmospheric gaseous elemental mercury (GEM) deposition, but a dearth of direct GEM exchange measurements causes major uncertainties about processes that determine GEM sinks. Here we present three years of forest-level GEM deposition measurements in a coniferous forest and a deciduous forest in northeastern USA, along with flux partitioning into canopy and forest floor contributions. Annual GEM deposition is 13.4 ± 0.80 μg m⁻²(coniferous forest) and 25.1 ± 2.4 μg m⁻²(deciduous forest) dominating mercury inputs (62 and 76% of total deposition). GEM uptake dominates in daytime during active vegetation periods and correlates with CO₂assimilation, attributable to plant stomatal uptake of mercury. Non-stomatal GEM deposition occurs in the coniferous canopy during nights and to the forest floor in the deciduous forest and accounts for 24 and 39% of GEM deposition, respectively. Our study shows that GEM deposition includes various pathways and is highly ecosystem-specific, which complicates global constraints of terrestrial GEM sinks.

 

Mercury (Hg) is an environmental toxicant dangerous to human health and the environment. Its anthropogenic emissions are regulated by global, regional, and local policies. Here, we investigate Hg sources in the coastal city of Boston, the third largest metropolitan area in the Northeastern United States. With a median of 1.37 ng m −3 , atmospheric Hg concentrations measured from August 2017 to April 2019 were at the low end of the range reported in the Northern Hemisphere and in the range reported at North American rural sites. Despite relatively low ambient Hg concentrations, we estimate anthropogenic emissions to be 3–7 times higher than in current emission inventories using a measurement-model framework, suggesting an underestimation of small point and/or nonpoint emissions. We also test the hypothesis that a legacy Hg source from the ocean contributes to atmospheric Hg concentrations in the study area; legacy emissions (recycling of previously deposited Hg) account for ∼60% of Hg emitted annually worldwide (and much of this recycling takes place through the oceans). We find that elevated concentrations observed during easterly oceanic winds can be fully explained by low wind speeds and recirculating air allowing for accumulation of land-based emissions. This study suggests that the influence of nonpoint land-based emissions may be comparable in size to point sources in some regions and highlights the benefits of further top-down studies in other areas. 

Mercury is toxic to wildlife and humans, and forests are thought to be a globally important sink for gaseous elemental mercury (GEM) deposition from the atmosphere. Yet there are currently no annual GEM deposition measurements over rural forests. Here we present measurements of ecosystem–atmosphere GEM exchange using tower-based micrometeorological methods in a midlatitude hardwood forest. We measured an annual GEM deposition of 25.1 µg ⋅ m −2 (95% CI: 23.2 to 26.7 1 µg ⋅ m −2 ), which is five times larger than wet deposition of mercury from the atmosphere. Our observed annual GEM deposition accounts for 76% of total atmospheric mercury deposition and also is three times greater than litterfall mercury deposition, which has previously been used as a proxy measure for GEM deposition in forests. Plant GEM uptake is the dominant driver for ecosystem GEM deposition based on seasonal and diel dynamics that show the forest GEM sink to be largest during active vegetation growing periods and middays, analogous to photosynthetic carbon dioxide assimilation. Soils and litter on the forest floor are additional GEM sinks throughout the year. Our study suggests that mercury loading to this forest was underestimated by a factor of about two and that global forests may constitute a much larger global GEM sink than currently proposed. The larger than anticipated forest GEM sink may explain the high mercury loads observed in soils across rural forests, which impair water quality and aquatic biota via watershed Hg export. 

Understanding the processes that influence and control carbon cycling in Arctic tundra ecosystems is essential for making accurate predictions about what role these ecosystems will play in potential future climate change scenarios. Particularly, air–surface fluxes of methane and carbon dioxide are of interest as recent observations suggest that the vast stores of soil carbon found in the Arctic tundra are becoming more available to release to the atmosphere in the form of these greenhouse gases. Further, harsh wintertime conditions and complex logistics have limited the number of year-round and cold season studies and hence too our understanding of carbon cycle processes during these periods. We present here a two-year micrometeorological data set of methane and carbon dioxide fluxes that provides near-continuous data throughout the active summer and cold winter seasons. Net emission of methane and carbon dioxide in one of the study years totalled 3.7 and 89 g C m−2 a−1 respectively, with cold season methane emission representing 54% of the annual total. In the other year, net emission totals of methane and carbon dioxide were 4.9 and 485 g C m−2 a−1 respectively, with cold season methane emission here representing 82% of the annual total – a larger proportion than has been previously reported in the Arctic tundra. Regression tree analysis suggests that, due to relatively warmer air temperatures and deeper snow depths, deeper soil horizons – where most microbial methanogenic activity takes place – remained warm enough to maintain efficient methane production whilst surface soil temperatures were simultaneously cold enough to limit microbial methanotrophic activity. These results provide valuable insight into how a changing Arctic climate may impact methane emission, and highlight a need to focus on soil temperatures throughout the entire active soil profile, rather than rely on air temperature as a proxy for modelling temperature–methane flux dynamics. 

To simulate global mercury (Hg) dynamics in chemical transport models (CTMs), surface-atmosphere exchange of gaseous elemental mercury, Hg 0 , is often parameterized based on resistance-based dry deposition schemes coupled with a re-emission function, mainly from soils. Despite extensive use of this approach, direct evaluations of this implementation against field observations of net Hg 0 exchange are lacking. In this study, we evaluate an existing net exchange parameterization (referred to here as the base model) by comparing modeled fluxes of Hg 0 to fluxes measured in the field using micrometeorological techniques. Comparisons were performed in two terrestrial ecosystems: a grassland site in Switzerland and an Arctic tundra site in Alaska, U.S., each including summer and winter seasons. The base model included the dry deposition and soil re-emission parameterizations from Zhang et al. (2003) and the global CTM GEOS-Chem, respectively. Comparisons of modeled and measured Hg 0 fluxes showed large discrepancies, particularly in the summer months when the base model overestimated daytime net deposition by approximately 9 and 2 ng m −2 h −1 at the grassland and tundra sites, respectively. In addition, the base model was unable to capture a measured nighttime net Hg 0 deposition and wintertime deposition. We conducted a series of sensitivity analyses and recommend that Hg simulations using CTMs: (i) reduce stomatal uptake of Hg 0 over grassland and tundra in models by a factor 5–7; (ii) increase nighttime net Hg 0 deposition, e.g. , by increasing ground and cuticular uptake by reducing the respective resistance terms by factors of 3–4 and 2–4, respectively; and (iii) implement a new soil re-emission parameterization to produce larger daytime emissions and lower nighttime emissions. We also compared leaf Hg 0 uptake over the growing season estimated by the dry deposition model against foliar Hg measurements, which revealed good agreement with the measured leaf Hg concentrations after adjusting the base model as suggested above. We conclude that the use of resistance-based models combined with the new soil re-emission flux parameterization is able to reproduce observed diel and seasonal patterns of Hg 0 exchange in these ecosystems. This approach can be used to improve model parameterizations for other ecosystems if flux measurements become available. 

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.