Cumulative Anthropogenic Impacts of Past and Future Emissions and Releases on the Global Mercury Cycle

Humans have intentionally mined and released mercury (Hg) from the Earth's lithosphere over millennia. Here, we synthesize past, present, and future anthropogenic Hg emissions and releases and use a global geochemical box model to characterize accumulation in the atmosphere, land, and ocean. We project an upper-bound for emissions and releases between 2010 and 2300 (Shared Socioeconomic Pathway (SSP)5-8.5; 1.7 Tg) that surpasses the historical total over the past half millennium (1.5 Tg). In contrast, the lower-bound for emissions and releases (SSP1-2.6; 0.7 Tg) is substantially smaller than the historical total. Observational constraints on global modeling suggest that most Hg released to land and water prior to 2010 remains sequestered at contaminated sites. Substantial oceanic enrichment by anthropogenic Hg (270% ca. 2010) has been driven mainly by atmospheric emissions. Cumulative future releases to land and water are projected to be approximately six-times greater than primary anthropogenic emissions to the atmosphere. This Hg is mainly sequestered in legacy Hg waste pools and is unlikely to impact Hg pollution in the global ocean unless it is mobilized by climate change. Modeling results suggest that by 2100 atmospheric Hg concentrations may be similar to present levels if society follows SSP5-8.5. Declines in the surface ocean (-19%) and atmosphere (-45%) are expected under SSP1-2.6, emphasizing the benefits of reductions in future Hg releases.

Mobilization of mercury by sediment transport after a prescribed fire in NE Portugal: Insight into size classes and temporal variation

Terrestrial ecosystems are important sinks for atmospheric mercury (Hg). It is well known that high severity wildfires can mobilize Hg in the surroundings of burned areas due to changes in ecosystem stability, but it is unclear whether this also occurs after lower severity fires, such as prescribed fires. The present study determined Hg concentrations and mobilization rates in different size fractions of sediments collected after a prescribed fire in a scrubland area. Sediments, collected from eight erosion plots on six occasions, were analysed for total Hg, C and N in several size classes (<0.2 mm, 0.2-0.5 mm, 0.5-2 mm and >2 mm) and Hg mobilization rates (HgST) were calculated for each size fraction. Average total Hg were 38, 57, 94 and 126 µg kg-1 for size fractions > 2, 0.5-2, 0.2-0.5 and < 0.2 mm, respectively. Total Hg was negatively correlated with C/N ratio, involving the humification degree of organic matter of sediments in Hg retention. In the last event (eight months after fire), sediments had 45-106 % more Hg, depending on size fraction, compared to the initial event. Mercury mobilization rates varied between 32 and 78 mg ha-1, with the fraction 0.5-2 mm accounting for 46 % of the mobilized Hg. The results revealed that prescribed fires can mobilize Hg, so their use to prevent wildfires must be done with caution.

Combined Impacts of Climate and Tree Physiology on Mercury Accumulation in Tropical and Subtropical Foliage and Robust Model Parametrization

Atmospheric elemental mercury (Hg0) assimilation by foliage contributes prevalently to the global atmospheric Hg0 sink in forests. Today, little is known about the mechanisms of foliar Hg accumulation and how climate factors and tree physiology interact to impact it. Here, we examined meteorological factors, foliar physiological traits, and Hg accumulation rates from leaf emergence to senescence in a tropical rainforest, tropical savanna, and subtropical evergreen broadleaf forest. Also, robust models for foliar Hg accumulation were parametrized. Generally, foliar Hg accumulation rate in subtropical evergreen forest was highest (16.4 ± 12.3 ng m-2 day-1), followed by the tropical rainforest (14.2 ± 9.8 ng m-2 day-1), and lowest in the tropical savanna (4.7 ± 4.9 ng m-2 day-1). Atmospheric relative humidity, stomatal conductance, and leaf photosynthesis are key drivers of spatial-temporal variations in foliar Hg accumulation. The canopy-structure-induced specific leaf physiological traits drive temporal variations in foliar Hg accumulation, and climate-controlled leaf physiological traits account for spatial variations among three forests. Finally, our robust models enable precise simulation of foliar Hg accumulation rates at both tree species and ecosystem scales facilitating particularly regional and global Hg transport and chemical models to quantify the vegetation's role as a sink for atmospheric Hg0 uptake.

Oceanic evasion fuels Arctic summertime rebound of atmospheric mercury and drives transport to Arctic terrestrial ecosystems

Mercury (Hg) contamination poses a persistent threat to the remote Arctic ecosystem, yet the mechanisms driving the pronounced summer rebound of atmospheric gaseous elemental Hg (Hg0) and its subsequent fate remain unclear due to limitations in large-scale seasonal studies. Here, we use an integrated atmosphere-land-sea-ice-ocean model to simulate Hg cycling in the Arctic comprehensively. Our results indicate that oceanic evasion is the dominant source (~80%) of the summer Hg0 rebound, particularly driven by seawater Hg0 release facilitated by seasonal ice melt (~42%), with further contributions from anthropogenic deposition and terrestrial re-emissions. Enhanced Hg0 dry deposition across the Arctic coastal regions, especially in the Arctic tundra, during the summer rebound highlights the potential transport of Hg from the pristine Arctic Ocean to Arctic terrestrial ecosystems. Arctic warming, with a transition from multi-year to first-year ice and tundra greening, is expected to amplify oceanic Hg evasion and intensify Hg0 uptake by the Arctic tundra due to increased vegetation growth, underlining the urgent need for continued research to evaluate Hg mitigation strategies effectively in the context of a changing Arctic.

Wildfire effects on mercury fate in soils of North-Western Siberia

Arctic soils store 49 Gg mercury (Hg) - an extremely toxic heavy metal, whereas soil Hg can be released to the atmosphere by wildfires. For the first time we investigated the effects of wildfires on the fate of soil Hg in North-Western (NW) Siberia based on GIS maps of areas burned during the last 38 years and a field paired comparison of unburned and burned areas in tundra (mosses, lichens, some grasses, and shrubs) and forest-tundra (multi-layered canopy of larch trees, shrubs, mosses, and lichens). These field surveys were deepened by soil controlled burning to assess the Hg losses from organic horizon and mineral soil. The soil Hg stocks in the organic horizon and in the top 10 cm of the mineral soil were 3.3 ± 0.6 and 16 ± 3 mg Hg m-2 for unburned tundra and forest-tundra, respectively. After the burning by wildfires, the soil Hg stocks decreased to 2.4 ± 0.1 and 6.6 ± 0.2 mg Hg m-2 for tundra and forest-tundra, respectively. By the averages annual burned areas in NW Siberia 527 km2, wildfires in tundra and forest-tundra released 0.19 and 2.9 Mg soil Hg per year, respectively, corresponding to 28 % and 59 % of the initial soil Hg stocks. These direct effects of wildfires on Hg volatilization are raised by indirect post-pyrogenic consequences on Hg fate triggered by the vegetation succession and adsorption of atmospheric Hg on the surface of charred biomass. Charred lichens and trees accumulated 4-16 times more Hg compared to the living biomass. Blackened burned vegetation and soil reduced surface albedo and slowly increased soil temperatures in Arctic after wildfires. This created favorable conditions for seeding grasses and shrubs after wildfire and transformed burned high-latitude ecosystems into greener areas, increasing their capacity to trap atmospheric Hg by vegetation, which partly compensate the burning losses of soil Hg.

Isotope-Based Characterization of Soil Elemental Mercury Emissions from Historical Mercury Mining Areas: Driving Pathways and Relative Contributions

Photo-, microbial, and abiotic dark reduction of soil mercury (Hg) may all lead to elemental mercury (Hg(0)) emissions. Utilizing lab incubations, isotope signatures of Hg(0) emitted from mining soils were characterized to quantify the interplay and contributions of various Hg reduction pathways, which have been scarcely studied. At 15 °C, microbial reduced Hg(0) showed a negative mass-dependent fractionation (MDF) (δ202Hg = -0.30 ± 0.08‰, 1SD) and near-zero mass-independent fractionation (MIF) (Δ199Hg = 0.01 ± 0.04‰, 1SD), closely resembling dark reduced Hg(0) (δ202Hg = -0.18 ± 0.05‰, Δ199Hg = -0.01 ± 0.03‰, 1SD). In comparison, photoreduced Hg(0) exhibited significant MDF and MIF (δ202Hg = -0.55 ± 0.05‰, Δ199Hg = -0.20 ± 0.07‰, 1SD). In the dark, Hg isotopic signatures remained constant over the temperature range of 15-35 °C. Nonetheless, light exposure and temperature changes together altered Hg(0) MIF signatures significantly. Isotope mixing models along with Hg(0) emission flux data highlighted photo- and microbial reduction contributing 79-88 and 12-21%, respectively, of the total Hg(0) emissions from mining soils, with negligible abiotic dark reduction. Microorganisms are the key driver of soil Hg(0) emissions by first dissolving HgS and then promoting ionic Hg formation, followed by facilitating the photo- and microbial reduction of organically bound Hg. These insights deepen our understanding of the biogeochemical processes that influence Hg(0) releases from surface soils.

Combating air pollution significantly reduced air mercury concentrations in China

In the past decade, China has motivated proactive emission control measures that have successfully reduced emissions of many air pollutants. For atmospheric mercury, which is a globally transported neurotoxin, much less is known about the long-term changes in its concentrations and anthropogenic emissions in China. In this study, over a decade of continuous observations at four Chinese sites show that gaseous elemental mercury (GEM) concentrations continuously increased until the early 2010s, followed by significant declines at rates of 1.8%-6.1% yr-1 until 2022. The GEM decline from 2013 to 2022 (by 38.6% ± 12.7%) coincided with the decreasing concentrations of criteria air pollutants in China and were larger than those observed elsewhere in the northern hemisphere (5.7%-14.2%). The co-benefits of emission control measures contributed to the reduced anthropogenic Hg emissions and led to the GEM decline in China. We estimated that anthropogenic GEM emissions in China were reduced by 38%-50% (116-151 tons) from 2013 to 2022 using the machine-learning and relationship models.

Exploring the origins and cleanup of mercury contamination: a comprehensive review

Mercury is a global pollutant that poses significant risks to human health and the environment. Natural sources of mercury include volcanic eruptions, while anthropogenic sources include industrial processes, artisanal and small-scale gold mining, and fossil fuel combustion. Contamination can arise through various pathways, such as atmospheric deposition, water and soil contamination, bioaccumulation, and biomagnification in food chains. Various remediation strategies, including phytoremediation, bioremediation, chemical oxidation/reduction, and adsorption, have been developed to address mercury pollution, including physical, chemical, and biological approaches. The effectiveness of remediation techniques depends on the nature and extent of contamination and site-specific conditions. This review discusses the challenges associated with mercury pollution and remediation, including the need for effective monitoring and management strategies. Overall, this review offers a comprehensive understanding of mercury contamination and the range of remediation techniques available to mitigate its adverse impacts.

Warming-Induced Vegetation Greening May Aggravate Soil Mercury Levels Worldwide

Mercury, a neurotoxic substance, circulates globally, significantly stored in soils through atmospheric deposition and plant decay. Despite being deposited, mercury can be remobilized and released into the atmosphere and water, enhancing its global cycle. Recent research suggests that climate warming may amplify the remobilization of soil mercury, facilitating its incorporation into food webs that humans exploit. However, the potential geospatial feedback of soil mercury levels in response to warming remains unclear. By leveraging up-to-date soil measurements and observation-driven models, we determined the amount of mercury stored in global 0-100 cm soils to be 4.3 Tg (interquartile range: 2.5-6.3 Tg). Furthermore, our analysis indicates that warming likely aggravates global soil mercury levels, particularly in many temperate areas in East Asia, North Europe, and North America (>20 ng g-1 increase by 2100) due to warming-induced vegetation greening. Critically, observation-driven models raise the possibility that implementing ambitious mercury-emission-control schemes alone may be insufficient to counterbalance the positive feedback of soil mercury concentration, while process-based biogeochemical modeling demonstrates consistent patterns that reinforce this concern. These findings hold broad implications; for example, such feedback may catalyze mercury remobilization in land-ocean continuums and exacerbate human risks, stressing the necessity for continued reductions in greenhouse gas and mercury emissions.

Quantifying soil accumulation of atmospheric mercury using fallout radionuclide chronometry

Soils are a principal global reservoir of mercury (Hg), a neurotoxic pollutant that is accumulating through anthropogenic emissions to the atmosphere and subsequent deposition to terrestrial ecosystems. The fate of Hg in global soils remains uncertain, however, particularly to what degree Hg is re-emitted back to the atmosphere as gaseous elemental mercury (GEM). Here we use fallout radionuclide (FRN) chronometry to directly measure Hg accumulation rates in soils. By comparing these rates with measured atmospheric fluxes in a mass balance approach, we show that representative Arctic, boreal, temperate, and tropical soils are quantitatively efficient at retaining anthropogenic Hg. Potential for significant GEM re-emission appears limited to a minority of coniferous soils, calling into question global models that assume strong re-emission of legacy Hg from soils. FRN chronometry poses a powerful tool to reconstruct terrestrial Hg accumulation across larger spatial scales than previously possible, while offering insights into the susceptibility of Hg mobilization from different soil environments.

Using Mercury Stable Isotopes to Quantify Directional Soil-Atmosphere Hg(0) Exchanges in Rice Paddy Ecosystems: Implications for Hg(0) Emissions to the Atmosphere from Land Surfaces

Gaseous elemental mercury [Hg(0)] emissions from soils constitute a large fraction of global total Hg(0) emissions. Existing studies do not distinguish biotic- and abiotic-mediated emissions and focus only on photoreduction mediated emissions, resulting in an underestimation of soil Hg(0) emissions into the atmosphere. In this study, directional mercury (Hg) reduction pathways in paddy soils were identified using Hg isotopes. Results showed significantly different isotopic compositions of Hg(0) between those produced from photoreduction (δ202Hg = -0.80 ± 0.67‰, Δ199Hg = -0.38 ± 0.18‰), microbial reduction (δ202Hg = -2.18 ± 0.25‰, Δ199Hg = 0.29 ± 0.38‰), and abiotic dark reduction (δ202Hg = -2.31 ± 0.25‰, Δ199Hg = 0.50 ± 0.22‰). Hg(0) exchange fluxes between the atmosphere and the paddy soils were dominated by emissions, with the average flux ranging from 2.2 ± 5.7 to 16.8 ± 21.7 ng m-2 h-1 during different sampling periods. Using an isotopic signature-based ternary mixing model, we revealed that photoreduction is the most important contributor to Hg(0) emissions from paddy soils. Albeit lower, microbial and abiotic dark reduction contributed up to 36 ± 22 and 25 ± 15%, respectively, to Hg(0) emissions on the 110th day. These novel findings can help improve future estimation of soil Hg(0) emissions from rice paddy ecosystems, which involve complex biotic-, abiotic-, and photoreduction processes.

Potential decoupling of CO2 and Hg uptake process by global vegetation in the 21st century

Mercury (Hg), a potent neurotoxin posing risks to human health, is cycled through vegetation uptake, which is susceptible to climate change impacts. However, the extent and pattern of these impacts are largely unknown, obstructing predictions of Hg's fate in terrestrial ecosystems. Here, we evaluate the effects of climate change on vegetation elemental Hg [Hg(0)] uptake using a state-of-the-art global terrestrial Hg model (CLM5-Hg) that incorporates plant physiology. In a business-as-usual scenario, the terrestrial Hg(0) sink is predicted to decrease by 1870 Mg yr-1 in 2100, that is ~60% lower than the present-day condition. We find a potential decoupling between the trends of CO2 assimilation and Hg(0) uptake process by vegetation in the 21st century, caused by the decreased stomatal conductance with increasing CO2. This implies a substantial influx of Hg into aquatic ecosystems, posing an elevated threat that warrants consideration during the evaluation of the effectiveness of the Minamata Convention.

Deforestation as an Anthropogenic Driver of Mercury Pollution

Deforestation reduces the capacity of the terrestrial biosphere to take up toxic pollutant mercury (Hg) and enhances the release of secondary Hg from soils. The consequences of deforestation for Hg cycling are not currently considered by anthropogenic emission inventories or specifically addressed under the global Minamata Convention on Mercury. Using global Hg modeling constrained by field observations, we estimate that net Hg fluxes to the atmosphere due to deforestation are 217 Mg year-1 (95% confidence interval (CI): 134-1650 Mg year-1) for 2015, approximately 10% of global primary anthropogenic emissions. If deforestation of the Amazon rainforest continues at business-as-usual rates, net Hg emissions from the region will increase by 153 Mg year-1 by 2050 (CI: 97-418 Mg year-1), enhancing the transport and subsequent deposition of Hg to aquatic ecosystems. Substantial Hg emissions reductions are found for two potential cases of land use policies: conservation of the Amazon rainforest (92 Mg year-1, 95% CI: 59-234 Mg year-1) and global reforestation (98 Mg year-1, 95% CI: 64-449 Mg year-1). We conclude that deforestation-related emissions should be incorporated as an anthropogenic source in Hg inventories and that land use policy could be leveraged to address global Hg pollution.

Mercury sources and budget for the Snake River above a hydroelectric reservoir complex

Understanding sources of mercury (Hg) and methylmercury (MeHg) to a water body is critical for management but is often complicated by poorly characterized Hg inputs and in situ processes, such as inorganic Hg methylation. In this study, we determined inorganic Hg and MeHg concentrations and loads (filter-passing and particulate fractions) for a semi-arid 164-kilometer stretch of the Snake River above the Hells Canyon Complex, a Hg-impaired hydroelectric reservoir complex on the Idaho-Oregon border, and used water quality measurements and Hg stable isotope ratios to create a comprehensive Hg source budget for the river. Results show that whereas most of the streamflow to the study reach comes from the main branch of the Snake River (i.e., the upstream watershed), major tributaries within the study reach contribute a greater proportion of inorganic Hg and MeHg loads. Mercury stable-isotope analyses highlight that Hg within the tributaries is predominantly associated with geologic deposits and snowmelt sources, the latter reflecting wet deposition. Surprisingly, irrigation return drains contribute 40-50 % of particulate inorganic Hg loads despite being ≤4.3 % of the overall water budget. Together, tributaries and irrigation return drains account for 97-100 % of the inorganic Hg and streamflow to the study reach, but ~65 % of the MeHg, indicating in-stream and riparian methylation may be an important and previously unrecognized source of MeHg. Streamflow, total suspended solids, dissolved organic carbon, and agricultural land cover were found to be important controls on the mobilization and transport of different Hg species and fractions. This study represents the first fluvial budget for Hg in the Snake River that accounts for particulate and filter-passing Hg species from both major tributaries and irrigation return drains, and expands our understanding of Hg sources and methylation processes within semi-arid environments. This information is critical to inform management decisions related to elevated Hg burdens in biota.

Mercury-induced excitotoxicity in presynaptic brain nerve terminals: modulatory effects of carbonaceous airborne particulate simulants

Multipollutant approach is a breakthrough in up-to-date environmental quality and health risk estimation. Both mercury and carbonaceous air particulate are hazardous neurotoxicants. Here, the ability of carbonaceous air particulate simulants, i.e. carbon dots obtained by heating of organics, and nanodiamonds, to influence Hg2+-induced neurotoxicity was monitored using biological system, i.e. presynaptic rat cortex nerve terminals. Using HgCl2 and classical reducing/chelating agents, an adequate synaptic parameter, i.e. the extracellular level of key excitatory neurotransmitter L-[14C]glutamate, was selected for further analysis. HgCl2 starting from 5 µM caused an acute and concentration-dependent increase in the extracellular L-[14C]glutamate level in nerve terminals. Combined application of Hg2+ and carbon dots from heating of citric acid/urea showed that this simulant was able to mitigate in an acute manner excitotoxic Hg2+-induced increase in the extracellular L-[14C]glutamate level in nerve terminals by 37%. These carbon dots and Hg2+ acted as a complex in nerve terminals that was confirmed with fluorimetric data on Hg2+-induced changes in their spectroscopic features. Nanodiamonds and carbon dots from β-alanine were not able to mitigate a Hg2+-induced increase in the extracellular L-[14C]glutamate level in nerve terminals. Developed approach can be applicable for monitoring capability of different particles/compounds to have Hg2+-chelating signs in the biological systems. Therefore, among testing simulants, the only carbon dots from citric acid/urea were able to mitigate acute Hg2+-induced neurotoxicity in nerve terminals, thereby showing a variety of effects of carbonaceous airborne particulate in situ and its potential to interfere and modulate Hg2+-associated health hazard.

Mercury mobilization in shrubland after a prescribed fire in NE Portugal: Insight on soil organic matter composition and different aggregate size

Soils constitute the major reservoir of mercury (Hg) in terrestrial ecosystems, whose stability may be threatened by wildfires. This research attempts to look at the effect of prescribed fire on the presence of Hg in a shrubland ecosystem from NE Portugal, delving into its relationship with soil aggregate size and the molecular composition of soil organic matter (SOM). During the prescribed fire, on average 347 mg Hg ha-1 were lost from the burnt aboveground biomass of shrubs and 263 mg Hg ha-1 from the combustion of the soil organic horizon. Overall, Hg concentration and pools in the mineral soil did not show significant changes due to burning, which highlights their role as long-term Hg reservoirs. The higher Hg concentrations found in smaller aggregates (<0.2 mm) compared to coarser ones (0.5-2 mm) are favored by the higher degree of organic matter decomposition (low C/N ratio), rather than by greater total organic C contents. The Hg-enriched finest fraction of soil (<0.2 mm) could be more prone to be mobilized by erosion, whose potential arrival to water bodies increases the environmental concern for the Hg present in fire-affected soils. The SOM quality (molecular composition) and the main organic families, analyzed by Fourier-transform infrared spectroscopy in combination with multivariate statistical analysis, significantly conditioned the retention/emission of Hg in the uppermost soil layers. Thus, before the fire, Hg was strongly linked to lipid and protein fractions, while Hg appeared to be linked to aromatic-like compounds in fire-affected SOM.

Multidimensional Drivers of Mercury Distribution in Global Surface Soils: Insights from a Global Standardized Field Survey

Soil stores a large amount of mercury (Hg) that has adverse effects on human health and ecosystem safety. Significant uncertainties still exist in revealing environmental drivers of soil Hg accumulation and predicting global Hg distribution owing to the lack of field data from global standardized analyses. Here, we conducted a global standardized field survey and explored a holistic understanding of the multidimensional environmental drivers of Hg accumulation in global surface soils. Hg content in surface soils from our survey ranges from 3.8 to 618.2 μg kg-1 with an average of 74.0 μg kg-1 across the globe. Atmospheric Hg deposition, particularly vegetation-induced elemental Hg0 deposition, is the major source of surface soil Hg. Soil organic carbon serves as the major substrate for sequestering Hg in surface soils and is significantly influenced by agricultural management, litterfall, and elevation. For human activities, changing land-use could be a more important contributor than direct anthropogenic emissions. Our prediction of a new global Hg distribution highlights the hot spots (high Hg content) in East Asia, the Northern Hemispheric temperate/boreal regions, and tropical areas, while the cold spots (low Hg content) are in arid regions. The holistic understanding of multidimensional environmental drivers helps to predict the Hg distribution in global surface soils under a changing global environment.

International trade shapes global mercury-related health impacts

Mercury (Hg) is a strong neurotoxin with substantial dangers to human health. Hg undergoes active global cycles, and the emission sources there of can also be geographically relocated through economic trade. Through investigation of a longer chain of the global biogeochemical Hg cycle from economic production to human health, international cooperation on Hg control strategies in Minamata Convention can be facilitated. In the present study, four global models are combined to investigate the effect of international trade on the relocation of Hg emissions, pollution, exposure, and related human health impacts across the world. The results show that 47% of global Hg emissions are related to commodities consumed outside of the countries where the emissions are produced, which has largely influenced the environmental Hg levels and human exposure thereto across the world. Consequently, international trade is found to enable the whole world to avoid 5.7 × 10⁵ points for intelligence quotient (IQ) decline and 1,197 deaths from fatal heart attacks, saving a total of $12.5 billion (2020 USD) in economic loss. Regionally, international trade exacerbates Hg challenges in less developed countries, while resulting in an alleviation in developed countries. The change in economic loss therefore varies from the United States (-$4.0 billion) and Japan (-$2.4 billion) to China (+$2.7 billion). The present results reveal that international trade is a critical factor but might be largely overlooked in global Hg pollution mitigation.

Buffering effect of global vegetation on the air-land exchange of mercury: Insights from a novel terrestrial mercury model based on CESM2-CLM5

The vegetation uptake of atmospheric elemental mercury [Hg(0)] and its subsequent littering are critical processes of the terrestrial Hg cycles. There is a large uncertainty in the estimated global fluxes of these processes due to the knowledge gap in the underlying mechanisms and their relationship with environmental factors. Here, we develop a new global model based on the Community Land Model Version 5 (CLM5-Hg) as an independent component of the Community Earth System Model 2 (CESM2). We explore the global pattern of gaseous elemental Hg [Hg(0)] uptake by vegetation and the spatial distribution of litter Hg concentration constrained by observed datasets as well as its driving mechanism. The annual vegetation uptake of Hg(0) is estimated as 3132 Mg yr^-1, which is considerably higher than previous global models. The scheme of dynamic plant growth including stomatal activities substantially improves the estimation for global terrestrial distribution of Hg, compared to the leaf area index (LAI) based scheme that is often used by previous models. We find the global distribution of litter Hg concentrations driven by vegetation uptake of atmospheric Hg(0), which are simulated to be higher in East Asia (87 ng/g) than in the Amazon region (63 ng/g). Meanwhile, as a significant source for litter Hg, the formation of structural litter (cellulose litter + lignin litter) results in a lagging effect between Hg(0) deposition and litter Hg concentration, implying the buffering effect of vegetation on the air-land exchange of Hg. This work highlights the importance of vegetation physiology and environmental factors in understanding the vegetation sequestration of atmospheric Hg globally, and calls for greater efforts to protect forests and afforestation.

Mercury accumulation and dynamics in montane forests along an elevation gradient in Southwest China

Understanding atmospheric mercury (Hg) accumulation in remote montane forests is critical to assess the Hg ecological risk to wildlife and human health. To quantify impacts of vegetation, climatic and topographic factors on Hg accumulation in montane forests, we assessed the Hg distribution and stoichiometric relations among Hg, carbon (C), and nitrogen (N) in four forest types along the elevation of Mt. Gongga. Our results show that Hg concentration in plant tissues follows the descending order of litter > leaf, bark > root > branch > bole wood, indicating the importance of atmospheric Hg uptake by foliage for Hg accumulation in plants. The foliar Hg/C (from 237.0 ± 171.4 to 56.8 ± 27.7 µg/kg) and Hg/N (from 7.5 ± 3.9 to 2.5 ± 1.2 mg/kg) both decrease along the elevation. These elevation gradients are caused by the heterogeneity of vegetation uptake of atmospheric Hg and the variation of atmospheric Hg° concentrations at different altitudes. Organic soil Hg accumulation is controlled by forest types, topographic and climatic factors, with the highest concentration in the mixed forest (244.9 ± 55.7 µg/kg) and the lowest value in the alpine forest (151.9 ± 44.5 µg/kg). Further analysis suggests that soil Hg is positively correlated to C (r² = 0.66) and N (r² = 0.57), and Hg/C and Hg/N both increase with the soil depth. These stoichiometric relations highlight the combined effects from environmental and climatic factors which mediating legacy Hg accumulation and selective Hg absorption during processes of organic soil mineralization.

Global Health Impact of Atmospheric Mercury Emissions from Artisanal and Small-Scale Gold Mining

Artisanal and small-scale gold mining (ASGM) is the leading source of mercury (Hg), a global neurotoxin. Past research has focused on the health impacts on miners and nearby residents; here, we estimate the risk for global general populations by employing a comprehensive atmosphere-land-ocean-ecosystem and exposure-risk-valuation model framework. Our results suggest that ASGM sources contribute 12%, 10%, and 0.63% to the atmospheric Hg deposition, plankton methylmercury concentrations, and soil total Hg concentrations at present day, respectively, and cause 5.8×10⁵ points of intelligence quotient decrements and 1,430 deaths for global general populations per year. The monetized global health impact of ASGM ($154 billion) is 1.5 times its local impact and accounts for half of the total revenue of ASGM ($319 billion). A major spatial decoupling between the health impact and economic gains is also revealed, suggesting that intervention measures such as awareness-raising, capacity-building, and technology transfer funded by the Global North are cost-effective.

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Contributions of ASGM to MeHg concentrations in different organisms are calculated

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The global health impact of atmospheric Hg from ASGM is first revealed

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The health impacts and economic gains of ASGM are compared

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Intervention measures for ASGM are proved to be cost-effective

Permafrost Degradation and Its Hydrogeological Impacts

Under a warming climate, permafrost degradation has resulted in profound hydrogeological consequences. Here, we mainly review 240 recent relevant papers. Permafrost degradation has boosted groundwater storage and discharge to surface runoffs through improving hydraulic connectivity and reactivation of groundwater flow systems, resulting in reduced summer peaks, delayed autumn flow peaks, flattened annual hydrographs, and deepening and elongating flow paths. As a result of permafrost degradation, lowlands underlain by more continuous, colder, and thicker permafrost are getting wetter and uplands and mountain slopes, drier. However, additional contribution of melting ground ice to groundwater and stream-flows seems limited in most permafrost basins. As a result of permafrost degradation, the permafrost table and supra-permafrost water table are lowering; subaerial supra-permafrost taliks are forming; taliks are connecting and expanding; thermokarst activities are intensifying. These processes may profoundly impact on ecosystem structures and functions, terrestrial processes, surface and subsurface coupled flow systems, engineered infrastructures, and socioeconomic development. During the last 20 years, substantial and rapid progress has been made in many aspects in cryo-hydrogeology. However, these studies are still inadequate in desired spatiotemporal resolutions, multi-source data assimilation and integration, as well as cryo-hydrogeological modeling, particularly over rugged terrains in ice-rich, warm (>−1 °C) permafrost zones. Future research should be prioritized to the following aspects. First, we should better understand the concordant changes in processes, mechanisms, and trends for terrestrial processes, hydrometeorology, geocryology, hydrogeology, and ecohydrology in warm and thin permafrost regions. Second, we should aim towards revealing the physical and chemical mechanisms for the coupled processes of heat transfer and moisture migration in the vadose zone and expanding supra-permafrost taliks, towards the coupling of the hydrothermal dynamics of supra-, intra- and sub-permafrost waters, as well as that of water-resource changes and of hydrochemical and biogeochemical mechanisms for the coupled movements of solutes and pollutants in surface and subsurface waters as induced by warming and thawing permafrost. Third, we urgently need to establish and improve coupled predictive distributed cryo-hydrogeology models with optimized parameterization. In addition, we should also emphasize automatically, intelligently, and systematically monitoring, predicting, evaluating, and adapting to hydrogeological impacts from degrading permafrost at desired spatiotemporal scales. Systematic, in-depth, and predictive studies on and abilities for the hydrogeological impacts from degrading permafrost can greatly advance geocryology, cryo-hydrogeology, and cryo-ecohydrology and help better manage water, ecosystems, and land resources in permafrost regions in an adaptive and sustainable manner.

Mercury Isotope Fractionation during the Exchange of Hg(0) between the Atmosphere and Land Surfaces: Implications for Hg(0) Exchange Processes and Controls

Atmosphere-surface exchange of elemental mercury (Hg(0)) is a vital component in global Hg cycling; however, Hg isotope fractionation remains largely unknown. Here, we report Hg isotope fractionation during air-surface exchange from terrestrial surfaces at sites of background (two) and urban (two) character and at five sites contaminated by Hg mining. Atmospheric Hg(0) deposition to soils followed kinetic isotope fractionation with a mass-dependent (MDF) enrichment factor of -4.32‰, and negligible mass-independent fractionation (MIF). Net Hg(0) emission generated average MDF enrichment factors (ε²⁰²Hg) of -0.91, -0.59, 1.64, and -0.42‰ and average MIF enrichment factors (E¹⁹⁹Hg) of 0.07, -0.20, -0.14, and 0.21‰ for urban, background, and Hg mining soils and cinnabar tailing, respectively. Positive correlations between ε²⁰²Hg and ambient Hg(0) concentration indicate that the co-occurring Hg(0) deposition (accounting for 10-39%) in a regime of net soil emission grows with ambient Hg(0). The MIF of Hg(0) emission from soils (E¹⁹⁹Hg range -0.27 to 0.14‰, n = 8) appears to be overall controlled by the photochemical reduction of kinetically constrained Hg(II) bonded to O ligands in background soils, while S ligands may have been more important in Hg mining area soils. In contrast, the small positive MIF of Hg(0) emission from cinnabar ore tailing (mean E¹⁹⁹Hg = 0.21‰) was likely controlled by abiotic nonphotochemical reduction and liquid Hg(0) evaporation. This research provides critical observational constraints on understanding the Hg(0) isotope signatures released from and deposited to terrestrial surfaces and highlight stable Hg isotopes as a powerful tool for resolving atmosphere-surface exchange processes.

Improved Mechanistic Model of the Atmospheric Redox Chemistry of Mercury

We present a new chemical mechanism for Hg⁰/Hg^I/Hg^II atmospheric cycling, including recent laboratory and computational data, and implement it in the GEOS-Chem global atmospheric chemistry model for comparison to observations. Our mechanism includes the oxidation of Hg⁰ by Br and OH, subsequent oxidation of Hg^I by ozone and radicals, respeciation of Hg^II in aerosols and cloud droplets, and speciated Hg^II photolysis in the gas and aqueous phases. The tropospheric Hg lifetime against deposition in the model is 5.5 months, consistent with observational constraints. The model reproduces the observed global surface Hg⁰ concentrations and Hg^II wet deposition fluxes. Br and OH make comparable contributions to global net oxidation of Hg⁰ to Hg^II. Ozone is the principal Hg^I oxidant, enabling the efficient oxidation of Hg⁰ to Hg^II by OH. BrHg^IIOH and Hg^II(OH)₂, the initial Hg^II products of Hg⁰ oxidation, respeciate in aerosols and clouds to organic and inorganic complexes, and volatilize to photostable forms. Reduction of Hg^II to Hg⁰ takes place largely through photolysis of aqueous Hg^II-organic complexes. 71% of model Hg^II deposition is to the oceans. Major uncertainties for atmospheric Hg chemistry modeling include Br concentrations, stability and reactions of Hg^I, and speciation and photoreduction of Hg^II in aerosols and clouds.

Biomonitoring of Mercury Contamination in Poland Based on Its Concentration in Scots Pine (Pinus sylvestris L.) Foliage

This work evaluates current mercury (Hg) contamination in Poland, represented by the Hg concentrations in Scots pine foliage. Samples were collected over 295 investigation plots in monitoring grids throughout Poland, from pines aged between 12 and 147 years. Analyses were conducted with consideration of bioclimatic factors and soil properties. Concentrations in the pine foliage did not exceed the values characteristic of an ecosystem unaffected by industrial pollution, ranging from 0.0032 to 0.0252 mg kg⁻¹ dry mass. However, pine stands located in western and central Poland, and in the northwest near the Baltic Sea, exhibited higher Hg concentrations in foliage than in eastern regions. Hg content in foliage depends on the mean temperature of the driest quarter, as well as on Hg content in soils. This indicates that the periods of drought observed in recent years in Poland may affect Hg concentrations in pine foliage.

Quantification of Atmospheric Mercury Deposition to and Legacy Re-emission from a Subtropical Forest Floor by Mercury Isotopes

Air-soil exchange of elemental mercury vapor (Hg⁰) is an important component in the budget of the global mercury cycle. However, its mechanistic detail is poorly understood. In this study, stable Hg isotopes in air, soil, and pore gases are characterized in a subtropical evergreen forest to understand the mechanical features of the air-soil Hg⁰ exchange. Strong Hg^II reduction in soil releases Hg⁰ to pore gas during spring-autumn but diminishes in winter, limiting the evasion in cold seasons. Δ¹⁹⁹Hg in air modified by the Hg⁰ efflux during flux chamber measurement exhibit seasonality, from -0.33 ± 0.05‰ in summer to -0.08 ± 0.05‰ in winter. The observed seasonal variation is caused by a strong pore-gas driven soil efflux caused by photoreduction in summer, which weakens significantly in winter. The annual Hg⁰ gross deposition is 42 ± 33 μg m^-2 yr^-1, and the corresponding Hg⁰ evasion from the forest floor is 50 ± 41 μg m^-2 yr^-1. The results of this study, although still with uncertainty, offer new insights into the complexity of the air-surface exchange of Hg⁰ over the forest land for model implementation in future global assessments.

Global health effects of future atmospheric mercury emissions

Mercury is a potent neurotoxin that poses health risks to the global population. Anthropogenic mercury emissions to the atmosphere are projected to decrease in the future due to enhanced policy efforts such as the Minamata Convention, a legally-binding international treaty entered into force in 2017. Here, we report the development of a comprehensive climate-atmosphere-land-ocean-ecosystem and exposure-risk model framework for mercury and its application to project the health effects of future atmospheric emissions. Our results show that the accumulated health effects associated with mercury exposure during 2010-2050 are $19 (95% confidence interval: 4.7-54) trillion (2020 USD) realized to 2050 (3% discount rate) for the current policy scenario. Our results suggest a substantial increase in global human health cost if emission reduction actions are delayed. This comprehensive modeling approach provides a much-needed tool to help parties to evaluate the effectiveness of Hg emission controls as required by the Minamata Convention.

Soil properties influencing Hg vertical pattern in temperate forest podzols

Mercury content of twelve podzols from NW Spain was studied to elucidate the main soil properties involved in the Hg accumulation of these soils. The highest average Hg concentrations (Hg_T) were found in the Bh and Bs horizons (64 and 105 μg kg^-1), whereas the lowest occurred in the E horizons (15 μg kg^-1). Moderate values of Hg_T were obtained for the A and C horizons (38 and 52 μg kg^-1). The Hg enrichment factors revealed that the predominant origin of Hg in these soils is the atmosphere instead of the parent material. As it was shown by the PCA performed (which explained 82% of the variance of the data), the main soil characteristics involved in the pedogenetic processes of the studied podzols are organic matter and Al and Fe compounds. The stepwise linear regressions made described between 54% and 84% of the predicted Hg depending on the soil horizon. Besides a complex ensemble of biogeochemical reactions involved in the balance between input and outputs of Hg, the most influencing variable in the A horizons was organic C, moderate stability Al-humus complexes in the E horizons, Fe-humus complexes and pH_w in the Bh horizons, Al-humus compounds in the Bs horizons and crystalline Al and Fe compounds in the C horizons. Therefore, Hg is mobilized from the A and E horizons bound to dissolved organic matter and precipitated in the illuvial horizons due to the saturation of the organic matter with metals. The immobilization of Hg in the subsuperficial horizons of podzols leads to different environmental benefits derived from the removal of Hg from the A horizons, more exposed to climate-induced and land use/cover changes that could potentially modify the dynamics of Hg in those superficial horizons.

Potential impacts of mercury released from thawing permafrost

Mercury (Hg) is a naturally occurring element that bonds with organic matter and, when converted to methylmercury, is a potent neurotoxicant. Here we estimate potential future releases of Hg from thawing permafrost for low and high greenhouse gas emissions scenarios using a mechanistic model. By 2200, the high emissions scenario shows annual permafrost Hg emissions to the atmosphere comparable to current global anthropogenic emissions. By 2100, simulated Hg concentrations in the Yukon River increase by 14% for the low emissions scenario, but double for the high emissions scenario. Fish Hg concentrations do not exceed United States Environmental Protection Agency guidelines for the low emissions scenario by 2300, but for the high emissions scenario, fish in the Yukon River exceed EPA guidelines by 2050. Our results indicate minimal impacts to Hg concentrations in water and fish for the low emissions scenario and high impacts for the high emissions scenario.

Permafrost locks away the largest reservoir of mercury on the planet, but climate warming threatens to thaw these systems. Here the authors use models to show that unconstrained fossil fuel burning will dramatically increase the amount of mercury released into future ecosystems.

Stable Mercury Isotope Transition during Postdepositional Decomposition of Biomass in a Forest Ecosystem over Five Centuries

Organic soil is an important transient reservoir of mercury (Hg) in terrestrial ecosystems, but the fate of deposited Hg in organic forest soil is poorly understood. To understand the dynamic changes of deposited Hg on forest floor, the composition of stable Hg and carbon (C) isotopes in decomposing litters and organic soil layer was measured to construct the 500 year history of postdepositional Hg transformation in a subtropical evergreen broad-leaf forest in Southwest China. Using the observational data and a multiprocess isotope model, the contributions of microbial reduction, photoreduction, and dark reduction mediated by organic matter to the isotopic transition were estimated. Microbial reduction and photoreduction play a dominant role in the initial litter decomposition during first 2 years. Dark redox reactions mediated by organic matter become the predominant process in the subsequent 420 years. After that, the values of Hg mass dependent fractionation (MDF), mass independent fractionation (MIF), and Δ¹⁹⁹Hg/Δ²⁰¹Hg ratio do not change significantly, indicating sequestration and immobilization of Hg in soil. The linear correlations between the isotopic signatures of Hg and C suggest that postdepositional transformation of Hg is closely linked to the fate of natural organic matter (NOM). Our findings are consistent with the abiotic dark reduction driven by nuclear volume effect reported in boreal and tropical forests. We recommend that the dark reduction process be incorporated in future model assessment of the global Hg biogeochemical cycle.

The potential wildfire effects on mercury remobilization from topsoils and biomass in a smelter-polluted semi-arid area

Wildfires can be responsible for significant mercury (Hg) emissions especially in contaminated areas. Here, we investigated the Hg distribution in topsoils and vegetation samples and temperature-dependent Hg mobilization from biomass-rich topsoils collected near a copper (Cu) smelter in Tsumeb (semi-arid Namibia), where Hg-rich Cu concentrates are processed. The thermo-desorption (TD) experiments conducted on representative biomass-rich topsoils (3.9-7.7 mg Hg/kg) indicated that more than 91% of the Hg was released at ∼340 °C, which corresponds to the predominant grassland-fire conditions. The mineralogical investigation indicated that the Hg comes mainly from the deposited smelter emissions because no distinct Hg-rich microparticles corresponding to the windblown dust from the nearby disposal sites of the technological materials (concentrates, slags, tailings) were found. A comparison with the TD curves of the Hg reference compounds confirmed that the Hg in the biomass-rich topsoils occurs as a mixture of Hg bound to the organic matter and metacinnabar (black HgS), which exhibits similarities with the TD pattern of smelter flue dust residue. Despite the installation of a sulfuric acid plant in the smelter in 2015 and a calculated drop in the estimated Hg emissions (from 1301 ± 457 kg/y for the period 2004-2015 to 67 ± 5 kg/y after 2015), the Hg legacy pool in the smelter surroundings can potentially be re-emitted back to the atmosphere by wildfire. Using the Hg spatial distribution data in the area (184 km²), the estimates indicate that up to 303 kg and 1.3 kg can be remobilized from the topsoils and vegetation, respectively.

Mobility of mercury in soil and its transport into the sea

Mercury (Hg) is deposited temporarily in soil and can be remobilised into rivers and seas. Given that rivers are a significant part of the mercury budget in the southern Baltic region (inland sea located in northern Europe) and meteorological changes (e.g. intense rain, drought) are observed more frequently, it is important to recognize the factors affecting the cycling of bioavailable Hg forms. The aim of this study was to identify the processes influencing the changes of labile and stabile mercury proportion in soil and the potential impact on the outflow of labile Hg into fluvial systems. For this purpose, soil samples, river sediments, and river water were collected from the Reda River (southern Baltic Sea catchment area) during the 2015 hydrologic year. The material was analysed for total and particulate mercury content and Hg forms, by a thermo-desorption method. The analysis showed that due to changes of meteorological and hydrological conditions Hg can enter rivers and then be introduced into the marine environment in various forms. On the one hand due to high precipitation events washing out of labile (i.e. bond with halogenides, MeHg, HgSO₄), Hg forms into the river can be enhanced which affects increasing of availability of the most dangerous Hg form in the water systems. On the other hand the same event can cause the limitation of bioavailable mercury forms by a conversion of labile Hg into the most stable one (HgSO₄ ➔ HgS) under anaerobic conditions.

Mercury accumulation in soil from atmospheric deposition in temperate steppe of Inner Mongolia, China

Mercury (Hg) is a toxic and persistent pollutant and has long-term impacts on ecological systems and human health. Coal-fired power plants (CFPPs) are the main source of anthropogenic Hg emission, and the emitted atmospheric Hg is deposited to the surrounding environments which causes soil pollution. To assess the effects of atmospheric Hg from CFPPs in China on the temperate steppe, Hg contents in the topsoil and subsoil were analyzed for samples collected from 80 sites in central Inner Mongolia during 2012-2015. The average content of Hg in topsoil and subsoil were 14.9 ± 10.4 μg kg^-1 and 8.9 ± 5.8 μg kg^-1, respectively. The principal components analysis (PCA) indicated that the soil organic matter content and atmospheric deposition were the main factors determining soil Hg content in Inner Mongolia. We used the power plant impact factor (PPIF) to evaluate the impacts of the surrounding CFPPs. The PPIF results showed the most positive correlation with Hg content in topsoil at more than 400 km distances, indicating that the contribution of the long-range transport of Hg emitted from CFPPs is regional in scale. Considering the potential of Hg accumulation in soil, long-term and regional measurements of soil Hg and stricter emission-limit standards for power plants should be implemented to control soil Hg pollution in China.

Atmosphere-terrestrial exchange of gaseous elemental mercury: parameterization improvement through direct comparison with measured ecosystem fluxes

To simulate global mercury (Hg) dynamics in chemical transport models (CTMs), surface-atmosphere exchange of gaseous elemental mercury, Hg0, 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 Hg0 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 Hg0 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 Hg0 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 Hg0 deposition and wintertime deposition. We conducted a series of sensitivity analyses and recommend that Hg simulations using CTMs: (i) reduce stomatal uptake of Hg0 over grassland and tundra in models by a factor 5-7; (ii) increase nighttime net Hg0 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 Hg0 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 Hg0 exchange in these ecosystems. This approach can be used to improve model parameterizations for other ecosystems if flux measurements become available.

Climate and Vegetation As Primary Drivers for Global Mercury Storage in Surface Soil

Soil is the largest Hg reservoir globally. Data of Hg concentration in surface soil are fundamental to understanding environmental Hg cycling. However, present knowledge on the quantity and global distribution of Hg in soil remains deficient. Using stable Hg isotopic analyses and geospatial data, the concentration and global spatial distribution of Hg in surface soil of 0-20 cm depth have been developed. It is estimated that 1088 ± 379 Gg of Hg is stored in surface soil globally. Thirty-two percent of the surface Hg storage resides in tropical/subtropical forest regions, 23% in temperate/boreal forest regions, 28% in grassland and steppe and shrubland, 7% in tundra, and 10% in desert and xeric shrubland. Evidence from Hg isotopic signatures points to atmospheric Hg⁰ dry deposition through vegetation uptake as the primary source of Hg in surface soil. Given the influence of changing climate on vegetative development, global climate change can act as an important forcing factor for shaping spatial distribution of Hg in surface soil. This active forcing cycle significantly dilutes the impacts caused by Hg release from anthropogenic sources, and needs to be considered in assessing the effectiveness of reducing Hg use and emissions as specified in Minamata Convention on Mercury.

Occurrence, Distribution and Risk Assessment of Mercury in Multimedia of Soil-Dust-Plants in Shanghai, China

The urban environment is a complex ecosystem influenced by strong human disturbances in multi-environmental media, so it is necessary to analyze urban environmental pollutants through the comprehensive analysis of different media. Soil, road dust, foliar dust, and camphor leaves from 32 sample sites in Shanghai were collected for the analysis of mercury contamination in soil–road dust–leaves–foliar dust systems. Mercury concentrations in surface soils in Shanghai were the highest, followed by road dust, foliar dust, and leaves, successively. The spatial distribution of mercury in the four environmental media presented different distribution patterns. Except for the significant correlation between mercury concentrations in road dust and mercury concentrations in leaves (r = 0.56, p < 0.001), there was no significant correlation between the other groups in the four media. Besides this, there was no significant correlation between mercury concentrations and land types. The LUR (Land use regression) model was used to assess the impact of urbanization factors on mercury distribution in the environment. The results showed that soil mercury was affected by factories and residential areas. Foliar dust mercury was affected by road density and power plants. Leaf mercury was affected by power plants and road dust mercury was affected by public service areas. The highest average HI (Hazard index) value of mercury in Shanghai was found in road dust, followed by surface soil and foliar dust. The HI values for children were much higher than those for adults. However, the HI values of mercury exposure in all sampling sites were less than one, suggesting a lower health risk level. The microscopic mechanism of mercury in different environmental media was suggested to be studied further in order to learn the quantitative effects of urbanization factors on mercury concentrations.

Geochemical Factors Controlling Dissolved Elemental Mercury and Methylmercury Formation in Alaskan Wetlands of Varying Trophic Status

The transformations of aqueous inorganic divalent mercury (Hg(II)_i) to volatile dissolved gaseous mercury (Hg(0)_(aq)) and toxic methylmercury (MeHg) govern mercury bioavailability and fate in northern ecosystems. This study quantified concentrations of aqueous mercury species (Hg(II)_i, Hg(0)_(aq), MeHg) and relevant geochemical constituents in pore waters of eight Alaskan wetlands that differ in trophic status (i.e., bog-to-fen gradient) to gain insight on processes controlling dark Hg(II)_i reduction and Hg(II)_i methylation. Regardless of wetland trophic status, positive correlations were observed between pore water Hg(II)_i and dissolved organic carbon (DOC) concentrations. The concentration ratio of Hg(0)_(aq) to Hg(II)_i exhibited an inverse relationship to Hg(II)_i concentration. A ubiquitous pathway for Hg(0)_(aq) formation was not identified based on geochemical data, but we surmise that dissolved organic matter (DOM) influences mercury retention in wetland pore waters by complexing Hg(II)_i and decreasing the concentration of volatile Hg(0)_(aq) relative to Hg(II)_i. There was no evidence of Hg(0)_(aq) abundance directly limiting mercury methylation. The concentration of MeHg relative to Hg(II)_i was greatest in wetlands of intermediate trophic status, and geochemical data suggest mercury methylation pathways vary between wetlands. Our insights on geochemical factors influencing aqueous mercury speciation should be considered in context of the long-term fate of mercury in northern wetlands.

A Coupled Global Atmosphere-Ocean Model for Air-Sea Exchange of Mercury: Insights into Wet Deposition and Atmospheric Redox Chemistry

Air-sea exchange of mercury (Hg) is the largest flux between Earth system reservoirs. Global models simulate air-sea exchange based either on an atmospheric or ocean model simulation and treat the other media as a boundary condition. Here we develop a new modeling capability (NJUCPL) that couples GEOS-Chem (atmospheric model) and MITgcm (ocean model) at the native hourly model time step. The coupled model is evaluated against high-frequency simultaneous measurements of elemental mercury (Hg⁰) in both the atmosphere and surface ocean obtained during five published cruises in the Atlantic, Pacific, and Southern Oceans. Results indicate that the calculated global Hg net evasion flux is 12% higher for the online model than the offline model. We find that the coupled online model captures the spatial pattern of the observations; specifically, it improves the representation of peak seawater Hg⁰ (Hg⁰_aq) concentration associated with enhanced precipitation in the intertropical convergence zone in both the Atlantic and the Pacific Oceans. We investigate the causes of the observed Hg⁰_aq peaks with two sensitivity simulations and show that the high Hg⁰_aq concentrations are associated with elevated convective cloud mass flux and bromine concentrations in the tropical upper troposphere. Observations of elevated Hg⁰_aq concentrations in the western tropical Pacific Ocean merit further study involving BrO vertical distribution and cloud resolving models.

Assessing Air-Surface Exchange and Fate of Mercury in a Subtropical Forest Using a Novel Passive Exchange-Meter Device

A novel passive exchange meter (EM) device was developed to assess air-surface exchange and leaching of Hg in a forest floor. Flux measurements were carried out in a subtropical forest ecosystem during a full year. Over 40% of the Hg fixed in fresh forest litter was remobilized in less than 60 days in warm and humid conditions as a response to rapid turnover of labile organic matter (OM). A two-block experiment including understory and clearing showed that losses of Hg covaried with seasonal conditions and was significantly affected by forest coverage. The process controlling the bulk loss of total Hg from the litter was volatilization, which typically represented 76-96% of the loss processes (F_loss). The F_loss ranges were 520-1370 and 165-942 ng m^-2 d^-1 in the understory and clearing, respectively. On a yearly basis, deposition of airborne Hg exceeded total losses by a factor of 2.5 in the clearing and 1.5 in the understory. The vegetation litter in this subtropical forest therefore represented a net sink of atmospheric Hg. This study provided a novel approach to Hg air-soil exchange measurements and further insights on the link between Hg remobilization and OM turnover along with its environmental drivers.

Mercury contents in rice and potential health risks across China

Rice samples were collected at 560 sites in 15 provinces across China in areas without known point mercury (Hg) sources. Total mercury (THg) and methylmercury (MeHg) concentrations were analyzed in these rice samples for risk assessment. Relatively low THg and MeHg concentrations were found in the majority of the white rice samples with an overall mean of 4.74 (1.06-22.7) μg kg^-1 and 0.682 (0.03-8.71) μg kg^-1, respectively. The means (range of) THg concentration of rice in each geographical region were 5.23 (1.07-19.5) μg kg^-1, 5.14 (1.06-17.2) μg kg^-1, 4.45 (1.41-17.2) μg kg^-1, 4.20 (1.48-19.4) μg kg^-1, 3.49 (1.49-10.7) μg kg^-1, and 4.53 (1.30-19.4) μg kg^-1 in east, centre, south, southwest, northwest and northeast, China, respectively, and the corresponding values for MeHg concentrations were 0.898 (0.127-8.35) μg kg^-1, 0.603 (0.207-2.48) μg kg^-1, 0.516 (0.032-1.50) μg kg^-1, 0.615 (0.050-5.03) μg kg^-1, 0.704 (0.148-2.41) μg kg^-1 and 0.565 (0.035-8.71) μg kg^-1, respectively. Hg contents in rice across China were found to be at background levels. Both the probable daily intakes (PDIs) of inorganic Hg (IHg) and MeHg from rice consumption showed low risks for general population in the investigated regions.

A multidimensional concept for mercury neuronal and sensory toxicity in fish - From toxicokinetics and biochemistry to morphometry and behavior

BACKGROUND

Neuronal and sensory toxicity of mercury (Hg) compounds has been largely investigated in humans/mammals with a focus on public health, while research in fish is less prolific and dispersed by different species. Well-established premises for mammals have been governing fish research, but some contradictory findings suggest that knowledge translation between these animal groups needs prudence [e.g. the relative higher neurotoxicity of methylmercury (MeHg) vs. inorganic Hg (iHg)]. Biochemical/physiological differences between the groups (e.g. higher brain regeneration in fish) may determine distinct patterns. This review undertakes the challenge of identifying sensitive cellular targets, Hg-driven biochemical/physiological vulnerabilities in fish, while discriminating specificities for Hg forms.

SCOPE OF REVIEW

A functional neuroanatomical perspective was conceived, comprising: (i) Hg occurrence in the aquatic environment; (ii) toxicokinetics on central nervous system (CNS)/sensory organs; (iii) effects on neurotransmission; (iv) biochemical/physiological effects on CNS/sensory organs; (v) morpho-structural changes on CNS/sensory organs; (vi) behavioral effects. The literature was also analyzed to generate a multidimensional conceptualization translated into a Rubik's Cube where key factors/processes were proposed.

MAJOR CONCLUSIONS

Hg neurosensory toxicity was unequivocally demonstrated. Some correspondence with toxicity mechanisms described for mammals (mainly at biochemical level) was identified. Although the research has been dispersed by numerous fish species, 29 key factors/processes were pinpointed.

GENERAL SIGNIFICANCE

Future trends were identified and translated into 25 factors/processes to be addressed. Unveiling the neurosensory toxicity of Hg in fish has a major motivation of protecting ichtyopopulations and ecosystems, but can also provide fundamental knowledge to the field of human neurodevelopment.

Tracing the Uptake, Transport, and Fate of Mercury in Sawgrass ( Cladium jamaicense) in the Florida Everglades Using a Multi-isotope Technique

The role of macrophytes in the biogeochemical cycle of mercury (Hg) in the Florida Everglades is poorly understood. Stable isotope tracer techniques were employed to investigate Hg uptake by sawgrass ( Cladium jamaicense) from soil and atmospheric pathways and the fate of Hg after absorption. Our results suggest that soil spiked ²⁰¹Hg²⁺ was rapidly taken up by roots and transported to aboveground parts. The spiked ²⁰¹Hg that was transported to the aboveground parts was trapped; no release of the spiked ²⁰¹Hg from the leaf to the air was detected. Atmospheric ¹⁹⁹Hg⁰ exposure experiments revealed that the majority of the previously deposited ¹⁹⁹Hg⁰ taken into the leaf was fixed, with a very limited proportion (1.6%) available for re-emission to the atmosphere. The percentage of ¹⁹⁹Hg⁰ fixed in the leaf will help reduce the model uncertainty in estimating the Hg⁰ exchange over the air-vegetation surface. We propose that sawgrass needs to be viewed as an important sink for atmospheric Hg⁰ in the regional Hg mass balance; this would have important implications for the critical loads of Hg to the Everglades. The multi-isotope tracer technique could be an effective tool to identify the role of plants in biogeochemical cycling of Hg in other ecosystems.

A review of global environmental mercury processes in response to human and natural perturbations: Changes of emissions, climate, and land use

We review recent progress in our understanding of the global cycling of mercury (Hg), including best estimates of Hg concentrations and pool sizes in major environmental compartments and exchange processes within and between these reservoirs. Recent advances include the availability of new global datasets covering areas of the world where environmental Hg data were previously lacking; integration of these data into global and regional models is continually improving estimates of global Hg cycling. New analytical techniques, such as Hg stable isotope characterization, provide novel constraints of sources and transformation processes. The major global Hg reservoirs that are, and continue to be, affected by anthropogenic activities include the atmosphere (4.4-5.3 Gt), terrestrial environments (particularly soils: 250-1000 Gg), and aquatic ecosystems (e.g., oceans: 270-450 Gg). Declines in anthropogenic Hg emissions between 1990 and 2010 have led to declines in atmospheric Hg0 concentrations and HgII wet deposition in Europe and the US (- 1.5 to - 2.2% per year). Smaller atmospheric Hg0 declines (- 0.2% per year) have been reported in high northern latitudes, but not in the southern hemisphere, while increasing atmospheric Hg loads are still reported in East Asia. New observations and updated models now suggest high concentrations of oxidized HgII in the tropical and subtropical free troposphere where deep convection can scavenge these HgII reservoirs. As a result, up to 50% of total global wet HgII deposition has been predicted to occur to tropical oceans. Ocean Hg0 evasion is a large source of present-day atmospheric Hg (approximately 2900 Mg/year; range 1900-4200 Mg/year). Enhanced seawater Hg0 levels suggest enhanced Hg0 ocean evasion in the intertropical convergence zone, which may be linked to high HgII deposition. Estimates of gaseous Hg0 emissions to the atmosphere over land, long considered a critical Hg source, have been revised downward, and most terrestrial environments now are considered net sinks of atmospheric Hg due to substantial Hg uptake by plants. Litterfall deposition by plants is now estimated at 1020-1230 Mg/year globally. Stable isotope analysis and direct flux measurements provide evidence that in many ecosystems Hg0 deposition via plant inputs dominates, accounting for 57-94% of Hg in soils. Of global aquatic Hg releases, around 50% are estimated to occur in China and India, where Hg drains into the West Pacific and North Indian Oceans. A first inventory of global freshwater Hg suggests that inland freshwater Hg releases may be dominated by artisanal and small-scale gold mining (ASGM; approximately 880 Mg/year), industrial and wastewater releases (220 Mg/year), and terrestrial mobilization (170-300 Mg/year). For pelagic ocean regions, the dominant source of Hg is atmospheric deposition; an exception is the Arctic Ocean, where riverine and coastal erosion is likely the dominant source. Ocean water Hg concentrations in the North Atlantic appear to have declined during the last several decades but have increased since the mid-1980s in the Pacific due to enhanced atmospheric deposition from the Asian continent. Finally, we provide examples of ongoing and anticipated changes in Hg cycling due to emission, climate, and land use changes. It is anticipated that future emissions changes will be strongly dependent on ASGM, as well as energy use scenarios and technology requirements implemented under the Minamata Convention. We predict that land use and climate change impacts on Hg cycling will be large and inherently linked to changes in ecosystem function and global atmospheric and ocean circulations. Our ability to predict multiple and simultaneous changes in future Hg global cycling and human exposure is rapidly developing but requires further enhancement.

Historical and future trends in global source-receptor relationships of mercury

Growing concern about the risk associated with increasing environmental mercury (Hg) concentrations has resulted in a focus on the relationships between intercontinental emitted and accumulated Hg. We use a global biogeochemical Hg model with 8 continental regions and a global ocean to evaluate the legacy impacts of historical anthropogenic releases (2000BCE to 2008AD) on global source-receptor relationships of Hg. Legacy impacts of historical anthropogenic releases are confirmed to be significant on the source-receptor relationships according to our results. Historical anthropogenic releases from Asia account for 8% of total soil Hg in North America, which is smaller than the proportion (~17%) from previous studies. The largest contributors to the global oceanic Hg are historical anthropogenic releases from North America (26%), Asia (16%), Europe (14%) and South America (14%). Although anthropogenic releases from Asia have exceeded North America since the 1970s, source contributions to global Hg receptors from Asia have not exceeded North America so far. Future projections indicate that if Hg emissions are not effectively controlled, Asia will exceed North America as the largest contributor to the global ocean in 2019 and this has a long-term adverse impact on the future environment. For the Arctic Ocean, historical anthropogenic release from North America contributes most to the oceanic Hg reservoir and future projections reveal that the legacy impacts of historical releases from mid-latitudes would lead to the potential of rising Hg in the Arctic Ocean in the future decades, which calls for more effective Hg controls on mid-latitude releases.

Emerging investigator series: the effect of wildfire on streamwater mercury and organic carbon in a forested watershed in the southeastern United States

Wildfires alter forested ecosystems, which include large stores of mercury (Hg) and organic carbon, two compounds that are closely linked in vegetation, soils, and streamwater. Studies have shown that wildfires release elevated levels of mercury to the atmosphere which can be locally redeposited and leave charred organic material (vegetation and litter) on the soil surface. Both can contribute to the elevated mobilization of Hg into lakes and streams. However, no studies have conducted a detailed examination of hydrological transport of Hg following a wildfire. This study investigates the coupled transport of mercury and carbon at Twomile Run, a headwater stream located in the forested mountains of Shenandoah National Park, in the year following a low-severity wildfire. Weekly baseflow samples and bi-hourly high-flow storm samples were analyzed for dissolved and particulate mercury (Hg_D and Hg_P, respectively), dissolved organic carbon (DOC), UV absorbance at 254 nm (UV₂₅₄, surrogate for DOC quantity and character), and total suspended solids (TSS), and were compared with identical measurements taken from a nearby unburned watershed. For all flow conditions sampled at the burned site (which did not include the 2 months following the fire), streamwater Hg_D and DOC concentrations, and corresponding UV₂₅₄, were similar to the unburned system. TSS concentrations varied between sites but overall differences were relatively small in magnitude and likely attributable to site differences rather than fire effects. Notably, the Hg_P per unit of TSS at the burned site was an order of magnitude higher than the unburned site (2.66 and 0.13 ng Hg_P per mg TSS, respectively) for 8 months following the fire, resulting in elevated Hg_P concentrations for the range of flow conditions, after which there was a rapid return to non-disturbed conditions. Streamwater total Hg fluxes roughly doubled (0.55 to 1.04 μg m^-2 yr^-1) as a consequence of the fire, indicating that in addition to changing atmospheric and terrestrial Hg cycling, fires can rapidly and significantly alter the streamwater Hg which has implication for downstream ecosystems. These findings are particularly relevant as the occurrence and severity of wildfires are expected to increase in the mid-latitudes in response to climate change.

Mercury concentrations in China's coastal waters and implications for fish consumption by vulnerable populations

We assessed mercury (Hg) pollution in China's coastal waters, including the Bohai Sea, the Yellow Sea, the East China Sea and the South China Sea, based on a nationwide dataset from 301 sampling sites. A methylmercury (MeHg) intake model for humans based on the marine food chain and human fish consumption was established to determine the linkage between water pollutants and the pollutant intake by humans. The predicted MeHg concentration in fish from the Bohai Sea was the highest among the four seas included in the study. The MeHg intake through dietary ingestion was dominant for the fish and was considerably higher than the MeHg intake through water respiration. The predicted MeHg concentrations in human blood in the coastal regions of China ranged from 1.37 to 2.77 μg/L for pregnant woman and from 0.43 to 1.00 μg/L for infants, respectively, based on different diet sources. The carnivorous fish consumption advisory for pregnant women was estimated to be 288-654 g per week to maintain MeHg concentrations in human blood at levels below the threshold level (4.4 μg/L established by the US Environmental Protection Agency). With a 50% increase in Hg concentrations in water in the Bohai Sea, the bioaccumulated MeHg concentration (4.5 μg/L) in the fish consumers will be higher than the threshold level. This study demonstrates the importance in controlling Hg pollution in China's coastal waters. An official recommendation guideline for the fish consumption rate and its sources will be necessary for vulnerable populations in China.

Organic horizon and mineral soil mercury along three clear-cut forest chronosequences across the northeastern USA

Mercury (Hg) is a globally distributed pollutant trace metal that has been increasing in terrestrial environments due to rising anthropogenic emissions. Vegetation plays an important role in Hg sequestration in forested environments, but increasing tree removal for biofuels and wood products may affect this process. The long-term effect of clear-cutting on forest soil Hg remains uncertain, since most studies are limited to measuring changes for < 10 years following a single harvest event. The chronosequence approach, which substitutes space for time using forest stands of different ages since clear-cutting, allows for investigation of processes occurring over decades to centuries. Here, we utilized three clear-cut forest soil chronosequences across the northeastern USA to understand Hg accumulation and retention over several decades. Total Hg concentrations and pools were quantified for five soil depth increments along three chronosequences. Our results showed Hg concentrations and pools decreased in the initial 20 years following clear-cutting. Mineral soil Hg pools decreased 21-53% (7-14 mg m^-2) between 1-5-year-old stands and 15-25-year-old stands but mineral soil Hg pools recovered in 55-140-year-old stands to similar values as measured in 1-5-year-old stands. Our study is one of the first to demonstrate a decrease and recovery in Hg pool size. These changes in Hg did not correspond with changes in bulk density, soil C, or pH. We utilized a simple two-box model to determine how different Hg fluxes affected organic and mineral soil horizon Hg pools. Our simple model suggests that changes in litterfall and volatilization rates could have caused the observed changes in organic horizon Hg pools. However, only increases in leaching could reproduce observed decreases to mineral soil Hg pools. Further studies are needed to determine the mechanism of Hg loss from forest soils following clear-cutting.

Tundra uptake of atmospheric elemental mercury drives Arctic mercury pollution

Anthropogenic activities have led to large-scale mercury (Hg) pollution in the Arctic. It has been suggested that sea-salt-induced chemical cycling of Hg (through 'atmospheric mercury depletion events', or AMDEs) and wet deposition via precipitation are sources of Hg to the Arctic in its oxidized form (Hg(ii)). However, there is little evidence for the occurrence of AMDEs outside of coastal regions, and their importance to net Hg deposition has been questioned. Furthermore, wet-deposition measurements in the Arctic showed some of the lowest levels of Hg deposition via precipitation worldwide, raising questions as to the sources of high Arctic Hg loading. Here we present a comprehensive Hg-deposition mass-balance study, and show that most of the Hg (about 70%) in the interior Arctic tundra is derived from gaseous elemental Hg (Hg(0)) deposition, with only minor contributions from the deposition of Hg(ii) via precipitation or AMDEs. We find that deposition of Hg(0)-the form ubiquitously present in the global atmosphere-occurs throughout the year, and that it is enhanced in summer through the uptake of Hg(0) by vegetation. Tundra uptake of gaseous Hg(0) leads to high soil Hg concentrations, with Hg masses greatly exceeding the levels found in temperate soils. Our concurrent Hg stable isotope measurements in the atmosphere, snowpack, vegetation and soils support our finding that Hg(0) dominates as a source to the tundra. Hg concentration and stable isotope data from an inland-to-coastal transect show high soil Hg concentrations consistently derived from Hg(0), suggesting that the Arctic tundra might be a globally important Hg sink. We suggest that the high tundra soil Hg concentrations might also explain why Arctic rivers annually transport large amounts of Hg to the Arctic Ocean.

Source tracing of natural organic matter bound mercury in boreal forest runoff with mercury stable isotopes

Terrestrial runoff represents a major source of mercury (Hg) to aquatic ecosystems. In boreal forest catchments, such as the one in northern Sweden studied here, mercury bound to natural organic matter (NOM) represents a large fraction of mercury in the runoff. We present a method to measure Hg stable isotope signatures of colloidal Hg, mainly complexed by high molecular weight or colloidal natural organic matter (NOM) in natural waters based on pre-enrichment by ultrafiltration, followed by freeze-drying and combustion. We report that Hg associated with high molecular weight NOM in the boreal forest runoff has very similar Hg isotope signatures as compared to the organic soil horizons of the catchment area. The mass-independent fractionation (MIF) signatures (Δ¹⁹⁹Hg and Δ²⁰⁰Hg) measured in soils and runoff were in agreement with typical values reported for atmospheric gaseous elemental mercury (Hg⁰) and distinctly different from reported Hg isotope signatures in precipitation. We therefore suggest that most Hg in the boreal terrestrial ecosystem originated from the deposition of Hg⁰ through foliar uptake rather than precipitation. Using a mixing model we calculated the contribution of soil horizons to the Hg in the runoff. At moderate to high flow runoff conditions, that prevailed during sampling, the uppermost part of the organic horizon (Oe/He) contributed 50-70% of the Hg in the runoff, while the underlying more humified organic Oa/Ha and the mineral soil horizons displayed a lower mobility of Hg. The good agreement of the Hg isotope results with other source tracing approaches using radiocarbon signatures and Hg : C ratios provides additional support for the strong coupling between Hg and NOM. The exploratory results from this study illustrate the potential of Hg stable isotopes to trace the source of Hg from atmospheric deposition through the terrestrial ecosystem to soil runoff, and provide a basis for more in-depth studies investigating the mobility of Hg in terrestrial ecosystems using Hg isotope signatures.

Boreal Forests Sequester Large Amounts of Mercury over Millennial Time Scales in the Absence of Wildfire

Alterations in fire activity due to climate change and fire suppression may have profound effects on the balance between storage and release of carbon (C) and associated volatile elements. Stored soil mercury (Hg) is known to volatilize due to wildfires and this could substantially affect the land-air exchange of Hg; conversely the absence of fires and human disturbance may increase the time period over which Hg is sequestered. Here we show for a wildfire chronosequence spanning over more than 5000 years in boreal forest in northern Sweden that belowground inventories of total Hg are strongly related to soil humus C accumulation (R² = 0.94, p < 0.001). Our data clearly show that northern boreal forest soils have a strong sink capacity for Hg, and indicate that the sequestered Hg is bound in soil organic matter pools accumulating over millennia. Our results also suggest that more than half of the Hg stock in the sites with the longest time since fire originates from deposition predating the onset of large-scale anthropogenic emissions. This study emphasizes the importance of boreal forest humus soils for Hg storage and reveals that this pool is likely to persist over millennial time scales in the prolonged absence of fire.