Hg Evasion Research Articles

The Marginal Ice Zone as a dominant source region of atmospheric mercury during central Arctic summertime

Atmospheric gaseous elemental mercury (GEM) concentrations in the Arctic exhibit a clear summertime maximum, while the origin of this peak is still a matter of debate in the community. Based on summertime observations during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition and a modeling approach, we further investigate the sources of atmospheric Hg in the central Arctic. Simulations with a generalized additive model (GAM) show that long-range transport of anthropogenic and terrestrial Hg from lower latitudes is a minor contribution (~2%), and more than 50% of the explained GEM variability is caused by oceanic evasion. A potential source contribution function (PSCF) analysis further shows that oceanic evasion is not significant throughout the ice-covered central Arctic Ocean but mainly occurs in the Marginal Ice Zone (MIZ) due to the specific environmental conditions in that region. Our results suggest that this regional process could be the leading contributor to the observed summertime GEM maximum. In the context of rapid Arctic warming and the observed increase in width of the MIZ, oceanic Hg evasion may become more significant and strengthen the role of the central Arctic Ocean as a summertime source of atmospheric Hg.

Deposition and Re-Emission of Atmospheric Elemental Mercury over the Tropical Forest Floor.

Significant knowledge gaps exist regarding the emission of elemental mercury (Hg0) from the tropical forest floor, which limit our understanding of the Hg mass budget in forest ecosystems. In this study, biogeochemical processes of Hg0 deposition to and evasion from soil in a Chinese tropical rainforest were investigated using Hg stable isotopic techniques. Our results showed a mean air-soil flux as deposition of -4.5 ± 2.1 ng m-2 h-1 in the dry season and as emission of +7.4 ± 1.2 ng m-2 h-1 in the rainy season. Hg re-emission, i.e., soil legacy Hg evasion, induces negative transitions of Δ199Hg and δ202Hg in the evaded Hg0 vapor, while direct atmospheric Hg0 deposition does not exhibit isotopic fractionation. Using an isotopic mass balance model, direct atmospheric Hg0 deposition to soil was estimated to be 48.6 ± 13.0 μg m-2 year-1. Soil Hg0 re-emission was estimated to be 69.5 ± 10.6 μg m-2 year-1, of which 63.0 ± 9.3 μg m-2 year-1 is from surface soil evasion and 6.5 ± 5.0 μg m-2 year-1 from soil pore gas diffusion. Combined with litterfall Hg deposition (∼34 μg m-2 year-1), we estimated a ∼12.6 μg m-2 year-1 net Hg0 sink in the tropical forest. The fast nutrient cycles in the tropical rainforests lead to a strong Hg0 re-emission and therefore a relatively weaker atmospheric Hg0 sink.

Enhanced daytime atmospheric mercury in the marine boundary layer in the South Oceans

Understanding the spatial and temporal variations of atmospheric mercury (Hg) in the marine boundary layer could advance our knowledge on ocean evasion of Hg. Here, we conducted continuous measurements of total gaseous mercury (TGM) in the marine boundary layer during a round-the-world cruise from August 2017 to May 2018. We observed the highest and lowest TGM concentrations in Southern Indian Ocean (1.29 ± 0.22 ng m−3) and Southern Atlantic Ocean (0.61 ± 0.28 ng m−3), respectively. During the daytime, enhanced TGM was observed with the diurnal amplitude difference reaching its maximum in the range of 0.30–0.37 ng m−3 in Southern Indian Ocean and Southern Ocean. The positive correlation between TGM (R2 = 0.68–0.92) and hourly solar radiation in each ocean suggested that the daytime enhanced TGM was likely driven by Hg photoreduction in seawater, after excluding the influence of other meteorological factors. The diurnal amplitude of TGM in the marine boundary layer might be impacted by the microbial productivity and the ratio of ultraviolet radiation. Our study highlights that ocean acts as a net TGM source during the daytime in the Southern Hemisphere and aqueous photoreduction process may play an important role in the biogeochemical cycling of Hg.

Dissolved gaseous mercury production and sea-air gaseous exchange in impacted coastal environments of the northern Adriatic Sea

The northern Adriatic Sea is well known for mercury (Hg) contamination mainly due to historical Hg mining which took place in Idrija (Slovenia). The formation of dissolved gaseous mercury (DGM) and its subsequent volatilisation can reduce the amount of Hg available in the water column. In this work, the diurnal patterns of both DGM production and gaseous elemental Hg (Hg0) fluxes at the water-air interface were seasonally evaluated in two selected environments within this area, a highly Hg-impacted, confined fish farm (VN: Val Noghera, Italy) and an open coastal zone less impacted by Hg inputs (PR: Bay of Piran, Slovenia). A floating flux chamber coupled with a real-time Hg0 analyser was used for flux estimation in parallel with DGM concentrations determination through in-field incubations. Substantial DGM production was observed at VN (range = 126.0–711.3 pg L−1) driven by both strong photoreduction and possibly dark biotic reduction, resulting in higher values in spring and summer and comparable concentrations throughout both day and night. Significantly lower DGM was observed at PR (range = 21.8–183.4 pg L−1). Surprisingly, comparable Hg0 fluxes were found at the two sites (range VN = 7.43–41.17 ng m−2 h−1, PR = 0–81.49 ng m−2 h−1), likely due to enhanced gaseous exchanges at PR thanks to high water turbulence and to the strong limitation of evasion at VN by water stagnation and expected high DGM oxidation in saltwater. Slight differences between the temporal variation of DGM and fluxes indicate that Hg evasion is more controlled by factors such as water temperature and mixing conditions than DGM concentrations alone. The relative low Hg losses through volatilisation at VN (2.4–4.6% of total Hg) further confirm that static conditions in saltwater environments negatively affect the ability of this process in reducing the amount of Hg retained in the water column, therefore potentially leading to a greater availability for methylation and trophic transfer.

Effects of redox variability and early diagenesis on marine sedimentary Hg records

Volcanism is a dominant natural source of mercury (Hg) to the atmosphere, biosphere, ocean and sediments. In recent years, sedimentary Hg contents have emerged as a tool to reconstruct volcanic activity, and particularly activity of (subaerially emplaced) large igneous provinces in geological deep time. More specifically, Hg has shown potential as a useful proxy to illuminate the previously elusive impact of such large-scale volcanism on marine and terrestrial paleo-environments. While Hg is now widely applied as volcanism tracer, non-volcanic factors controlling sedimentary Hg content are generally not well constrained. Part of this uncertainty stems from our inability to directly observe a natural unperturbed “steady-state” environment as a baseline, as the modern Hg cycle is heavily influenced by anthropogenic activity. Here we focus on the effects of ambient redox conditions in the water column and shallow sediments (early diagenesis), quantify their influence on the geological Hg record and thereby contribute to constraining their potential impact on the use of Hg as a proxy for deep-time volcanic activity. Constraining these factors is of critical importance for the application of Hg as such a proxy. Many periods in the geological past for which records have been generated, such as the Mesozoic Oceanic Anoxic Events, are marked by a variety of high-amplitude environmental perturbations, including widespread deoxygenation and deposition of organic-rich sediments. We estimate the impact of redox changes and early diagenesis on the geological Hg record using a suite of (sub)recent–Pleistocene and Upper Cretaceous sediments representing oxic to euxinic marine conditions. Our sample set includes a transect through an oxygen minimum zone and cores that record transient shifts in oxygenation state, as well as post-depositional effects – all unrelated to volcanism, to the best of our knowledge. We find substantial alterations to the Hg record and the total organic carbon and total sulfur content, which are typically assumed to be the most common carrier phases of Hg in marine sediments. Moreover, these biases can lead to signal alteration on a par with those interpreted to result from volcanic activity. Geochemical modifications are ubiquitous and their potential magnitude implies that the factors leading to biases in the geological record warrant careful consideration before interpretation. Factors of particular concern to proxy application are (1) the disproportionate loss of organic carbon and sulfur relative to Hg during oxidation that strongly modulates normalized Hg records, (2) the evasion of Hg in anoxic and mildly euxinic sediments and (3) sharp focusing of Hg during post-depositional oxidation of organic matter. We suggest that paired analyses of additional redox-sensitive trace elements such as molybdenum, and organic-matter characteristics, particularly the type of organic matter, could provide first-order constraints on the role that redox and diagenetic changes played in shaping the Hg record as part of checking the attribution of enrichments to volcanic activity.

National-Scale Assessment of Total Gaseous Mercury Isotopes Across the United States.

With the 2011 promulgation of the Mercury and Air Toxics Standards by the U.S. Environmental Protection Agency, and the successful negotiation by the United Nations Environment Program of the Minamata Convention, emissions of mercury (Hg) have declined in the United States. While the declines in atmospheric Hg concentrations in North America are encouraging, linking the declines to changing domestic and global source portfolios remains challenging. To address these research gaps, the U.S. Geological Survey initiated the first national-scale effort to establish a baseline of total gaseous mercury stable isotope values at 31 sites distributed across the United States. Results indicated that unique Hg sources, such as Hg evasion from an elemental Hg contaminated site or free tropospheric intrusions in high altitude sites, were distinguishable from background atmospheric values. Minor gradients were observed across the nation, with regions of heavy industrial activity demonstrating lower , but no consistent changes in other isotopes such as and were observed. Furthermore, was impacted by foliar uptake and senescence but trends varied between forested regions in the northeastern and midwestern United States. These data demonstrate regional emission sources and other environmental variables can impact total gaseous Hg (TGM) isotope values, highlighting the need to characterize atmospheric Hg isotopes over larger geographical areas to evaluate changes related to national and international Hg regulations.

Climate-driven changes of global marine mercury cycles in 2100

Human exposure to monomethylmercury (CH3Hg), a potent neurotoxin, is principally through the consumption of seafood. The formation of CH3Hg and its bioaccumulation in marine food webs experience ongoing impacts of global climate warming and ocean biogeochemistry alterations. Employing a series of sensitivity experiments, here we explicitly consider the effects of climate change on marine mercury (Hg) cycling within a global ocean model in the hypothesized twenty-first century under the business-as-usual scenario. Even though the overall prediction is subjected to significant uncertainty, we identify several important climate change impact pathways. Elevated seawater temperature exacerbates elemental Hg (Hg0) evasion, while decreased surface wind speed reduces air-sea exchange rates. The reduced export of particulate organic carbon shrinks the pool of potentially bioavailable divalent Hg (HgII) that can be methylated in the subsurface ocean, where shallower remineralization depth associated with lower productivity causes impairment of methylation activity. We also simulate an increase in CH3Hg photodemethylation potential caused by increased incident shortwave radiation and less attenuation by decreased sea ice and chlorophyll. The model suggests that these impacts can also be propagated to the CH3Hg concentration in the base of the marine food web. Our results offer insight into synergisms/antagonisms in the marine Hg cycling among different climate change stressors.

Dynamics, distribution, and transformations of mercury species from pyrenean high-altitude lakes

While mercury (Hg) is a major concern in all aquatic environments because of its methylation and biomagnification pathways, very few studies consider Hg cycling in remote alpine lakes which are sensitive ecosystems. Nineteen high-altitude pristine lakes from Western/Central Pyrenees were investigated on both northern (France) and southern (Spain) slopes (1620–2600 m asl.). Subsurface water samples were collected in June 2017/2018/2019 and October 2017/2018 for Hg speciation analysis of inorganic mercury (iHg(II)), monomethylmercury (MMHg), and dissolved gaseous mercury (DGM) to investigate spatial and seasonal variations. In June 2018/2019 and October 2018, more comprehensive studies were performed in four lakes by taking water column depth profiles. Besides, in-situ incubation experiments using isotopically enriched Hg species (199iHg(II), 201MMHg) were conducted to investigate Hg transformation mechanisms in the water column. While iHg(II) (0.08–1.10 ng L−1 in filtered samples; 0.11–1.19 ng L−1 in unfiltered samples) did not show significant seasonal variations in the subsurface water samples, MMHg (<0.03–0.035 ng L−1 in filtered samples; <0.03–0.062 ng L−1 in unfiltered samples) was significantly higher in October 2018, mainly because of in-situ methylation. DGM (0.02–0.68 ng L−1) varies strongly and can exhibit higher levels in comparison with other pristine areas. Depth profiles and incubation experiments highlighted the importance of in-situ biotic methylation triggered by anoxic conditions in bottom waters. In-situ incubations confirm that significant methylation, demethylation and photoreduction extents are taking place in the water columns. Overall, drastic environmental changes occurring daily and seasonally in alpine lakes are providing conditions that can both promote Hg methylation (stratified anoxic waters) and MMHg photodemethylation (intense UV light). In addition, light induced photoreduction is a major pathway controlling significant gaseous Hg evasion. Global warming and potential eutrophication may thus have direct implications on Hg turnover and MMHg burden in those remote ecosystems.

Chemical Oxidation and Reduction Pathways of Mercury Relevant to Natural Waters: A Review

Mercury (Hg) pollution in the environment is a global issue and the toxicity of mercury depends on its speciation. Chemical redox reactions of mercury in an aquatic environment greatly impact on Hg evasion to the atmosphere and the methylation of mercury in natural waters. Identifying the abiotic redox pathways of mercury relevant to natural waters is important for predicting the transport and fate of Hg in the environment. The objective of this review is to summarize the current state of knowledge on specific redox reactions of mercury relevant to natural waters at a molecular level. The rate constants and factors affecting them, as well as the mechanistic information of these redox pathways, are discussed in detail. Increasing experimental evidence also implied that the structure of natural organic matter (NOM) play an important role in dark Hg(II) reduction, dark Hg(0) oxidation and Hg(II) photoreduction in the aquatic environment. Significant photooxidation pathways of Hg(0) identified are Hg(0) photooxidation by hydroxyl radical (OH•) and by carbonate radical (CO3−•). Future research needs on improving the understanding of Hg redox cycling in natural waters are also proposed.

A Simplified Drying Procedure for Analysing Hg Concentrations

Mercury (Hg) in peatlands remains a problem of global interest. To mitigate the risks of this neurotoxin, accurate assessments of Hg in peat are needed. Treatment of peat that will be analysed for Hg is, however, not straightforward due to the volatile nature of Hg. The drying process is of particular concern since Hg evasion increases with the temperature. Samples are, therefore, often freeze-dried to limit Hg loss during the drying processes. A problem with freeze-drying is that cost and equipment resources can limit the number of samples analysed in large projects. To avoid this bottleneck, we tested if drying in a 60 °C-degree oven could be an acceptable alternative to freeze-drying. We both freeze-dried and oven-dried (60 °C) 203 replicate pairs of peat samples, and then examined the differences in total Hg concentration. The Hg concentration differed significantly between the two drying methods with a median Hg deficit in oven-dried samples of 4.2%. Whether a 4.2% deficit of Hg depends on one’s purpose. The lower median Hg concentration in oven-dried samples has to be weighed against the upside efficiently drying large sets of peat samples. By freeze-drying a subset of the samples, we fitted a function to correct for Hg loss during oven-drying (y=0.96x+0.08). By applying this correction, the freeze-drying bottleneck could oven-dry large-scale inventories of total Hg in peatlands with results equivalent to freeze-drying, but only have to freeze-dry a subset.

Arctic atmospheric mercury: Sources and changes

Global anthropogenic and legacy mercury (Hg) emissions are the main sources of Arctic Hg contamination, primarily transported there via the atmosphere. This review summarizes the state of knowledge of the global anthropogenic sources of Hg emissions, and examines recent changes and source attribution of Hg transport and deposition to the Arctic using models. Estimated global anthropogenic Hg emissions to the atmosphere for 2015 were ~2220 Mg, ~20% higher than 2010. Global anthropogenic, legacy and geogenic Hg emissions were, respectively, responsible for 32%, 64% (wildfires: 6–10%) and 4% of the annual Arctic Hg deposition. Relative contributions to Arctic deposition of anthropogenic origin was dominated by sources in East Asia (32%), Commonwealth of Independent States (12%), and Africa (12%). Model results exhibit significant spatiotemporal variations in Arctic anthropogenic Hg deposition fluxes, driven by regional differences in Hg air transport routes, surface and precipitation uptake rates, and inter-seasonal differences in atmospheric circulation and deposition pathways. Model simulations reveal that changes in meteorology are having a profound impact on contemporary atmospheric Hg in the Arctic. Reversal of North Atlantic Oscillation phase from strongly negative in 2010 to positive in 2015, associated with lower temperature and more sea ice in the Canadian Arctic, Greenland and surrounding ocean, resulted in enhanced production of bromine species and Hg(0) oxidation and lower evasion of Hg(0) from ocean waters in 2015. This led to increased Hg(II) (and its deposition) and reduced Hg(0) air concentrations in these regions in line with High Arctic observations. However, combined changes in meteorology and anthropogenic emissions led to overall elevated modeled Arctic air Hg(0) levels in 2015 compared to 2010 contrary to observed declines at most monitoring sites, likely due to uncertainties in anthropogenic emission speciation, wildfire emissions and model representations of air-surface Hg fluxes.

Dissolved Gaseous Mercury (DGM) in the Gulf of Trieste, Northern Adriatic Sea

Continuous dissolved gaseous mercury (DGM) measurements were performed during the summer months (May to September 2019) in the Gulf of Trieste (northern Adriatic Sea), a well-studied contaminated site due to releases of mercury from the former mercury mine Idrija in Slovenia. Continuous DGM data were regularly checked by the discrete manual method to assure traceability and comparability of the results and used for the calculation of the upward flux of Hg(0) between the water and the air compartment, using the gas exchange model applied in previous studies in the Mediterranean Sea. DGM concentrations measured by continuous and discrete methods showed good agreement, 68.7 and 73.5 ng·m−3, respectively. The diurnal DGM variability examined by sorting the DGM concentrations in 24 1-h intervals was extremely low (68.3–69.2 ng·m−3). Various environmental parameters measured at oceanographic buoy Vida, and the nearby stations were used to determine the relationship between DGM and the individual environmental parameters. The correlation with the oxygen saturation was pronounced during the July high DGM event (R2 = 0.70, p &lt; 0.05), and the gradient between the bottom and surface temperature was correlated with both DGM peaks in June and July (R2 = 0.42 and R2 = 0.43, p &lt; 0.05). Transport from the more polluted northern part of the Gulf was determined as the most probable source of both high DGM events. The computed average annual Hg(0) flux across the water–air interface (5.13 ng·m−2·h−1) was lower than those reported in recent studies. We assume that for an appropriate assessment of the Hg evasion flux and of the temporal DGM variability in such heterogeneously polluted coastal areas, both spatial and temporal coverage are required.

Climate change and mercury in the Arctic: Abiotic interactions

Dramatic environmental shifts are occuring throughout the Arctic from climate change, with consequences for the cycling of mercury (Hg). This review summarizes the latest science on how climate change is influencing Hg transport and biogeochemical cycling in Arctic terrestrial, freshwater and marine ecosystems. As environmental changes in the Arctic continue to accelerate, a clearer picture is emerging of the profound shifts in the climate and cryosphere, and their connections to Hg cycling. Modeling results suggest climate influences seasonal and interannual variability of atmospheric Hg deposition. The clearest evidence of current climate change effects is for Hg transport from terrestrial catchments, where widespread permafrost thaw, glacier melt and coastal erosion are increasing the export of Hg to downstream environments. Recent estimates suggest Arctic permafrost is a large global reservoir of Hg, which is vulnerable to degradation with climate warming, although the fate of permafrost soil Hg is unclear. The increasing development of thermokarst features, the formation and expansion of thaw lakes, and increased soil erosion in terrestrial landscapes are increasing river transport of particulate-bound Hg and altering conditions for aquatic Hg transformations. Greater organic matter transport may also be influencing the downstream transport and fate of Hg. More severe and frequent wildfires within the Arctic and across boreal regions may be contributing to the atmospheric pool of Hg. Climate change influences on Hg biogeochemical cycling remain poorly understood. Seasonal evasion and retention of inorganic Hg may be altered by reduced sea-ice cover and higher chloride content in snow. Experimental evidence indicates warmer temperatures enhance methylmercury production in ocean and lake sediments as well as in tundra soils. Improved geographic coverage of measurements and modeling approaches are needed to better evaluate net effects of climate change and long-term implications for Hg contamination in the Arctic.

Atmospheric gaseous elemental mercury and its sea-air evasion fluxes from the Sea of Okhotsk to the South China Sea: results of cruise studies in 2019

Abstract. At present, the behavior of mercury in the sea-atmosphere system in the north western Pacific Ocean is poorly understood. From September to December 2019, continuous measurements of atmospheric gaseous elemental mercury (Hg(0)) and point measurements of Hg(0) evasion fluxes from the sea to the atmosphere were carried out during two adjacent cruises from the South China Sea to the Sea of Okhotsk. The median concentration of Hg(0) (1.1 ng/m3) was below both the background of the Northern Hemisphere and the average values previously recorded in these areas. There was a trend of increasing Hg(0) concentration with decreasing latitude. The Yellow Sea with adjacent territories and northeast China were the Hg(0) source area for the Sea of Japan and the Sea of Okhotsk. The Kuril sector of the Pacific Ocean was identified as a source of Hg(0) for the Sea of Okhotsk. East China, Southeast China and Indochina were sources of Hg(0) for the East China and South China seas. A winter increase in Hg(0) concentrations was observed in the Sea of Japan and the East China Sea. Our data showed significant relationships between Hg(0) fluxes, latitude, and water temperature.

Multiscale Temporal Variations of Atmospheric Mercury Distinguished by the Hilbert-Huang Transform Analysis Reveals Multiple El Niño-Southern Oscillation Links.

Atmospheric mercury (Hg) cycling is sensitive to climate-driven changes, but links with various teleconnections remain unestablished. Here, we revealed the El Niño-Southern Oscillation (ENSO) influence on gaseous elemental mercury (GEM) concentrations recorded at a background station in East Asia using the Hilbert-Huang transform (HHT). The timing and magnitude of GEM intrinsic variations were clearly distinguished by ensemble empirical mode decomposition (EEMD), revealing the amplitude of the GEM concentration interannual variability (IAV) is greater than that for diurnal and seasonal variability. We show that changes in the annual cycle of GEM were modulated by significant IAVs at time scales of 2-7 years, highlighted by a robust GEM IAV-ENSO relationship of the associated intrinsic mode functions. With confirmation that ENSO modulates the GEM annual cycle, we then found that weaker GEM annual cycles may have resulted from El Niño-accelerated Hg evasion from the ocean. Furthermore, the relationship between ENSO and GEM is sensitive to extreme events (i.e., 2015-2016 El Niño), resulting in perturbation of the long-term trend and atmospheric Hg cycling. Future climate change will likely increase the number of extreme El Niño events and, hence, could alter atmospheric Hg cycling and influence the effectiveness evaluation of the Minamata Convention on Mercury.

Assessment of Mercury Concentrations and Fluxes Deposited from the Atmosphere on the Territory of the Yamal-Nenets Autonomous Area

The problem of mercury input and its further distribution in the Arctic environment is actively debated, especially in recent times, due to the observed processes of permafrost thawing causing the enhanced release of mercury into the Arctic atmosphere and further distribution in the terrestrial and aquatic ecosystem. The atmospheric mercury deposition occurs via dry deposition and wet scavenging by precipitation events. Here we present a study of Hg in wet precipitation on the remote territory of the Russian Arctic; the data were obtained at the monitoring stations Nadym and Salekhard in 2016–2018. Mercury pollution of the Salekhard atmosphere in cold time is mainly determined by regional and local sources, while in Nadym, long-range transport of mercury and local fuel combustion are the main sources of pollutants in the cold season, while internal regional sources have a greater impact on the warm season. Total mercury concentrations in wet precipitation in Nadym varied from &lt;0.5 to 63.3 ng/L. The highest Hg concentrations in the springtime were most likely attributed to atmospheric mercury depletion events (AMDE). The contributions of wet atmospheric precipitation during the AMDE period to the annual Hg deposition were 16.7% and 9.8% in 2016/2017 and 2017/2018, respectively. The average annual volume-weighted Hg concentration (VWC) in the atmospheric precipitation in Nadym is notably higher than the values reported for the remote regions in the Arctic and comparable with the values obtained for the other urbanized regions of the world. Annual Hg fluxes in Nadym are nevertheless close to the average annual fluxes for remote territories of the Arctic zone and significantly lower than the annual fluxes reported for unpolluted sites of continental-scale monitoring networks of the different parts of the world (USA, Europe, and China). The increase of Hg deposition flux with wet precipitation in Nadym in 2018 might be caused by regional emissions of gas and oil combustion, wildfires, and Hg re-emission from soils due to the rising air temperature. The 37 cm increase of the seasonally thawed layer (STL) in 2018 compared to the 10-year average reflects that the climatic changes in the Nadym region might increase Hg(0) evasion, considering a great pool of Hg is contained in permafrost.

Mercury accumulation in leaves of different plant types – the significance of tissue age and specific leaf area

Abstract. Mercury, Hg, is one of the most problematic metals from an environmental perspective. To assess the problems caused by Hg in the environment, it is crucial to understand the processes of Hg biogeochemistry, but the exchange of Hg between the atmosphere and vegetation is not sufficiently well characterized. We explored the mercury concentration, [Hg], in foliage from a diverse set of plant types, locations and sampling periods to study whether there is a continuous accumulation of Hg in leaves and needles over time. Measurements of [Hg] were made for deciduous and conifer trees in Gothenburg, Sweden (botanical garden and city area), as well as for evergreen trees in Rwanda. In addition, data for wheat from an ozone experiment conducted at Östad, Sweden, were included. Conifer data were quantitatively compared with literature data. In every case where older foliage was directly compared with younger, [Hg] was higher in older tissue. Covering the range from the current year up to 4-year-old needles in the literature data, there was no sign of Hg saturation in conifer needles with age. Thus, over timescales of approximately 1 month to several years, the Hg uptake in foliage from the atmosphere always dominated over Hg evasion. Rwandan broadleaved trees had generally older leaves due to lack of seasonal abscission and higher [Hg] than Swedish broadleaved trees. The significance of atmospheric Hg uptake in plants was shown in a wheat experiment where charcoal-filtrated air led to significantly lower leaf [Hg]. To search for general patterns, the accumulation rates of Hg in the diverse set of tree species in the Gothenburg area were related to the specific leaf area (SLA). Leaf-area-based [Hg] was negatively and non-linearly correlated with SLA, while mass-based [Hg] had a somewhat weaker positive relationship with SLA. An elaborated understanding of the relationship behind [Hg] and SLA may have the potential to support large-scale modelling of Hg uptake by vegetation and Hg circulation.

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 HgII 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.

Interannual Variability of Air-Sea Exchange of Mercury in the Global Ocean: The "Seesaw Effect" in the Equatorial Pacific and Contributions to the Atmosphere.

Air-sea exchange of gaseous elemental mercury (Hg(0)) is influenced by different meteorological factors and the availability of Hg in seawater. Here, we use the MITgcm ocean model to explore the interannual variability of this flux and the influence of oceanographic and atmospheric dynamics. We apply the GEOS-Chem model to further simulate the potential impact of the evasion variability on the atmospheric Hg levels. We find a latitudinal pattern in Hg(0) evasion with a relatively small variability in mid-latitudes (3.1-6.7%) and a large one in the high latitudes and Equator (>10%). Different factors dominate the patterns in the equatorial (wind speed), mid- (oceanic flow and temperature), and high-latitudinal (sea-ice, temperature, and dynamic processes) oceans. A seesaw pattern of Hg(0) evasion anomaly (±5-20%) in the equatorial Pacific is found from November to next January between El Niño and La Niña years, owing to the anomalies in wind speed, temperature, and vertical mixing. Higher atmospheric Hg level (2%-5%) are simulated for Hg(0) evasion fluxes with three-month lag, associated with the suppression of upwelling in the beginning of the El Niño event. Despite of the uncertainties, this study elucidates the spatial patterns of the interannual variability of the ocean Hg(0) evasion flux and its potential impact on atmospheric Hg levels.

Mercury Reduction by Nanoparticulate Vivianite.

Mercury (Hg) is a toxic trace element of global environmental concern which has been increasingly dispersed into the environment since the industrial revolution. In aquatic and terrestrial systems, Hg can be reduced to elemental Hg (Hg0) and escape to the atmosphere or converted to methylmercury (MeHg), a potent neurotoxin that accumulates in food webs. FeII-bearing minerals such as magnetite, green rusts, siderite, and mackinawite are recognized HgII reducers. Another potentially Hg-reducing mineral, which commonly occurs in Fe- and organic/P-rich sediments and soils, is the ferrous iron phosphate mineral vivianite (FeII3(PO4)2·8H2O), but its reaction with HgII has not been studied to date. Here, nanoparticulate vivianite (particle size ∼ 50 nm; FeII content > 98%) was chemically synthesized and characterized by a combination of chemical, spectroscopic, and microscopic analyses. Its ability to reduce HgII was investigated at circumneutral pH under anoxic conditions over a range of FeII/HgII ratios (0.1–1000). For FeII/HgII ratios ≥1, which are representative of natural environments, HgII was very quickly and efficiently reduced to Hg0. The ability of vivianite to reduce HgII was found to be similar to those of carbonate green rust and siderite, two of the most effective Hg-reducing minerals. Our results suggest that vivianite may be involved in abiotic HgII reduction in Fe and organic/P-rich soils and sediments, potentially contributing to Hg evasion while also limiting MeHg formation in these ecosystems.