August 2015. Mercury isotope measurements at Toolik Field Station
By Martin Jiskra. During the visit to Toolik Field Station in August, Daniel and Martin installed a new setup to collect air samples for mercury isotope measurements. Martin works as a Postdoc at Geosciences Environment Toulouse in France (CNRS) on a project called: "Understanding the fate of Arctic atmospheric mercury (Hg) deposition – A Hg stable isotope investigation of redox processes and Hg re-emissions" in collaboration with Daniel’s group. The sampling campaign will run through the winter for one year. The goal of the project is to measure the variation of stable mercury isotopes at Toolik Field station in the Arctic tundra in order to gain additional insights into the processes governing the mercury exchange between the atmosphere and snow and soils.
Funding provided by EU Horizon 2020 (Marie Curie fellowship to Martin Jiskra)
August 2015. New constraints on terrestrial surface-atmosphere fluxes of gaseous elemental mercury using a global database
By Yannick Agnan. Gaseous elemental mercury (Hg0) is a semi volatile chemical easily exchanged between terrestrial surfaces and the atmosphere. In spite of nearly 40 years of research, major uncertainties still exist concerning factors controlling this exchange and the influence of methodology used (dynamic flux chambers and micrometeorological methods). 132 published papers were compiled in a global database, including ca. 200,000 estimated Hg0 flux measurements. The authors (Y. Agnan, T. Le Dantec, C. Moore, G. Edwards, and D. Obrist) showed that measurements were unevenly distributed. Fluxes measured in Hg enriched sites were driven by substrate Hg concentrations, as observed by a log relationship which is absent in background areas. Flux over litter- and snow-covered soils were lower than measurements over bare soils. The annual worldwide estimates based on land covers were 523 Mg a−1 (with a large 25% uncertainties from −592 to 1,557 Mg a−1), including 129 Mg a−1 for background landscapes, 261 Mg a−1 for atmospherically-influenced sites (low substrate Hg concentration, but atmospheric Hg0 concentration > 3 ng m−3), and 133 Mg a−1 for contaminated, naturally-enriched, and mining areas. These results were recently published for critical review in Environmental Science & Technology.
Funding provided by NFS (Coupled Natural and Human Systems)
August 2015. Dalton Highway transect sampling in northern Alaska to provide new insights into spatial distribution of mercury in vegetation and soils of tundra ecosystems
By Christine Hedge. Christine traveled to northern Alaska for her first field visit with Daniel to perform sampling for her PhD research. In addition to sampling vegetation, soil, and runoff at the Toolik Field Station, sampling was done along a northern transect extending from Toolik Field Station to the Arctic Ocean along the Dalton Highway. Eight sites in total were sampled along the transect in areas where previous soils data on carbon, nitrogen, and other components had been measured to leverage this investigation upon. Analysis of Hg in the terrestrial landscape of these tundra sites will allow Christine to investigate the spatial distribution of mercury from an inland to coastal region in an area where almost no data on Hg within soils exists. Additional investigations from this transect sampling will include understanding how atmospheric deposition of Hg may change along this gradient and how other factors such as soil age, soil depth, carbon content, soil chemistry, and plant cover may influence storage and cycling of Hg within tundra ecosystems.
Soil pit (top) and portion of a permafrost core (bottom) sampled along the Dalton Highway transect.
Funding provided by NFS (Polar Programs) and Alaska National Park Service
August 2015. Arctic Hg cycling project at Toolik Field station in northern Alaska enters second year of continuous field measurement
By Daniel Obrist. Christine and Daniel recently visited Toolik Field station to re-set and calibrate systems and instruments to begin a second year of field measurements on the north-slope of the Brooks Range in northern Alaska. As part of a project entitled “Collaborative Research: Soil−Snow−Atmosphere Exchange of Mercury in the Interior Arctic Tundra” (collaboration with D. Helmig’s group at the University of Colorado, Boulder), we have been measuring trace gas exchanges of Hg and other trace gases between soils, snow, plants, and the atmosphere for over a year. In addition, we are characterizing other deposition fluxes, soil and snow chemistry, and soil solution and stream characterization. We are excited to enter the second winter of our observations, and are in the process of analysis of a first full year of unique observations in the arctic tundra.
Funding provided by NFS (Polar Programs)
July 2015. Participation and publication with the Western North American Mercury Synthesis Working Group
By Daniel Obrist. Daniel was involved in a synthesis effort by the “Western North American Mercury Synthesis Working Group” to assess mercury impacts across the west. As part of this effort, he attended two workshops at the John Wesley Powell Center for Analysis and Synthesis in Fort Collins, CO, and took the lead on a focus paper describing terrestrial mercury distribution patterns across nine states of the contiguous Western United States. His manuscript entitled “Terrestrial mercury in the western United States: spatial distribution defined by land cover and plant productivity” and authored by several colleagues (Daniel Obrist, Chris Pearson, Jackson Webster, Tyler Kane; Che-Jen Lin; George R. Aiken, and Charles N. Alpers) was recently submitted to Science of the Total Environment. A key result of the analysis is that vegetation patterns have strong effects on spatial distribution patterns of soil mercury across the Western U.S., likely driven by plant productivity. These effects result in 2.5 times higher soil Hg concentrations in forested areas compared to barren locations.
Funding provided by the DRI (Division of Atmospheric Sciences) and the USGS
June 2015. Participation in the 12th annual International Conference of Mercury as a Global Pollutant in Jeju, South Korea
By Christine Hedge. Christine attended the 12th International Conference of Mercury as a Global Polluant in Jeju, South Korea where she gave an oral presentation entitled “Soil and Plant Mercury Concentration and pools in northern Alaska. Her presentation included initial results of analysis on soil and vegetation from the Toolik Field Station (TFS) in northern Alaska, the main study site of her project. Highlights from the initial results showed surprisingly high plant Hg deposition rates of 17.3 µg m−2 yr−1 which is comparable to rates found in temperate regions. Within the soils, organic Hg concentrations at TFS are similar to what is observed in temperate regions however the mineral horizons show concentrations of 2−3 times what is observed in the lower 48 states. Over 90% of the mercury is stored in the mineral layers of soil at Toolik which is a very high pool of mercury that we may have to be concerned about with thawing of permafrost soils.
June 2015. New paper on snowpack nutrient and pollutant loads in the Lake Tahoe basin
By Daniel Obrist. Chris Pearson (former Graduate Student in Graduate Program of Hydrologic Sciences) is the first author of a new article published in Biogeosciences entitled “Nutrient and mercury deposition and storage in an alpine snowpack of the Sierra Nevada, USA”. Chris and his co-authors (Rina Schumer, Ben Trustman, Karl Rittger, Dale Johnson, and Daniel Obrist) chemically characterized snowpack in the Lake Tahoe basin over the duration of two winters. Their results show a key importance of organic nitrogen composing on average almost 50% of total nitrogen observed in snowpack. Detailed spatial and temporal snowpack sampling revealed that nitrate was supplied relatively evenly across the basin via w et deposition, and that ammonia/ nitrogen was driven by dry deposition with was particularly enhanced in late winter and spring and along the western side of the basin. Snowpack phosphorus was strongly enhanced near urban areas in the Lake Tahoe basin. Mercury showed low but detectable snowpack levels, and the fact that snowpack concentrations were significantly lower lower compared to measured wet deposition revealed an important role of photochemical re-emissions after snow-related deposition.
Funding provided by the USGS (State Water Research Program)
May 2015. Part-time appointment as scientific collaborator in Switzerland
March 2015. Paper on polycyclic aromatic hydrocarbons in forest soils accepted in Chemosphere
By Daniel Obrist. A new article was published in Chemosphere. entitled “Accumulation of polycyclic aromatic hydrocarbons (PAHs) and oxygenated PAHs (OPAHs) in organic and mineral soil horizons from four U.S. remote forests”. In this article, authored by Daniel Obrist, Barbara Zielinksa (both DRI), and Judith Perlinger (Michigan Technological University), we describe the distribution of 23 PAH and 9 OPAH compounds across four remote forests of the United States. We found that forest soils accumulate significant levels of PAHs in litter and soil horizons, and that the accumulation was related to the water solubility of compounds. Highly toxic OPAHs, which are also emitted during combustion processes and in addition form in-situ in soils, show higher accumulation compared to their parent-PAHs, particularly in decomposed litter and in soil layers. These patterns suggest that OPAHs have a high persistency in litter and soils and a strong sorption behavior to organic matter.
Funding provided by the DRI (Division of Atmospheric Sciences) and EPA STAR
March 2015. Funding provided by the Alaska National Park Service to extend research on the terrestrial cycling of mercury to national parks in northern Alaska
By Christine Hedge. Christine was awarded a scholarship from the Alaska National Park Service and Murie Science and Learning Center to extend her research on mercury pollutant cycling in tundra ecosystems to include areas in and near parks in northern Alaska. Sampling locations include Denali National Park, the Noatak National Preserve, and the Arctic National Wildlife Refuge. The first research objective is to understand how Hg is distributed spatially across different tundra systems of Alaska, in particular in soils and plants, to determine what areas are affected most by atmospheric deposition. Second, is to quantify the pools of Hg stored in tundra ecosystems to understand if tundra ecosystems serve as important repositories, storing current and past atmospheric Hg pollution that has been transported to the Arctic. Finally, she will study the link between soil Hg concentrations and Hg in aqueous phase (soil solution and adjacent streams) to assess how runoff is linked to upland soil concentrations and understand what controls the transport and mobilization of this pollutant across different tundra regions.
Funding provided by NFS (Polar Programs) and Alaska National Park Service
February 2015. First-ever eddy covariance measurements of gaseous elemental mercury published in Environmental Science & Technology
By Chris Moore. Ashley Pierce (former graduate student) was first author and led the publication in Environmental Science & Technology of results from a field campaign that we conducted in Palomino Valley, NV using our newly developed cavity ring-down spectrometer system. Publication of this data was extremely challenging and involved a whole team of international scientists, including Chris Moore (DRI), Georg Wohlfahrt (University of Innsbruck, Austria), Lukas Hortnagl (ETH Zurich, Switzerland), Natascha Kljun (Swansea University, U.K.) and Daniel Obrist (DRI). The results are extremely important as until this system measurement of gaseous elemental mercury fluxes via the highly effective eddy covariance technique was out of reach of current instrumentation. However, these measurements were made over contaminated sources and more work is needed to get the sensitivity needed for background soils/surfaces.
Funding provided by NFS (Major Instrumentation)
December 2014. Large chamber to investigate oxidation of mercury
By Dean Howard. Measurements taken in Australia by Macquarie University have shown localised, overnight depletion of atmospheric mercury in the presence of fog and dew. The lack of available photochemical mechanisms and of strong halogen sources during these events suggest that they are separate in nature to atmospheric mercury depletion events (AMDEs) that have been observed elsewhere around the globe. Previous laboratory studies have suggested that ozone may be a significant oxidant of mercury in the presence of liquid water and field studies undertaken elsewhere have shown that mercury may be contained within fog droplets. This research utilises a large reaction chamber in order to investigate oxidation of atmospheric mercury by ozone in the presence of fog and dew in order to provide a possible mechanism for the events observed in Australia.
December 2014. DRI partners with Michigan Tech, MIT, and Boston University to address the northbound flow of airborne pollutants
By Daniel Obrist. Half of the people in Greenland have toxic levels of PCBs in their blood. A harmful cocktail of contaminants, including mercury and dioxin, has led to fish consumption advisories in all of the Great Lakes, including Superior. Pollutants like these find their way north via a complex web of human and natural systems. Now, a team of scientists, led by Michigan Technological University’s Judith Perlinger, are working on project to better understand how those systems interact and find ways to address the problem. This project combines the complex behavior of these chemicals in the environment with social adaptations and governance issues to ultimately reduce the impacts of these pollutants. Known as atmosphere-surface exchangeable pollutants, or ASEPs, the scientists will study mercury, polychlorinated biphenyl compounds (PCBs) and polycyclic aromatic hydrocarbons (PAHs). These pollutants, originating from the Northern Hemisphere at lower latitudes, migrate to higher latitudes and stay in the environment for a long time and, finally, tend to accumulate in people and wildlife. First, the team members will estimate where the pollutants originate, describe the natural systems that transport them north, and identify where the pollutants finally land. Their models will offer predictions through the year 2050 and will account for the effects of climate change and changes in land use and cover and government policy relating to ASEPs. Secondly, they will model the economic impacts of the pollutants throughout the US, focusing especially on the Northeast and Great Lakes. They will examine specific human health effects and associated economic consequences of consuming mercury-contaminated fish. Finally, they will identify the many stakeholders in the Great Lakes region, from community and sportsman’s groups to government agencies and tribes, and propose strategies for them to work together to help address the problem. The project also has an educational component. The team is teaching an inter-University distributed course this spring called Communicating Wicked Environmental Problems.
Funding provided by NFS (Coupled Natural and Human Systems)
August 2014. Review on air impacts of natural gas acquisition, processing, and use published in Environmental Science & Technology
By Chris Moore. Due to advances in drilling technology, there has been a rush to develop many new, previously unattainable natural gas reservoirs. However, in the rush drill many environmental impacts of these activities have been ignored. We recently published article entitled “Air Impacts of Increased Natural Gas Acquisition, Processing and Use: A Critical Review” in Environmental Science & Technology to summarize these important effects for the atmosphere. The paper, led by Chris Moore and including Barbara Zielinska (DRI), Gabrielle Petron (NOAA), and Robert Jackson (Stanford) summarizes the very slim scientific information on the air quality impacts of these activities and offers specific recommendations for future study of these impacts. This work has been well received and as of August 2015 has already been cited 16 times (according to Web of Science).
Funding provided by NFS (AirWaterGas Sustainability Reserach Network)
July 2014. Soil-snow-atmosphere exchange of mercury in the interior arctic tundra
By Daniel Obrist. The goal of this project is to characterize soil-snow-atmosphere dynamics of mercury (Hg) in the snow-dominated Arctic tundra. Chemical conversion of Hg in snowpack from non-volatile forms to gaseous elemental mercury (GEM) can lead to substantial degassing of Hg from snow, thereby reducing the impact of atmospheric deposition. Contrary to the GEM chemistry seen in the mid-latitude snowpack, preliminary observations from Toolik Lake, on the north slope of the Brooks Range, Alaska, provide evidence that photochemical GEM formation and degassing are suppressed in tundra snow and that for much of the winter, interstitial GEM is actually converted into non-volatile Hg. These patterns result in extended periods when interstitial snowpack air is depleted in GEM. If confirmed, this chemistry would likely signify a net transfer of atmospheric GEM to snow or underlying soils, thereby increasing Hg deposition to tundra ecosystems. Proposed project objectives are to investigate (1) the frequency and underlying processes that determine GEM depletion and formation in arctic snowpack and tundra soils; (2) the degree to which GEM dynamics cause vertical Hg exchange between soils, snow, and the atmosphere; and (3) how these processes provide additional sources − or sinks − of Hg via atmosphere-surface transfer and snowmelt input. GEM concentrations in soils, snow, and air, as well as vertical exchanges, will be characterized at Toolik Field Station. Measurements will be made by means of a snow-sampling manifold system allowing for fully automated and continuous all-winter measurements of trace gases at multiple depths in the undisturbed snowpack and the atmosphere. These experiments will be supplemented by flux chamber measurements to assess the contribution of the underlying tundra soils. Other trace gas observations, and chemical characterization of soil, snow, melt water, and soil water will be incorporated to assess the environmental and biogeochemical controls on GEM dynamics and the Hg budget. This proposed research will leverage ongoing LTER and NEON projects at the Toolik Field station, providing linkages between in-snow processes, tundra soil and freshwater biogeochemical cycling, pollution import into the Arctic, and ecosystem processes. The project will directly involve high school, undergraduate, graduate students, and a postdoctoral scientist. It will expand an existing partnership with local high school chemistry classes through research presentations in classrooms, laboratory tours, and data analyses using study results. Dissemination to the scientific community will be accomplished through peer-reviewed publications and conference presentations, and by communication with U.S. and international regulatory agencies. The general public will be reached through news releases, institutional publications, open house events, and a web site. Data will be archived at the National Snow and Ice Data Center at the University of Colorado for distribution to the national and international polar research community.
Funding provided by NFS (Polar Programs)
November 2013. Impacts of sea ice dynamics on atmospheric cycling of mercury and ozone
By Chris Morre. As the Minamata Convention to curb global mercury (Hg) pollution remains to be signed, understanding Hg processes is important to the science support of this global treaty. In the Arctic, Hg and ozone (O3) is rapidly removed from the boundary layer during atmospheric Hg and O3 depletion events (AMDEs and ODEs), leading to destruction of O3 along with oxidation of gaseous elemental Hg0 to form oxidized HgII and its subsequent deposition to snow and ice. Our studies have used the results from two Arctic field campaigns near Barrow, Alaska, to show that coastal AMDEs and ODEs are directly linked to sea ice dynamics. We have used ground-based measurements, satellite imagery, and transport modeling to show that consolidated ice cover facilitates AMDEs and ODEs, which then immediately cease upon interactions with open sea ice leads (long fractures in sea ice exposing open water). We attribute the highly dynamic responses of Hg0 and O3 to lead-induced convection in the stable Arctic boundary layer forcing Hg0 and O3 from un-depleted air masses aloft into the boundary layer. This convective forcing provides fresh Hg0 to the surface layer where it is subject to renewed oxidation and deposition, while supplying additional O3 that can alter atmospheric chemistry. The ongoing regime shift of Arctic sea ice from perennial to mostly seasonal ice promotes sea ice lead activity, thereby increasing net Hg0 deposition to fragile Arctic ecosystems. Results of this study are published in Nature.
Funding provided by NASA (Cryospheric Sciences Program) and the DRI (Division of Atmospheric Sciences )
July 2013. Atmospheric simulation chamber for studying mercury chemistry
By Steven Darby/Ashley Pierce. Mercury is a pollutant that can be transported easily through the atmosphere. Typically the concentration in the atmosphere is low – the problems start when mercury is deposited to the surface where it may bioaccumulate through the food chain. The mechanism of deposition is not well known, but some studies have indicated that reactions involving bromine are key. These studies are often carried out at room temperature, with very large concentrations that are not found in the environment. In this project we tested various mercury reactions directly at low concentrations and low temperatures that are more representative of the conditions in the environment. To do this we constructed a 300 L FEP chamber, temperature controlled between −10 to +30°C. A Cavity Ring-Down Spectroscopy (CRDS) system quantifies gaseous elemental mercury (GEM) in situ with a time resolution of 25 Hz. Two more optical cavities use Incoherent Broadband Cavity-Enhanced Absorption Spectroscopy (IBBCEAS) to measure Br2, NO2, and BrO. Measurements also included the addition of NO2, O3 and sea salt aerosols into the chamber with GEM and Br2. The goal of this project was to better quantify the reaction kinetics for these compounds.
Funding provided by Science Foundation Ireland and NSF (Major Instrumentation)
April 2013. Development of a cavity ring-down spectroscopy sensor for measuring atmospheric gaseous elemental mercury concentration
By Ashley Pierce. Due to our lack of understanding of parts of the global Hg cycle it is important to improve available measurement techniques. Current instruments for measuring Hg and specifically gaseous elemental mercury (GEM) take several minutes to analyze and sever liters of air per sample. GEM has a longer atmospheric residence time than other forms of Hg and this coupled with its semi-volatile nature means GEM can travel long distances where it can then be deposited to remote ecosystems and eventually transformed into bioavailable toxic forms of Hg (i.e. methylmercury). Deposition is the main source of Hg to remote ecosystems. Our lab has developed a cavity ring-down spectroscopy (CRDS) sensor for measuring atmospheric GEM concentrations. Using a laser system, that outputs the wavelength that GEM absorbs (253.65 nm), we coupled the laser beam into a measurement cavity with a mirror at each end. The laser pulse bounces between the two mirrors effectively creating a long path length and high sensitivity while keeping the sample size small. Differential measurements allow for continuous correction of system baseline extinction losses. The CRDS sensor is used to measure GEM in ambient air and laboratory studies at 25 Hz, as well as to measure GEM fluxes using the eddy covariance (EC) flux measurement technique which is the first application of EC for GEM. With these measurements we aim to improve the quantification of dry deposition of GEM which is confounded by the bi-directional movement of GEM between surfaces and the atmosphere, to better measure fast concentration fluctuations from sources and sinks and during chemical transformations. Results of this study are published in Atmospheric Measurement Techniques.
Funding provided by NSF (Major Instrumentation)
March 2013. Response of soil mercury stocks to climate change
By Oleksandra Hararuk. Most of the mercury contained in an ecosystem resides in soils and is closely associated with soil carbon. Soil carbon is sensitive to changes in environmental factors, such as atmospheric CO₂, temperature and rainfall, all of which will change in the coming 100 years as projected by multiple earth system models. Apart from association with soil carbon mercury is associated with precipitation, as it is a major source of mercury deposition into an ecosystem. Using a present-day empirical relationship between mercury to carbon ratio and latitude, precipitation, and clay, we explored the sensitivity of soil mercury to climate change using a global earth system model to run the climate change scenarios. The results, published in Biogeosciences, indicate that soil mercury may increase by as much as 150% from the present-day values after 100 years of increasing CO₂, temperatures, and precipitation; or decrease up to 50% under increased CO₂, temperatures, and decreased precipitation.
Funding provided by EPA STAR
November 2012. Quantifying nutrient and mercury concentrations and loads in Lake Tahoe snowpack
By Chris Pearson. Lake Tahoe clarity has declined from 30 m readings in the 1960s to approximately 20 m today. Eutrophication from atmospheric and terrestrial nitrogen (N) and phosphorus (P) as well as light scattering by particulate inputs are the main causes of this decline. Along with N and P, mercury (Hg) is well-known pollutant due to its toxicity and ability to bio-accumulate in aquatic ecosystems. Previous studies have focused on direct deposition of nutrients and pollutants to the lake surface and little temporal or spatial data exists on the dynamics of atmospheric inputs to the basin’s snowpack. High variability in atmospheric deposition rates throughout the basin caused by complex terrain, precipitation gradients, and pollutant origins make lake level or single measurements insufficient to estimate total basin wide loads. My research will clarify sources of atmospheric N, P, and Hg input to the Lake Tahoe watershed through measurement and modeling of concentrations and loads within the basin’s snowpack. From the first snowfall until the end of melting, snowpack, covering 314 sq. miles of the basin, acts as a temporary storage for water and constituents that accumulate throughout winter and spring. With approximately 70 percent of precipitation falling as snow during the winter and spring months, atmospheric deposition contained in the basin’s snowpack is potentially a large driver of runoff and ground water pollutant levels. We directly measure N, P, and Hg concentrations and loads in Tahoe snowpack throughout the basin. These measurements include integrated snowpack samples, wet deposition collection, and storm-based surface samples to help quantify pollutant sources (i.e. in-basin/out-of-basin, wet/dry deposition) and volatilization loses. With these measurements, basin wide loads from winter and spring atmospheric deposition will be spatially mapped, quantified, sourced, and connected to precipitation levels to allow for improved protection and management of lake quality.
Funding provided by the DRI and the USGS (State Water Research Institute Program)
March 2012. Development of a cold plate sampler to measure atmospheric mercury and volatile organic compounds
By Chris Moore. We have recently designed and initially tested a cold plate sampler (CPS) for daily collection of vapor-phase deposition (i.e., frost). The center piece of our CPS consists of a Peltier cooling element that cools an exposed stainless steel plate; warm air is dissipated away from the collection surface by a fan through a chimney on the warm side of the Peltier element, which prevents the fan from icing in cold conditions; and the cold plate is mounted upside-down in an enclosure for protection against the elements. As part of a NASA/JPL-funded study – "The Bromine, Ozone, and Mercury Experiment (BROMEX)" – in Barrow, Alaska, we performed initial testing of this CPS to force deposition of water vapor onto the stainless steel surface cooled far below the dew point. At the cold exposed surface, water vapor deposits from ambient air and generates an ice (frost) layer that can be sampled daily using trace-metal protocols. Our results showed a linear relationship (r2 = 0.32) between total Hg concentrations in collected ice deposition and atmospheric gaseous oxidized mercury (GOM) concentrations. While not perfect, the correlations do not account for environmental variables (such as wind speed and surface temperature) that are expected to influence GOM collection efficiency of the CPS (see below). Because GOM is the most water-soluble form of atmospheric Hg, we hypothesize that Hg accumulation may be due to dissolution of GOM in a quasi-liquid layer during formation of new ice. Although these initial tests were performed in the Arctic, we believe after further extensive testing that this technology may be deployed at almost any ambient temperature to act as surrogate surface measurement technique.
Funding provided by the DRI (Division of Atmospheric Sciences – EDGES)
September 2011. Accumulation and fate processes of mercury in terrestrial ecosystems
By Daniel Obrist. Terrestrial ecosystems serve as a key link between atmospheric deposition of Hg – the main pathway for Hg input in remote ecosystems – and mobilization to streams, lakes, and oceans where Hg pollution impacts wildlife and humans through consumption of Hg-laden fish. Soils account for more than 90% of Hg stored in terrestrial ecosystems, and top soil Hg pools (approximately 40 cm) are estimated to be as high as 15,230 metric tons in the U.S., or, if extrapolated globally, may account for more than 300,000 metric tons. This large pool size stems in part from natural, geologic sources but also is due to a large legacy of past anthropogenic pollution released across centuries that has accumulated in soils. Atmospheric Hg deposition is efficiently retained in upper soils, in particular bound in humus-rich layers. Due to the semi-volatile nature of elemental mercury (Hg0), however, terrestrial Hg also partially volatilizes into the atmosphere, a process accounting for up to one-third of total atmospheric Hg emissions. Our studies aim to characterize fate processes of mercury in soils, litter, and vegetation to understand the effects of large terrestrial Hg pools for future atmospheric mercury loads and mobilization to watersheds. This is done by field investigations and experimental laboratory studies to assess the behavior of mercury in soils, litter, and vegetation. More information in published articles: Environmental Science & Technology, Biogeosciences, and Science of the Total Environment.
Funding provided by EPA STAR
November 2010. Mercury in the Dead Sea
By Daniel Obrist. The atmosphere over the Dead Sea is laden with oxidized mercury. Some of the highest levels of oxidized mercury ever observed outside the polar regions exist there. The results published in Nature Geoscience. In the research, Daniel Obrist and colleagues at DRI and at Hebrew University in Isreal measured several periods of extremely high atmospheric oxidized mercury. Mercury exists in the atmosphere in an elemental and in an oxidized state. High levels of oxidized mercury are a concern because this form is deposited quickly in the environment after its formation. Observations of high naturally-occurring oxidized mercury levels had been limited to the polar atmosphere. There, oxidized mercury is formed during a process called atmospheric mercury depletion events: elemental mercury is converted to oxidized mercury, which is then readily deposited on surfaces. Now, we have found near-complete depletion of elemental mercury – and formation of some of the highest oxidized mercury levels ever seen – above the Dead Sea, a place where temperatures reach 45˚C. Such pronounced mercury depletion events were unexpected outside the frigid poles. High temperatures were thought to impede this chemical process. "Elemental mercury is somewhat resistant to oxidation, so it's been difficult to explain levels of oxidized mercury measured in the atmosphere outside polar regions," says Alex Pszenny, director of NSF's Atmospheric Chemistry Program, which funded the research. These new results provide an explanation. The mechanisms involved in the conversion of mercury above the Dead Sea appear similar, however, to those in polar regions: both start with halogens. Halogens, or halogen elements, are non-metal elements such as fluorine, chlorine, bromine and iodine. Observations and modeling results indicate that at the Dead Sea, the conversion of elemental mercury is driven by bromine. The new results show that bromine levels observed above oceans may be high enough to initiate mercury oxidation. We discovered that bromine can oxidize mercury in the mid-latitude atmosphere far from the poles. That points to an important role of bromine-induced mercury oxidation in mercury deposition over the world's oceans. What goes into the ocean may eventually wind up in its fish. And in those who eat them.
Funding provided by the DRI and the USGS (State Water Research Institute Program)