Mercury (Hg) researchers have made progress in understanding atmospheric Hg, especially with respect to oxidized Hg (Hg-II) that can represent 2 to 20% of Hg in the atmosphere. Knowledge developed over the past similar to 10 years has pointed to existing challenges with current methods for measuring atmospheric Hg concentrations and the chemical composition of Hg-II compounds. Because of these challenges, atmospheric Hg experts met to discuss limitations of current methods and paths to overcome them considering ongoing research. Major conclusions included that current methods to measure gaseous oxidized and particulate-bound Hg have limitations, and new methods need to be developed to make these measurements more accurate. Developing analytical methods for measurement of Hg-II chemistry is challenging. While the ultimate goal is the development of ultrasensitive methods for online detection of Hg-II directly from ambient air, in the meantime, new surfaces are needed on which Hg-II can be quantitatively collected and from which it can be reversibly desorbed to determine Hg-II chemistry. Discussion and identification of current limitations, described here, provide a basis for paths forward. Since the atmosphere is the means by which Hg is globally distributed, accurately calibrated measurements are critical to understanding the Hg biogeochemical cycle.

Anthropogenic releases of mercury (Hg)1-3 are a human health issue4 because the potent toxicant methylmercury (MeHg), formed primarily by microbial methylation of inorganic Hg in aquatic ecosystems, bioaccumulates to high concentrations in fish consumed by humans5,6. Predicting the efficacy of Hg pollution controls on fish MeHg concentrations is complex because many factors influence the production and bioaccumulation of MeHg7-9. Here we conducted a 15-year whole-ecosystem, single-factor experiment to determine the magnitude and timing of reductions in fish MeHg concentrations following reductions in Hg additions to a boreal lake and its watershed. During the seven-year addition phase, we applied enriched Hg isotopes to increase local Hg wet deposition rates fivefold. The Hg isotopes became increasingly incorporated into the food web as MeHg, predominantly from additions to the lake because most of those in the watershed remained there. Thereafter, isotopic additions were stopped, resulting in an approximately 100% reduction in Hg loading to the lake. The concentration of labelled MeHg quickly decreased by up to 91% in lower trophic level organisms, initiating rapid decreases of 38-76% of MeHg concentration in large-bodied fish populations in eight years. Although Hg loading from watersheds may not decline in step with lowering deposition rates, this experiment clearly demonstrates that any reduction in Hg loadings to lakes, whether from direct deposition or runoff, will have immediate benefits to fish consumers.

This review focuses on providing the history of measurement efforts to quantify and characterize the compounds of reactive mercury (RM), and the current status of measurement methods and knowledge. RM collectively represents gaseous oxidized mercury (GOM) and that bound to particles. The presence of RM was first recognized through measurement of coal-fired power plant emissions. Once discovered, researchers focused on developing methods for measuring RM in ambient air. First, tubular KCl-coated denuders were used for stack gas measurements, followed by mist chambers and annular denuders for ambient air measurements. For ~15 years, thermal desorption of an annular KCl denuder in the Tekran® speciation system was thought to be the gold standard for ambient GOM measurements. Research over the past ~10 years has shown that the KCl denuder does not collect GOM compounds with equal efficiency, and there are interferences with collection. Using a membrane-based system and an automated system—the Detector for Oxidized mercury System (DOHGS)—concentrations measured with the KCl denuder in the Tekran speciation system underestimate GOM concentrations by 1.3 to 13 times. Using nylon membranes it has been demonstrated that GOM/RM chemistry varies across space and time, and that this depends on the oxidant chemistry of the air. Future work should focus on development of better surfaces for collecting GOM/RM compounds, analytical methods to characterize GOM/RM chemistry, and high-resolution, calibrated measurement systems.

ADVERTISEMENT RETURN TO ISSUEPREVViewpointNEXTProgress on Understanding Atmospheric Mercury Hampered by Uncertain MeasurementsDaniel A. Jaffe*†‡, Seth Lyman§, Helen M. Amos∥, Mae S. Gustin⊥, Jiaoyan Huang⊥, Noelle E. Selin#, Leonard Levin∇, Arnout ter Schure○, Robert P. Mason◆, Robert Talbot¶, Andrew Rutter∞, Brandon Finley†, Lyatt Jaeglé‡, Viral Shah‡, Crystal McClure‡, Jesse Ambrose†, Lynne Gratz†, Steven Lindberg$, Peter Weiss-Penzias⊗, Guey-Rong Sheu∀, Dara Feddersen⧓, Milena Horvat◘, Ashu DastoorЯ, Anthony J. Hynes@, Huiting Mao∏, Jeroen E. Sonke★, Franz Slemr⧖, Jenny A. Fisher∫, Ralf Ebinghaus∮, Yanxu Zhang×, and Grant Edwards⪫View Author Information† School of Science, Technology, Engineering and Mathematics, University of Washington, Bothell, Washington 98011, United States‡ Department of Atmospheric Sciences, University of Washington, Seattle, Washington 98195, United States§ Bingham Entrepreneurship and Energy Research Center, Utah State University, Vernal, Utah 84078, United States∥ Department of Earth and Planetary Science, Harvard University, Cambridge, Massachusetts 02138, United States⊥ Department of Natural Resources and Environmental Sciences, University of Nevada, Reno, Nevada 89557, United States# Engineering Systems Division and Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 89557, United States∇ Palo Alto, California 94304, United States○ Electric Power Research Institute, Palo Alto, California 94304, United States◆ Department of Marine Sciences & Chemistry, University of Connecticut, Groton, Connecticut 06340, United States¶ Institute for Climate and Atmospheric Science, University of Houston, Houston, Texas 77004, United States∞ Department of Chemistry, Carroll University, Waukesha, Wisconsin 53186, United States$ Graeagle, California 96103, United States⊗ Department of Microbiology and Environmental Toxicology, University of California, Santa Cruz, California 95064, United States∀ Department of Atmospheric Sciences, National Central University, Jhongli, 320, Taiwan⧓ Department of Chemistry, University of New Hampshire, Durham, New Hampshire 03824, United States◘ Department of Environmental Sciences, Jozef Stefan Institute, Ljubljana, SloveniaЯ Air Quality Research Division, Environment Canada, Dorval, Quebec M3H 5T4, Canada@ Division of Marine and Atmospheric Chemistry, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149, United States∏ Department of Chemistry, State University of New York, College of Environmental Science and Forestry, Syracuse, New York 13210, United States★ Observatoire Midi-Pyrénées, Université Paul Sabatier, 31062, Toulouse, France⧖ Atmospheric Chemistry Division, Max Planck Institute for Chemistry, Mainz, Germany∫ Centre for Atmospheric Chemistry, University of Wollongong, Wollongong, New South Wales 2522, Australia∮ Department of Environmental Chemistry, Institute of Coastal Research, Geesthacht, Germany× School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States⪫ Faculty of Science, Macquarie University, Sydney, New South Wales 2109, Australia*E-mail: [email protected]Cite this: Environ. Sci. Technol. 2014, 48, 13, 7204–7206Publication Date (Web):June 18, 2014Publication History Received3 June 2014Published online18 June 2014Published inissue 1 July 2014https://pubs.acs.org/doi/10.1021/es5026432https://doi.org/10.1021/es5026432newsACS PublicationsCopyright © 2014 American Chemical Society. This publication is available under these Terms of Use. Request reuse permissions This publication is free to access through this site. Learn MoreArticle Views2098Altmetric-Citations81LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail PDF (644 KB) Get e-AlertscloseSUBJECTS:Calibration,Fluorescence,Mercury,Organic reactions,Quality management Get e-Alerts

From August 22 to September 16, 2012, atmospheric mercury (Hg) was measured from a common manifold in the field during the Reno Atmospheric Mercury Intercomparison eXperiment. Data were collected using Tekran systems, laser induced fluorescence, and evolving new methods. The latter included the University of Washington-Detector for Oxidized Mercury, the University of Houston Mercury instrument, and a filter-based system under development by the University of Nevada-Reno. Good transmission of total Hg was found for the manifold. However, despite application of standard protocols and rigorous quality control, systematic differences in operationally defined forms of Hg were measured by the sampling systems. Concentrations of reactive Hg (RM) measured with new methods were at times 2-to-3-fold higher than that measured by Tekran system. The low RM recovery by the latter can be attributed to lack of collection as the system is currently configured. Concentrations measured by all instruments were influenced by their sampling location in-the-manifold and the instrument analytical configuration. On the basis of collective assessment of the data, we hypothesize that reactions forming RM were occurring in the manifold. Results provide a new framework for improved understanding of the atmospheric chemistry of Hg.

As part of the Mercury Experiment to Assess Atmospheric Loading in Canada and the U.S. (METAALICUS), different stable Hg(II) isotope spikes were applied to the upland and wetland areas of a boreal catchment between 2001 and 2006 to examine retention of newly deposited Hg(II). In the present study, a Geographical Information Systems (GIS)‐based approach was used to quantify canopy and ground vegetation pools of experimentally applied upland and wetland spike Hg within the METAALICUS watershed over the terrestrial loading phase of the experiment. A chemical kinetic model was also used to describe the changes in spike Hg concentrations of canopy and ground vegetation over time. An examination of the fate of spike Hg initially present on canopy vegetation using a mass balance approach indicated that the largest percentage flux from the canopy over one year post‐spray was emission to the atmosphere (upland: 45%; wetland: 71%), followed by litterfall (upland: 14%; wetland: 10%) and throughfall fluxes (upland: 12%; wetland: 9%) and longer term retention of spike in the forest canopy (11% for both upland and wetland). Average half‐lives (t1/2) of spike on deciduous (110 ± 30 days) and coniferous (180 ± 40 days) canopy and ground vegetation (890 ± 620 days) indicated that retention of new atmospheric Hg(II) on terrestrial (especially ground) vegetation delays downward transport of new atmospheric Hg(II) into the soil profile and runoff into lakes.

Ecosystems that have low mercury (Hg) concentrations (i.e., not enriched or impacted by geologic or anthropogenic processes) cover most of the terrestrial surface area of the earth yet their role as a net source or sink for atmospheric Hg is uncertain. Here we use empirical data to develop a rule-based model implemented within a geographic information system framework to estimate the spatial and temporal patterns of Hg flux for semiarid deserts, grasslands, and deciduous forests representing 45% of the continental United States. This exercise provides an indication of whether these ecosystems are a net source or sink for atmospheric Hg as well as a basis for recommendation of data to collect in future field sampling campaigns. Results indicated that soil alone was a small net source of atmospheric Hg and that emitted Hg could be accounted for based on Hg input by wet deposition. When foliar assimilation and wet deposition are added to the area estimate of soil Hg flux these biomes are a sink for atmospheric Hg.

Although there is now a general consensus among mercury (Hg) biogeochemists that increased atmospheric inputs of inorganic Hg(II) to lakes and watersheds can result in increased methylmercury (MeHg) concentrations in fish, researchers still lack kinetic data describing the movement of Hg from the atmosphere, through watershed and lake ecosystems, and into fish. The use of isotopically enriched Hg species in environmental studies now allows experimentally applied new Hg to be distinguished from ambient Hg naturally present in the system. Four different enriched stable Hg(II) isotope "spikes" were applied sequentially over four years to the ground vegetation of a microcatchment at the Experimental Lakes Area (ELA) in the remote boreal forest of Canada to examine retention of Hg(II) following deposition. Areal masses of the spikes and ambient THg (all forms of Hg in a sample) were monitored for eight years, and the pattern of spike retention was used to estimate retention of newly deposited ambient Hg within the ground vegetation pool. Fifty to eighty percent of applied spike Hg was initially retained by ground vegetation. The areal mass of spike Hg declined exponentially over time and was best described by a first-order process with constants(k) ranging between 9.7 x 10(-40 day(-1) and 11.6 x 10(-4) day(-1). Average halflife (t1/2) of spike Hg within the ground vegetation pool (+/-S.D.) was 704 +/- 52 days. This retention of new atmospheric Hg(II) by vegetation delays movement of new Hg(II) into soil, runoff, and finally into adjacent lakes. Ground-applied Hg(II) spikes were not detected in tree foliage and litterfall, indicating that stomatal and/or root uptake of previously deposited Hg (i.e., "recycled" from ground vegetation or soil Hg pools) were likely not large sources of foliar Hg under these experimental conditions.

A systematic investigation of the impact of current uncertainties in Hg emissions from specific source categories on global air Hg concentrations is presented. First, the uncertainties in different emission source categories are discussed and then the results of a base simulation and three sensitivity simulations conducted with a global chemical transport model for mercury (CTM-Hg) are presented. The total Hg emissions in the four scenarios range from 6600 to 9400Mg/a. The sensitivity studies investigate the impact of the range in uncertainty in natural emissions, emissions of previously deposited Hg, and anthropogenic emissions both in China and worldwide, while taking into account constraints imposed by available data (current/pre-industrial emission ratio of 2–4). In one case, natural emissions and emissions of previously deposited Hg were changed to represent a mid point of the range of values found in the literature. This lead to a 16% increase in background emissions, i.e., natural emissions and emissions of previously deposited Hg combined. Increasing natural emissions by 16% or Chinese anthropogenic emissions by 100% yielded atmospheric Hg concentrations comparable with those measured across the globe without any changes to the atmospheric chemistry. Increasing natural emissions and emissions of previously deposited Hg by 16% and all anthropogenic emissions by 100% as compared to the base scenario yielded atmospheric Hg concentrations that were not compatible with measurements and changes in the chemical behavior of Hg in the atmosphere would be required to yield results that are consistent with observed Hg concentrations. The current uncertainty in total Hg emissions at the global scale is placed at about a factor of two.

This study focused on the development of a seasonal data set of the Hg air/surface exchange over soils associated with low Hg containing surfaces in a deciduous forest in the southern USA. Data were collected every month for 11 months in 2004 within Standing Stone State Forest in Tennessee using the dynamic flux chamber method. Mercury air/surface exchange associated with the litter covered forest floor was very low with the annual mean daytime flux being 0.4±0.5ngm−2h−1 (n=301). The daytime Hg air/surface exchange over the year oscillated between emission (81% of samples with positive flux) and deposition (19% of samples with negative flux). A seasonal trend of lower emission in the spring and summer (closed canopy) relative to the fall and winter (open canopy) was observed. Correlations were found between the air/surface exchange and certain environmental factors on specific days sampled but not collectively over the entire year. The very low magnitude of Hg air/surface exchange as observed in this study suggests that an improved methodology for determining and reporting emission fluxes is needed when the values of fluxes and chamber blanks are both very low and comparable. This study raises questions and points to a need for more research regarding how to scale the Hg air/surface exchange for surfaces with very low emissions.

Mercury air/surface exchange was measured over litter-covered soils with low Hg concentrations within various types of forests along the eastern seaboard of the USA. The fieldwork was conducted at six forested sites in state parks in South Carolina, North Carolina, New Jersey, Pennsylvania, New York and Maine from mid-May to early June 2005. The study showed that the Hg air/surface exchange was consistently very low and similar (overall daytime mean flux=0.2±0.9ngm−2h−1, n=310, for all six sites monitored) with the various forest types. These flux values are comparable with those found in a year-long study in Tennessee (yearly daytime mean=0.4±0.5ngm−2h−1), but lower than many previous flux results reported for background soils. The Hg fluxes at all sites oscillated around zero, with many episodes of deposition (negative fluxes) occurring in both daytime and nighttime. While there were particular days showing significant correlations among the Hg air/surface exchange and certain environmental parameters, perhaps because of the low fluxes encountered, few significant correlations were found for any particular day of sampling between the Hg flux and environmental parameters such as solar radiation, soil temperature, air temperature (little variability seen), relative humidity, and ambient air Hg concentrations. Factors driving the Hg exchange as previously found for enriched soils may not hold for these background litter-covered forest soils. The results suggest that spatial variations of the Hg air/surface exchange were small among these different forest types for this particular time of year.

This paper summarizes recent advances in the understanding of the exchange of Hg between the atmosphere and natural terrestrial surfaces including substrates (soil, rocks, litter-covered surfaces and weathered lithological material) and foliage. Terrestrial landscapes may act as new sources of atmospheric Hg, and as repositories or temporary residences for anthropogenically and naturally derived atmospheric Hg. The role of terrestrial surfaces as sources and sinks of atmospheric Hg must be quantified in order to develop regional and global Hg mass balances, and to assess the efficacy of regulatory controls on anthropogenic point sources in reduction of human Hg exposure.

This paper describes springtime atmospheric mercury (Hg) speciation and snow pack mercury concentration measurements in the McMurdo/Ross Island sea ice region of Antarctica. Near-surface gaseous elemental mercury (GEM) depletions (to concentrations below our detection limit, <0.01ngm−3), similar to those shown to occur in the springtime Arctic, were observed and reactive gaseous mercury (RGM) and fine particulate mercury (FPM) were produced in significant quantities (average 116 and 49pg(Hg)m−3, respectively). GEM concentrations in the near-surface air were significantly enhanced during brief afternoon terrestrial snowmelt events. Snow pack total mercury was significantly elevated (40–430ngl−1), with a maximum at the northern extent of the fast-ice (adjacent to the grease ice/freezing ocean surface), and lesser values towards the coast and on Ross Island, suggesting that, similarly again to recent Arctic results, marine halogens, released by the freezing sea surface, induce localized mercury depletion events. A possible secondary contributing source of local halogens and mercury are direct emissions from the active Ross Island volcano, Mt. Erebus.

Although a positive relationship between atmospheric loadings of inorganic mercury (Hg(II)) to watersheds and concentrations of methyl mercury (MeHg) in fish has now been established, net wet and dry deposition of Hg(II) and MeHg to watersheds remains challenging to quantify. In this study, concentrations and loadings of total mercury (THg; all forms of Hg in a sample) and MeHg in open area wet deposition, throughfall, and litterfall were quantified atthe remote Experimental Lakes Area in the boreal ecoregion, NW Ontario, Canada. Between 1992 and 2006, mean annual THg and MeHg loadings in the open were 36 +/- 17 and 0.5 +/- 0.2 mg ha(-1), respectively. Throughfall THg and MeHg loadings were generally 2-4 times and 0.8-2 times higher, respectively, than loadings in the open. Loadings of both THg and MeHg were highest under an old growth spruce/fir canopy and lowest under a deciduous maple canopy, whereas loadings under young jack pine and wetland spruce/pine/alder canopies were intermediate. Litterfall generally represented the largest input of THg (86-105 mg ha(-1)) and MeHg (0.7-0.8 mg ha(-1)) to the landscape on an annual basis. Using the "direct" method of estimating dry deposition (thoughfall + litterfall - open loadings), we calculated that annual dry deposition of THg and MeHg under forest canopies ranged from 105 to 201 mg ha(-1), whereas dry deposition of MeHg ranged from 0.7 to 1.2 mg ha(-1). Photoreduction and emission of wet-deposited Hg(ll) from canopy foliage were accounted for, resulting in 3-5% (5-6 mg ha(-1)) higher annual estimates of dry deposition than via the direct method alone. NetTHg and MeHg loadings to this remote landscape were lower than at any other previously studied forested site globally. This study shows that THg and MeHg loading can be extremely variable within a heterogeneous boreal landscape and that processes such as Hg photoreduction and emission from foliage should be considered when estimating dry deposition of Hg.

It is well known that mercury (Hg) emission from soils is largely controlled by solar radiation and soil temperature, exhibiting diel cycles that closely follow diel variations of solar radiation. To study soil Hg emission processes, we conducted experiments by measuring soil Hg emission fluxes under controlled conditions in the laboratory with a dynamic flux chamber using outside ambient air as flushing air. Unexpectedly, we observed consistent, recurring diel cycles of Hg emissions from dry soils held at constant temperature in the dark in our laboratory. The peaks of the emissions also seemed subject to some seasonal variation and to respond to local weather conditions with lower flux peaks in wintertime and on cloudy or rainy days. Finally, much lower soil Hg emission fluxes were observed in the presence of Hg-free zero air than in the presence of outside ambient air. It is hypothesized that some unidentified air-borne substance(s) in the ambient air might be responsible for the observed diel cycles of soil Hg emission. Further elaborate mechanistic investigations are clearly needed to test the initial working hypotheses and uncover the cause for this interesting, mysterious phenomenon. The present work and recent finding of enhancement of Hg emissions from soil and mineral particles by O3 seem to point to a research need to probe the possible role of near-ground atmospheric chemistry in Hg air/soil exchange.