Fate of elemental mercury in the Arctic during atmospheric mercury depletion episodes and the load of atmospheric mercury to the Arctic

Atmospheric mercury depletion episodes (AMDEs) were studied at Station Nord, Northeast Greenland, 81 degrees 36' N, 16 degrees 40' W, during the Arctic Spring. Gaseous elemental mercury (GEM) and ozone were measured starting from 1998 and 1999, respectively, until August 2002. GEM was measured with a TEKRAN 2735A automatic mercury analyzer based on preconcentration of mercury on a gold trap followed by detection using fluorescence spectroscopy. Ozone was measured by UV absorption. A scatter plot of GEM and ozone concentrations confirmed that also at Station Nord GEM and ozone are linearly correlated during AMDEs. The relationship between ozone and GEM is further investigated in this paper using basic reaction kinetics (i.e., Cl, ClO, Br, and BrO have been suggested as reactants for GEM). The analyses in this paper show that GEM in the Arctic troposphere most probably reacts with Br. On the basis of the experimental results of this paper and results from the literature, a simple parametrization for AMDE was included into the Danish Eulerian Hemispheric Model (DEHM). In the model, GEM is converted linearly to reactive gaseous mercury (RGM) over sea ice with temperature below -4 degrees C with a lifetime of 3 or 10 h. The new AMDE parametrization was used together with the general parametrization of mercury chemistry [Petersen, G.; Munthe, J.; Pleijel, K.; Bloxam, R.; Vinod Kumar, A. Atmos. Environ. 1998, 32, 829-843]. The obtained model results were compared with measurements of GEM at Station Nord. There was good agreement between the start and general features periods with AMDEs, although the model could not reproduce the fast concentration changes, and the correlation between modeled and measured values decreased from 2000 to 2001 and further in 2002. The modeled RGM concentrations over the Arctic in 2000 were found to agree well with the temporal and geographical variability of the boundary column of monthly average BrO observed by the GOME satellite. Scenario calculations were performed with and without AMDEs. For the area north of the Polar Circle, the mercury deposition increases from 89 tons/year for calculations without an AMDE to 208 tons/year with the AMDE. The 208 tons/year represent an upper limit for the mercury load to the Artic.     

Rapid reduction and reemission of mercury deposited into snowpacks during atmospheric mercury depletion events at churchill, Manitoba, Canada

Mercury (Hg) in some Arctic marine mammals has increased to levels that may be toxic to Northern peoples consuming them as traditional food. It has been suggested that sunlight-induced atmospheric reactions called springtime atmospheric Hg depletion events (AMDEs) result in the loading of -150-300 tons of Hg to the Canadian Arctic archipelago each spring and that AMDEs are the ultimate source of Hg to Arctic foodwebs. AMDEs result from the oxidation of gaseous elemental Hg0 (GEM) in Arctic atmospheres to reactive gaseous Hg (RGM) and particulate Hg (pHg), both of which fall out of the atmosphere to snowpacks. We studied the springtime cycling of Hg between air and snowpacks near Churchill, Manitoba, for 2 years to determine the net input of Hg to Hudson Bay from AMDEs. In 2004, we monitored atmospheric concentrations of GEM, pHg, and RGM while simultaneously measuring concentrations of total Hg (THg) in surface snow collected over the sea ice on Hudson Bay. During numerous springtime AMDEs, concentrations of THg in surface snow increased, often to over 60 ng/L, demonstrating that AMDEs resulted in deposition of oxidized Hg (Hg(II)) to snowpacks. However, immediatelyfollowing AMDEs, average concentrations of THg in snow declined drastically from between 67.8+/-7.7 ng/L during AMDEs to only 4.25+/-1.85 ng/L four or more days following them. In 2003, we measured THg in surface snow collected daily over the sea ice and total gaseous Hg (TGM) concentrations in the interstitial airspaces of snowpacks. When concentrations of THg in the surface snow decreased, concentrations of TGM in interstitial airspaces of the snowpack increased sharply from between approximately 1.4-3.4 ng/m(3) to between approximately 20-150 ng/m(3), suggesting thatthere was a reduction of deposited Hg(II) to GEM, which then diffused out of snowpacks. At snowmelt in both 2003 and 2004, average concentrations of THg in meltwater collected over Hudson Bay were only 4.04+/-2.01 ng/L. Using concentrations of THg in meltwater and snow water equivalent, we estimated a net springtime loading of only 2.1+/-1.7 mg/ha of Hg to Hudson Bay from AMDEs, indicating that only a small portion of the Hg(II) deposited during AMDEs enters Hudosn Bay each spring.     

Modern and historic atmospheric mercury fluxes in northern Alaska: Global sources and Arctic depletion

We reconstruct from lake-sediment archives atmospheric Hg deposition to Arctic Alaska over the last several centuries and constrain a contemporary lake/watershed mass-balance with real-time measurement of Hg fluxes in rainfall, runoff, and evasion. Results indicate that (a) anthropogenic Hg impact in the Arctic is of similar magnitude to that at temperate latitudes; (b) whole-lake Hg sedimentation determined from 55 210Pb-dated cores from the five small lakes demonstrates a 3-fold increase in atmospheric Hg deposition since the advent of the Industrial Revolution; (c) because of high soil Hg concentrations and relatively low atmospheric deposition fluxes, erosional inputs to these lakes are more significant than in similar temperate systems; (d) volatilization accounts for about 20% of the Hg losses (evasion and sedimentation); and (e) another source term is needed to balance the evasional and sedimentation sinks. This additional flux (1.21+/-0.74 microg m(-2) yr(-1)) is comparable to direct atmospheric Hg deposition and may be due to some combination of springtime Arctic depletion and more generalized deposition of reactive gaseous Hg species.     

Measurements of atmospheric mercury species at a coastal site in the Antarctic and over the south Atlantic Ocean during polar summer

Mercury and many of its compounds behave exceptionally in the environment because of their volatility, capability for methylation, and subsequent biomagnification in contrast with most of the other heavy metals. Long-range atmospheric transport of elemental mercury, its transformation to more toxic methylmercury compounds, the ability of some to undergo photochemical reactions, and their bioaccumulation in the aquatic food chain have made it a subject of global research activities, even in polar regions. The first continuous high-time-resolution measurements of total gaseous mercury in the Antarctic covering a 12-month period were carried out at the German Antarctic research station Neumayer (70 degrees 39' S, 8 degrees 15' W) between January 2000 and February 2001. We recently reported that mercury depletion events (MDEs) occur in the Antarctic after polar sunrise, as was previously shown for Arctic sites. These events (MDEs) end suddenly during Antarctic summer. A possible explanation of this phenomenon is presented in this paper, showing that air masses originating from the sea-ice surface were a necessary prerequisite for the observations of depletion of atmospheric mercury at polar spring. Our extensive measurements at Neumayer of atmospheric mercury species during December 2000-February 2001 show that fast oxidation of gaseous elemental mercury leads to variable Hg0 concentrations during Antarctic summer, accompanied by elevated concentrations, up to more than 300 pg/m3, of reactive gaseous mercury. For the first time in the Southern Hemisphere, atmospheric mercury species measurements were also performed onboard of a research vessel, indicating the existence of homogeneous background concentrations over the south Atlantic Ocean. These new findings contain evidence for an enhanced oxidizing potential of the Antarctic atmosphere over the continent that needs to be considered for the interpretation of dynamic transformations of mercury during summertime.     

Measurements of mercury in dew: atmospheric removal of mercury species to a wetted surface

The importance of dew in the mercury cycle was investigated during three sampling periods in the Great Lakes region and one in the Florida Everglades. Mercury concentrations ranged from 1.0 to 22.6 ng/L in dew. Deposition per dew event was, on average, lowest at a remote site on Lake Superior (0.31 ng/m2) and highest in the Florida Everglades (1.4 ng/m2). The estimated mercury deposition to the canopy associated with dew approximately equaled that of precipitation during the wintertime Everglades study. Relative to other trace elements (Mg, Ti, V, Mn, Ni, Cu, As, Sr, Cd, Sb, La, Ce, Pb), mercury was found to be more enriched in rain than dew, suggesting the importance of gas scavenging for precipitation. The fraction of mercury in dew from particulate deposition was estimated to average 40%, with the remaining contribution from reactive gaseous Hg (RGM). RGM, for which little reliable data exists, was measured in the Everglades and was significantly reduced at the start of a dew event, indicating pronounced removal of this soluble mercury species to wetted surfaces. The first estimates of RGM deposition velocities based on mercury flux measurements are reported here and range up to 1.6 cm/s.     

Methylmercury in freshwater fish linked to atmospheric mercury deposition

A connection between accumulation of methylmercury (MeHg) in wild fish populations and atmospheric mercury deposition has not been made. Large databases for both MeHg in fish and atmospheric mercury deposition have been assimilated from monitoring efforts spanning the contiguous United States. Here, we compare results of these data sets and show that state-wide average concentrations of MeHg in a cosmopolitan freshwater fish, the largemouth bass Micropterus salmoides, are related positively to wet atmospheric Hg fluxes among most of the 25 states that are analyzed, which span a 5-fold range in Hg deposition. Differences in largemouth bass MeHg concentrations among states are unrelated to average precipitation depth, wet atmospheric acid deposition, or interstate variations in the type of water body (river, lake, reservoir) from which the fish were sampled. There are modest correlations between MeHg in bass and surface water pH, temperature, and wet atmospheric deposition of sulfate. However, when fish and atmospheric mercury results are combined at the state level, wet atmospheric Hg deposition accounts for about two-thirds of the variation in bass MeHg among most states, and stepwise multiple regression analysis reveals thatthese variables do not improve the linear model significantly. This suggests the accumulation of MeHg in wild fish populations is linked to atmospheric Hg loadings, two-thirds of which are estimated to be from anthropogenic sources.     

Methylmercury in mosquitoes related to atmospheric mercury deposition and contamination

A connection between loadings of inorganic Hg, especially from the atmosphere, and accumulation of methylmercury (MeHg) in aquatic biota has not been firmly established. Mosquitoes (Diptera: Culicidae) may be a useful indictor of Hg contamination or MeHg accumulation in aquatic ecosystems because they have aquatic life stages, and their ubiquitous distribution permits sampling across wide ranges of climate, biological productivity, and atmospheric Hg deposition. We examined MeHg in adult mosquitoes from subtropical (Florida), maritime (California), continental (Michigan), and arctic (Alaska) regions of North America that span a range in wet atmospheric Hg deposition (1.5-15 microg m(-2) y(-1)). More than 90% of the Hg in mosquitoes was MeHg, and concentrations varied among locations. Levels of MeHg differed among mosquito species at six sites in northwest Florida (Ochlerotatus atlanticus < Culex nigripalpus < Anopheles crucians); this may be related to differences in biogeochemical characteristics of the aquatic habitat that affect dietary accumulation of MeHg during the larval stage. Mosquito MeHg was related positively to wet atmospheric Hg deposition among locations where atmospheric deposition is the principal source of Hg, and it was greatly enhanced in Hg-polluted environs near the Sulphur Bank Mine in Lake County, California. These results suggest that MeHg in mosquitoes may be a useful and sensitive indicator of Hg loadings to aquatic systems, including that derived from atmospheric deposition.     

Enhanced deposition and bioaccumulation of mercury in Antarctic terrestrial ecosystems facing a coastal polynya

Mercury emitted by anthropogenic and natural sources occurs in the atmosphere mostly in the gaseous elemental form, which has a long lifetime in tropical and temperate regions. Once deposited in terrestrial and aquatic ecosystems the metal is partly re-emitted into the air, thus assuming the characteristics of global pollutants such as persistent volatile chemicals. In polar regions, during and after the sunrise, the photochemically driven oxidation of gaseous Hg by reactive halogens may result in areas of greatly enhanced Hg deposition. Mercury concentrations in soils, lichens, and mosses collected in a stretch between 74 degrees 30' S and 76 degrees 00' S, in ice-free coastal areas of Victoria Land facing the Terra Nova Bay coastal polynya, were higher than typical Antarctic baselines. The finding of enhanced Hg bioaccumulation in Antarctic terrestrial ecosystems facing a coastal polynya strongly supports recent speculations on the role of ice crystals ("frost flowers") growing in polynyas as a dominant source of sea salt aerosols and bromine compounds, which are involved in springtime mercury depletion events (MDEs). These results raise concern aboutthe possible environmental effects of changes in regional climate and sea ice coverage, and on the possible role of Antarctica as a sink in the mercury cycle.     

Sources of speciated atmospheric mercury at a residential neighborhood impacted by industrial sources

Speciated measurements of atmospheric mercury plumes were obtained at an industrially impacted residential area of East St. Louis, IL. These plumes were found to result in extremely high mercury concentrations at ground level that were composed of a wide distribution of mercury species. Ground level concentrations as high as 235 ng m(-3) for elemental mercury (Hg0) and 38 300 pg m(-3) for reactive mercury species (reactive gaseous (RGM) plus particulate (PHg) mercury) were measured. The highest mercury concentrations observed during the study were associated with plumes that contained high concentrations of all mercury species (Hg0, RGM, and PHg) and originated from a source located southwest of the sampling site. Variations in proportions of Hg0/RGM/PHg among plumes, with Hg0 dominating some plumes and RGM and/or PHg dominating others, were attributed to differences in emissions from different sources. Correlations between mercury plumes and elevated NO(x) were not observed; however, a correlation between elevated SO2 and mercury plumes was observed during some but not all plume events. Despite the presence of six coal-fired power plants within 60 km of the study site, wind direction data along with Hg/SO2 and Hg/NO(x) ratios suggest that high-concentration mercury plumes impacting the St. Louis-Midwest Particle Matter Supersite are attributable to local point sources within 5 km of the site.     

Estimation of dry deposition of atmospheric mercury in Nevada by direct and indirect methods

Atmospheric models and limited measurements indicate that dry deposition of atmospheric mercury is an important process by which mercury is input to ecosystems. To begin to fill the measurement data gap, multiple methods were used simultaneously during seasonal campaigns conducted in 2005 and 2006 to estimate dry deposition of atmospheric mercury at two Mercury Deposition Network (MDN) sites in rural Nevada and in Reno, Nevada. Gaseous elemental mercury (Hg0), reactive gaseous mercury (RGM), and particulate-bound mercury (Hgp) concentrations were measured using Tekran 2537A/1130/ 1135 systems. These speciated measurements were combined with on-site meteorological measurements to estimate depositional fluxes of RGM and Hgp using dry deposition models. Modeled fluxes were compared with more direct measurements obtained using polysulfone cation-exchange membranes and foliar surfaces. Dynamic flux chambers were used to measure soil mercury exchange. RGM concentrations were higher during warmer months at all sites, leading to seasonal variation in the modeled importance of RGM as a component of total depositional load. The ratio of dry to wet deposition was between 10 and 90%, and varied with season and with the methods used for dry deposition approximations. This work illustrates the variability of mercury dry deposition with location and time and highlights the need for direct dry deposition measurements.     

Impact of surface heterogeneity on mercury uptake by carbonaceous sorbents under UHV and atmospheric pressure

Chemical and morphological heterogeneities of carbon sorbents play important roles in gas-phase adsorption. However, the specific chemical complexes and topological structures of carbon that favor or impede elemental mercury uptake are not well understood and are the subject of this study. Temperature programmed desorption (TPD) with a model carbonaceous material (highly oriented pyrolytic graphite, HOPG) under ultrahigh vacuum (UHV) conditions and fixed bed adsorption by activated carbon (BPL) at atmospheric pressure were combined to investigate the effects of chemical and morphological heterogeneities on mercury adsorption by carbonaceous surfaces. TPD results show that mercury adsorption at 100 K onto HOPG surfaces with and without chemical functional groups and topological heterogeneity created by plasma oxidation occurs through physisorption. The removal of chemical functionalities from the HOPG surface enhances mercury physisorption. Plasma-oxidation of HOPG provides additional surface area for mercury adsorption. However, the pits created by plasma oxidation are more than 10 nm in diameter and do not simulate microporosity that predominates in activated carbons. Mercury adsorption by activated carbon at atmospheric pressure occurs through two distinct mechanisms. Physisorption governs mercury adsorption at lower temperatures (i.e., below 348 K), while chemisorption predominates at high adsorption temperatures (i.e., above 348 K). Presence of water on activated carbon surface enhances mercury uptake by both physisorption and chemisorption. Oxygen containing functional groups reduce mercury uptake by physisorption by blocking access to the micropores. No significant impact of oxygen functionalities was observed in the chemisorption regime. The key findings of this study open the possibility to apply scientific information obtained from the studies with simple surfaces such as HOPG under ideal conditions (UHV) to industrial sorbents under realistic process conditions.     

Estimating the natural background atmospheric deposition rate of mercury utilizing ombrotrophic bogs in southern Sweden

A critical gap in the understanding of the global cycling of mercury is the limited data describing the natural background atmospheric deposition rate of mercury before the advent of pollution. Existing estimates of the natural deposition rate are typically about 2-5 microg of Hg m(-2) year(-1) (see, for example, Swain et al. Science 1992, 257, 784-787), based on studies that generally rely on short, 210Pb-dated lake sediment and peat cores that span the past 150 years. Analyses of mercury in long peat cores in southcentral Sweden indicate that natural mercury deposition rates in the period 4000-500 BP were lower, about 0.5-1 microg of Hg m(-2) year(-1). This suggests that recent mercury accumulation rates in the peat (15-25 microg of Hg m(-2) year(-1)) and measured atmospheric deposition rates of mercury in Sweden over the past 3 decades (5-30 microg of Hg m(-2) year(-1)) (Munthe et al. Water, Air, Soil Pollut.: Focus 2001, 1, 299-310) are at least an order of magnitude greater than the prepollution deposition rate, rather than representing only a 3-5-fold increase, as has generally been estimated.     

Gas-phase HO-initiated reactions of elemental mercury: kinetics, product studies, and atmospheric implications

Mercury is an environmentally volatile toxic fluid metal that is assumed to have a long atmospheric residence time and hence is subject to long-range transport. The speciation and chemical transformation of mercury in the atmosphere strongly influences its bioaccumulation potential in the human food chain as well as its global cycling. To investigate the oxidation of Hg0 by HO, the dominantdaytime atmospheric oxidant, we performed kinetic and product studies over the temperature range 283-353 K under near atmospheric pressure (100+/-0.13 kPa) in air and N2 diluents. Experiments were carried out by the relative rate method using five reference molecules and monitored by gas chromatography with mass spectroscopic detection (GC-MS). The HO were generated using UV photolysis of isopropyl nitrite at 300 < or = lambda < or = 400 nm in the presence of NO. The room-temperature rate constant was found to be (9.0+/-1.3) x 10(-14) cm3 molecule(-1) s(-1). The temperature dependence of the reaction can be expressed as a simple Arrhenius expression (in unit of 10(-14) cm3 molecule(-1) s(-1)) using ethane as the reference molecule: kHg + HO = 3.55 x 10(-14) exp((294+/-16)/T). The major reaction product, HgO, was identified in the gaseous form, as aerosols and as deposits on the container walls, using chemical ionization mass spectrometry (CI-MS), electron impact mass spectrometry (EI-MS), GC-MS, and cold vapor atomic fluorescence spectrometry (CVAFS). Experimental results reveal that ca. 6% of the reaction products were collected on a 0.2 microm filter as suspended aerosol, ca. 10% were in the gaseous form, and about 80% were deposited on the reaction vessel wall. The potential implications of our results in the understanding of tropospheric mercury transformation are herein discussed.     

Impacts of the Canadian forest fires on atmospheric mercury and carbonaceous particles in Northern New York

The impact of Canadian forest fires in Quebec on May 31, 2010 on PM(2.5), carbonaceous species, and atmospheric mercury species was observed at three rural sites in northern New York. The results were compared with previous studies during a 2002 Quebec forest fire episode. MODIS satellite images showed transport of forest fire smoke from southern Quebec, Canada to northern New York on May 31, 2010. Back-trajectories were consistent with this regional transport. During the forest fire event, as much as an 18-fold increase in PM(2.5) concentration was observed. The concentrations of episode-related OC, EC, BC, UVBC, and their difference (Delta-C), reactive gaseous mercury (RGM), and particle-bound mercury (PBM) were also significantly higher than those under normal conditions, suggesting a high impact of Canadian forest fire emissions on air quality in northern New York. PBM, RGM, and Delta-C are all emitted from forest fires. The correlation coefficient between Delta-C and other carbonaceous species may serve as an indicator of forest fire smoke. Given the marked changes in PBM, it may serve as a more useful tracer of forest fires over distances of several hundred kilometers relative to GEM. However, the Delta-C concentration changes are more readily measured.     

Enhancing atmospheric mercury research in China to improve the current understanding of the global mercury cycle: the need for urgent and closely coordinated efforts

The current understanding of the global mercury (Hg) cycle remains uncertain because Hg behavior in the environment is very complicated. The special property of Hg causes the atmosphere to be the most important medium for worldwide dispersion and transformation. The source and fate of atmospheric Hg and its interaction with the surface environment are the essential topics in the global Hg cycle. Recent declining measurement trends of Hg in the atmosphere are in apparent conflict with the increasing trends in global anthropogenic Hg emissions. As the single largest country contributor of anthropogenic Hg emission, China's role in the global Hg cycle will become more and more important in the context of the decreasing man-made Hg emission from developed regions. However, much less Hg information in China is available. As a global pollutant which undergoes long-range transport and is persistence in the environment, increasing Hg knowledge in China could not only promote the Hg regulation in this country but also improve the understanding of the fundamental of the global Hg cycle and further push the abatement of this toxin on a global scale. Then the atmospheric Hg research in China may be a breakthrough for improving the current understanding of the global Hg cycle. However, due to the complex behavior of Hg in the atmosphere, a deeper understanding of the atmospheric Hg cycle in China needs greater cooperation across fields.