Atmospheric transport of mercury to the Tibetan Plateau

The Tibetan Plateau (including the Himalayas) is one of the most remote and cold regions in the world. It has very limited to nonexistent industry but is adjacent to the two most populous and rapidly industrializing countries (China and India) and thus provides a unique location for studying the atmospheric transport of mercury. Here we report the first study on the atmospheric transport of mercury to the Tibetan Plateau. The total mercury profiles in four snowpits from glaciers above 5700 m asl along a southwest-northeast transect across the central Tibetan Plateau were obtained in 2005-2006. In general, the total mercury concentrations in the snow samples ranged from < 1 to 9 ng L(-1), increasing northeastward from the southernmost site at Mount Everest. Higher total mercury concentrations were found in the snow deposited in the nonmonsoon season, as indicated by seasonal variation of delta18O values and major ions in the snowpack. The annual atmospheric depositional flux of total Hg was estimated to range from 0.74 to 2.97 microg m(-2) yr(-1) in the region, the majority of which occurred via particulate matter deposition.     

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.     

Processes influencing rainfall deposition of mercury in Florida

The primary goal of the Florida Atmospheric Mercury Study (FAMS) was to quantify the atmospheric deposition of Hg throughout Florida. Monthly integrated precipitation and weekly integrated particulate samples were collected at 10 sites in Florida for periods ranging from 2 to 5 yr. The monthly rainfall across the state and the concentrations of Hg in wet-only and bulk deposition increased by a factor of 2-3 during the summertime "wet season" (May-October). These parallel increases in rainfall amount and Hg concentration resulted in 5-8-fold increases in rainfall Hg deposition during the wet season. The annual volume-weighted Hg concentrations ranged from 14 +/- 2 to 16 +/- 2 ng/L across southern Florida, and the annual rainfall Hg fluxes ranged from 20 +/- 3 to 23 +/- 3 micrograms m-2 yr-1. The weekly integrated particulate Hg concentrations in southern Florida were low (4.9-9.3 pg/m3) and did not exhibit strong seasonal variability. Considering the pronounced seasonal pattern in rainfall Hg deposition, the relatively uniform summertime rainfall Hg concentrations, and the low concentrations of particulate Hg, we conclude that processes other than particulate Hg transport and scavenging govern rainfall Hg deposition in southern Florida. We hypothesize that long-range transport of reactive gaseous Hg (RGM) species coupled with strong convective thunderstorm activity during the summertime represents > 50% of the Hg deposition in southern Florida. Model calculations indicate that local anthropogenic particulate Hg and RGM emissions account for 30-46% of the summertime rainfall Hg deposition across the southern Florida peninsula.     

Methylated mercury species in Canadian high Arctic marine surface waters and snowpacks

We sampled seawater and snowpacks in the Canadian high Arctic for methylated species of mercury (Hg). We discovered that, although seawater sampled under the sea ice had very low concentrations of total Hg (THg, all forms of Hg in a sample; on average 0.14-0.24 ng L(-1)), 30-45% of the THg was in the monomethyl Hg (MMHg) form (on average 0.057-0.095 ng L(-1)), making seawater itself a direct source of MMHg for biomagnification through marine food webs. Seawater under the ice also contained high concentrations of gaseous elemental Hg (GEM; 129 +/- 36 pg L(-1)), suggesting that open water regions such as polynyas and ice leads were a net source of approximately 130 +/- 30 ng Hg m(-2) day(-1) to the atmosphere. We also found 11.1 +/- 4.1 pg L(-1) of dimethyl Hg (DMHg) in seawater and calculated that there could be a significant flux of DMHg to the atmosphere from open water regions. This flux could then resultin MMHg deposition into nearby snowpacks via oxidation of DMHg to MMHg in the atmosphere. In fact, we found high concentrations of MMHg in a few snowpacks near regions of open water. Interestingly, we discovered a significant log-log relationship between Cl- concentrations in snowpacks and concentrations of THg. We hypothesize that as Cl- concentrations in snowpacks increase, inorganic Hg(II) occurs principally as less reducible chloro complexes and, hence, remains in an oxidized state. As a result, snowpacks that receive both marine aerosol deposition of Cl- and deposition of Hg(II) via springtime atmospheric Hg depletion events, for example, may contain significant loads of Hg(II). Overall, though, the median wet/dry loads of Hg in the snowpacks we sampled in the high Arctic (5.2 mg THg ha(-1) and 0.03 mg MMHg ha(-1)) were far below wet-only annual THg loadings throughout southern Canada and most of the U.S. (22-200 mg ha(-1)). Therefore, most Arctic snowpacks contribute     

Differences in mercury bioaccumulation between polar bears (Ursus maritimus) from the Canadian high- and sub-Arctic

Polar bears (Ursus maritimus) are being impacted by climate change and increased exposure to pollutants throughout their northern circumpolar range. In this study, we quantified concentrations of total mercury (THg) in the hair of polar bears from Canadian high- (southern Beaufort Sea, SBS) and sub- (western Hudson Bay, WHB) Arctic populations. Concentrations of THg in polar bears from the SBS population (14.8 ± 6.6 μg g(-1)) were significantly higher than in polar bears from WHB (4.1 ± 1.0 μg g(-1)). On the basis of δ(15)N signatures in hair, in conjunction with published δ(15)N signatures in particulate organic matter and sediments, we estimated that the pelagic and benthic food webs in the SBS are ～ 4.7 and ～ 4.0 trophic levels long, whereas in WHB they are only ～ 3.6 and ～ 3.3 trophic levels long. Furthermore, the more depleted δ(13)C ratios in hair from SBS polar bears relative to those from WHB suggests that SBS polar bears feed on food webs that are relatively more pelagic (and longer), whereas polar bears from WHB feed on those that are relatively more benthic (and shorter). Food web length and structure accounted for ～ 67% of the variation we found in THg concentrations among all polar bears across both populations. The regional difference in polar bear hair THg concentrations was also likely due to regional differences in water-column concentrations of methyl Hg (the toxic form of Hg that biomagnifies through food webs) available for bioaccumulation at the base of the food webs. For example, concentrations of methylated Hg at mid-depths in the marine water column of the northern Canadian Arctic Archipelago were 79.8 ± 37.3 pg L(-1), whereas, in HB, they averaged only 38.3 ± 16.6 pg L(-1). We conclude that a longer food web and higher pelagic concentrations of methylated Hg available to initiate bioaccumulation in the BS resulted in higher concentrations of THg in polar bears from the SBS region compared to those inhabiting the western coast of HB.     

Importance of the forest canopy to fluxes of methyl mercury and total mercury to boreal ecosystems

The forest canopy was an important contributor to fluxes of methyl mercury (MeHg) and total mercury (THg) to the forest floor of boreal uplands and wetlands and potentially to downstream lakes, at the Experimental Lakes Area (ELA), northwestern Ontario. The estimated fluxes of MeHg and THg in throughfall plus litterfall below the forest canopy were 2 and 3 times greater than annual fluxes by direct wet deposition of MeHg (0.9 mg of MeHg ha(-1)) and THg (71 mg of THg ha(-1)). Almost all of the increased flux of MeHg and THg under the forest canopy occurred as litterfall (0.14-1.3 mg of MeHg ha(-1) yr(-1) and 110-220 mg of THg ha(-1) yr(-1)). Throughfall added no MeHg and approximately 9 mg of THg ha(-1) yr(-1) to wet deposition at ELA, unlike in other regions of the world where atmospheric deposition was more heavily contaminated. These data suggest that dry deposition of Hg on foliage as an aerosol or reactive gaseous Hg (RGM) species is low at ELA, a finding supported by preliminary measurements of RGM there. Annual total deposition from throughfall and litterfall under a fire-regenerated 19-yr-old jack pine/birch forest was 1.7 mg of MeHg ha(-1) and 200 mg of THg ha(-1). We found that average annual accumulation of MeHg and THg in the surficial litter/fungal layer of soils since the last forest fire varied between 0.6 and 1.6 mg of MeHg ha(-1) and between 130 and 590 mg of THg ha(-1) among sites differing in drainage and soil moisture. When soil Hg accumulation sites were matched with similar sites where litterfall and throughfall were collected, measured fluxes of THg to the forest floor (sources) were similar to our estimates of longterm soil accumulation rates (sinks), suggesting that the Hg in litterfall and throughfall is a new and not a recycled input of Hg to forested ecosystems. However, further research is required to determine the proportion of Hg in litterfall that is being biogeochemically recycled within forest and wetland ecosystems and, thus, does not represent new inputs to the forest ecosystem.     

Total and organic mercury concentrations in the white muscles of swordfish (Xiphias gladius) from the Indian and Atlantic oceans

A total of 226 swordfish samples collected from Taiwanese fishing vessels in the Indian and Atlantic oceans were examined for total mercury (THg) and organic Hg (OHg). Analysis of 56 pooled white muscle samples showed that THg and OHg concentrations ranged from 0.056 to 3.97 (1.3 ± 0.97) and from 0.043 to 3.92 (1.01 ± 0.82) µg g^-1 flesh mass, respectively. These values were similar to those from various previous studies during the past three decades. THg and OHg were significantly linearly correlated with fork length (FL, cm) of the fish from Indian and Atlantic oceans; however, there was no significant OHg%-FL relationship. OHg and THg also were significantly correlated. Fishes with FL ≤ 140 cm met the methyl Hg (meHg) regulatory standard set by the European Commission Decision (meHg ≤ 1.0); and fish with FL ≤ 211 cm met the Taiwanese Food and Hygiene Standard (meHg ≤ 2.0). Weekly swordfish consumption rates and amounts are recommended accordingly.     

Ambient mercury sources in Rochester, NY: results from Principle Components Analysis (PCA) of Mercury Monitoring Network Data

Continuous atmospheric measurements of speciated mercury (Hg) (elemental mercury (Hg?), reactive gaseous mercury (RGM), and particulate mercury (Hgp)) were made in Rochester, NY from Dec 2007 to May 2009. Continuous measurements of ozone (O?), sulfur dioxide (SO?), carbonmonoxide (CO), particulate matter (PM?.?), and meteorological data were also available. A principle components analysis (PCA) of 3886 observations of 13 variables for the period identified six major factors. Melting snow was observed to be a source of Hg?in winters. Positive correlations between RGM and O? in the spring and summer may be indicative of Hg? oxidation. RGM concentrations increased simultaneously with SO? suggesting the influence of coal fired power plants (CFPP). The ?fth factor (F5) containing O? (high negative loading), CO (positive loading), Hg? and Hg(p) (positive), and/or RGM (negative) was identified as a mobile source which was usually observed during morning rush hours (6:00-9:00 a.m.). The concentrations of the three mercury species from the direction of the CFPP were significantly reduced following the shutdown of this source.     

Mercury distribution across 14 U.S. forests. Part II: Patterns of methyl mercury concentrations and areal mass of total and methyl mercury

This study characterized distribution patterns of monomethyl mercury (MeHg) and areal mass of total mercury (THg) and MeHg across U.S. upland forests. MeHg concentrations increased from surface litter (average: 0.14 μg kg(-1)) to intermediate (0.47 μg kg(-1)) and deeper, decomposed litter (1.43 μg kg(-1)). MeHg concentrations were lower in soils (0.10 μg kg(-1) at 0-20 cm depth; 0.06 μg kg(-1) at >20 cm depth). Ratios of MeHg to THg were higher in litter compared to soils. In soils, MeHg concentrations positively correlated with THg across all sites, and MeHg concentrations also increased with C content and latitude. THg areal mass ranged from 41.6 g ha(-1) to 268.8 g ha(-1). Largest THg mass at all sites was sequestered in soils (average of 91%), followed by litter (8%) and aboveground biomass (<1%). MeHg mass (litter plus soils only) ranged from 75 to 443 mg ha(-1), of which 88% was found in soils. Both THg and MeHg mass correlated with latitude, with average mass increases of 10.6 g ha(-1) (THg) and 20 μg ha(-1) (MeHg) per degree latitude, indicating that highest THg and MeHg accumulation in upland forests are expected in northern sites.