Mass-Independent Fractionation of Even and Odd Mercury Isotopes during Atmospheric Mercury Redox Reactions.

Mass-independent fractionation (MIF) of stable even mass number mercury (Hg) isotopes is observed in rainfall and gaseous elemental Hg0 globally and is used to quantify atmospheric Hg deposition pathways. The chemical reaction and underlying even-Hg MIF mechanism are unknown however and speculated to be caused by Hg photo-oxidation on aerosols at the tropopause. Here, we investigate the Hg isotope composition of free tropospheric Hg0 and oxidized HgII forms at the high-altitude Pic du Midi Observatory. We find that gaseous oxidized Hg has positive Δ199Hg, Δ201Hg, and Δ200Hg and negative Δ204Hg signatures, similar to rainfall Hg, and we document rainfall Hg Δ196Hg to be near zero. Cloud water and rainfall Hg show an enhanced odd-Hg MIF of 0.3‰ compared to gaseous oxidized HgII, potentially indicating the occurrence of in-cloud aqueous HgII photoreduction. Diurnal MIF observations of free tropospheric Hg0 show how net Hg0 oxidation in high-altitude air masses leads to opposite even- and odd-MIF in Hg0 and oxidized HgII. We speculate that even-Hg MIF takes place by a molecular magnetic isotope effect during HgII photoreduction on aerosols that involves magnetic halogen nuclei. A Δ200Hg mass balance suggests that global Hg deposition pathways in models are likely biased toward HgII deposition. We propose that Hg cycling models could accommodate the Hg-isotope constraints on emission and deposition fluxes.

Automated Stable Isotope Sampling of Gaseous Elemental Mercury (ISO-GEM): Insights into GEM Emissions from Building Surfaces.

Atmospheric monitoring networks quantify gaseous elemental mercury (GEM) concentrations, but not isotopic compositions. Here, we present a new method for automated and quantitative stable isotope sampling of GEM (ISO-GEM) at the outlet of a commercial Hg analyzer. A programmable multivalve manifold selects Hg at the analyzer inlet and outlet based on specific criteria (location, time, GEM concentration, auxiliary threshold). Outlet Hg recovery was tested for gold traps, oxidizing acidic solution traps, and activated carbon traps. We illustrate the ISO-GEM method in an exploratory study on the effect of building walls on local GEM. We find that GEM concentrations directly at the building surface (wall inlet) are significantly enhanced (mean 3.8 ± 1.8 ng/m3) compared to 3 m from the building wall (free inlet) (mean 1.5 ± 0.4 ng/m3). GEM δ202Hg (-1.26‰ ± 0.41‰, 1 SD, n = 16) and Δ199Hg (-0.05‰ ± 0.10‰, 1 SD, n = 16) at the wall inlet were different from ambient GEM δ202Hg (0.76‰ ± 0.09‰, 1 SD, n = 16) and Δ199Hg (-0.21‰ ± 0.05‰, 1 SD, n = 16) at the free inlet. The isotopic fingerprint of GEM at the wall inlet suggests that GEM emission from the aluminum building surface affected local GEM concentration measurements. These results illustrate the versatility of the automated Hg isotope sampling.

Eurasian river spring flood observations support net Arctic Ocean mercury export to the atmosphere and Atlantic Ocean.

Midlatitude anthropogenic mercury (Hg) emissions and discharge reach the Arctic Ocean (AO) by atmospheric and oceanic transport. Recent studies suggest that Arctic river Hg inputs have been a potentially overlooked source of Hg to the AO. Observations on Hg in Eurasian rivers, which represent 80% of freshwater inputs to the AO, are quasi-inexistent, however, putting firm understanding of the Arctic Hg cycle on hold. Here, we present comprehensive seasonal observations on dissolved Hg (DHg) and particulate Hg (PHg) concentrations and fluxes for two large Eurasian rivers, the Yenisei and the Severnaya Dvina. We find large DHg and PHg fluxes during the spring flood, followed by a second pulse during the fall flood. We observe well-defined water vs. Hg runoff relationships for Eurasian and North American Hg fluxes to the AO and for Canadian Hg fluxes into the larger Hudson Bay area. Extrapolation to pan-Arctic rivers and watersheds gives a total Hg river flux to the AO of 44 ± 4 Mg per year (1σ), in agreement with the recent model-based estimates of 16 to 46 Mg per year and Hg/dissolved organic carbon (DOC) observation-based estimate of 50 Mg per year. The river Hg budget, together with recent observations on tundra Hg uptake and AO Hg dynamics, provide a consistent view of the Arctic Hg cycle in which continental ecosystems traffic anthropogenic Hg emissions to the AO via rivers, and the AO exports Hg to the atmosphere, to the Atlantic Ocean, and to AO marine sediments.

Tropospheric GOM at the Pic du Midi Observatory-Correcting Bias in Denuder Based Observations.

Gaseous elemental mercury (GEM, Hg) emissions are transformed to divalent reactive Hg (RM) forms throughout the troposphere and stratosphere. RM is often operationally quantified as the sum of particle bound Hg (PBM) and gaseous oxidized Hg (GOM). The measurement of GOM and PBM is challenging and under mounting criticism. Here we intercompare six months of automated GOM and PBM measurements using a Tekran (TK) KCl-coated denuder and quartz regenerable particulate filter method (GOMTK, PBMTK, and RMTK) with RMCEM collected on cation exchange membranes (CEMs) at the high altitude Pic du Midi Observatory. We find that RMTK is systematically lower by a factor of 1.3 than RMCEM. We observe a significant relationship between GOMTK (but not PBMTK) and Tekran flushTK blanks suggesting significant loss (36%) of labile GOMTK from the denuder or inlet. Adding the flushTK blank to RMTK results in good agreement with RMCEM (slope = 1.01, r2 = 0.90) suggesting we can correct bias in RMTK and GOMTK. We provide a bias corrected (*) Pic du Midi data set for 2012-2014 that shows GOM* and RM* levels in dry free tropospheric air of 198 ± 57 and 229 ± 58 pg m-3 which agree well with in-flight observed RM and with model based GOM and RM estimates.

Isotopic Composition of Gaseous Elemental Mercury in the Free Troposphere of the Pic du Midi Observatory, France.

Understanding the sources and transformations of mercury (Hg) in the free troposphere is a critical aspect of global Hg research. Here we present one year of observations of atmospheric Hg speciation and gaseous elemental Hg (GEM) isotopic composition at the high-altitude Pic du Midi Observatory (2860 m above sea level) in France. Biweekly integrated GEM from February 2012 to January 2013 revealed significant variations in δ(202)HgGEM (-0.04‰ to 0.52‰) but not in Δ(199)HgGEM (-0.17‰ to -0.27‰) or Δ(200)HgGEM (-0.10‰ to 0.05‰). δ(202)HgGEM was negatively correlated with CO and reflected air mass origins from Europe (high CO, low δ(202)HgGEM) and from the Atlantic Ocean (low CO, high δ(202)HgGEM). We suggest that the δ(202)HgGEM variations represent mixing of recent low δ(202)HgGEM European anthropogenic emissions with high δ(202)HgGEM northern hemispheric background GEM. In addition, Atlantic Ocean free troposphere air masses showed a positive correlation between δ(202)HgGEM and gaseous oxidized Hg (GOM) concentrations, indicative of mass-dependent Hg isotope fractionation during GEM oxidation. On the basis of atmospheric δ(202)HgGEM and speciated Hg observations, we suggest that the oceanic free troposphere is a reservoir within which GEM is readily oxidized to GOM.

Atmospheric Mercury Transfer to Peat Bogs Dominated by Gaseous Elemental Mercury Dry Deposition.

Gaseous elemental mercury (GEM) is the dominant form of mercury in the atmosphere. Its conversion into oxidized gaseous and particulate forms is thought to drive atmospheric mercury wet deposition to terrestrial and aquatic ecosystems, where it can be subsequently transformed into toxic methylmercury. The contribution of mercury dry deposition is however largely unconstrained. Here we examine mercury mass balance and mercury stable isotope composition in a peat bog ecosystem. We find that isotope signatures of living sphagnum moss (Δ(199)Hg = -0.11 ± 0.09‰, Δ(200)Hg = 0.03 ± 0.02‰, 1σ) and recently accumulated peat (Δ(199)Hg = -0.22 ± 0.06‰, Δ(200)Hg = 0.00 ± 0.04‰, 1σ) are characteristic of GEM (Δ(199)Hg = -0.17 ± 0.07‰, Δ(200)Hg = -0.05 ± 0.02‰, 1σ), and differs from wet deposition (Δ(199)Hg = 0.73 ± 0.15‰, Δ(200)Hg = 0.21 ± 0.04‰, 1σ). Sphagnum covered during three years by transparent and opaque surfaces, which eliminate wet deposition, continue to accumulate Hg. Sphagnum Hg isotope signatures indicate accumulation to take place by GEM dry deposition, and indicate little photochemical re-emission. We estimate that atmospheric mercury deposition to the peat bog surface is dominated by GEM dry deposition (79%) rather than wet deposition (21%). Consequently, peat deposits are potential records of past atmospheric GEM concentrations and isotopic composition.

Atmospheric mercury concentrations observed at ground-based monitoring sites globally distributed in the framework of the GMOS network.

Long-term monitoring of data of ambient mercury (Hg) on a global scale to assess its emission, transport, atmospheric chemistry, and deposition processes is vital to understanding the impact of Hg pollution on the environment. The Global Mercury Observation System (GMOS) project was funded by the European Commission (http://www.gmos.eu) and started in November 2010 with the overall goal to develop a coordinated global observing system to monitor Hg on a global scale, including a large network of ground-based monitoring stations, ad hoc periodic oceanographic cruises and measurement flights in the lower and upper troposphere as well as in the lower stratosphere. To date, more than 40 ground-based monitoring sites constitute the global network covering many regions where little to no observational data were available before GMOS. This work presents atmospheric Hg concentrations recorded worldwide in the framework of the GMOS project (2010-2015), analyzing Hg measurement results in terms of temporal trends, seasonality and comparability within the network. Major findings highlighted in this paper include a clear gradient of Hg concentrations between the Northern and Southern hemispheres, confirming that the gradient observed is mostly driven by local and regional sources, which can be anthropogenic, natural or a combination of both.

Total mercury and methylmercury in high altitude surface snow from the French Alps.

Surface snow samples were collected weekly from the 31st of December 2008 to the 21st of June 2009 from Lake Bramant in the French Alps. Total mercury (THg), total dissolved mercury (THgD), methylmercury (MeHg) and particle distributions in surface snow were analyzed. Results showed that THg concentrations, MeHg concentrations and particle load increased with snow surface temperature, which is an indicator of rising temperatures as the season progresses. Significant correlations between MeHg and snow surface temperature and MeHg and total particles greater than 10 µm were observed. This suggests that the MeHg found in the snow originates from atmospheric deposition processes rather than in situ snowpack sources. This study suggests that an important post-winter atmospheric deposition of MeHg and THg occurs on summital zones of the French Alps and it is likely that this contamination originates from the surrounding valleys.

Mercury and methylmercury concentrations in high altitude lakes and fish (Arctic charr) from the French Alps related to watershed characteristics.

Total mercury (THg) and methylmercury (MeHg) concentrations were measured in the muscle of Arctic charr (Salvelinus alpinus) and in the water column of 4 lakes that are located in the French Alps. Watershed characteristics were determined (6 coverage classes) for each lake in order to evaluate the influence of watershed composition on mercury and methylmercury concentrations in fish muscle and in the water column. THg and MeHg concentrations in surface water were relatively low and similar among lakes and watershed characteristics play a major role in determining water column Hg and MeHg levels. THg muscle concentrations for fish with either a standardized length of 220mm, a standardized age of 5 years or for individualuals did not exceed the 0.5mg kg(-1) fish consumption advisory limit established for Hg by the World Health Organization (WHO, 1990). These relatively low THg concentrations can be explained by watershed characteristics, which lead to short Hg residence time in the water column, and also by the short trophic chain that is characteristic of mountain lakes. Growth rate did not seem to influence THg concentrations in fish muscles of these lakes and we observed no relationship between fish Hg concentrations and altitude. This study shows that in the French Alps, high altitude lakes have relatively low THg and MeHg concentrations in both the water column and in Arctic charr populations. Therefore, Hg does not appear to present a danger for local populations and the fishermen of these lakes.

Bioavailable mercury cycling in polar snowpacks.

Polar regions are subject to contamination by mercury (Hg) transported from lower latitudes, severely impacting human and animal health. Atmospheric Mercury Depletion Events (AMDEs) are an episodic process by which Hg is transferred from the atmospheric reservoir to arctic snowpacks. The fate of Hg deposited during these events is the subject of numerous studies, but its speciation remains unclear, especially in terms of environmentally relevant forms such as bioavailable mercury (BioHg). Here, using a bacterial mer-lux biosensor, we report the fraction of newly deposited Hg at the surface and at the bottom of the snowpack that is bioavailable. Snow samples were collected over a two-month arctic field campaign in 2008. In surface snow, BioHg is related to atmospheric Hg deposition and snow fall events were shown to contribute to higher proportions of BioHg than AMDEs. Based on our data, AMDEs represent a potential source of 20 t.y(-1) of BioHg, while wet and dry deposition pathways may provide 135-225 t.y(-1) of BioHg to Arctic surfaces.