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 (GOM_TK, PBM_TK, and RM_TK) with RM_CEM collected on cation exchange membranes (CEMs) at the high altitude Pic du Midi Observatory. We find that RM_TK is systematically lower by a factor of 1.3 than RM_CEM. We observe a significant relationship between GOM_TK (but not PBM_TK) and Tekran flush_TK blanks suggesting significant loss (36%) of labile GOM_TK from the denuder or inlet. Adding the flush_TK blank to RM_TK results in good agreement with RM_CEM (slope = 1.01, <i>r</i>² = 0.90) suggesting we can correct bias in RM_TK and GOM_TK. 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

Solution Speciation Controls Mercury Isotope Fractionation of Hg(II) Sorption to Goethite

The application of Hg isotope signatures as tracers for environmental Hg cycling requires the determination of isotope fractionation factors and mechanisms for individual processes. Here, we investigated Hg isotope fractionation of Hg­(II) sorption to goethite in batch systems under different experimental conditions. We observed a mass-dependent enrichment of light Hg isotopes on the goethite surface relative to dissolved Hg (ε²⁰²Hg of −0.30‰ to −0.44‰) which was independent of the pH, chloride and sulfate concentration, type of surface complex, and equilibration time. Based on previous theoretical equilibrium fractionation factors, we propose that Hg isotope fractionation of Hg­(II) sorption to goethite is controlled by an equilibrium isotope effect between Hg­(II) solution species, expressed on the mineral surface by the adsorption of the cationic solution species. In contrast, the formation of outer-sphere complexes and subsequent conformation changes to different inner-sphere complexes appeared to have insignificant effects on the observed isotope fractionation. Our findings emphasize the importance of solution speciation in metal isotope sorption studies and suggest that the dissolved Hg­(II) pool in soils and sediments, which is the most mobile and bioavailable, should be isotopically heavy, as light Hg isotopes are preferentially sequestered during binding to both mineral phases and natural organic matter

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 (GOM_TK, PBM_TK, and RM_TK) with RM_CEM collected on cation exchange membranes (CEMs) at the high altitude Pic du Midi Observatory. We find that RM_TK is systematically lower by a factor of 1.3 than RM_CEM. We observe a significant relationship between GOM_TK (but not PBM_TK) and Tekran flush_TK blanks suggesting significant loss (36%) of labile GOM_TK from the denuder or inlet. Adding the flush_TK blank to RM_TK results in good agreement with RM_CEM (slope = 1.01, <i>r</i>² = 0.90) suggesting we can correct bias in RM_TK and GOM_TK. 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

Deforestation as an Anthropogenic Driver of Mercury Pollution

Deforestation reduces the capacity of the terrestrial biosphere to take up toxic pollutant mercury (Hg) and enhances the release of secondary Hg from soils. The consequences of deforestation for Hg cycling are not currently considered by anthropogenic emission inventories or specifically addressed under the global Minamata Convention on Mercury. Using global Hg modeling constrained by field observations, we estimate that net Hg fluxes to the atmosphere due to deforestation are 217 Mg year–1 (95% confidence interval (CI): 134–1650 Mg year–1) for 2015, approximately 10% of global primary anthropogenic emissions. If deforestation of the Amazon rainforest continues at business-as-usual rates, net Hg emissions from the region will increase by 153 Mg year–1 by 2050 (CI: 97–418 Mg year–1), enhancing the transport and subsequent deposition of Hg to aquatic ecosystems. Substantial Hg emissions reductions are found for two potential cases of land use policies: conservation of the Amazon rainforest (92 Mg year–1, 95% CI: 59–234 Mg year–1) and global reforestation (98 Mg year–1, 95% CI: 64–449 Mg year–1). We conclude that deforestation-related emissions should be incorporated as an anthropogenic source in Hg inventories and that land use policy could be leveraged to address global Hg pollution

Deforestation as an Anthropogenic Driver of Mercury Pollution

Deforestation reduces the capacity of the terrestrial biosphere to take up toxic pollutant mercury (Hg) and enhances the release of secondary Hg from soils. The consequences of deforestation for Hg cycling are not currently considered by anthropogenic emission inventories or specifically addressed under the global Minamata Convention on Mercury. Using global Hg modeling constrained by field observations, we estimate that net Hg fluxes to the atmosphere due to deforestation are 217 Mg year–1 (95% confidence interval (CI): 134–1650 Mg year–1) for 2015, approximately 10% of global primary anthropogenic emissions. If deforestation of the Amazon rainforest continues at business-as-usual rates, net Hg emissions from the region will increase by 153 Mg year–1 by 2050 (CI: 97–418 Mg year–1), enhancing the transport and subsequent deposition of Hg to aquatic ecosystems. Substantial Hg emissions reductions are found for two potential cases of land use policies: conservation of the Amazon rainforest (92 Mg year–1, 95% CI: 59–234 Mg year–1) and global reforestation (98 Mg year–1, 95% CI: 64–449 Mg year–1). We conclude that deforestation-related emissions should be incorporated as an anthropogenic source in Hg inventories and that land use policy could be leveraged to address global Hg pollution

Corrections to Methyl Mercury Formation in Hillslope Soils of Boreal Forests: The Role of Forest Harvest and Anaerobic Microbes

Corrections to Methyl Mercury Formation in Hillslope Soils of Boreal Forests: The Role of Forest Harvest and Anaerobic Microbe

Kinetics of Hg(II) Exchange between Organic Ligands, Goethite, and Natural Organic Matter Studied with an Enriched Stable Isotope Approach

The mobility and bioavailability of toxic Hg­(II) in the environment strongly depends on its interactions with natural organic matter (NOM) and mineral surfaces. Using an enriched stable isotope approach, we investigated the exchange of Hg­(II) between dissolved species (inorganically complexed or cysteine-, EDTA-, or NOM-bound) and solid-bound Hg­(II) (carboxyl-/thiol-resin or goethite) over 30 days under constant conditions (pH, Hg and ligand concentrations). The Hg­(II)-exchange was initially fast, followed by a slower phase, and depended on the properties of the dissolved ligands and sorbents. The results were described by a kinetic model allowing the simultaneous determination of adsorption and desorption rate coefficients. The time scales required to reach equilibrium with the carboxyl-resin varied greatly from 1.2 days for Hg­(OH)₂ to 16 days for Hg­(II)–cysteine complexes and approximately 250 days for EDTA-bound Hg­(II). Other experiments could not be described by an equilibrium model, suggesting that a significant fraction of total-bound Hg was present in a non-exchangeable form (thiol-resin and NOM: 53–58%; goethite: 22–29%). Based on the slow and incomplete exchange of Hg­(II) described in this study, we suggest that kinetic effects must be considered to a greater extent in the assessment of the fate of Hg in the environment and the design of experimental studies, for example, for stability constant determination or metal isotope fractionation during sorption

Methyl Mercury Formation in Hillslope Soils of Boreal Forests: The Role of Forest Harvest and Anaerobic Microbes

Final harvest (clear-cutting) of coniferous boreal forests has been shown to increase streamwater concentrations and export of the neurotoxin methyl mercury (MeHg) to freshwater ecosystems. Here, the spatial distribution of inorganic Hg and MeHg in soil as a consequence of clear-cutting is reported. A comparison of soils at similar positions along hillslopes in four 80 years old Norway spruce (Picea abies) stands (REFs) with those in four similar stands subjected to clear-cutting (CCs) revealed significantly (<i>p</i> < 0.05) enhanced MeHg concentrations (ng g^–1), MeHg areal masses (g ha^–1), and percent MeHg of Hg_TOT in O horizons of CCs located between 1 and 41 m from streams. Inorganic Hg measures did not differ between REFs and CCs at any position. The O horizon thickness did not differ between CCs and REFs, but the groundwater table and soil water content were significantly higher at CCs than at REFs. The largest difference in percent MeHg of Hg_TOT (12 times higher at CCs compared to REFs, <i>p</i> = 0.003) was observed in concert with a significant enhancement in soil water content (<i>p</i> = 0.0003) at intermediate hillslope positions (20−38 m from stream), outside the stream riparian zone. Incubation experiments demonstrated that soils having significantly enhanced soil pools of MeHg after clear-cutting also showed significantly enhanced methylation potential as compared with similarly positioned soils in mature reference stands. The addition of inhibitors demonstrated that sulfate-reducing bacteria (SRB) and methanogens were key methylators. Rates of demethylation did not differ between CCs and REFs. Our results suggest that enhanced water saturation of organic soils providing readily available electron donors stimulate Hg-methylating microbes to net formation and buildup of MeHg in O horizons after forest harvest

Mercury Deposition and Re-emission Pathways in Boreal Forest Soils Investigated with Hg Isotope Signatures

Soils comprise the largest terrestrial mercury (Hg) pool in exchange with the atmosphere. To predict how anthropogenic emissions affect global Hg cycling and eventually human Hg exposure, it is crucial to understand Hg deposition and re-emission of legacy Hg from soils. However, assessing Hg deposition and re-emission pathways remains difficult because of an insufficient understanding of the governing processes. We measured Hg stable isotope signatures of radiocarbon-dated boreal forest soils and modeled atmospheric Hg deposition and re-emission pathways and fluxes using a combined source and process tracing approach. Our results suggest that Hg in the soils was dominantly derived from deposition of litter (∼90% on average). The remaining fraction was attributed to precipitation-derived Hg, which showed increasing contributions in older, deeper soil horizons (up to 27%) indicative of an accumulation over decades. We provide evidence for significant Hg re-emission from organic soil horizons most likely caused by nonphotochemical abiotic reduction by natural organic matter, a process previously not observed unambiguously in nature. Our data suggest that Histosols (peat soils), which exhibit at least seasonally water-saturated conditions, have re-emitted up to one-third of previously deposited Hg back to the atmosphere. Re-emission of legacy Hg following reduction by natural organic matter may therefore be an important pathway to be considered in global models, further supporting the need for a process-based assessment of land/atmosphere Hg exchange