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Understanding the fate of Arctic atmospheric mercury (Hg) deposition – A Hg stable isotope investigation of redox processes and Hg re-emissions

Periodic Reporting for period 1 - MEROXRE (Understanding the fate of Arctic atmospheric mercury (Hg) deposition – A Hg stable isotope investigation of redox processes and Hg re-emissions)

Reporting period: 2015-04-01 to 2017-03-31

Mercury (Hg) is a pollutant of global concern for human and ecosystem health. This is particularly true in the Arctic where indigenous populations are excessively exposed to dietary Hg from fish and marine mammal consumption. The deposition of Hg from the atmosphere to Earth surfaces and its re-emission via biogeochemical reduction processes determine Hg concentrations in Earth surface reservoirs, such as soils, snow, and runoff into Arctic lakes and surface Ocean. Stable Hg isotopes are a promising new tool to identify the dominant pathways of atmospheric Hg deposition and potential re-emission processes. We investigated the Hg isotope signatures of different plant species, organic and mineral soil horizons and bedrock of the Arctic tundra at Toolik Field Station, Alaska. We measured stable Hg isotope signatures of atmospheric Hg0, Hg0 in interstitial snow and soil air and Hg in snow, representing atmospheric HgII scavenged during snowfall or deposited during halogen-driven atmospheric mercury depletion events (AMDE). The results suggest that vegetation uptake of gaseous elemental mercury from the atmosphere is the dominant deposition pathway to the terrestrial ecosystem, and contributes about 70% to Hg in organic soils. We have observed large mass-independent Hg isotope anomalies in snow during spring, with minimal D199Hg values of -1.4‰, which are in agreement with previously observed Hg isotope signatures during the AMDE season at the Arctic coast. However, the transfer of HgII from AMDE's to the terrestrial ecosystem does not represent a significant source (0-5 %), suggesting that most HgII deposited during AMDE's was re-emitted prior to snow-melt. The Hg isotope signatures in the soils and the atmosphere showed no indication for substantial re-emission of gaseous elemental mercury from the soils. The Hg stable isotope results agreed well with micro-metrological flux measurements conducted at the same location. Both independent approaches suggest that vegetation uptake of gaseous elemental Hg represents the dominant Hg flux between the atmosphere and Arctic tundra soils and they thus represent a net sink for atmospheric Hg. Changes in environmental conditions, such as warming and thawing of permafrost due to climate change or fires could however lead to a remobilization of the large Hg pool stored in Arctic tundra soils.
- Between 1.4.2015 and 30.7.2015 ER Martin Jiskra (MJ) purcheased the componants to build a Hg isotope samling system, able to collect atmospheric Hg0 at six individual lines (Figure 2 center). The system was built in the lab of GET in Toulouse and tested.

- Between 2.8.2015 and 22.8.2015 MJ travelled to Toolik Field Station (TFS) in Alaska, to install the atmospheric Hg0 sampling system and collect soil and vegetation samples.

- From 5.8.2015 to 10.9.2016 the Hg isotope sampling system was operating and sampling gaseous Hg0 at 6 Inlets (1 Atmosphere inlet, 3 interstitial snow air inlet and 2 soil inlets). Samples were manually changed every 6 to 8 weeks during regular field visits of the team. In total 8 periods of 6 samples were collected and analyzed for Hg isotope composition.

- Between 20.3.2016 - 8.4.2016 MJ visited TFS for sampling of snow and sample pre-oncentration in the field using a newly developed purge-and-trap system (Figure 2, top right). Samples were brought back to Toulouse France for Hg stable isotope analysis.

- Between 21.6.2016 and 1.7.2016 MJ visited TFS for the installation of a new Hg isotope sampling system (Prototype MIMO), capable to sample Hg at the outlet of a commercial Hg concentration analyzer (Tekran 2637X) (Figure 2 bottom).

- September 2015 and January 2017 MJ processed the soil, vegetation and air samples uning a two-stage oven and analyzed the samples for it's Hg stable isotope composition using the MC-ICPMS of GET, CNRS in Toulouse.

- From January 2016 to March 2017, MJ was working together with RS (Tianjin University), on the validation of a new 1D-box model. The Hg isopope fractionation factor during foliar uptake of Hg0 from the atmosphere, determined from the Hg isotope measurements of atmospheric and vegetation samples at Toolik represented an important componant of the global Hg isotope model.
Mercury is a top priority pollutant of the European comission. The European Union has signed the Minamata convention on Mercury in October 2013 and the ratification is under progress. In the MEROXRE Project we find that tundra uptake of atmospheric elemental Hg is driving arctic Hg pollution and that variations in atmospheric Hg concentrations are largely influenced by plant uptake of atmospheric Hg. These findings are novel and are currently in press with the journal Nature. On the long-term MEROXRE findings will help to improve the monitoring efforts by countries required from the Minamata convention. We ask the environmental agencies to addapt their Hg monitoring strategies and intensify efforts in assessing Hg0 deposition fluxes to terrestrial surfaces.