Measurement and modeling the mercury depletion events in the arctic at the Ny-Alesund Site (MEDEX)
Funded by: US- Environmental Protection Agency – Washington D.C., USA
The toxicity of mercury and the fragility of polar environments is a combination of much concern. Current knowledge suggests that elemental mercury has an atmospheric residence time quite long enough for circumnavigating the globe before depositing. Recent mercury (Hg) measurements in the Arctic by Canadian and United States scientists showed the depletion of atmospheric elemental Hg concentrations with the simultaneous formation of a reactive form of Hg (RGM), which being involatile either rapidly deposits or condenses onto particulate matter already present. These events occurred from the onset of polar dawn (March) until early June in conjunction with tropospheric ozone depletion events in the Arctic springtime, pointing to a seasonal variation in mercury input to the Arctic ecosystem at the exact time that microorganisms begin to flourish thus starting the Hg bioaccumulation process.
Hg depletion events could therefore be, the main source of Hg contamination in wildlife both locally (at the point of deposition) and regionally (as a result of transport by surface water). The exact mechanism of the transformation of elemental Hg to RGM is as yet undetermined, the known gas phase Hg oxidation reactions appear to be too slow to account for the observed changes, but may be similar to the photochemical pathways that deplete ozone in Polar Regions.
A number of different sampling techniques will be used to assess the level of mercury and its compounds in the Arctic air. Two monitoring stations are foreseen in the measurement strategy: the first, on Mount Zeppelin, 474m a.s.l. where it will be possible to continuously sample mercury species in the free troposphere above the boundary layer inversion, and the second station, near the sea on the coast of the fjord, in order to collect air samples in the lower troposphere on the sea/ice pack. Following this strategy, it will be possible to compare the results obtained contemporaneously at two sites in order to evaluate the influences of different chemical-physics variables on the mercury concentrations. The Hg compound and other species measurements are summarised and described below:
- The gas phase (TGM and RGM) and aerosol concentrations of mercury;
- Oxidant species O3 and H2O2 that are responsible for the transformation of the elemental Hg to RGM, and free radicals (i.e. BrO, ClO that may be mercury oxidants;
- Complete suite of air pollutants including the oxides of nitrogen;
- Meteorological data;
- Aerosol samples for subsequent scanning electron microscopy of individual particles, trace elements and organic species;
- During the field campaign snow and rain samples will be collected and stored in clean containers for subsequent analysis. Representative snow core samples will be taken using prescribed procedures for subsequent chemical characterization.
Sampling and Analytical Techniques for Atmospheric Measurements
|PARAMETER||INSTRUMENTATION|| SAMPLING TIME
|Hg0||Tekran Analyser Mod.2537A||5-minutes|
|RGM (automat./man.)||KCl-Coated-Annular Denuders / Tekran 2537A-Tekran 1130|| 2-hours (automatic method)
6-hours (manual method)
|TPM||Mini Quartz Traps or/and 47mm Quartz fibber filter / CVAFS|
|PM2.5||Teflon/quartz-filters (outlet-annular denuders)||24-hour|
|Major Ions||Annular Denuders/Filter Pack/ Sequential System||12-24 hours|
|IC Laboratory (Dionex 100 / Dionex 90)||Air and snow samples||Continuous|
|O3||API-Ozone Analyser Mod. 400A||5-minutes|
The CNR-IIA tropospheric photochemical model including mercury can be used as a stand alone box model or in conjunction with dispersion / meteorological models. Used as a box model it is useful to study the importance of individual processes in mercury cycling, and can be used to evaluate the significance of individual reactions under given meteorological conditions. The identification of the most important processes and reaction pathways allows the database to be trimmed down to provide a streamlined chemical module to be used in conjunction with circulation models, for example. If the dispersion models used are not excessively time consuming it is possible to link the full chemistry module and follow the physico-chemical evolution of an air mass along its trajectory. Thus it will be possible to perform the detailed simulations required of mercury depletion events, because these occur when the Arctic troposphere behaves very much as if it were a closed ‘box’ (there is a stable inversion), thus the box model approach is valid. It will also be possible to perform the dispersion / trajectory analyses coupled with a slightly simplified chemistry model to determine where the Hg present before depletion comes from, and where the Hg that replenishes the gas phase Hg has been transported from.
- CNR – Istitute of Environmental Pollution (CNR-IIA) – COORDINATORE
- United States Environmental Protection Agency (US-EPA)
Gruppo di lavoro
Scientific Coordinator CNR IIA
Staff CNR IIA
Staff CNR IIA
Staff CNR IIA
Scientific Coordinator US-EPA