Atmospheric and catchment mercury concentrations and fluxes in Fennoscandia

image of Atmospheric and catchment mercury concentrations and fluxes in Fennoscandia

Measurements in Southern Fennoscandia show a weak declining trend in mercury deposition which can be attributed to reduction controls in EU countries. Deposition of mercury in Arctic areas is likely to be governed by the amount of mercury in background air and therefore largely dependent on mercury emissions from mercury sources in the entire northern hemisphere. Hence, further reduction in anthropogenic emissions of mercury will require control measures in the entire northern hemisphere. The so called atmospheric mercury depletion events (AMDEs) are occurring during polar spring. How much of the deposited mercury that remains contra is re-emitted to the atmosphere is, however, crucial for assessing the importance of AMDE in the Arctic environment. Forest soils are an important sink for mercury deposited from the atmosphere. However, this sink can be affected by perturbations in conjunction to common forestry practices and lead to mobilization of the stored mercury and enhanced methyl mercury formation. Similar effects can be expected in areas where climate change results in large increases in precipitation amounts. The processes governing these changes in mercury mobilization are to some extent unknown and general predictions of the magnitude of the changes are thus associated with a large degree of uncertainty




One of the most specific properties of mercury regarding environmental concerns is its ability to appear as a diluted gas in the atmosphere. Both anthropogenic and natural mercury emissions constitute to the most part of Hg0, gaseous elemental mercury (GEM). The atmospheric residence time of mercury has been estimated to be around 0.7-1.4 year (Schroeder and Munthe, 1995; Selin et al., 2007 and references therein) which is long enough for distribution on hemispherical scales before it eventually is oxidised and deposited to ground and water surfaces (Schroeder and Munthe, 1998). According to measurements GEM appears to be fairly uniformly distributed with concentrations of around 1.7 ng m-3 in the northern hemisphere and 1.3 ng m-3 in the south (Slemr et al., 2003). The higher values in the north are consistent with major mercury sources being predominately located in the northern hemisphere. This makes mercury a global pollutant in contrast to other heavy metals that have more local or regional impact. GEM is only, to a small extent, dry deposited to ground and vegetation or taken up in cloud water and removed from the atmosphere via wet deposition. On the other hand, mercury is slowly oxidised in the atmosphere forming divalent mercury species (Hg(II)) which are more easily dry deposited and also prone to wet deposition. Divalent mercury is found in the gas phase as well as in rainwater and snow. The exact chemical compositions of these oxidation products are not yet known, however. The gaseous fraction of oxidised mercury is referred to as Reactive Gaseous Mercury (RGM) or Gaseous Oxidised Mercury (GOM). RGM is operationally defined, as the fraction of gaseous mercury that can be sampled using a certain measurement method. Although not yet proven, it is likely that RGM is constituted of species like HgBr2, HgCl2 or mixed halides and related compounds. Total Gaseous Mercury (TGM) is another operational defined mercury fraction. It includes GEM plus other possible gaseous mercury species, such as RGM, that may also be detected when measuring gaseous mercury by the commonly used gold-trap method. Although there are exceptions, TGM and GEM are often equal within a few percent. Mercury is also found in aerosols. Particles emitted from coal fired power plants, for example, contain mercury. Volcanoes and biomass burning are examples of natural sources that emit particulate mercury. GEM and, preferentially, RGM may also be adsorbed on already existing particles in the atmosphere. The abbreviation TPM (Total Particulate Mercury) is used in the literature and it normally denotes the concentration of particulate mercury obtained with open face air filter samplers. Total particle mercury means that the sampling is not made in a size fractionated manner. If mercury in the fine mode (< 2.5 μm particle size) is sampled it is sometimes denoted Fine Particulate Mercury (FPM).


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