Trace metals in the atmosphere : the wet deposition of trace metals : the selenium cycle

Sammanfattning: Cd, Cu, Fe, Mn, Pb and Zn concentrations in wet deposition have been monitored in Sweden since the autumn of 1983. Concentrations of these elements are highest in southern Sweden and decrease northward. Examination of enrichment factors, relative to crustal material, indicate that the major source of Cd, Cu, Pb and Zn is anthropogenic emissions. However, Cd and Zn anthropogenic emissions from Swedish sources are 2-5 times smaller than wet deposition fluxes. Therefore, to explain the relatively high temporal and spatial correlation of Cd, Pb and Zn concentrations, it is concluded that their wet deposition in Sweden is mainly governed by long range transport of pollutants from the rest of Europe. It is also postulated that Cd, Pb and Zn may be useful tracers for long range transport of sub-micron anthropogenic aerosols. Assessment of toxicity potentials (TP) indicate that Cu, Mn, and Zn monthly mean concentrations are not potentially toxic to freshwater and coniferous forest ecosystems. In contrast, Cd and Pb concentrations in southern Sweden can reach levels which are suspected to be deliterious to freshwater ecosystems.It is postulated that the relatively even distribution of selenium in the atmosphere is partly due to an internal cycle between selenium on particles and in the gas phase. Budget calculations for the region 30°N to 90°N indicate that anthropogenic emissions are 3-4 times larger than natural sources. In addition, it is suggested that terrestrial ecosystems may be an important source of atmospheric selenium.