Trends In The Base Cation Deposition In Finland

Abstract

The deposition of the base cations is an important element in determining the total deposited acidity in the critical loads approach used in the protocols of the Convention on Long-range Transboundary Air Pollution. In Finland, where the mineral content and the weathering rate of the soils are low, the atmospheric deposition forms a significant source for base cations. The concentration of the base cations, especially calcium, in both wet and dry deposition has declined widely in Finland since 1985. The southern parts of the country also receive less potassium and magnesium in precipitation than a decade ago. Particulate concentration measurements show a downward trend for calcium, magnesium and sodium in the southeast and for calcium and potassium in the far north. The observed reduction of non-sea-salt base cations is supposed partly to be the result of declining emissions of fly ash from energy production and industrial plants, In the south-eastern Finland, the sharp decline reflects the strongly decreasing emissions of the Estonian oil shale power plants. The changes in the base cation deposition are particularly interesting in comparison to the deposition of acidifying components. In the southern and central Finland, the sulphur deposition decreased'rapidly during the 1990's. This is due to the heavy emission reduction actions in Finland and elsewhere in Europe. An especially large reduction of base cations occurred in the south-eastern part of Finland, where the wet deposition of base cations decreased almost as much as the wet deposition of sulphur. This cuts off part of the positive effect of the reduced sulphur deposition. At stations in other parts of the country except the southeast and the far north, the sulphur deposition decreased much more than the base cation deposition.