Deposition of nitrogen and other major inorganic compounds in Norway, 1992–1996

Abstract

Representative measurements of air and precipitation chemistry have been used to estimate atmospheric deposition for the period 1992–1996. In coastal areas of SW Norway, large precipitation amounts with high concentrations of nitrate and ammonium resulted in annual nitrogen (N) inputs of more than 2 g m −2year −1 (20 kg ha −1year −1). Dry deposition is relatively unimportant in the areas with high precipitation intensities, whereas in some areas with less precipitation, forested areas may receive up to 30–50% of the total deposition in the form of dry deposition. Inputs of dry deposition to non-forested areas are much smaller, particularly during the period of snow cover. Comparison with similar estimates for the periods 1983–1987 and 1988–1992 show that there has been a decrease in the deposition of reduced N, by 2% and 14%, and in the deposition of oxidized N, by 6% and by 23%, relative to the two periods. The trend for oxidized N is somewhat smaller than expected based on reported emission reductions in Northern Europe. Changes in the emission rates from off-shore activities in the North Sea may partly explain the observed discrepancies. For ammonium, the trend may be influenced by both reduced emissions and by the reductions of sulphur dioxide and nitrogen oxides, which may have reduced the rate of conversion from ammonia to ammonium aerosol, thereby decreasing the atmospheric residence time and the transport across the North Sea. Inter annual variations in the meteorological conditions may also have influenced the amounts of deposition.