Acid deposition trends at the Finnish Integrated Monitoring catchments in relation to emission reductions

The effects of transboundary air pollutants on deposition levels and precipitation concentrations have been monitored within the UN/ECE Integrated Monitoring Programme since 1988 at four Finnish background stations. A monotonic decreasing trend was detected for the concentration and deposition of sulphate, nitrate, ammonium and potassium at the southern stations. Further to the north, fewer compounds showed a significant reduction. For sulphur, model calculations with the operational deposition model DAIQUIRI also indicate a clear decrease in deposition. At the southern stations this reflected rather well the reduction of sulphur emissions in Finland and elsewhere in Europe. In the northern part of the country, where the distance to the major emission sources is longer and transport is more affected by meteorological parameters, wind direction especially, the annual amounts of sulphur deposition varied more independently of the changes in emission levels. The proportion of domestic oxidised nitrogen and total ammonium deposition decreased on moving northwards. The downward trend for nitrogen compounds throughout the country seems to be due to international emission reductions.