Airborne sulfur and nitrogen in Finland-trends and exposure in relation to air transport sector

We present the concentration trends and the atmospheric exposure of sulfur and nitrogen compounds in different transport sectors during the period 1981-2000, based on the air quality data of the Finnish Meteorological Institute. Sulfur and nitrogen concentrations in air and precipitation were assessed from background stations covering the whole country. A significant decrease of more than 60% was observed in the concentrations of all sulfur compounds throughout the country during 1981-2000. In the 1990's, significant trends were detected in all sulfur time series, with the exception of sulfate in precipitation at the northernmost stations. On the other hand, the concentrations of oxidized nitrogen compounds in air and precipitation have declined only slightly or not at all, especially in the 1990's. This is attributed to the fairly small domestic and European NO(x) emission reductions after 1981. The ammonium concentration in precipitation has declined by more than 50% during 1981-2000. The atmospheric sulfur exposure was found to be dominated by transport from the West and South-West sectors during summer, whereas in winter all the southward sectors were important. The largest decline in the sectoral sulfur concentrations has taken place already before 1991, with a more pronounced reduction during winter than during summer. The atmospheric nitrogen exposure was mainly dominated by transport from the sectors between west and south, but in the latter half of the study period there seems to have been a slight shift towards increased transport from the eastern and northern sectors.