Changes in the concentration of reduced nitrogen in the air in Finland between 1990 and 2007

The concentration of reduced nitrogen in the air declined in 1990―2007 at the Finnish background stations in the southern and central parts of the country. The annual mean values, the summer mean values and the winter mean values declined by 35%―60%. In 2007 the annual mean values were 0.1―0.4 μg N m ―3 . The data from Uto and Oulanka were analysed in transport sectors. At both stations, the foreign contribution from the southern, southwestern and western sectors was dominant in the reduced nitrogen concentrations. In Uto, the ammonium concentration decreased in the air arriving from the sectors extending from the south to the west and in the air transported from the east. In Oulanka, the concentration did not decrease in any of the transport sectors. However, the exposure of reduced nitrogen from the west declined at both sites. In Uto, this was due to the decline in both the ammonium concentration and the frequency of western transport. In Oulanka, the reduced exposure was due to a shift of air transport from the west to the north.