ATMOSPHERIC DEPOSITION OF NITROGEN, SULFUR AND CHLORIDE IN 2 WATERSHEDS LOCATED IN SOUTHERN NORWAY

The deposition of atmospheric pollutants in southern Norway is relativ ely high, and exceeds the critical load for acid deposition in sensiti ve ecosystems. While the effects of the sulfur deposition are relative ly well known and documented, the fate and effects of oxidized and red uced nitrogen compounds are much less well known. In order to study th e cycling of nitrogen in southern Norway, the inter-institutional rese arch program Nitrogen from Mountains to Fjords was launched in 1991. T he program aims to describe the nitrogen mass balance and the effects of nitrogen deposition in two different watersheds in southern Norway. In this respect, quantification of the atmospheric input for comparis on with runoff and other transfer data has been an important task. The atmospheric input has been estimated on the basis of available data f rom the national monitoring of air and precipitation chemistry, and ad ditional measurement activities within the study areas, for the period 1993-1995. In the Bjerkreim watershed topographical effects result in large variation in precipitation amounts within the study area, and b etween the individual 44 hydrological subcatchments. Annual nitrogen d eposition varied between 1400 and 2300 mg N m(-2) yr(-1), with the hig hest figures obtained for the mountainous areas receiving most precipi tation. Similar observations were made for the atmospheric deposition of sulfur and chloride, which were determined in order to compare inpu t and runoff data as well as a basis for calculation of exceedances of the critical loads. Annual deposition of sulfur and chloride were 900 -1500 mg S m(-2) yr(-1) and 12000-21000 mg Cl- m(-2) yr(-1). The Auli watershed shows much less variation in the amount of precipitation and , consequently, in the deposition of nitrogen, sulfur and chloride to the 12 subcatchments for which input data were estimated. Annual avera ge deposition of nitrogen was estimated to be 1100 to 1250 mg N m(-2) yr(-1), the deposition of sulfur and chloride between 730 and 830 mg S m(-2) yr(-1), and 1780 and 2200 mg Cl- m(-2) yr(-1), respectively. Th e relative contribution of the dry deposition to the total deposition figures above is low, generally, of the order of 10%. This occurs beca use of the high precipitation frequencies and relatively low airborne concentrations.