ABSTRACT. Movements of air masses over several thousand kilometres affect the quality of arctic air in both summer and winter. This paper summarizes the results of a programme by the Norwegian Institute for Air Research (NILU) to identify and characterize sources of pollutants measured in the Norwegian Arctic, and to model the movement of pollutants within the air masses. Aircraft-based data on scattering coefficient, particle size distribution and chemical composition of aerosols were used to trace the origins of polluted layers of arctic air. Polluted air masses were observed at altitudes above 2,000 m in summer and 2,500 m in winter. Polluted layers below 1,500 m were related to episodic movements of air masses from areas where temperatures were similar to those of the Arctic, for example the northern Soviet Union: these layers also gave chemical indications of their origins. Profiles derived from airborne data were extended downward by measurements from the ground. From information on (1) total and size-differentiated chemical composition of surface aerosol, (2) enrichment factors of chemical constituents, (3) emission inventories for potential sources of air pollution in the Norwegian Arctic, and (4) wind trajectories at 850 mb, we conclude that emissions from anthropogenic sources mainly in Eurasia, and occasionally in North America, contribute to the pollution of arctic air over the whole year. Good agreement between these measurements and estimates from Lagrangian transport model estimates supports this conclusion.