The Arctic air mass contains gaseous and particulate compounds that originate mainly from fossil fuel combustion at mid-latitudes and especially from the Eurasian sector. Observations of the temporal variation of SO 2, SO 42- and V concentrations in the North American and Norwegian Arctic are presented. At Igloolik, Canada, 3-7-day average SO 2 concentrations ranged from 2.3 to 4.3 μg m -3 in February to much lower values in spring and fall. The most probable cause of similar strong variations in the ratio of SO 4-2 to V observed throughout the North American Arctic is a seasonally varying SO 2 oxidation rate. Interpreted in the light of a Lagrangian transport model, observations indicate that the mean SO 2-oxidation rate between Eurasian sources and the North American Arctic is 0.1 % h -1 in early December, 0.04 % h -1 in late February, and 0.1-0.2% h -1 in early April. The residence time of SO 2, controlled not only by chemical conversion to sulphate but also by dry deposition, is 14-20 days in late fall, 16-32 days at mid-winter and 10-19 days in April. The estimated rates of SO 2 oxidation cannot be explained by photochemical oxidation mechanisms at least when reactive hydrocarbons are ignored.