Aerosol size distributions were measured in the Finnish Arctic over a continuous period of 1-year using diffusion battery and condensation particle counter. Numerous nucleation bursts were observed. Some of the nucleation events were associated with marine air clean of SO2, while others were associated with SO2-pollution events originating from smelters located in the Kola Peninsula. Six nucleation events were analyzed with a combined gas phase chemistry-aerosol dynamical model that used meteorological input data taken from calculated back trajectories. The assumed homogeneous nucleation mechanism of sulfuric acid and water explains the observed pollution-related nucleation events well but does not explain the marine nucleation events. ¿ 1998 American Geophysical Union