Ozone concentrations measured at the Pallas-Sodankylä Global Atmosphere Watch station, located north of the Arctic Circle on a low mountain, were analysed together with data on nitrogen oxides concentrations and cloudiness. Air parcel trajectories were used to characterise the source area of air masses. The monthly mean ozone concentrations at Pallas showed a maximum in April and a broad minimum in late summer/autumn. In summer, the highest concentrations were observed in southern air masses on average, while the Atlantic and Arctic cases dominated in the earlier and latter part of the year, respectively. In autumn/early winter, there were large differences between the source areas. Throughout the year, ozone concentrations were depleted in the presence of clouds. In summer, the reduction was larger in unpolluted than polluted cases. It is suggested that the formation of nitrate radical and the scavenging of dinitrogen pentoxide act as an efficient ozone sink in winter, while in summer the difference between clear and cloudy conditions is due to the direct and indirect effects of aqueous chemistry.