| Summary: | During 21 months (April 2000–December 2001) of continuous aerosol particle number size distribution measurements in a subarctic area in northern Finland over 90 new particle formation events were observed. Measurements were done at two stations separated from each other by 250 kilometres. The Pallas station is located in western Lapland and Värriö station in eastern Lapland. New particle formation events were compared between the stations. Trajectories, trace gas concentrations and various weather parameters were investigated to explain the reasons for observed differences. According to trajectory analyses, all events were associated with marine/polar air masses originating from the northern Atlantic or the Arctic Ocean. 55% of particle formation events were observed at the same day on both sites, while on 45% of the cases particle formation was observed only at one of the stations. Most of these differences were explained by air masses of different origin or rain/fog on the other station. Among events observed only at the Värriö site, experiencing air masses from Kola Peninsula industrial area, about 80% were associated with high SO2 concentrations. At the Pallas site no increase in SO2 concentrations was observed. This is an indication that in plumes originating from the Kola industrial area, new particles are formed by nucleation involving H2SO4 from SO2 oxidation. The distance from Kola Peninsula to Pallas is long enough to reduce the SO2 concentration level lower than needed to launch the nucleation process at Pallas. Also on some occasions the air masses arriving at Pallas had been reasonably higher up in the atmosphere as compared with air masses at the site closer to Kola Peninsula. Air masses at higher altitudes are not influenced so much by the Kola industry. Based on this work about one third of the nucleation events observed at Värriö are effected or caused by the Kola Peninsula industry.
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