A modelling analysis of observations of aerosol concentrations during the LAPBIAT measurement campaign is presented and possible sources of the aerosols and their precursors are discussed. The LAPBIAT measurement campaign at the SMEAR I station at Varrio in eastern Lapland took place from 26 April to 11 May 2003 and registered a series of particle formation events. The SILAM model was applied in adjoint (inverse) mode for back-tracing the air masses observed during several selected episodes. It was also applied in forward mode using the EMEP sulphur emission data in order to qualitatively evaluate the contribution of anthropogenic sulphur to the corresponding events. As shown by adjoint simulations, the air masses corresponding to the observed peaks of concentrations of the nucleation-size aerosol often originated from areas different from the source regions responsible for the observed aged pollution plumes. We examined the origins of air masses during three nucleation events registered during the Varrio campaign. Observations of the first nucleation event were interrupted when polluted plume from the Nikel metallurgy plant (Russia) was transported to the site, replacing the cleaner air masses with the on-going particle formation. The second (most intensive) event occurred in the air that was transported for substantial distances over the central part of Finland in prevailing dry and relatively warm conditions. Since such conditions correspond to large biogenic VOC emissions, one can expect a major contribution of these species in the observed event. During the third episode, the air masses were transported directly from the Arctic Sea, thus suggesting a significant impact of marine aerosols. Also, three episodes with elevated concentrations of accumulation and coarse range particles were found. According to adjoint simulations, urban and industrial emissions were responsible for these events.