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60 Co activity distribution in the air and deposition in the Ignalina NPP region. The solubility in water of the aerosol-carriers of radionuclides in the ground-level air is investigated. It is shown that modelling results may be improved using different washout rates for water-soluble and insoluble radionuclides. The backward trajectories of air masses transport calculated using a HYSPLIT model show the Chernobyl NPP accident polluted area to be an additional source of 137 Cs in the INPP region.

Results of the137 Cs activity concentration measurements in the ground-level air in the Ignalina Nuclear Power Plant (Ignalina NPP) region in 1978-2006 are presented and discussed. The peculiarities of the trend of mean annual radiocaesium activity concentrations in the air during the studied period are explained and described empirically. The analysis of the radiocaesium concentration course after the Chernobyl accident shows that mean annual 137 Cs activity concentrations in the air are decreasing slower than it is expected due to its radioactive decay and aerosol, as the 137 Cs carrier, removal processes in the atmosphere. The Hybrid Single-Particle Lagrangian Integrated Trajectory model of the global dispersion and deposition of the admixture in the air is used to study episodic increases in the 137 Cs activity concentration in the ground-level air of more than 8 µBq m −3 . Modelling results show the transport of resuspended radiocaesium with air mass from the Chernobyl NPP accident polluted regions to be an important source of the radionuclide to the environment of the Ignalina NPP. It is shown that 137 Cs emissions from its generation sources (nuclear industry objects) and, probably, the depositional flux of the radiocaesium from the stratospheric reservoir, can contribute up to 10% to the mean annual 137 Cs activity concentration value. Mean arithmetic and mean geometric 137 Cs activity concentrations in the air during a year are calculated and compared.

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