Long-term variations of radionuclides in the Bratislava air.

Variations of aerosol radionuclides (2001-2015) in the ground-level air in Bratislava (Slovakia) showed Be maxima in spring/early summer and minima in winter, however, an inverse trend was observed for Pb, Cs and K. A decreasing amplitude and splitting of summer maxima for Be in the last years has been found. A temporal behavior of the Be/Pb activity ratio showed higher levels during warm seasons due to vertical convection of air masses from higher altitudes. The Cs activity concentration in the surface air between 2003 and 2010 was decreasing with an effective half-life of 1.9 ± 0.3 years. The yearly average Cs concentrations during 2009-2014 were almost constant, disturbed only by the Fukushima accident in 2011. The increased atmospheric Cs and K levels observed during the autumn-winter season may be due to surface soil resuspension, biomass burning and radionuclide transport by winds. Seasonal variations of Rn activity concentrations were found with maxima at the end of autumn and in winter, and minima in spring. The variability of the average annual course of Rn has been larger than that of Pb. The Pb/Rn activity ratio was highest at the end of winter and in the spring, while from June to December remained nearly constant. More intensive atmospheric mixing in spring months caused a decrease in the Rn activity concentration, while the aerosol component of the atmosphere has been affected mainly during the autumn and winter seasons. The mean residence time of aerosols in the atmosphere was calculated using the Pb/Rn method to be 4.5 ± 0.9 days.