Temporal Variability of Bismuth-214 Activity Concentration in the Atmospheric Surface Layer of Sevastopol Region

The article presents the results of monitoring of bismuth-214 activity concentration in atmospheric aerosols in the atmospheric surface layer of the Sevastopol region in 2007– 2020. Bismuth-214 is a daughter product of the decay of radon-222. It is one of the main radionuclides that form the natural gamma background of the surface atmosphere. The aim of this work is to obtain quantitative characteristics of the temporal variability of the concentration of bismuth-214 in the atmospheric surface layer of the Sevastopol region and to estimate the annual effective dose due to outdoor radon-222 exposure. During the study period, 2701 aerosol samples were taken and processed. Measurements of the activity of bismuth-214 in atmospheric aerosol samples were performed using a lowbackground gamma-spectrometer with a NaI (Tl) scintillation detector. The concentration of the radionuclide in individual samples varied from 0.1 to 11.4 Bq m –3 , the average value was 2.7 ± 1.5 Bq m –3 . There is periodicity of 29, 66, 110 days and 1 year in the temporal variability of the activity concentration of bismuth-214. Higher values of bismuth-214 activity concentration are typical for the period from July to October (3.1– 3.5 Bq m –3 ), lower values are typical for the period from December to April (2.1– 2.2 Bq m –3 ). It is shown that the seasonal variation in the origin of atmospheric aerosol in the region under study can be the factor determining the change in the seasonal cycle of bismuth-214 concentration in comparison with the expected cycle of radon-222. In the last year of observations, there is an increase in the concentration of bismuth-214 by an average of 22% compared with the average long-term value of its concentration and a change in its annual cycle, which is probably associated with construction work carried out in the central part of the city. Quantitative estimates of the effective dose due to outdoor radon-222 exposure have been obtained.