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This study investigates the temporal dynamics of outdoor radon activity concentration (RAC) in Bratislava, Slovakia, using a four-year dataset of continuous measurements. The fast Fourier Transform (FFT) analysis of the RAC data revealed the presence of periodicities at 24, 12, and 8 hours. A typical diurnal RAC cycle was observed, with peak concentrations occurring in the early morning and the lowest levels in the late afternoon. However, the diurnal RAC pattern exhibited distinct seasonal variations, being more pronounced during spring, summer, and autumn compared to winter. Seasonal variations in RAC were analyzed in the context of their daily minimum (background) and maximum values as well as their amplitudes. In order to gain insight into the underlying factors influencing RAC variability, we examined the relationships between RAC and a number of selected meteorological parameters, boundary layer height, and radon flux modeled based on uranium content and soil properties. The results of the correlation analysis indicated that the primary factors influencing the variation of background radon are wind direction, boundary layer height, and relative humidity. Wind direction was also the dominant factor affecting maximum daily RAC. The amplitude of RAC variation was strongly correlated with radon flux, temperature, and wind speed. The effect of other meteorological factors such as precipitation and atmospheric pressure on RAC was found to be statistically insignificant. Our results indicate that seasonal fluctuations in RAC are primarily influenced by changes in radon exhalation rate and atmospheric dispersion conditions caused by turbulence and thermal convection. These findings enhance our understanding of atmospheric radon dynamics.
This work was supported by the Scientific Grant Agency of the Ministry of Education, Science, Research and Sport of the Slovak Republic and the Slovak Academy of Sciences (VEGA projects No. 1/0019/22 and 1/0086/22), the Slovak Research and Development Agency (project No. APVV-21-0356), and the excellent grant of Comenius University (No. UK/3056/2024).