Diurnal variation of radon progeny in indoor and outdoor air of a subtropical city

Abstract

Two monitors that collect radon progeny on a 0.8 μm filter at a 150 mL/min pumping rate, and detect gross alpha counts by a silicon surface barrier detector were used to continuously measure the working levels indoors and outdoors in Kaohsiung city for 2 y. The average outdoor radon progency concentration was about 18.3 nJm −3, while the indoor average of two different sites was 57.0 nJm −3 for the basement and 47.2 nJm −3 for the ground floor. The indoor/outdoor ratio of radon progeny concentration was around 2.4. Fourier analysis, based on hourly data, showed a significant periodic cycle at 24 h. This daily cycle was significant in winter and spring but not as significant in summer and fall. There was a less significant 12-h cycle for outdoor radon progeny. The radon progeny level was high in the early morning and low in the afternoon. This phenomenon is related to the temperature of near surface air for outdoor with a correlation coefficient of −0.68. The crosscorrelation function shows a 4-h time lag between indoor and outdoor data. It may be due to the limited space for indoor radon and its progeny to disperse. The maximum correlation coefficient in the 0.3-0.4 range implies a weak correlation between indoor and outdoor radon progeny. This analysis shows the typical radon progeny profile in a city of oceanic climate in a subtropical zone.