Temporal variation of radon progeny ratio in outdoor air

This paper investigates the concentration ratio of radon progeny (218Po, 214Pb, and 214Bi) in outdoor air. From 1988 to 1998, alpha spectroscopy was used intermittently to collect progeny data outside the author's office located in the northeastern part of Japan. Let the quantity R(EEC) represent the ratio EEC(218Po)/[EEC(214Pb) + EEC(214Bi)], where, e.g., EEC(218Po) denotes the equilibrium equivalent radon concentration contributed by 218Po. The concentration ratio R(EEC) correlates with outdoor air stability. Statistical and time series analyses of R(EEC) indicate (1) outdoor air is more stable in the morning than in the afternoon, (2) outdoor air is more stable in summer/autumn than in winter/spring, and (3) in spite of no significant correlation between R(EEC) and wind speed, the power spectrum of R(EEC) appears similar to that of wind speed. This is probably due to the fact that near-surface mixing is not very sensitive to wind speed.