“Unattached” Fraction of Short-lived Rn Decay Products in Indoor and Outdoor Environments

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Abstract
The unattached fraction fp of potential .alpha. energy of short-lived Rn decay products was measured under realistic, natural conditions in different dwellings and in the open atmosphere by a single-screen technique. An improved data evaluation method was developed where the measured activities of 218Po (RaA) and 214Pb (RaB) were corrected by the screen-attached activities of 214Bi (214Po) [RaC (RaC'')]. This method is based on the experimental observation that the 214Bi(214Po) unattached activities are negligible under realistic living conditions and that the size distributions of the aerosol-attached activities of all short-lived Rn daughters are identical. In closed rooms without additional aerosol sources, a mean unattached fraction fp of the potential .alpha. energy of 0.096 was observed at a mean aerosol particle concentration of 6100 cm-3 and at a mean equilibrium factor F of 0.30. This mean fp value is about three times higher than the value used in the literature for the radiation exposure calculation of the human public. In closed rooms with additional aerosol sources (cigarette smoke, heating systems, aerosols from a burning candle), the aerosol particle concentrations ranged up to 106 cm-3 and the attachment rates, X, increased up to 1000 h-1. The fp values sometimes decreased below the detection limit of 0.005, and the F values increased to as high as 0.77. In the ambient atmosphere in the vicinity of Gottingen, a mean unattached fraction fp of 0.02 and a mean aerosol particle concentration of 3.4 .times. 104 cm-3 were measured at 1 m above the ground. The mean equilibrium factor F was determined to be 0.7.