Relationship between the Unattached Fraction of Airborne RaA and the Concentration of Condensation Nuclei

Abstract

THE decay of a radon atom results in the production of an atom of the first daughter, ²¹⁸Po, traditionally called RaA. This RaA atom may collide with and become attached to one of the condensation nuclei present in the atmosphere, and the fraction, f, of RaA atoms which are in the unattached state is of interest in any assessment of the hazard arising from the inhalation of radon daughter products¹. One might expect f to depend primarily on the concentration, Z, of condensation nuclei in the atmosphere (such nuclei nearly always form the major part, in terms of numbers of particles, of the atmospheric aerosol), but there are other parameters which are likely to influence f, for example, the size distribution of the condensation nuclei, the charge carried by these nuclei, the rate of change of the nucleus concentration, and the rate of change of the RaA concentration. In addition, in some circumstances the losses of unattached RaA atoms by collision with surfaces, such as walls, may be significant and such losses are yet another factor having some influence on f.