Wall Deposition of Radon Progeny and Particles in a Spherical Chamber

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Abstract
In indoor and mining environments, deposition to “plate-out” of radon progeny onto walls occurs simultaneously with the attachment of progeny of airborne particles. Attachment and plate-out processes affect the atmosphere in which radon exposure takes place by reducing concentrations and shifting activity size distributions. Deposition of fine particles on paintings and other art objects is also a concern in museums. Here we describe plate-out measurements of radon progeny and aerosol particles in a spherical chamber under controlled laboratory conditions. The temperature and velocity profiles in still and turbulent air were monitored. A laboratory mixer with variable speeds and speed control was used to increase turbulence in the chamber. During mixing, air velocity was detected when rotational speeds were higher than 500 rpm. Monodisperse silver aerosols and polystyrene latex particles in the size range of 5 nm to 2 μm were used in the deposition study. Nanometer particles between 0.88 to 1.80 nm were generated by passing 220Rn gas into the chamber and letting the gas decay into 212Pb. The deposition rates of particles and radon progeny (212Pb) in the chamber were determined by monitoring the concentration decay of the aerosol as a function of time. Our data confirmed that the homogeneous turbulence model can be used to describe the wall deposition rate in still and mixing conditions. Higher deposition rates were observed during increased air mixing. Higher rates were more significant for particles smaller than 1.0 μm, indicating that the turbulence produced by mixing increased the turbulent diffusional deposition. The coefficient of eddy diffusivity was predicted by the mass transfer equation. The coefficient was also reasonably predicted from a technique using velocity measurement and from an energy dissipation equation.

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