Aerosol progeny

Bandi, F., A. Khan, and C.R. Phillips, Effects of Aerosol Poly-dispersity on Theoretical Calculation of Unattached Fractions of Radon Progeny, this volume (1987). 

Knutson, E.O., A.C. George, L. Hinchliffe, and R. Sextro, Single Screen and Screen Diffusion Battery Method for Measuring Radon Progeny Size Distributions, 1-500 nm, presented to the 1985 Annual Meeting of the American Association for Aerosol Research,... 

Effects of Aerosol Folydispersity on Theoretical Calculations of Unattached Fractions of Radon Progeny... 

Theoretical calculations of unattached fractions of radon progeny require prediction of an attachment coefficient. Average attachment coefficients for aerosols of various count median diameters, CMD, and geometric standard deviations, ag, are calculated using four different theories. These theories are ... 

Theoretical calculations of unattached fractions of radon or thoron progeny involve four important parameters, namely, 1) the count median diameter of the aerosol, 2) the geometric standard deviation of the particle size distribution, 3) the aerosol concentration, and 4) the age of the air. All of these parameters have a significant effect on the theoretical calculation of the unattached fraction and should be reported with theoretical or experimental values of the unattached fraction. 

In addition to the quasi-steady state assumption, the other assumptions required to arrive at equation (1) are 1. the aerosol itself does not coagulate 2. there is a fully developed concentration gradient around each aerosol particle and 3. the concentration of unattached radon progeny atoms is much greater than the concentration of aerosol particles (in order that concentration gradients of radon progeny atoms may exist). This last assumption is usually not valid since the radon progeny concentration is usually much less than the aerosol concentration. 

From equation (1) it can be seen that application of the diffusion theory leads to the conclusion that the rate of attachment of radon progeny atoms to aerosol particles is directly proportional to the diameter of the aerosol particles. 

The kinetic theory of radon progeny attachment to aerosol particles assumes that unattached atoms and aerosol particles undergo random collisions with the gas molecules and with each other. The attachment coefficient, 3(d), is proportional to the mean relative velocities between progeny atoms and particles and to the collision cross section (Raabe, 1968a) ... 

The attachment coefficient is a function of the aerosol particle diameter, d, and mean velocity, v, as well as the unattached progeny diameter, d, and its mean velocity v. Since in most situations d d and v v, equation (2) reduces to... 

When the radius of an aerosol particle, r, is of the order of the mean free path, i, of gas molecules, neither the diffusion nor the kinetic theory can be considered to be strictly valid. Arendt and Kallman (1926), Lassen and Rau (1960) and Fuchs (1964) have derived attachment theories for the transition region, r, which, for very small particles, reduce to the gas kinetic theory, and, for large particles, reduce to the classical diffusion theory. The underlying assumptions of the hybrid theories are summarized by Van Pelt (1971) as follows 1. the diffusion theory applies to the transport of unattached radon progeny across an imaginary sphere of radius r + i centred on the aerosol particle and 2. kinetic theory predicts the attachment of radon progeny to the particle based on a uniform concentration of radon atoms corresponding to the concentration at a radius of r + L... 

In calculating 3(d) using Fuchs theory, D and v for radon progeny are used as above. It is also assumed that the aerosol particles are spherical and of unit density. It may be noted that assuming a higher density for the aerosols does not change the results appreciably. Fuchs model is calculated for... 

The rate of attachment of radon progeny to aerosols, X, is expressed as ... 

The most complete theory for aerosol coagulation is that of Fuchs (1964). Since the attachment of radon progeny to aerosols can be considered as the coagulation of radon progeny (small diameter particle) to aerosols (large diameter particle), it is reasonable to use Fuchs theory to describe this process. The hybrid theory is an approximation to Fuchs theory and thus can be used to describe the attachment of radon progeny to aerosols over the entire aerosol size spectrum. 

If it is accepted that the hybrid theory is the most complete theory for attachment of radon progeny to aerosols, the magnitude of the error involved in using exclusively either the kinetic or the diffusion theory can be seen from Figs. 3-7 and Table III. 

Unattached fractions of RaA (at t = °°) for two mine aerosols and for a typical room aerosol are shown in Table III. It is usually assumed that the attachment of radon progeny to aerosols of CMD < 0.1 ym follows the kinetic theory. In Table III it is apparent that the hybrid and kinetic theories predict similar unattached fractions for monodisperse aerosols. However, for more polydisperse aerosols, the kinetic theory predicts lower unattached fractions than the diffusion theory and thus the diffusion theory is the more appropriate theory to use. It is also evident that the kinetic-diffusion approximation predicts unattached fractions similar to those predicted by the hybrid theory in all cases. 

Calculation of the attachment coefficient is required for theoretical prediction of the unattached fraction of radon progeny. The hybrid theory, which is a form of Fuchs theory with certain justifiable assumptions, can be used to describe attachment to aerosols under all conditions of Og and CMD. 

Khan, A., F. Bandi, C.R. Phillips and P. Duport, Underground Measurements of Aerosol and Radon and Thoron Progeny Activity Distributions, to be published in Proc. 191st American Chemical Society National Meeting, New York, April 13-18 (1986). 

Underground Measurements of Aerosol and Activity Distributions of Radon and Thoron Progeny... 

Experiments were conducted in a large (-26 m3) radon/thoron test facility (RTTF) designed for calibration purposes and simulation studies (Bigu, 1984). A number of different materials were exposed in the RTTF to a radon/radon progeny or thoron/thoron progeny atmosphere. Exposure of the materials was carried out under laboratory-controlled conditions of radiation level, aerosol concentration, air moisture content and temperature. The materials used were in the form of circular discs of the same thickness (-0.5 mm) and diameter (-25 mm), and they were placed at different locations on the walls of the RTTF at about 1.6 m above the floor. Other samples were placed on horizontal trays. Samples (discs) of different materials were arranged in sets of 3 to 4 they were placed very close to one another to ensure exposure under identical conditions. Exposure time was at least 24 hours to ensure surface activity equilibrium, or near equilibrium, conditions. 

In order to verify the results reported above, an independent series of measurements in the large RTTF was carried out. Samples were exposed to a thoron progeny atmosphere, and surface gross a-particle activity was measured this time for periods of 30 min. The environmental conditions during this experimental phase were in the fol lowing range. Temperature 24-27°C, relative humidity 40-55%, aerosol concentration 1.2 x 103 - 3.4 x 103 cm-3. Some of the data obtained are reproduced in Table I. 

Porstendorfer, 1984). Knutson et al. (1983) measured similar results in their chamber investigation. The results show that the values of the deposition velocity of the free radon daughters are about 100 times those of the aerosol radon progeny. But there are no information about the effective deposition surface S of a furnished room for the calculation of the plateout rates qf and qa by means of Vg and Vg. For this reason the direct measurements of the plateout rates in rooms are necessary. Only Israeli (1983) determined the plateout rates in houses with values between qf = 3-12 h"1 and qa = 0.4-2.0 h"1, which give only a low value of the... 

The decay of Rn-222 and Rn-220 in the atmosphere produces low vapor pressure progeny that coagulate with other nuclei or condense on existing aerosols. These progeny include 3.0-min (radioactive half-life) Po-218, 26.8-min Pb-214, and 10.6-h Pb-212. A... 

Experiments in which mice, rats, and dogs were exposed to inhalation of high concentrations of radon progeny on natural aerosols for prolonged... 

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