Aerodynamic size distributions radionuclides

Aerodynamic Size Distributions of Naturally-Radioactive Aerosols. Measurements of radionuclide distributions using cascade impactors indicate that Be-7 and Pb-210 are associated with larger aerosols than Pb-212 and Pb-214 (Robig et al., 1980 Papastefanou and Bondietti, 1986). Measurements of Pb-210 associations over oceans indicated activity median aerodynamic diameters (AMAD) near 0.6 pm (Sanak et al., 1981). The impactor measurements of Moore et al. (1980) on Pb-210, Bi-210, and Sr-90 sizes in continental air indicated that about 80% of the activity from all three nuclides was associated with aerosols below 0.3 pm. That work also determined that the mean age of aerosol Pb-210 was about a week. Knuth et al. (1983) compared Pb-210 and stable Pb sizes at a continental location and found that 78% of the Pb-210 found below 1.73 pm was smaller than 0.58 pm. Young (1974) reported that the most of the Be-7 in the atmosphere was associated with submicron aerosols. 

This paper summarizes part of the results of an investigation designed to characterize the aerodynamic size distributions of natural radioactivity and to evaluate the results in the context of sulfate distributions and recent advances in the understanding of aerosol growth mechanisms. This paper, while emphasizing our results on Pb-212 and Pb-214, also summarizes our initial data for longer-lived radionuclides. 

Aerodynamic size distribution of radionuclide-associated aerosol particles (radioactive aerosols)... 

The aerodynamic size distribution of radionuclide-associated aerosol particles is, as mentioned in Section 2, a surface distribution, and so it is trimodal, the first mode being the so-called Aitken nuclei mode, the second the accumulation mode and the third the coarse particle mode. Analytically, these modes can be summarised as follows ... 

The aerodynamic size distributions of Pb and SO were quite different, reflecting the different dependencies of surface area and volume on aerosol diameter (Friedlander, 1977). Lead-212, like the other radionuclides, becomes associated with atmospheric aerosol particles by condensation or coagulation processes which are surface-area related. Sulfate, on the other hand, is the main solute in the accumulation mode aerosol so that its steady-state distribution is proportional to volume, even though condensation of H2SO4 may dominate its initial aerosol association. The difference also reflects residence times of 16.7 h for Pb -i- Bi and of about a week for sulfate. 

Rdbig et al. (1980) reported that the distribution of the long-lived radionuclide Be was shifted to large particle sizes due to long residence times of Be in the atmosphere. An equivalent aerodynamic diameter of about 0.65 pm for Be might have resulted from the plot of the activity size distribution of the ambient air obtained by a high volume cascade impactor... 

Fig. 2.8. Aerodynamic size (Dp) distributions of radionuclides R) and SO (Af) derived from a 7-day aerosol sampling made using two high-volume cascade impactors, HVI. Pb-212, and were only measured on...

A method for estimating the residence time of tropospheric aerosol particles associated with the cosmic-ray produced radionuclides, such as Be, is based on the aerosol particle growth rate, which is the change of particle diameter with time, which was estimated to be 0.004 to 0.005 pmh (McMurry and Wilson, 1982) and the difference between the activity median aerodynamic diameter, AMAD, of a radionuclide, e.g. Be, and the size of the Aitken nuclei in the size distribution of the aerosol particles, which is 0.015 pm (NRC, 1979). The AMAD of all radionuclides is in the accumulation mode of the size distribution of atmospheric aerosol particles which ranges between 0.1 and 2.0 pm (NRC, 1979 Papastefanou and Bondietti, 1987). 

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