Activity median diameter distributions

The calculated size distribution of newly attached decay products is shown as curve C in Fig. 1.9. The activity median diameter is 0.16 /zm. With passage of time, the distribution would be shifted to larger particle sizes, as coagulation proceeds. George (1972) used diffusion batteries to measure the size distribution of nuclei carrying radon decay products and found activity median diameters (AMD) averaging 0.18,0.11, and 0.30 /um in a city basement, fifth floor room, and rural outside air, respectively. 

Activity Median Aerodynamic Diameter (AMAD)—The diameter of a unit-density sphere with the same terminal settling velocity in air as that of the aerosol particle whose activity is the median for the entire size distribution of the aerosol. 

The mean values of the activity and aerosol median diameters together with the best estimate of the standard deviation, an, based on the total number of measurements made for each parameter, are listed in Table V. Figures 5-8 show representative size distributions ... 

Table IV. Activity and Aerosol Size Distributions, Count Median Diameter (CMD) and the Geometric Standard Deviation (indicated in brackets)...

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. 

Table II. Summary of Mean Monthly Activity Median Aerodynamic Diameters (AMAD) and Geometric Standard Deviations (tfg) of Radon and Thoron Daughter Size Distributions in Ambient Aerosols...

The aerodynamic size distributions of Pb-214, Pb-212, Pb-210, Be-7, P-32, S-35-SoJ , and stable SO4 were measured using cascade impactors. Pb-212 and Pb-214, measured by alpha spectroscopy, were largely associated with aerosols small than 0.52 11m. Based on over 46 low-pressure impactor measurements, the mean activity median aerodynamic diameter (AMAD) of Pb-212 was found to be 0.13 11m, while for Pb-214 the AMAD was larger—0.16 lim. The slightly larger size of Pb-214, confirmed with operationally different impactors, was attributed to a-recoil-driven redistribution of Pb-214 following decay of aerosol-associated Po-218. A recoil model was presented that explained this redistribution. Low-pressure impactor measurements indicated that the mass median aerodynamic diameter of SoJ ... 

Improved control devices now frequently installed on conventional coal-utility boilers drastically affect the quantity, chemical composition, and physical characteristics of fine-particles emitted to the atmosphere from these sources. We recently sampled fly-ash aerosols upstream and downstream from a modern lime-slurry, spray-tower system installed on a 430-Mw(e) coal utility boiler. Particulate samples were collected in situ on membrane filters and in University of Washington MKIII and MKV cascade impactors. The MKV impactor, operated at reduced pressure and with a cyclone preseparator, provided 13 discrete particle-size fractions with median diameters ranging from 0,07 to 20 pm with up to 6 of the fractions in the highly respirable submicron particle range. The concentrations of up to 35 elements and estimates of the size distributions of particles in each of the fly-ash fractions were determined by instrumental neutron activation analysis and by electron microscopy, respectively. Mechanisms of fine-particle formation and chemical enrichment in the flue-gas desulfurization system are discussed. 

NIR has been used to determine the mass median diameter of a micronized active compound contained in a lactose monohydrate at a concentration of 4% by weight and a size between 8 and 20 pm [223]. Multivariate statistical analysis was applied to zero order NIR spectra using particle size distributions by low angle laser light scattering as a reference technique. Due to its speed, simplicity and low operating costs it was demonstrated that this is a viable alternative to other methods used to carry out this type of analysis. 

From the 11 measurements carried out over a 2-year period including all seasons, Papastefanou and loannidou (1995) reported that the activity median aerodynamic diameter (AMAD) varied from 0.76 to 1.18 pm (average 0.90 pm) and the geometric standard deviation (a ) varied from 1.86 to 2.77 (average 2.24). The AMAD and (ag) calculations were made by plotting the cumulative distributions on log-normal probability paper. They also showed that 60% of the Be activity was associated with particles with diameter smaller than 1.1 pm. 

The " Pb activity median aerodynamic diameters, AMADs, determined with the low pressure impactors (LPI) varied from 0.10 to 0.37 pm (mean value 0.16 pm). For Pb, the AMADs varied from 0.07 to 0.245 pm (mean value, 0.13 pm). These AMAD calculations were made assuming log-normal distributions. An abbreviated version of these results is presented in Table 2.3 (Papastefanou and Bondietti, 1987). 

Summary of mean monthly activity median aerodynamic diameters (AMADs) and geometric standard deviations (ffg) of radon ( Rn) and thoron ( °Rn) decay products size distributions in ambient aerosols... 

Winkler et al. (1998) reported that the activity size distribution of Pb in ambient aerosols was unimodal (log-normal) and associated with submicron aerosols of about 0.5 to 0.6 pm. On average, the activity median aerodynamic diameter, AMAD, of Pb-aerosols (0.53 pm) has been found to be significantly lower than the average mass median aerodynamic diameter, MMAD (0.675 pm), and higher than or at most equal to the respective surface median aerodynamic diameter, SMAD, (0.465 pm) of the aerosols SMAD < AMAD < MMAD. Variation of the atmospheric processes resulted in a variability of the activity median aerodynamic diameter, AMAD, between 0.28 and 0.74 pm for Pb. While in the winter period (October to April) the AMAD of Pb averaged 0.595 pm, in the summer period Pb was associated with significantly smaller aerosols (AMAD 0.43 pm). 

Table 2.4 summarises the activity median aerodynamic diameter (AMAD) of Pb and Be and the mass median aerodynamic diameter (MMAD) of SO found in measurements made in the spring period. Beryllium-7 activity size distributions are substantially smaller than 804 , regardless of the time of year. The Pb data included in Table 2.4, while limited, suggest that summer aerosol particle sizes are larger than winter aerosol particle sizes. 

In contrast to the findings of Bondietti and Brantley (1986), lost et al. (1986) found no increase of the activity median aerodynamic diameter, AMAD, during the measurement period. They believed that this increase might not reflect the original release at Chernobyl, but was due to transport effects. Apart from this, the activity size distribution of Cs from the Chernobyl fallout was very similar to the activity size distribution in the fallout from nuclear weapons tests (Lockhart et al., 1965b), in contrast to the activity size distribution found in the stratosphere, which is shifted towards smaller particles (Persson and Sisefsky, 1971). 

Activity median aerodynamic diameters, AMADs, in tun and geometric standard deviations, Og, the activity size distributions for aerosol collected in different work environments... 

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). 

Average values of the relative size distribution of the unattached radon decay products in terms of potential alj a energy concentration (PAEC). The measured values were approximated by a sum of i log-normal distributions, char-actmsed by the activity median aerodynamic diameter, AMAD, noted as AMDui, geometric standard deviation, Ogui. and activity fraction, fpvd- n = numba- of measurements Z = aerosol parhcle concentrations Cq = radon concentration RH = relative humidity... 

Parameters of the activity size distribution of the aerosol-attached short-Uved radon decay products in air at different locations. Activity median aerodynamic diameter, AMAD, noted as AMD geometric standard deviation, Og fraction of the mode, fpi. The indices i = n, a and c represent the nucleation (Aitken nuclei), accumulation and coarse particles modes, Z = aerosol particle concentrations... 

As shown by Schumann et al. (1988), bounce-off effects do not play any role for particles smaller than 1 pm in diameter. For larger aerosol particles, the bounce-off effect can be minimised by coating the impaction plates or foils with oleic acid solutions. Additionally, the bounce-off problem is more severe for mass size distributions than for activity size distributions with median diameters between 200 and 600 pm. Nevertheless, the influence of blow-off effects on the activity size distribution for longer operation periods has still to be investigated. 

Using a method of weighed least-squares, a log-normal function describing the aerosol mass and activity size distribution with respect to aerodynamic (resistance) diameter has been routinely fitted to the data. It is also possible to approximate a log-normal function for the mass and activity distribution with the use of log-probability paper. The distribution is then describable by a mass or activity median aerodynamic diameter, MMAD or AMAD, and associated geometric standard deviation, Og. 

---