Active aerodynamic measure

Active aerodynamic measures are being used successfully in aeronautical engineering to suppress wing flutter. Using additional control surfaces, con-... 

Based on 46 measurements, the activity median aerodynamic diameter of Pb-212 averaged 0.13 pm (0g = 2.97), while Pb-214 averaged 0.16 pm (Og = 2.86). The larger median size of Pb-214 was attributed to a-recoil depletion of smaller aerosols following decay of aerosol-associated Po-218. 

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 I presents the average aerodynamic distributions of Pb-212 and Pb-214, as well as the frequency with which Pb-214 or Pb-212 was the dominant isotope in each size range. The Aitken nuclei fraction (below 0.08 pm) contained a higher percentage of Pb-212 activity compared with Pb-214 in 69.6% of the measurements. The predominance of Pb-212 in this fraction is also illustrated by the distributions reported in Figure 1. In the remaining measurements, where Pb-214 was fractionally more abundant below 0.08 um, the disparity between the relative amounts of each isotope was not nearly as dramatic. Conversely, Figure 1 and Table I illustrate that Pb-214 is generally enriched in the accumulation mode aerosol, particularly between 0.11 and 0.52 ]xm, where most of the surface area and mass occurs.

Figure 3. A comparison of mean Pb-212 activity (R), and SO4" mass (M) aerodynamic size (Dp) distributions from low-pressure impactor measurements made between Feb. and Sept. 1985. Lower Dp limits are arbitrary.

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 ... 

This treatment removed 92% of the Pu contamination, as measured by the activity of the 241 Am daughter product of241 Pu. It was even more effective in reducing the subsequent rate of re-suspension by the wind, which was 99% lower than before the treatment. As discussed later (Section 6.13), re-suspension depends strongly on the shearing stress exerted by the wind on the ground, and this was less after the treatment because the aerodynamic roughness of the surface was reduced. 

A mean value of 3 x 10-10 m-1 for Kr was deduced by Shinn et al. (1983) from measurements of airborne Pu over a bare field near the Savannah River Processing Plant. Here the activity median aerodynamic diameter was 3 //m. The main reason why Kr measured in Nevada and North Carolina was much lower than Kr measured at Maralinga and Monte Bello is the effect of ageing on the characteristics of the deposited material. 

Measurements of the urban aerosol mass distribution have shown that two distinct modes often exist in the 0.1 to 1.0 pm diameter range (Hering and Friedlander 1982 McMurry and Wilson 1983 Wall et al. 1988 John et al. 1990). These are referred to as the condensation mode (approximate aerodynamic diameter 0.2 pm) and the droplet mode (aerodynamic diameter around 0.7 pm). These two submicrometer mass distribution modes have also been observed in nonurban continental locations (McMurry and Wilson 1983 Hobbs et al. 1985 Radke et al. 1989). Hering and Friedlander (1982) and John et al. (1990) proposed that the larger mode could be the result of aqueous-phase chemical reactions. Meng and Seinfeld (1994) showed that growth of condensation mode particles by accretion of water vapor or by gas-phase or aerosol-phase sulfate production cannot explain existence of the droplet mode. Activation of condensation mode particles, formation of cloud/fog drops, followed by aqueous-phase chemistry, and droplet evaporation were shown to be a plausible mechanism for formation of the aerosol droplet mode. 

To study the potential health effects of pollutants in crowded spaces for recreational activities, particle mass versus size distribution of an ambient aerosol have been measured using a 10-stage Quartz Crystal Microbalance cascade impactor system (Junker et al. 2000). This impactor system consists of a series of 10 aerodynamic inertial impactors capturing particles at a mid-point cut-off ranging from 0.07 to 35 mm, arranged in a cascade with jets, which segregate the larger aerosol particles on top. 

The powder formulation contains the active substance in the correct aerodynamic size distribution, which for most currently marketed formulations is either obtained by micronisation or by spray drying. Both techniques produce polydisperse particles and their mass median aerodynamic diameter is preferably in the range between 1 and 5 pm, depending on the precise target area. Particles within this size range are extremely cohesive, whereas the powder masses to be measured are miniscule and mostly less than 5-500 micrograms for the active substances used in asthma and COPD treatment. Such small quantities of micronised powders cannot be delivered in a reproducible way without... 

A histogram of the activity size distribution of Be versus aerodynamic diameter Dp) is presented in Figure 2.1. This distribution was selected by Papastefanou and loannidou (1995) from 11 atmospheric aerosol sampling measurements made over an almost 2-year period at Thessaloniki, Greece (40°38 N, 22 58 E) by using Andersen 1 ACFM cascade impactors at a flow rate of 1.7 m h (28.31 min or 1 ft min ). 

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. 

Winkler et al. (1998) in 46 measurements in a period of 1 1/3 years (December 1994-March 1996) at Munich-Neuherberg, Germany (48 13 N, 11°36 E) showed that the activity median aerodynamic diameter, AMAD, of Be-aerosols ranged from 0.44 to 0.74 pm (average... 

Yu and Lee (2002) in 14 measurements in Hong Kong (22°18 N, 114 10 E) for a 3 1/2 month period (26 November 2001-8 March 2002) demonstrated that the activity median aerodynamic diameter, AMAD, of Be-aerosols varied from 0.33 to 1.15 pm (average 0.67 pm). They concluded that the AMADs of Be-aerosols are anticorrelated with Be concentrations in air, are correlated with relative humidity, RH and mean cloud cover, while temperature does not affect the AMADs of the Be-aerosols. 

The activity size distributions of Pb and Pb versus aerodynamic diameter (Dp) are represented by four subplots (Figure 2.5). These distributions were selected from 46 measurements made over a 10-month period at Oak Ridge, Tennessee (35°58 N, 84°17 W) by... 

The activity median aerodynamic diameters (AMADs) of Pb (Table 2.3) and mass median aerodynamic diameters (MMADs) of SO (Table 2.4) determined from a series of low- pressure (LPI) cascade impactor measurements made during the period January to October (1985) by Papastefanou and Bondietti (1987) are illustrated in Figure 2.7. The Pb data were derived from measurements made at the same time as and from measurements made to compare Pb versus Pb. The mean aerodynamic diameter of Pb was about three times smaller than that of SO ". Much less sulfate was found in the aerosol fraction below 0.08 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. 

Activity median aerodynamic diameters (AMADs, in jim) of Chernobyl fission products and cosmogenic Be measured at Oak Ridge, Tennessee... 

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). 

Using the data on radon decay product aerosols, Papastefanou and Bondietti (1991) reported a mean residence time, xr, of 8 days for aerosols of 0.3-pm activity median aerodynamic diameter (AMAD) size as determined from Bi/ Pb activity ratios. From the decay scheme of 2 Po T /2 = 3.05 min), because of the relatively short half-lives of the product radionuclides after two a-decays and two /S-decays, Pb-aerosols are produced. From 32 experiments for radon decay product aerosols ( °Pb-aerosols), Papastefanou and Bondietti (1991) calculated average values of fractions F and F2 of about 76.11 and 21.32, respectively. From 12 measurements of sulfate aerosols, Bondietti and Papastefanou (1993) calculated in the same manner average values of fractions F and F2 of about 68.67 and 12.63, respectively. According to Equation (4.12) and the above mentioned data, a mean residence time, xr, SO , of about 12 days would apply to sulfate aerosols of 0.3-pm mass median aerodynamic diameter (MMAD) size. 

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