Impactor aerosol measurements

For determination of the aerodynamic diameters of particles, the most commonly apphcable methods for particle-size analysis are those based on inertia aerosol centrifuges, cyclones, and inertial impactors (Lundgren et al.. Aerosol Measurement, University of Florida, Gainesville, 1979 and Liu, Fine Paiiicles—Aerosol Generation, Measurement, Sampling, and Analysis, Academic, New York, 1976). Impactors are the most commonly used. Nevertheless, impactor measurements are subject to numerous errors [Rao and Whitby, Am. Ind. Hyg. A.s.soc.]., 38, 174 (1977) Marple and WiUeke, "Inertial Impactors, in Lundgren et al.. Aerosol Measurement and Fuchs, "Aerosol Impactors, in Shaw, Fundamentals of Aerosol Sci-... 

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. 

The surface on which particles impact is also an important factor in determining impactor efficiency. Particles which bounce off the impaction surface can be carried through the impactor and can distort measurement data. Particle bounce will lower the collection efficiency of a given impactor stage and will lower the apparent mean diameter of the aerosol measured. 

The former approach, attempting to measure the droplets in their fully hydrated state, has enjoyed more widespread use. Experimentally, this has been accomplished in a number of ways. The use of low-flow impactors that entrain minimal quantities of ambient air and hence maintain the quasiequilibrium humidity exiting the nebulizer is one approach (30). Adding humidified dilution air, rather than ambient air, to a high-volume cascade impactor is another. Cooling the impactor to the same temperature as the nebulizer cloud (reservoir) has also been shown to be an effective way of providing sufficient stabilization for aerosol measurement purposes (31). However, a technique that has been used extensively in conjunction with a large number of deposition studies is laser diffraction (3). As described above, this technique measures the physical diameter... 

Marple, V.A., Willeke, K. (1979). Aerosol impactors. In Aerosol Measurement. University Presses of Horida,... 

B. W. Loo, J. M. JaMevic, and F. S. Goulding, "Dichotomous Virtual Impactors for Large Scale Monitoring of Airborne Particulate Matter," in B. Y. H. Liu, ed., Eine Particles, Aerosol Generation, Measurement, Sampling and Analysis, Academic Press, Inc., New York, 1976, pp. 311—350. 

Pb-210, Be-7, P-32, S-35 (as So ), and stable so - were measured using cascade impactors. The activity distribution of Pb-212 and Pb-214 was largely associated with aerosols smaller than 0.52 pm. 

Measurements on aerodynamic sizes of atmospheric aerosols and associated radionuclides were carried out with Anderson 2000, Inc., 1-ACFM Ambient Impactors with or without the Anderson low-pressure modification, as well as with Sierra model 236 (six-stage) high-volume impactors (HVI). The 1-ACFM design operated at 28... 

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 ... 

Aerosol samples for composition measurements were collected using the Caltech low pressure impactor (LPI) (21,22). This fully calibrated instrument classified particles into eight size fractions between 0.05 and 4 ym aerodynamic diameter. Collection of particles smaller than 0.3 ym is achieved by reducing the pressure and increasing the velocity of the jets. 

Approximate times of polluted (P), continental (C), and marine (M) air masses are indicated based on synoptic weather maps and consistent with aerosol composition measurements. Times of impactor samples, taken in duplicate concurrently with the streaker, are indicated at the top as midpoints of alternating 10- and 12-h sampling periods. 

In practice, when one measures the size distributions of aerosols using techniques discussed in Chapter 11, one normally measures one parameter, for example, number or mass, as a function of size. For example, impactor data usually give the mass of particles by size interval. From such data, one can obtain the geometric mass mean diameter (which applies only to the mass distribution), and crg, which, as discussed, is the same for all types of log-normal distributions for this one sample. Given the geometric mass mean diameter (/) ,) in this case and crg, an important question is whether the other types of mean diameters (i.e., number, surface, and volume) can be determined from these data or if separate experimental measurements are required. The answer is that these other types of mean diameters can indeed be calculated for smooth spheres whose density is independent of diameter. The conversions are carried out using equations developed for fine-particle technology in 1929 by Hatch and Choate. 

Van Vaeck, L K. Van Cauwenberghe, and J. Janssens, The Gas-Particle Distribution of Organic Aerosol Constituents Measurement of the Volatilization Artifact in Hi-Vol Cascade Impactor Sampling, Atmos. Environ., 18, 417-430 (1984). 

In an attempt to increase the amount of particles retained in the lungs, large porous particles with low density (p < 0.1 g/cm2) have been designed (Edwards et al. 1997). The particles were composed of 50% lactide and 50% glycolide. Porous and nonporous particles loaded with testosterone were aerosolized into a cascade impactor system from a dry powder inhaler (DPI) and the respirable fraction was measured. Nonporous particles (d = 3.5 pm, p = 0.8 g/cm3) exhibited a respirable fraction of 20.5 3.5%, whereas 50 + 10% of porous particles (d = 8.5 pm, p = 0.1 g/cm3) were respirable, even though the aerodynamic diameter of the two particle types were nearly identical. Porous particles as a consequence of their large size and low mass density can... 

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