Cascade impactors, aerosol measurements

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. 

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 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 ... 

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... 

Particle Number Concentration and Size Distribution. The development of aerosol science to its present state has been directly tied to the available instrumentation. The introduction of the Aitken condensation nuclei counter in the late 1800s marks the beginning of aerosol science by the ability to measure number concentrations (4). Theoretical descriptions of the change in the number concentration by coagulation quickly followed. Particle size distribution measurements became possible when the cascade impactor was developed, and its development allowed the validation of predictions that could not previously be tested. The cascade impactor was originally introduced by May (5, 6), and a wide variety of impactors have since been used. Operated at atmospheric pressure and with jets fabricated by conventional machining, most impactors can only classify particles larger... 

Chemical Composition Aerosol composition measurements have most frequently been made with little or no size resolution, most often by analysis of filter samples of the aggregate aerosol. Sample fractionation into coarse and fine fractions is achieved with a variety of dichotomous samplers. These instruments spread the collected sample over a relatively large area on a filter that can be analyzed directly or after extraction Time resolution is determined by the sample flow rate and the detection limits of the analytical techniques, but sampling times less than 1 h are rarely used even when the analytical techniques would permit them. These longer times are the result of experiment design rather than feasibility. Measurements of the distribution of chemical composition with respect to particle size have, until recently, been limited to particles larger than a few tenths of a micrometer in diameter and relatively low time resolution. One of the primary tools for composition-size distribution measurements is the cascade impactor. 

Particle Size Analysis Many particle-size-analysis methods suitable for dry-dust measurement are unsuitable for liquids because of coalescence and drainage after collection. Measurement of particle sizes in the flowing aerosol stream by using a cascade impactor is one... 

Most cascade impactors do not give data in real time. The collection surfaces must be removed from the device and subjected to chemical or gravimetric analysis. However, one impactor does give data in real time. The Model PC-2 Air Particle Analyzer (California Measurements, Inc., Sierra Madre, CA) achieves a real-time measurement by using piezoelectric quartz crystal microbalance (QCM) mass sensors to electronically weigh particles at each impactor stage [62,63], The device has 10 stages and separates the aerosol into... 

In addition, the cumulative size distribution obtained for pure micron-ized and supercritically produced powders, combined with computation of the aerodynamic diameter according to Eqs. (2) and (3), shows that FPF of supercritically produced particles is about 50%, twice as much as FPF for micronized powder and close to the dispersion efficiency found with lactose (76). The correct values for the total emitted dose of drug particles also were confirmed. In all cases, the state of dispersion for the different formulations was correctly predicted and the results obtained showed good agreement with the cascade impactor measurements made on the same samples. Therefore laser diffraction has distinct advantages over impactor techniques with respect to the speed and reproducibility of measurements and can be used as a complementary method for analysis of aerosol behaviour of different formulations. 

There are instruments capable of measuring the composition of individual particles (Chapter 6)t and eventually it will be possible to determine the size-composition p.d.f. Currently, such measurements are made on a research basis in practice the average composition of the particles in a discrete size interval is determined by collecting an aerosol sample over a period of several hours using a cascade impactor (Chapter 6) and analyzing the material on each stage chemically. The concentration measured in this way is related to g(u, wj,. rt. r, f) as follows ... 

In the applications of gas-solid flows, measurements of particle mass fluxes, particle concentrations, gas and particle velocities, and particle aerodynamic size distributions are of utmost interest. The local particle mass flux is typically determined using the isokinetic sampling method as the first principle. With the particle velocity determined, the isokinetic sampling can also be used to directly measure the concentrations of airborne particles. For flows with extremely tiny particles such as aerosols, the particle velocity can be approximated as the same as the flow velocity. Otherwise, the particle velocity needs to be measured independently due to the slip effect between phases. In most applications of gas-solid flows, particles are polydispersed. Determination of particle size distribution hence becomes important. One typical instrument for the measurement of particle aerodynamic size distribution of particles is cascade impactor or cascade sampler. In this chapter, basic principles, applications, design and operation considerations of isokinetic sampling and cascade impaction are introduced. 

The Ni contents of the four fractions are determined by ETAAS. Vincent et al. (2001) used a similar approach based on differential step-wise leaching of nickel from aerosol particles that had been fractionated with a cascade impactor. For fractionation of nickel species in natural waters, Mandal et al. (2002) used Chelex-100 as a competing ligand to measure the rate of Nk released by dissociation of nickel in dissolved organic carbon complexes. 

For a spherical particle of nonunit density the classical aerodynamic diameter is different from its physical diameter and it depends on its density. Aerosol instruments like the cascade impactor and aerodynamic particle sizer measure the classical aerodynamic diameter of atmospheric particles, which is in general different from the physical diameter of the particles even if they are spherical. 

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 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... 

In nearly all cases, cascade impactors with inlet throats are used for the analysis of DPI clouds. In most instances this is because DPI clouds contain excipients as well as drug particles, and chemical drug-specific assays are required in order to measure the drug size distribution. Although inlet bends are not strictly necessary, because the aerosol particles are moving at the same speed as the inhaled air, they are usually used as a matter of experimental convenience. 

Pritchard J, Layzell G, Miller J. Correlation of cascade impactor data with measurements of lung deposition for pharmaceutical aerosols. In Dmg Delivery to the Lungs VII, The Aerosol Society, 1996 101-104. 

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