Cascade impactor distribution

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 ... 

Program faculty members are developing an automated cascade impactor for collection of task-based size distribution data of beryllium-containing aerosols. Based on the size distribution, the fraction of beryllium-containing aerosol penetrating a respirator and the inhalation and deposition in different regions of the lungs can be estimated. 

It must also be emphasized that the major mass of a heterodispersed aerosol may be contained in a few relatively large particles, since the mass of a particle is proportional to the cube of its diameter. Therefore, the particle-size distribution and the concentration of the drug particles in the exposure atmosphere should be sampled using a cascade impactor or membrane filter sampling technique, monitored using an optical or laser particle-size analyzer, and analyzed using optical or electron microscopy techniques. 

The size distribution of the particulate matter in the 0.01-5 ym size range is analyzed on line using an electrical mobility analyzer and an optical particle counter. Samples of particles having aerodynamic diameters between 0.05 and 4 ym are classified according to size using the Caltech low pressure cascade impactor. A number of analytical procedures have been used to determine the composition distribution in these particles. A discrete mode of particles is observed between 0.03 and 0.1 ym. The major components of these particles are volatile elements and soot. The composition of the fine particles varies substantially with combustor operating conditions. 

Most of the published composition/size distribution data have been obtained by analyzing cascade impactor samples. Some of these data suffer from poor size classification as a result of particle bounce or reentrainment, seriously limiting size resolution. Even when this problem is overcome, the data obtained with conventional cascade impactors are not capable of resolving many details of the distribution of submicron particles. These instruments typically classify only those particles larger than 0.3-0.5 tam aerodynamic diameter. All smaller particles are collected on a filter downstream of the impactor. Some measurements of the variation of composition with size below this limit have been attempted by aerodynamically classifying resuspended ash ( ). These data suffer from incomplete disapregation as well as poor classification of the smaller particles. 

The aerosol produced by a laboratory pulverized coal combustor was size classified in the range 0.03 to 4 ym Stokes equivalent diameter using a low-pressure cascade impactor. The samples thus collected were analyzed using a focussed beam particle induced X-ray emission technique. This combination of techniques was shown to be capable of resolving much of the structure of the submicron coal ash aerosol. Two distinct modes in the mass distribution were observed. The break between these modes was at a particle size of about 0.1... 

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. 

The average particle size distributions for four predominantly crustal elements, Al, Si, Ca, and Ti, are shown in Figure 3. They are essentially identical. It should be pointed out that the downturn of the relative concentrations above 8 ymad (impactor stage 6) is the combined result of the actual distribution of particle sizes in the atmosphere and the efficiency with which these very coarse particles can enter (upward) into the cascade impactor. This efficiency must decrease with increasing particle size and generally depend on inlet design and wind speed. Nevertheless, it is important to note here that the patterns of the four elements are similar, implying a common aerosol source. 

Van Vaeck, L., and K. Van Cauwenberghe, Cascade Impactor Measurements of the Size Distribution of the Major Classes of Organic Pollutants in Atmospheric Particulate Matter, Atmos. Environ., 12, 2229-2239 (1978). 

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). 

Stein SW. Size distribution measurements of metered dose inhalers using Andersen Mark II cascade impactors. Int J Pharma 1999 186(1) 43—52. 

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. 

JET IMPACTION. A rapidly moving particle, striking a suitably coated surface, will leave an impression whose size is a function of the original drop diameter. Pilcher (7S) illustrates the operation of the jet impactor. The higher the jet velocity, the smaller the particle size which will impinge. An instrument called the cascade impactor consists of a series of these slides. The jet velocity is increased from slide to slide thus, the cascade is useful in determining droplet size distributions. 

The principal problems in determining size distribution parameters with cascade impactors are wall losses, inefficient collection due to particle bounce, deposition of gas-phase species on impaction substrates, and deposition of fine particles from boundary layers. 

FIGURE 17.4. Differential particle size distrihution of B(a)P carhon black aerosol (100 pg/m ). Subsequent to exposure, substrate post-weights were recorded and entered into the Win-CIDRS (Windows-Cascade Impactor Data Reduction Program) to generate the particle size distribution for the particulate aerosol. From Hood et al. (2000). 

Cascade impactors and cyclones have been used in order to determine the aerodynamic size distribution. Impactors allow the classification of particles with an aerodynamic diameter Dp between 0.1 pm < Dp < 5 pm, while cyclones work in the 2 to 20 pm range. 

Fig 4 shows the distribution obtained with the cascade impactors. It can be seen, as in the Fig. 2, that in the 0.1-1 pm size range the mass concentration is higher in Test 2 than in Test 4 although considering the total measurement interval, the size distribution in Test 4 is higher than in Test 2. 

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