Particulates cascade impactors

Fig. 14-6. Schematic diagram of a four-stage cascade impactor. Source Giever, P. M., Particulate matter sampling and sizing, in "Air Pollution," 3rd ed., Vol. lil (A. C. Stem, ed.). Academic Press, New York, 1976, p. 41,...

Cascade impactor An instrument used to sample and separate particulates into a number of successive fractions of different sizes. 

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. 

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. 

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). 

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). 

Kawanaka, Y., Matsumoto, E., Sakamoto, K., Wang, N., and Yun, S.-J., Size distribution of mutagenic compounds and mutagenicity in atmospheric particulate matter collected with a low-pressure cascade impactor, Atmos. Environ., 38, 2125 -2132, 2004. 

A series filter was used to separate atmospheric particulate iodine, HI and I2, HOI and organic iodine. The particulate iodine is usually separated and collected using micropore filters or glass microfiber. The distribution of iodine in the different sizes of particulates is collected by a multi-stage cascade impact collector. Wimschneider and Heumann (1995) used a six-stage slot cascade impactor to collect particulates... 

Particles that can be inhaled, those less than 10 p,m (10,000 nm PMIO) can be separated by the high volume cascade impactor (HVCI) into four fractions. When the HVCI was used to collect organic urban aerosols presumably from transportation, combustion, and the Earth s crust, the breathable particulate matter (PM) was divided into PM 10 to 2.5 p.m (coarse aerosols, which are mechanically produced), and the PM 2.5 to 1 p.m (intermodal) fiaclion, which is expected to have particles that contain properties of both coarse (larger) and fine (smaller) aerosols. In addition it separates the PM 1 to 0.2 p,m (1000 to 200 nm accumulation) fraction (just larger than nano- or ultrafine particles with properties similar to those particles) and PM 0.2 (particles <200 run diameter in air) firaction. The cutoff size (200 nm) was chosen for convenience. 

May BCR (1975) An ultimate cascade impactor for aerosol assessment. J Aerosol Sci 6 1-7 Nieuwenhuijsena MJ, Gomez-Perales JE, Colvile RN (2007) Levels of particulate air pollution, its elemental composition, determinants and health effects in metro systems. Atmos Environ 41 7995-8006... 

Neutron activation analysis is one of the major techniques for the determination of many minor and trace elements in a large variety of solid environmental and pollution samples, such as atmospheric aerosols, particulate emissions, fly ash, coal, incineration ash, and sewage sludge. Instrumental neutron activation analysis of total, inhal-able, or respirable airborne particulate matter collected on a cellulose or membrane filter, or in a cascade impactor on some organic substrate, allows the determination of up to 45 elements by an irradiation - counting scheme similar to the one given in Figure 3. Radiochemical NAA is applied only when extremely low limits of determination are required. Instrumental photon activation analysis is also complementary to INAA. 

Particle size irrfluences the deposition of inhaled particulates in the respiratory tract and so irrformation about the distribution of particle sizes is needed for the correct interpretation of bioassay results and subsequent dose assessmertL In mar sitirations the airborne particle size distribution should be determined using cascade impactors or other methods. As a minimum, air sample measuremerrts shorrld include measurement of the concentration of the respirable fraction of airborne particulates. Some models for interpreting PAS results discriminate agairrst non-respirable particles [15], In general, the more site and material specific information that is available, the better will be the dose assessment. 

Gases can be sampled by standardised procedures and apparatuses gases themselves can be absorbed in liquids or adsorbed on solid tube-like supports. Particulate matter in gases is precipitated either as a whole in electrofilters or other types of precipitators, or can be fragmented according to particle sizes by cascade impactors. 

Figure 1 Four commercially available devices for the size separation and/or collection of airborne particulates. From left to right lOM sampler, button sampler, cascade multistage impactor, cyclone.

Impactor samplers, which permit a broader range of measurable particulate size distributions, are constructed so that air entering at high flow rates encounters a series of trapping surfaces and particles are trapped based on size. First trapped are the coarsest particles followed sequentially by smaller and smaller fractions. The cascade samplers typically allow fractionations within narrow ranges, such as 0.01—0.1 (im (Dzubay et al., 1976). 

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