Aerosol containers particles

Monodisperse All particles exactly the same size. A monodisperse aerosol contains particles of only a single size. As might be expected, this condition is extremely rare in nature. 

For many purposes it is important to know the actual chemical species, and determining specia-tion from bulk compositions is a formidable challenge, especially when the aerosol contains particles of different composition, as invariably occurs in ambient air. At best, bulk analysis yields accurate information on the actual species present only for major particle types having simple compositions. Consequently, there has been an increasing interest in the analysis of individual aerosol particles. However, the attendant problems in such analyses are formidable because of the small sizes and, in many cases, the chemical complexity of individual particles. 

Although aerosols containing particles of uniform size (monodisperse aerosols) can be... 

Let us suppose that the radius of raindrops is uniform and equal to R. One raindrop obviously sweeps out an air volume of R2nh between the cloud base and the surface if the height of the cloud base is h. Let this air volume be filled with a monodisperse aerosol containing particles of unit density with radius r at a number concentration N(r). Then the total mass of aerosol particles in the air volume swept out will be ... 

In this chapter, size classification refers to the separation of particles by size so that the resulting aerosol contains particles of a given size, whereas size characterization is the determination of the size distribution of the aerosol. Size characterization of nanoflbers and nanotubes is commonly performed by transmission electron microscopy (TEM). TEM analysis has proven invaluable for examining the structure and composition of individual particles and is also valuable for size characterization if immediate feedback is not needed. Inasmuch as the particles must first be collected and then analyzed, the process is slow and laborious. 

Classically, aerosols are particles or droplets that range from about 0.15 to 5 p.m ia size and are suspended or dispersed ia a gaseous medium such as air. However, the term aerosol, as used ia this discussion, identifies a large number of products which are pressure-dispensed as a Hquid or semisohd stream, a mist, a fairly dry to wet spray, a powder, or even a foam. This definition of aerosol focuses on the container and the method of dispensiag, rather than on the form of the product. 

Fig. 6. Size distribution of an urban aerosol showing the three modes containing much of the aerosol mass. The fine mode contains particles produced by condensation of low volatility gases. The mid-range, or accumulation mode, results from coagulation of smaller aerosols and condensation of gases on preexisting particles. Coarse particulates, the largest aerosols, are usually generated mechanically.

Available information from human exposures indicates that airborne americium-containing particles are deposited in the respiratory tract, cleared to some extent via mucociliary action, and swallowed or expelled (Edvardsson and Lindgren 1976 Fry 1976 Newton et al. 1983 Sanders 1974 Toohey and Essling 1980). Descriptions of human respiratory tract models that can be used for radiation protection also include relevant information regarding biokinetics of inhaled particles (ICRP 1994b, 1995 NCRP 1997). Quantitative data are not available, however. Supporting animal studies include inhalation exposure to aerosols of americium (Buldakov et al. 1972 DOE 1978 Gillett et al. 1985 Sanders and Mahaffey 1983 Talbot et al. 1989 Thomas et al. 1972) or intratracheal instillation of americium compounds (Moushatova et al. 1996). 

From a public health point of view, the concentration of nickel associated with small particles that can be inhaled into the lungs is of greatest concern. The nickel content of aerosols from power plant emissions is not strongly correlated with particle size (Hansen and Fisher 1980). In one modem coal plant, 53% and 32% of nickel in emissions were associated with particles <3 and <1.5 pm in diameter, respectively (Sabbioni et al. 1984). Other studies found that only 17-22% of nickel emissions from coal-fired power plants were associated with particles of >2 pm, and that the mass medium diameter (MMD) of nickel-containing particles from a plant with pollution control devices was 5. 4 pm (Gladney et al. 1978 Lee et al. 1975). In one study, 40% of the nickel in coal fly ash was adsorbed on the surface of the particles rather than being embedded in the aluminosilicate matrix (Hansen and Fisher 1980). Surface-adsorbed nickel would be more available than embedded nickel. 

Hence such aerosols, internal mixtures, are usually not good representation for a polluted atmosphere containing particles from many different sources. 

To determine the influence of wind, velocity, and particle size on the toxicity of oil aerosols containing DDT, Latta and coworkers (31) conducted experiments in a wind tunnel at wind speeds of 2, 4, 8, and 16 miles per hour. The median lethal dose for a female mosquito was 1 particle 83 microns in diameter from a 10% DDT solution. 

Carbonaceous aerosols in the atmosphere are complex in nature and are found in both coarse particles (> about 2.5 pm) and fine particles (< about 2.5 pm). Sources of carbon-containing particles are varied and include resuspended soil particles, pollen, plant waxes, etc. in the coarse fraction, and soot particles, sorbed organics including PAHs, and secondary aerosols resulting from... 

The preparation of synthetic atmospheres for nonreactive gases and vapors is relatively straightforward, but the preparation of fumes, aerosols, and particulates is considerably more difficult. For purposes of industrial hygiene sampling, a polydisperse aerosol containing respirable-size particles is required. 

Using the Hatch-Choate equation, compute the mass median diameter from the information developed in Prob. 5. If the aerosol contains 1 million particles per cubic foot and the particle density is 1 g/cm3, find the aerosol concentration in micrograms per cubic meter. 

Example 10.2 Estimate the number of 0.1-p.m-diameter particles deposited per square centimeter per hour on a wall placed next to a semi-infinite aerosol containing 100 particles per cubic centimeter. 

For an aerosol containing many particles of the same size,... 

Smog A combination of smoke and fog usually containing photochemical reaction products combined with water vapor to produce an irritating aerosol Smog particle sizes are usually quite small being somewhat less than 1 pm in diameter. 

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