Aerosol Particle Diameters

The attachment coefficient is a function of the aerosol particle diameter, d, and mean velocity, v, as well as the unattached progeny diameter, d, and its mean velocity v. Since in most situations d d and v v, equation (2) reduces to... 

As particle number size distributions can be complex, and the instruments used generate large amount of size distribution data, which can be hard to effectively describe, a common method is to calculate integrated particle number concentrations for specific aerosol particle diameter ranges, depending on which part of the particle number size spectrum is needed for the application. In this chapter, three different ranges are used (Fig. la) ... 

At normal pressure and temperature, lB reaches a minimum at an aerosol particle diameter of 2 x 10 e cm, but increases only by about a factor of 5 for particles 2 orders of magnitude larger or smaller than this size. Thus, the pseudo mean free path is essentially constant over the size range of interest, having a value of about 10 6 cm. 

Example 17.7 A monodisperse aerosol is viewed in an aerosol owl, and four red bands are observed as the sample is scanned over approximately 160°. Determine the aerosol particle diameter indicated by these data. 

Process parameters of importance for mist and fume control include measures to minimize the moisture content of the combustion air fed to the phosphorus burners. A high moisture content in this gas stream tends to increase mist formation. Solid phosphorus pentoxide has been found to be very difficult to dissolve in either water or phosphoric acid. This prompts temperatures in the hydrator to be maintained high enough so that absorption takes place from the vapor phase. Further studies have shown that uptake of phosphorus pent-oxide vapor in 70% phosphorus acid is only about 60% and climbs with increasing acid concentrations up to about 88% phosphoric acid. Operating with two fiber beds operated in series, the first bed as an agglomerator, and the second as a collector, can also efficiently control mists and fumes from furnace acid plants [30]. Mass containment efficiencies of better than 99.9% were reported for a median aerosol particle diameter of 1.1 to 1.6 fim. 

Nanoaerosol, Fig. 2 Filtration efficiency versus aerosol particle diameter (not in scale)... 

Fig. 3.3. Relationship between rain scavenging rates and aerosol particle diameter.

Figures 5.2 and 5.3 show the effective dose per unit exposure of the tracheo-bronchial (bronchial and bronchiolar) and pulmonary or alveolar region as a function of aerosol particle diameter calculated for the radon decay products Po, " Pb, and " Po and the thoron decay products Pb, Bi and Po. A tissue weighting factor of the lung and the radiation weighting factor of 0.12 and 20, respectively, are taken into account. The effective dose from a radioactive mixture can be deduced by adding the effective doses of each decay product.

Appendix 3. Slip correction factor for standard and non-standard conditions (a) Slip correction factor minus one versus aerosol particle diameter at standard conditions (curve A for aerosol particles of 0.1 pm diameter, curve B for aerosol particles of less than 0.1 pm diameter), and (b) slip correction factor versus aerosol particle diameter times pressure for temperatures from 233 to 893 K (—40 to 600 °C). 

Molecular and aerosol particle diameters, copyright P.C. Reist. Molecular diameters calculated from viscosity data. From Altwicker, E.R. et al.. Air pollution, in Environmental Engineers Handbook, 2nd ed., Liu, D.H.F. and Liptak, B.G., Eds., CRC Press, Boca Raton, FL, 1997, p. 334. Originally adapted from Lapple, 1961, Stanford Research Institute Journal, 3rd quarter, and J.S. Eckert and R.F. Strigle, Jr., 1974, JAPCA, 24 961-965. 

Acid mist eliminators use three aerosol collection mechanisms inertial impaction, interception, and Brownian motion. Inertial impaction works well for aerosols having particle diameters larger than 3 p.m Brownian motion and interception work well with aerosols having smaller particle diameters. 

Particles in the atmosphere come from different sources, e.g., combustion, windblown dust, and gas-to-particle conversion processes (see Chapter 6). Figure 2-2 illustrates the wide range of particle diameters potentially present in the ambient atmosphere. A typical size distribution of ambient particles is shown in Fig. 2-3. The distribution of number, surface, and mass can occur over different diameters for the same aerosol. Variation in chemical composition as a function of particle diameter has also been observed, as shown in Table 4-3. 

Mean particle diameter The mean value of the particle size distribution of the test aerosol. 

Fig. 7-12 Schematic of an atmospheric aerosol size distribution. This shows the three mass modes, the main sources of mass for each mode, and the principal processes involved in inserting mass into and removing mass from each mode (m = mass concentration. Dp = particle diameter). (Reproduced with permission from K. T. Whitby and G. M. Sverdrup (1983). California aerosols their physical and chemical characteristics. In "The Character and Origin of Smog Aerosols" (G. M. Hidy, P. K. Mueller, D. Grosjean, B. R. Appel, and J. J. Wesolowski, eds), p. 483, John Wiley, New York.)...

Airborne particulate matter may comprise liquid (aerosols, mists or fogs) or solids (dust, fumes). Refer to Figure 5.2. Some causes of dust and aerosol formation are listed in Table 4.3. In either case dispersion, by spraying or fragmentation, will result in a considerable increase in the surface area of the chemical. This increases the reactivity, e.g. to render some chemicals pyrophoric, explosive or prone to spontaneous combustion it also increases the ease of entry into the body. The behaviour of an airborne particle depends upon its size (e.g. equivalent diameter), shape and density. The effect of particle diameter on terminal settling velocity is shown in Table 4.4. As a result ... 

Activity Median Aerodynamic Diameter (AMAD)—The diameter of a unit-density sphere with the same terminal settling velocity in air as that of the aerosol particle whose activity is the median for the entire size distribution of the aerosol. 

Equation (1) points to a number of important particle properties. Clearly the particle diameter, by any definition, plays a role in the behavior of the particle. Two other particle properties, density and shape, are of significance. The shape becomes important if particles deviate significantly from sphericity. The majority of pharmaceutical aerosol particles exhibit a high level of rotational symmetry and consequently do not deviate substantially from spherical behavior. The notable exception is that of elongated particles, fibers, or needles, which exhibit shape factors, kp, substantially greater than 1. Density will frequently deviate from unity and must be considered in comparing aerodynamic and equivalent volume diameters. 

The two fundamental theories for calculating the attachment coefficient, 3, are the diffusion theory for large particles and the kinetic theory for small particles. The diffusion theory predicts an attachment coefficient proportional to the diameter of the aerosol particle whereas the kinetic theory predicts an attachment coefficient proportional to the aerosol surface area. The theory... 

From equation (1) it can be seen that application of the diffusion theory leads to the conclusion that the rate of attachment of radon progeny atoms to aerosol particles is directly proportional to the diameter of the aerosol particles. 

From equation (3) is is clear that the kinetic theory predicts an attachment rate of radon daughters to aerosol particles proportional to the square of the diameter of the aerosol particle. 

The most complete theory for aerosol coagulation is that of Fuchs (1964). Since the attachment of radon progeny to aerosols can be considered as the coagulation of radon progeny (small diameter particle) to aerosols (large diameter particle), it is reasonable to use Fuchs theory to describe this process. The hybrid theory is an approximation to Fuchs theory and thus can be used to describe the attachment of radon progeny to aerosols over the entire aerosol size spectrum. 

In poorly ventilated rooms the average value of the attachment coefficient with 7.4 10 3 cm h 1 (Table lb) was significantly higher than the value in rooms with moderate ventilation (2.4 10 3 cm It1 ) (Table lib) corresponding to diameters d = 117 nm and d = 6.5 nm, respectively. This aged aerosols in poorly ventilated rooms did not only show lower particle concentrations but also greater particle diameters. 

The chemical characterization of aerosol particles currently is of great interest in the field of atmospheric chemistry. A major goal is the development of a method for continuous elemental analysis of aerosols, especially for the elements C, N, and S. Chemiluminescence reactions described in this chapter have adequate sensitivity and selectivity for such analyses. In fact, considering that a 1- j.m-diameter particle has a mass of =0.5-1.0 pg, online analysis of single aerosol particles should be achievable, especially for larger particles. 

In exposures of humans to artificially generated aerosols, where the information is to be relevant to ambient aerosols, several factors are important the particle diameter distribution must be fairly constant and fall within size ranges typical for the given compound in the ambient air, the chemical composition of the aerosol must be stable and predictable, and the electric charge distribution of the aerosol must simulate that of normal atmospheric aerosols. 

Both from deposition studies and force balances it can be derived that the optimum (aerodynamic) particle size lies between 0.5 and 7.5 pm. Within this approximate range many different subranges have been presented as most favourable, e.g. 0.1 to 5 pm [24], 0.5 to 8.0 pm [25], 2 to 7 pm [26] and 1-5 pm [27-29]. Particles of 7.5 pm and larger mainly deposit in the oropharynx [30] whereas most particles smaller than 0.5 pm are exhaled again [31]. All inhalation systems for drug delivery to the respiratory tract produce polydisperse aerosols which can be characterized by their mass median aerodynamic diameter (MMAD) and geometric standard deviation (oq). The MMAD is the particle diameter at 50% of the cumulative mass curve. 

Aerosol oil particles of SGF-1, a distillate oil chemically similar to fuel oil no. 2, used by the military to generate oil fogs (particle diameter of 0.5-1.2 m) may remain aloft for approximately 1 hour, may be transported 1-10 km (average of 5 km) downwind during this time, and, with the exception of evaporative losses, will be deposited to soil or surface waters (Army 1986 Liss-Suter et al. 1978). 

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