Aerosol Deposition Mechanisms

Comparison of the results for the four major drcuit components is shown in figure (24). It is apparent that tiie total deposition computed >y RAFT is condderabty higher, almost a factor of two, than the deposition predicted by A CTORIA and TRAPFRANCE. TTie VICTORIA results show slightly greater deposition than TRAPFRANCE. However, the dominant aerosol deposition mechanisms are the same for all three codes, as the location of most depodtion. The deposition profiles and the dominant aerosol deposition processes are anal3TOd in terms of the fonr primary components in the drcuit... 

The basic operations in dust collection by any device are (1) separation of the gas-borne particles from the gas stream by deposition on a collecting surface (2) retention of the deposit on the surface and (3) removal of the deposit from the surface for recovery or disposal. The separation step requires (1) application of a force that produces a differential motion of a particle relative to the gas and (2) a gas retention time sufficient for the particle to migrate to the coUecting surface. The principal mechanisms of aerosol deposition that are apphed in dust collectors are (1) gravitational deposition, (2) flow-line interception, (3) inertial deposition, (4) diffusional deposition, and (5) electrostatic deposition. Thermal deposition is only a minor factor in practical dust-collectiou equipment because the thermophoretic force is small. Table 17-2 lists these six mechanisms and presents the characteristic... 

TABLE 17-2 Summary of Mechanisms and Parameters in Aerosol Deposition... 

In addition to the deposition mechanisms themselves, methods for preliminary conditioning of aerosols may be used to increase the effectiveness of the deposition mechanisms subsequently apphed. One such conditioning method consists of imposing on the gas nigh-intensity acoustic vibrations to cause collisions and flocculation of the aerosol particles, producing large particles that can be separated by simple inertial devices such as cyclones. This process, termed sonic (or acoustic) agglomeration, has attained only hmited commercial acceptance. 

The major effects of air pollution on fabrics are soiling and loss of tensile strength. Sulfur oxides are considered to cause the greatest loss of tensile strength. The most widely publicized example of this type of problem has been damage to women s nylon hose by air pollution, described in newspaper accounts. The mechanism is not understood, but it is postulated that fine droplets of sulfuric acid aerosol deposit on the very thin nylon... 

The deposition rate of the attached fraction, plotted in Figure 3, is calculated from the aerosol size distribution assuming diffusion and electrophoresis to be the most important deposition mechanisms (Raes et al.,1985a). The accuracy of the absolute values was checked by forming the aerosol mass balance after the generation of a high aerosol concentration.In Table II is compared the decay of the... 

The available data on retention in the respiratory tract, as summarized by Mitchell (M6) indicate that upwards of 20% of inhaled aerosols are retained in the respiratory tract, approaching 100% for particles over 5 microns in diameter. The order of retention follows reasonably well what would be expected from the physics of the various deposition mechanisms assuming that once deposited a particle is retained by the surface. This would,... 

An ideal in vitro model for the characterization of aerosol formulations would incorporate cell types from various regions of the lung (tracheal, bronchial, and alveolar) and would facilitate simulation of deposition mechanisms by impaction, sedimentation, and diffusion of a high-metered singlebolus inhalation. In the future, such systems may reduce the need for animal studies and may offer to correlate in a predictive way the results from such in vitro tests to clinical bioavailability data after pulmonary drug delivery in vivo. 

Aerosols for pulmonary drag delivery are delivered via the mouth. Three principal deposition mechanisms operate within the lower respiratory tract (Figure 10.4) ... 

O Riordan, T.G. Pahner, L.B. Smaldone, G.C. Aerosol deposition in mechanically ventilated patients optimizing nebulizer delivery. Amer. J. Resp. Crit. Care Med. 1994, 149, 214-219. 

Accurately estimating the total amount of acid deposited on a receptor, such as a lake, or a forest, is problematic because acids are deposited by difficult-to-quantify dry processes as well as wet processes. The wet deposition of both sulfuric and nitric acids is believed to account for only about half of the total deposition of these acids on surface waters, soils, and vegetation. Data from whole watershed mass balance studies (e.g., Table 4-13) support the hypothesis that total deposition of sulfate considerably exceeds what is measured in the form of wet deposition alone. A significant amount of H2S04 is deposited as sulfate aerosols, such as ammonium sulfate ](NH4)2S04]. The direct absorption of S03, followed by oxidation of S03 to H2S04 at the absorbing surface, is another deposition mechanism. Dry deposition of nitric acid includes sorption of nitric acid vapor onto surfaces, as well as deposition... 

In considering the mechanisms of aerosol deposition within the lung and the factors that may influence them, it is of some importance to consider first the anatomy and air velocities within the respiratory tract. The temporal aspects of the passage of air through the various anatomic regions and the point during the breathing cycle are also relevant factors. 

As with most questions on particle size, the answer is very dependent on the definition used and the experimental technique. For a dynamic aerosol cloud, the correct definition is the aerodynamic particle size, which is the diameter of an equivalent sphere of unit density. An equivalent sphere is a conventional assumption in particle sizing, but for the aerodynamic size, the density is included to account for the momentum of the particle, i.e., both mass and velocity are important. The technique chosen for measurement must include these parameters, and impaction is the normally chosen technique, which also reflects the major deposition mechanism in the lung. A schematic of an impaction plate is given in Figure 10.3. 

Figure 2-30. Summary of possible mechanisms for generation of YBCO films using aerosol deposition conditions. From left to right spray pyrolysis, particle deposition arising form unevaporated or involatile reagent(s), chemical vapor deposition, and particle deposition arising from reagent(s) evaporating prior to deposition. (From Salazar et al. [189].)...

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