Aerosols shape

In many practical cases Leith s approach to the definition of the aerosol shape factor has greatly simplified the understanding of this correction to Stokes law. For example, consider again the aerodynamic diameter of a fiber having a cross-sectional diameter df, length L, and density p. This can be approximated by using Eqs. 5.17 and 5.18 for the case of long axis motion parallel to the flow as... 

A sample solution is drawn or pumped into a V-shaped groove cut into the end of a capillary tube. The crossed gas and liquid streams form an aerosol. An impactor bead can be used to provide an even smaller droplet size. 

Finally, in yet another variant, the sample liquid stream and the gas flow are brought together at a shaped nozzle into which the liquid flows (parallel-path nebulizer). Again, the intersection of liquid film and gas flow leads to the formation of an aerosol. Obstruction of the sample flow by formation of deposits is not a problem, and the devices are easily constructed from plastics, making them robust and cheap. 

An interesting appHcation is the study of aerosols, such as sprayed paint. A flash hologram of the paint spray at a convenient magnification records positions and sizes of the particles ia the hologram. This image can then be studied at leisure at higher magnification for size and shape of iadividual particles. 

In addition to the packaging of foods and beverages in regular containers, a large quantity of tinplate is used in the form of aerosol containers for cosmetics, paint, insecticides, poHshes, and other products. Decorative trays, Hthographed boxes, and containers of unusual shape are additional oudets for tinplate. 

Suspended particles are the most important factor in visibility reduction. In most instances, the visual quality of air is controlled by partide scattering and is characterized by the extinction coeffident The size of particles plays a crucial role in their interaction with light. Other factors are the refractive index and shape of the particles, although their effect is harder to measure and is less well understood. If we could establish these properties, we could calculate the amount of light scattering and absorption. Alternatively, the extinction coeffident associated with an aerosol can be measured directly. 

Airborne partieulate matter may eomprise liquid (aerosols, mists or fogs) or solids (dust, fumes). Refer to Figure 5.2. Some eauses of dust and aerosol formation are listed in Table 4.3. In either ease dispersion, by spraying or fragmentation, will result in a eonsiderable inerease in the surfaee area of the ehemieal. This inereases the reaetivity, e.g. to render some ehemieals pyrophorie, explosive or prone to spontaneous eombustion it also inereases the ease of entry into the body. The behaviour of an airborne partiele depends upon its size (e.g. equivalent diameter), shape and density. The effeet of partiele diameter on terminal settling veloeity is shown in Table 4.4. As a result ... 

A venturi scrubber is a venturi-shaped air passage with water introduced just ahead of or into the venturi throat. The liquid-gas contact is at a maximum in the venturi throat. The relative velocity between gas and liquid aerosol droplets is high, with the gas velocities in the range of 50-100 m/s. The particles are conditioned in the throat, and condensation is the important collection mechanism. After the particles in the gas have been deposited on droplets, a comparatively simple device such as a cyclone collector can be used to collect the wetted dust. 

Aerosol dynamics are based on spherical particles, a premise which almost never exists in practice. However, if there is consistency in handling the aerosol dynamics calculations, the aerodynamic diameter (see Section 13.5.2.2) that is measured gives fairly accurate predictions of aerodynamic behavior. As a result, the difference between the real shape and size of the particles and the aerodynamic shape and size is unimportant for most practical purposes. 

Fig. 16-4 pH sensitivity to SO4- and NH4. Model calculations of expected pH of cloud water or rainwater for cloud liquid water content of 0.5 g/m. 100 pptv SO2, 330 ppmv CO2, and NO3. The abscissa shows the assumed input of aerosol sulfate in fig/m and the ordinate shows the calculated equilibrium pH. Each line corresponds to the indicated amoimt of total NH3 + NH4 in imits of fig/m of cloudy air. Solid lines are at 278 K, dashed ones are at 298 K. The familiar shape of titration curves is evident, with a steep drop in pH as the anion concentration increases due to increased input of H2SO4. (From Charlson, R. J., C. H. Twohy and P. K. Quinn, Physical Influences of Altitude on the Chemical Properties of Clouds and of Water Deposited from the Atmosphere." NATO Advanced Research Workshop Acid Deposition Processes at High Elevation Sites, Sept. 1986. Edinburgh, Scotland.)... 

Surfactants employed for w/o-ME formation, listed in Table 1, are more lipophilic than those employed in aqueous systems, e.g., for micelles or oil-in-water emulsions, having a hydrophilic-lipophilic balance (HLB) value of around 8-11 [4-40]. The most commonly employed surfactant for w/o-ME formation is Aerosol-OT, or AOT [sodium bis(2-ethylhexyl) sulfosuccinate], containing an anionic sulfonate headgroup and two hydrocarbon tails. Common cationic surfactants, such as cetyl trimethyl ammonium bromide (CTAB) and trioctylmethyl ammonium bromide (TOMAC), have also fulfilled this purpose however, cosurfactants (e.g., fatty alcohols, such as 1-butanol or 1-octanol) must be added for a monophasic w/o-ME (Winsor IV) system to occur. Nonionic and mixed ionic-nonionic surfactant systems have received a great deal of attention recently because they are more biocompatible and they promote less inactivation of biomolecules compared to ionic surfactants. Surfactants with two or more hydrophobic tail groups of different lengths frequently form w/o-MEs more readily than one-tailed surfactants without the requirement of cosurfactant, perhaps because of their wedge-shaped molecular structure [17,41]. 

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 slip correction factors are important for particles smaller than 1 pm in diameter, which is rarely the case for pharmaceutical aerosols. Slip correction is required for the Stokes equation to remain predictive of particle behavior for these small particles. Therefore, assuming the absence of shape effects for particles in the Stokes regime of flow, Eq. (1) collapses into the following expression ... 

Inertial impaction is the method of choice for evaluating particle or droplet size delivery from pharmaceutical aerosol systems. This method lends itself readily to theoretical analysis, ft has been evaluated in general terms [39] and for specific impactors [40]. Inertial impaction employs Stokes law to determine aerodynamic diameter of particles being evaluated. This has the advantage of incorporating shape and density effects into a single term. 

A rate law that shows some of the peculiarities of reactions in solids arises in the following way. A solid particle having a spherical shape is assumed to react only on the surface. This rate law has been found to model the shrinking of solid particles in aerosols as well as other reactions that take place on the surface of solid particles. 

It is an aerobic, gram-negative, motile, nonsporing, rod-shaped bacterium. It can survive for many months in surface water and up to 3 months in shaded soil. The natural reservoir is soil and water. This is a biosafety level 2 agent. Additional primary containment and personnel precautions may be indicated for activities with a high potential for aerosol or droplet production. 

Fuchs (F4, pp. 102-105) also points out that, for a unipolar aerosol, the rate of concentration change, as given by Eq. (9) or (10), is an intrinsic property of the aerosol, depending on neither size nor shape of cloud. Some of the other relationships [Eqs. (8) and (11)], however, are subject to the cloud symmetry specified above. Fuchs also points out that in an ambipolar cloud the particles of minority sign tend to be driven toward the center or core where the cloud approaches neutrality asymptotically. The outer edge of the cloud tends to become unipolar with the sign of the initially most prevalent charge. 

D3. Dawkins, G. S., Electrostatic effects in the deposition of aerosols on cylindrical shapes, Univ. of Illinois Eng. Exptl. Sta. Tech. Rept. 15, AEC Rept. COO-1017, Contr. AT(ll-l)-276 (1958). 

---