Small absorbing particles

As usual, N is the particle number density and xe is the extinction cross section per particle. It is assumed that both the particles and the atmosphere in which they are imbedded are nonmagnetic, which requires that tii = tii = tio, the magnetic permeability of free space. 

Eimb-tangent observations are much better suited for inferring the vertical structure of an absorbing aerosol. Density rather than spectral variations are responsible for providing vertical contrast. As long as the particles are sufficiently small and absorbing, Eq. (8.3.9) can be used to infer their vertical distribution and abundance. 

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Absorption dominates over scattering for sufficiently small absorbing particles. Volumetric extinction by such particles is independent of their size but not of their shape we shall discuss shape effects further in the following chapter. 

Figure 12.7 Calculated extinction cross section per unit volume of a silicon carbide ellipsoid with geometrical factors 0.1, 0.3, and 0.6. Cext - Cabs for sufficiently small absorbing particles.

HPLC (Chapter 10 Lagniappe) High-pressure liquid chromatography a variant of column chromatography using high pressure to force solvent through very small absorbent particles. 

Since different materials approach the Rayleigh limit at different values of the Mie parameter, it is possible to recognize the particle material, in a mixture of particles with substantially different refractive indices. The present technique is particularly suitable for sizing of very small absorbing particles, which reach the Rayleigh limit at a very small particle diameter. 

Iron particles are considered as an example of small absorbing particles. The real and imaginary parts of the refractive index of these particles are 1.51 and 1.63 see Born and Wolf °. The response of the iron particles is compared with that of small glass beads, which are taken as dielectric particles with refractive index 1.51. A comparison between these two cases demonstrates how the material of a small particle may be recognized in a mixture of particles with different optical properties. 

At pH 5.3 and at low salt concentrations polymer addition has practically no influence on the aggregation and on the sedimentation behaviour. At pH 2.3 and at the highest salt concentration (0.1 m) the polymer addition results in infinite absorbances immediately after mixing these reduce to very small absorbances E., and E, - within a short time. This behaviour is attributed to the significant flocculation and the very fast sedimentation of the flocculated particles. 

The catalysts used in low temperature fuel cells are usually based on small Pt particles dispersed on a carbon support with typical particle sizes in the range 1 — 10 nm in diameter. The XAS provides a measure of the average electronic state and local coordination of the absorbing atom, for example, Pt, on a per-atom basis, as described above. Thus, the XAS, for both the XANES and EXAFS regions, of such Pt/C catalysts reflects the size of the particles. 

In tobacco smoke, nicotine is present as a constituent of small tar particles. It is rapidly absorbed through bronchi and lung alveoli, and is detectable in the brain only 8 s after the first inhalation. 

Clark La Motta (Ref 7) showed that LOX made with gas black or lamp black are more sensitive to impact than the standard Bureau of Mines 40% straight Dynamite. Impact sensitivity increased when small amounts of iron oxides, aluminum dust or ferro-silicon were added to the LOX. Impact sensitivity also increased as absorbent particle size was reduced. As the oxygen evapd, impact sensitivity, as expected, decreased... 

We have mentioned that the region of negative e is of special importance for the optics of small particles, which can absorb and scatter strongly at frequencies that depend on their shape. In particular, strong absorption by spheres occurs at the frequency where c = —2. Figure 9.11 shows that there can be an extensive region of shape-dependent absorption and scattering by small metallic particles this will be discussed more fully in Chapter 12. 

There are some notable differences apparent in Fig. 11.14 between the extinction curves for aluminum spheres and those for water droplets. For example, av is still constant for sufficiently small aluminum particles but the range of sizes is more restricted. The large peak is not an interference maximum aluminum is too absorbing for that. Rather it is the dominance of the magnetic dipole term bx in the series (4.62). Physically, this absorption arises from eddy current losses, which are strong when the particle size is near, but less than, the skin depth. At X = 0.1 jam the skin depth is less than the radius, so the interior of the particle is shielded from the field eddy current losses are confined to the vicinity of the surface and therefore the volume of absorbing material is reduced. 

A brief treatment of scattering by large, absorbing particles and the concept of absorption and scattering cross sections are presented in Section 5.7 along with two examples of applications of the Mie theory (to absorbing, but small, particles) and a discussion of Tyndall spectra. 

There are several types of mechanisms that could account for this behavior. (A) There could be abnormal flow paths, such as fissures which allow a small amount of activity to migrate more quickly than the main body. (b) This activity could be carried on colloidal, non-absorbing particles. Or (C) the flow rate could be too fast for equilibrium to occur for the adsorption and desorption processes. These possibilities can be examined to determine which appears the most important. 

The apparatus consisted of a flow reactor containing the coal and an oxygen absorber. A fixed bed reactor was used for studies on large coal particles, while a spout reactor was used for studies on small coal particles. Both reactors had a volume of about 400 cc. and were designed so that both wall and end effects were eliminated. Experimental residence time distributions indicated that the fixed bed reactor approximated a plug flow reactor, while the fluidized bed spout reactor had perfect mixing. 

Antenna-coupled bolometers (Schwarz and Ulrich, 1977) offer another way to achieve a small heat capacity and a small area sensitive to charged particles. Radiation is absorbed by an antenna and then coupled into a transmission line which brings it to a very small absorbing thermometer. 

Particulates are commonly classified into micro- and nanoparticles based on the size of the particles. Nanoparticles are colloidal particles ranging from 10 to 1,000 run, in which drag may be entrapped, encapsulated, and/or absorbed. Microparticulates are drag-containing small polymeric particles (erodible, non-erodible or ion-exchange resins) within the size of 1-10 /on, which are suspended in a liquid carrier medium. 

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