Optical scattering aerosols

FIGURE 9.23 (a) Aerosol particle size distribution measured at Pomona during the 1972 State of California Air Resources Board ACHEX program, (b) Calculated optical scattering by particles, bsp, for measured size distribution (adapted from Waggoner and Charl-son, 1976). 

Fig. 7-1. Left Idealized particle size distributions for the rural continental and the maritime aerosols. The distribution of sea-salt particles that contribute to the maritime aerosol is shown separately. The transition from the rural to the urban aerosol is indicated. Right Determination of remote tropospheric aerosol size distribution by a combination of instrumental techniques. [ Single-stage and free-wing impactors, O—O set of five double-stage impactors singleparticle optical scattering analyzer these data were obtained at the observatory lzana, Tenerife,...

Iskander MF, Chen HY, Penner JE Optical scattering and absorption by branched chains of aerosols, Appl Opt 28(15) 3083—3091, 1989. 

Finally, alternate forms of substrate pumping were considered in some detail. The energy escape fraction is largest from small spheres of uranium metal or oxide. Such spheres carmotbe incorporated directly into the laser medium because of the optical scattering loss they introduce, even if they were overcoated with a thin reflective coating. However, concepts have been developed to use such reflective aerosols to pump a fluorescer medium, which produces intense incoherent light usable for optically prrmping a laser medium. 

Cooke, D. D., and Kerker, M., Response Calculations for Light Scattering Aerosol Particle Counters, Optics, 14, 734—739 (1975). 

FIGURE 7.13 Schematic of an evaporative light scattering detector. The three stages are nebulization, in which column effluent is aerosolized evaporation, in which the mobile phase is vaporized and optical detection, in which the light scattering of the residual solute particles is recorded. Some detectors also include an obstacle in the flow path for droplet discrimination, which leads to a more homogenous distribution of droplet sizes. 

It should be noted that scattering of light by particles can be measured using remote sensing techniques on satellites, from which such parameters as total aerosol optical thickness i.e., the exponent (bcxlL) in... 

An important factor in deriving 03 concentrations is the presence of aerosol particles, which also scatter light at 0.6 ixm. Thus, correction for their contribution to extinction at this wavelength must by applied to derive the ozone concentrations. This requires some assumptions regarding aerosol particle properties such as the size distribution, which is not known. It is also commonly assumed that the optical properties of particles do not change with altitude. Such problems introduce uncertainties into the calculation of the particle contribution (e.g., Steele and Turco, 1997a, 1997b Thomason et al., 1997 Fussen, 1998) and hence into the ozone concentrations extracted from such data. 

There are also more limited treatments of scattering. McCartney (1976, Chaps. 4-6) confines his attention to scattering by atmospheric particles. This is also discussed by Twomey (1977, Chaps. 9-10) in his treatise on atmospheric aerosols. In Goody (1964, Chap. 7) there are discussions of absorption by gases and, in less detail, extinction by molecules and by droplets. Parts of books on electromagnetic theory or optics include the theory of scattering by a sphere, most notably Stratton (1941, pp. 563-573) and Born and Wolf (1965, pp. 633-664). The latter also derive the Ewald-Oseen extinction theorem and apply it to reflection and refraction at a plane interface (pp. 98-104). 

Another kind of effective or average optical constants involves mixtures of different particles such as atmospheric aerosols or soils. Effective optical constants for compacted samples of these mixtures might be inferred from reflectance and transmittance measurements as if the samples were homogeneous. But scattering or extinction calculations based on these optical constants would not necessarily be correct. 

An example of practical importance in atmospheric physics is the inference of effective optical constants for atmospheric aerosols composed of various kinds of particles and the subsequent use of these optical constants in other ways. One might infer effective n and k from measurements—made either in the laboratory or remotely by, for example, using bistatic lidar—of angular scattering fitting the experimental data with Mie theory would give effective optical constants. But how effectual would they be Would they have more than a limited applicability Would they be more than merely consistent with an experiment of limited scope It is by no means certain that they would lead to correct calculations of extinction or backscattering or absorption. We shall return to these questions in Section 14.2. 

Because the single-scattering albedo depends sensitively on the imaginary part of the refractive index there has been keen interest in determining optical constants of atmospheric particles. These are used to calculate the important parameters in the heat balance problem for present and predicted aerosol... 

Aerosols—Optical properties 2. Absorption of light. 3. Light—Scattering. 1. Huffman, Donald R.,... 

Optical Methods. Optical methods, based on the scattering of light by dispersed droplets, provide a relatively simple and rapid measure of particle size. However, optical techniques give data concerning the average drop size or the predominant size only, and size-distribution data cannot be obtained. Optical methods are more suited to the size analysis of aerosols and extremely fine mists than to the analysis of typical fuel sprays. 

The effect of aerosol on the UV flux reaching the earth s surface is reviewed. Under cloudless conditions UV is scattered or absorbed by aerosol. The net effect depends on the optical properties of the aerosol (scattering or absorbing). In cloudy conditions the UV flux attenuation is more pronounced but depends also on the cloud type. In most instances of clear or cloudy skies an attenuation occurs, with a noteworthy departure the case of partly cloudy sky in a rural area. 

Figure 6. From figure 11 of Kylling et al. 1998. The ratio between simulated Brewer and Bentham UVB dose rates with and without aerosols as a function of the aerosol optical depth at 355 nm. Ratios of model results with aerosol single scattering albedo of (0.95 solid line), 0.87(dotted line) and 0.80 (dashed line) versus aersosol free model results are shown for solar zenith angle of 10° and an ozone column of 340 DU.

The correlation between photolysis rates and optical depths is obvious for the JN02 case in the UV-A region, it is not clear for JO( D). The model sensitivity study supports the expectation that aerosol optical depth and single scattering albedo are the two decisive parameters to describe the radiative effects of aerosols. 

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