R

/„ A2(/, where 70 is the intensity of the incident light beam (taken as unpolarized) and i, and /,, are known as the Mie intensity parameters for the perpendicular-polarized and parallel-polarized components of the scattered light, respectively. The Mie intensity parameters are a complex function of the refractive index (m) of the scatterer, the size parameter (a), and the scattering angle (6). For further mathematical details and descriptions of Mie scattering, the reader is referred to the books by Van de Hulst (1957), Kerker (1969), and Bohren and Huffman (1983).

The refractive index of a material, m, is defined as the ratio of the speed of light (c) in a vacuum to that (v) in the material, that is, m c v. Because light travels more slowly in materials than in air, m > 1. The index of refraction, m.d, of materials that absorb light as well as scatter it is expressed in the form of a complex number (see Hinds, f982)

8TT2R2

where i V f, mr is the real refractive index, and a is a constant that depends on the absorption coefficient of the material as well as to the wavelength. Values of the index of refraction of A 589 nm of some materials either found in the atmosphere or used to calibrate instruments that measure particle size using light scattering (see later) are given in Table 9.6. A typical refractive index for a dry aerosol that absorbs light in the atmosphere is f .5 0.02/ (Covert et al., 1980).

Unlike Rayleigh scattering, which occurs equally in the forward and backward directions, Mie scattering is predominantly in the forward direction, except for the smallest particles. This can be seen in Fig. 9.19, which shows the Mie intensity parameters /, and /,, as a function of the scattering angle 6 for three different values of the size parameter a defined by Eq. (Y), assuming the droplets are composed entirely of liquid water (i.e., m 1.333). For a 2.0 and 10 (i.e., D A

TABLE 9.6 Index of Refraction at 589 nm for Some Species Found in the Atmosphere or Used for Instrument Calibration"

Species

Index of refraction

Vacuum

0 0

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