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Scattering angle (deg)

FIGURE 9.19 Mie intensity parameters versus scattering angle for water droplets (m 1.333) having a 0.8, 2.0, and 10.0. Solid lines are /,, and dashed lines are iu (adapted from Hinds, 1982).

The dependence of Mie scattering on particle size can be seen in Fig. 9.20, which shows the sum of the Mie intensity parameters (/, in) as a function of the size parameter a for two scattering angles, 6 30° and 6 90°, respectively. It is seen that Mie scattering generally increases with size over this range of values of a and, as seen in Fig. 9.19, scattering is more pronounced in the forward direction (i.e., at smaller values of 6).

It is noteworthy that in Fig. 9.20 the function becomes smooth and approaches a variation with Db as the size parameter decreases toward small values. This is expected, since in the limit of very small particles or molecules, Mie theory reduces to Rayleigh scattering, which, as seen in Chapter 3, varies with D'\

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 (bcxiL) in I /„ exp( bcnL), Eq. (V), albedo, etc. can be determined. However, as discussed in detail by Mishchenko et al. (1995), application of conventional Mie theory can lead to significant errors in the aerosol optical thickness if the particles are not spherical, as is assumed in development of Mie scattering theory.

Particles can also absorb light in the atmosphere; the radiant energy absorbed is then converted to heat. As discussed later, graphitic carbon is believed to be the species responsible for most of the light absorption occurring in typical urban atmospheres, although there

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