Page 176 - Fundamentals of Air Pollution

142 10. Effects on the Atmosphere, Soil, and Water Bodies

Light and suspended particles interact in the four basic ways shown in
Fig. 10-6: refraction, diffraction, phase shift, and absorption. For particles
with a diameter of 0.1-1.0 /xm, scattering and absorption can be calculated
by using the Mie equations (7). Figure 10-7 shows the relative scattering
and absorption efficiency per unit volume of particle for a typical aerosol
containing some light-absorbing soot (8). This clearly shows the importance
of atmospheric particles in the diameter range 0.1 to 1.0 /urn as efficient
light-scattering centers. With particles of larger and smaller diameters,
scattering decreases. Absorption generally contributes less to the extinction
coefficient than does the scattering processes. Atmospheric particles of
different chemical composition have different refractive indices, resulting
in different scattering efficiencies. Figure 10-8 shows the scattering-to-mass
ratio for four different materials (9). Clearly, carbon or soot aerosols, and
aerosols of the same diameter with water content, scatter with different
efficiencies at the same diameter.
Visibility is also affected by alteration of particle size due to hydroscopic
particle growth, which is a function of relative humidity. In Los Angeles,
California, the air, principally of marine origin, has numerous sea salt
particles. Visibility is noticeably reduced when humidity exceeds about
67%. In a study of visibility related to both relative humidity and origin of

Fig. 10-6. Four forms of particle light interaction. Light scattering by coarse particles
(>2 /im) is the combined effect of diffraction and refraction, (a) Diffraction is an edge effect
whereby the light is bent to fill in the shadow behind the particle, (b) The speed of a wavefront
entering a particle with refractive index n >1 (for water, n = 1.33) is reduced, (c) Refraction
produces a lens effect. The angular dispersion resulting from bending incoming rays increases
with n. (d) For absorbing media, the refracted wave intensity decays within the particle. When
the particle size is comparable to the wavelength of light (0.1-1.0 /im), these interactions
(a-d) are complex and enhanced. Source: U.S. Environmental Protection Agency, "Protecting
Visibility," EPA-450/5-79-008. Office of Air Quality Planning Standards, Research Triangle
Park, NC, 1979.