Page 173 - Fundamentals of Air Pollution

I. The Physics of Visibility 139

types of fine particles such as sulfates, although not good light absorbers,
are very efficient at scattering light.

A. Light Extinction in the Atmosphere
The interaction of light in the atmosphere is described mathematically
in Eq. (10-1):

where - dl is the decrease in intensity, b ext the extinction coefficient, I the
original intensity of the beam of light, and dx the length of the path traveled
by the beam of light.
Figure 10-3(a) shows a beam of light transmitted through the atmosphere.
The intensity of the beam I(x) decreases with the distance from the illumina-
tion source as the light is absorbed or scattered out of the beam. For a short
period, this decrease is proportional to the intensity of the beam and the
length of the interval at that point. Here b exi is the extinction or attenuation
coefficient and is a function of the degree of scattering and absorption of
the particles and gases which are present in the beam path.
Figure 10-3(b) illustrates a slightly more complicated case, but one more
applicable to atmospheric visibility. In this example, the observer still de-

Fig. 10-3. (a) A diagram of extinction of light from a source such as an electric light in a
reflector, illustrating (i) transmitted, (ii) scattered, and (iii) absorbed light, (b) A diagram of
daylight visibility, illustrating (i) residual light from a target reaching an observer, (ii) light
from a target scattered out of an observer's line of sight, (iii) air light from the intervening
atmosphere, and (iv) air light constituting horizon sky. Source: U.S. Environmental Protection
Agency, "Protecting Visibility," EPA-450/5-79-008. Office of Air Quality Planning and Stan-
dards, Research Triangle Park, NC, 1979.