Page 174 - Fundamentals of Air Pollution

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

pends on the ability to perceive light rays emanating from the target object
and on the scattering and absorption of those rays out of the beam. In
addition, however, the observer must contend with additional light scat-
tered into the line of sight from other angles. This extraneous light is
sometimes called air light. Equation (10-1) is modified to account for this
phenomenon by adding a term to represent this background intensity.

This air light term contributes to the reduced visibility we call atmospheric
haze.
A simplified relationship developed by Koschmieder which relates the
visual range and the extinction coefficient is given by Eq. (10-3),

where L v is the distance at which a black object is just barely visible (3).
Equation (10-3) is based on the following assumptions:
1. The background behind the target is uniform.
2. The object is black.
3. An observer can detect a contrast of 0.02.
4. The ratio of air light to extinction is constant over the path of sight.
While the Koschmieder relationship is useful as a first approximation for
determining visual range, many situations exist in which the results are
only qualitative.
The extinction coefficient b ext is dependent on the presence of gases and
molecules that scatter and absorb light in the atmosphere. The extinction
coefficient may be considered as the sum of the air and pollutant scattering
and absorption interactions, as shown in Eq. (10-4):

where fr rg is scattering by gaseous molecules (Rayleigh scattering), ib ag ab-
sorption by NO 2 gas, b scat scattering by particles, and b ap absorption by
particles. These various extinction components are a function of wave-
length. As extinction increases, visibility decreases.
The Rayleigh scattering extinction coefficient for particle-free air is
1
0.012 km" for "green" light (-y = 0.05 /xm) at sea level (4). This permits
a visual range of ~320 km. The particle-free, or Rayleigh scattering, case
represents the best visibility possible with the current atmosphere on earth.
The absorption spectrum of NO 2 shows significant absorption in the
visible region (see Fig. 10-4) (5). As a strong absorber in the blue region, NO 2
can color plumes red, brown, or yellow. Figure 10-5 shows a comparison of
extinction coefficients of 0.1 ppm NO 2 and Rayleigh scattering by air (6).
In urban areas, some discoloration can be due to areawide NO 2 pollution.
In rural areas, the biggest problem with NO 2 is that in coherent plumes
from power plants, it contributes to the discoloration of the plume.