Page 175 - Fundamentals of Air Pollution

I. The Physics of Visibility 141

Fig. 10-4. Absorption spectrum of NO 2. Source: Hall, T. C, Jr., and Blacet, F. E., /. Ckem.
Phys. 20, 1745 (1952).

Suspended particles are the most important factor in visibility reduction.
In most instances, the visual quality of air is controlled by particle scattering
and is characterized by the extinction coefficient fr scat. The size of particles
plays a crucial role in their interaction with light. Other factors are the
refractive index and shape of the particles, although their effect is harder
to measure and is less well understood. If we could establish these proper-
ties, we could calculate the amount of light scattering and absorption.
Alternatively, the extinction coefficient associated with an aerosol can be
measured directly.

Fig. 10-5. Comparison of b^, for 0.1 ppm NC*2 and Rayleigh scattering by air. The photopic
eye response represents the range of wavelengths over which the eye detects light. Source:
Husar, R., White, W. H., Paterson, D. E., and Trijonis, }., "Visibility Impairment in the
Atmosphere," Draft report prepared for the U.S. Environmental Protection Agency under
Contract No. 68022515, Task Order No. 28.