Page 172 - Fundamentals of Air Pollution

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

Thus, it is closely related to our understanding of good versus bad visibility
for a particular set of environmental conditions.
Threshold contrast is illustrated in Fig. 10-2. / is the intensity of light
received by the eye from the object, and / + AJ represents the intensity
coming from the surroundings. The threshold contrast can be as low as
0.018-0.03 and the object can still be perceptible. Other factors, such as
the physical size of the visual image on the retina of the eye and the brain's
response to the color of the object, influence the perception of contrast.
Let's consider the influence of gases and particles on the optical properties
of the atmosphere. Reduction in visibility is caused by the following interac-
tions in the atmosphere: light scattering by gaseous molecules and particles,
and light absorption by gases and particles (2).
Light-scattering processes involve the interaction of light with gases or
particles in such a manner that the direction or frequency of the light is
altered. Absorption processes occur when the electromagnetic radiation
interacts with gases or particles and is transferred internally to the gas or
particle.
Light scattering by gaseous molecules is wavelength dependent and is
the reason why the sky is blue. This process is dominant in atmospheres
that are relatively free of aerosols or light-absorbing gases. Light scattering
by particles is the most important cause of visibility reduction. This phenom-
enon is dependent on the size of the particles suspended in the atmosphere.
Light absorption by gases in the lower troposphere is limited to the
absorption characteristics of nitrogen dioxide. This compound absorbs the
shorter, or blue, wavelengths of visible light, causing us to observe the
red wavelengths. We therefore perceive a yellow to reddish-brown tint in
atmospheres containing quantities of NO 2. Light absorption by particles is
related principally to carbonaceous or black soot in the atmosphere. Other

Fig. 10-2. Threshold contrast in distinguishing an object from its surroundings. The eye
responds to an increment in light intensity by increasing the number of signals (pulses) sent
to the brain. The detection of threshold contrast involves the ability to discriminate between
the target (I) and the brighter background (/ + A/). Source: Gregory, R. L., Eye and Brain:
"The Psychology of Seeing." Weidenfeld and Nicolson, London, 1977.