Geoscience Reference
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gas scattering all had similar small effects. On the pol-
luted day, the most important visibility-reducing pro-
cesses were particle scattering, particle absorption, gas
absorption, and gas scattering, in that order.
Equation 7.12 relates a theoretical quantity, mete-
orological range, to a measured extinction coefficient.
When prevailing visibility, a subjective quantity, is mea-
sured simultaneously with an extinction coefficient,
they usually do not satisfy Equation 7.12. Instead,
the relationship between prevailing visibility and the
measured extinction coefficient must be obtained
empirically. Griffing (1980) studied measurements of
prevailing visibility ( V ,inkm) and extinction coef-
ficients (km 1 ) overa5-year period and derived the
empirical relationship
1
9
σ t
.
V
(7.13)
Figure 7.24 shows a map of estimated extinction
coefficients in the United States between 1991 and 1995
derived from prevailing visibility measurements substi-
tuted into Equation 7.13. During winters, visibility was
poorest (extinction coefficients were highest) in central
and Southern California, Illinois, Indiana, Iowa, Ken-
tucky, and Michigan. During summers, visibility was
poorest in Los Angeles and much of the midwestern and
southern United States. Winter visibility loss in central
California was likely due to a combination of low-lying
winter inversions, high relative humidity, and heavy par-
ticle loadings. Visibility loss in Los Angeles was due
to the presence of smog all year long. Poor visibility in
the winter in the Midwest was due to power plant emis-
sions combined with high relative humidity. Summer
visibility degradation in the midwestern and southern
United States was due to a combination of power plant
emissions, organic particles from photochemical smog
and vegetation, and high relative humidity.
Figure 7.25. Haze over Los Angeles, May 1972. Gene
Daniels, U.S. EPA, May 1972, Still Pictures Branch,
U.S. National Archives.
intensity, producing a whitish haze. Hazes resulting
from water growth on pollution particles are common
in urban areas under sunny skies (Figure 7.25).
Cloud and fog drops, which contain more liquid water
than do aerosol particles, appear white because they
scatter all wavelengths of visible light with equal inten-
sity. When pollution is heavy and the relative humidity
is high, clouds and fogs are difficult to distinguish from
hazes, as shown in Figure 7.26.
7.3. Colors in the Atmosphere
Red sunsets and blue skies arise from selective scatter-
ing of visible light wavelengths by gas molecules. Rain-
bows arise from interactions of light with raindrops.
Some additional optical phenomena are discussed next.
7.3.2. Reddish and Brown Colors in Smog
Reddish and brown colors in smog, such as those seen
in Figure 7.27, are due to three factors. The first is
preferential absorption of blue and some green light
by NO 2 (g), which allows most green and red to be
transmitted, giving smog a yellow, brown, or reddish-
green color. Brown layers resulting from NO 2 (g) can
be seen most frequently from 9 to 11 AM in polluted
7.3.1. White Hazes and Clouds
When the relative humidity is high (but less than 100
percent), aerosol particles increase in size by absorb-
ing liquid water. As their diameters approach the wave-
length of visible light, the particles enter the Mie regime
(Section 7.1.5) and scatter all wavelengths with equal
 
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