Geoscience Reference
In-Depth Information
Figure 7.28. Red sky in late afternoon over transmission lines near Salton Sea, California, May 1972. The smog
causing the optical effect originated from Los Angeles. Charles O'Rear, U.S. EPA, May 1972, Still Pictures
Branch, U.S. National Archives.
preferentially absorb blue and green light over red light.
In the absence of a dust storm and away from desert
regions, the concentration of soil dust particles is gen-
erally low, and the effect of soil dust on atmospheric
optics is limited.
the horizon. Gas molecules then scatter the blue and
green wavelengths out of the viewer's line of sight,
allowing only the red to transmit from the horizon to the
viewer. When heavy smog is present, particle concen-
trations near the surface and aloft increase sufficiently
to allow this phenomenon to occur in the afternoon, as
seen in Figure 7.28. Smog particles similarly enhance
the brilliance of the red horizon at sunset, as seen in
Figure 6.21.
7.3.3. Black Colors in Smog
Black colors in smog (e.g., Figures 5.7 and 5.8) are due
solely to absorption of solar radiation by black carbon,
a component of soot. Soot is emitted primarily during
coal, diesel and jet fuel, biofuel, and biomass burning.
Black carbon absorbs all wavelengths of white light,
transmitting none, thereby appearing black against the
background sky.
7.3.5. Purple Glow in the Stratosphere
After a strong volcanic eruption, a purple glow may
appear in the stratosphere, as shown in Figure 7.29. This
glow results because volcanos emit sulfur dioxide gas,
which converts to sulfuric acid-water particles, many of
which reach the stratosphere. Such particles scatter light
through the stratospheric ozone layer. Ozone weakly
absorbs green and some red wavelengths (Figure 7.3),
transmitting blue and some red, which combine to form
purple. The purple light is scattered back to a viewer's
eyebythe enhanced stratospheric particle layer, causing
the stratosphere to appear purple.
7.3.4. Red Skies and Brilliant Horizons
in Smog
Whereas the sun itself appears red at sunset and sun-
rise, and the horizon appears red during sunset and
sunrise when aerosol particles are present (Figure 7.8),
the sky can also appear red in the afternoon, and red
horizons can become more brilliant in the presence
of air pollution. As discussed in Section 7.1.2.2, red
horizons occur because aerosol particles scatter the
sun's white (blue, green, plus red) light, providing a
source of white light to a viewer from all points along
7.4. Summary
In this chapter, processes that affect radiation were dis-
cussed with an emphasis on the effects of gases and
 
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