Effects of Pollution on Visibility, Ultraviolet Radiation, and Colors in the Sky - Air Pollution and Global Warming: History, Science, and Solutions - page 158

Figure 7.7. Yellow sun in the early evening off the
island of Maui in the South Pacific Ocean. Photo by
Mark Z. Jacobson.

Figure 7.8. Red horizon after the sun dips below a
deck of stratus clouds over the Pacific Ocean,
signifying the beginning of twilight. Photo by Mark Z.
Jacobson.

enough to scatter preferentially blue light and give the
sky a deep blue appearance.
In the afternoon, light takes a longer path through
the air than it does at noon; thus, more blue and some
green light is scattered out of the direct solar beam in the
afternoon than at noon. Although a single gas molecule
is less likely to scatter a green than a blue wavelength,
the number of gas molecules along a viewer's line of
sight is so large in the afternoon that the probability
of green light scattering is sizable. Nearly all red and
some green are still transmitted to the viewer's eye in
the afternoon, causing the sun to appear yellow. In clean
air, the sun can remain yellow until just before it reaches
the horizon, as shown in Figure 7.7.
When the sun reaches the horizon at sunset, sunlight
traverses its longest distance through the atmosphere,
and all blue and green and some red wavelengths are
scattered out of the sun's direct beam. Only some red
light transmits, and a viewer looking at the sun sees
a red sun . Sunlight can be seen after sunset because
sunlight refracts as it enters Earth's atmosphere, as
illustrated in Figure 7.6. Refraction ,discussed in Sec-
tion 7.1.4.2, is the bending of light as it passes from
amedium of one density to a medium of a different
density.
During and after sunset, the whole horizon often
appears red (Figure 7.8) due to the presence of aerosol
particles along the horizon. Aerosol particles scatter
blue, green, and red wavelengths of direct sunlight,
sending such light in all directions along the hori-
zon. At all points along the horizon, additional parti-
cles rescatter some of the scattered blue, green, and red
light toward the viewer's eye. As these wavelengths pass

through the air, blue and green are scattered preferen-
tially out of the viewer's line of sight by gas molecules,
whereas most red wavelengths are transmitted, causing
the horizon to appear red. Red horizons are common
overthe ocean, where sea spray particles are present,
and over land, where soil and pollution particles are
present.
The time after sunset during which the sky is still
illuminated due to refraction is called twilight .Twi-
light also occurs before sunrise. The length of twilight
increases with increasing latitude. At midlatitudes in
summer, the length of twilight is about one-half hour.
At higher latitudes in summer, twilight in the morning
may merge with that in the evening, creating twilight
that lasts all night, known as a white night .

7.1.3. Aerosol and Hydrometeor Particle
Absorption
All aerosol and hydrometeor particles absorb thermal-
and solar-IR radiation, but only a few absorb visible
and near-UV radiation. Next, visible and UV absorption
properties of aerosol particles and the effect of aerosol
particle absorption on UV radiation and pollution are
discussed.

7.1.3.1. Important Absorbers of Visible
and Ultraviolet Radiation
The strongest aerosol particle absorber across the solar
spectrum (UV, visible, and solar-IR wavelengths) is
black carbon .Itsubstantially absorbs all wavelengths
of light and does not transmit any, thereby appearing