Page 290 - Fundamentals of Air Pollution
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246 17. The Physics of the Atmosphere
vapor radiates very efficiently. Unlike solids and liquids, many gases absorb
(and reradiate) selectively in discrete wavelength bands, rather than
smoothly over a continuous spectrum.
6. Incoming Solar Radiation
The sun radiates approximately as a blackbody, with an effective tempera-
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ture of about 6000 K. The total solar flux is 3.9 x 10 W. Using Wien's
law, it has been found that the frequency of maximum solar radiation
l
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intensity is 6.3 x 10 s~ (X = 0.48 /am), which is in the visible part of
the spectrum; 99% of solar radiation occurs between the frequencies of
15
13
l
!
7.5 x 10 s~ (X = 4 /am) and 2 x 10 s~ (X = 0.15 /am) and about 50%
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1
in the visible region between 4.3 x 10 s" (X = 0.7 /am) and 7.5 x 10 14
1
s"" (X = 0.4 /am). The intensity of this energy flux at the distance of the
2
earth is about 1400 W m~ on an area normal to a beam of solar radiation.
This value is called the solar constant. Due to the eccentricity of the earth's
orbit as it revolves around the sun once a year, the earth is closer to the
sun in January (perihelion) than in July (aphelion). This results in about a
7% difference in radiant flux at the outer limits of the atmosphere between
these two times.
2
Since the area of the solar beam intercepted by the earth is TrE , where
E is the radius of the earth, and the energy falling within this circle is
2
spread over the area of the earth's sphere, 47r£ , in 24 hr, the average
2
energy reaching the top of the atmosphere is 338 W m" . This average
radiant energy reaching the outer limits of the atmosphere is depleted as
it attempts to reach the the earth's surface. Ultraviolet radiation with a
wavelength less than 0.18 /am is strongly absorbed by molecular oxygen
in the ionosphere 100 km above the earth; shorter x-rays are absorbed at
even higher altitudes above the earth's surface. At 60-80 km above the
earth, the absorption of 0.2-0.24 /am wavelength radiation leads to the
formation of ozone; below 60 km there is so much ozone that much of the
0.2-0.3 /tm wavelength radiation is absorbed. This ozone layer in the lower
mesosphere and the top of the stratosphere shields life from much of
the harmful ultraviolet radiation. The various layers warmed by the the
absorbed radiation reradiate in wavelengths dependent on their tempera-
ture and spectral emissivity. Approximately 5% of the total incoming solar
radiation is absorbed above 40 km. Under clear sky conditions, another
10-15% is absorbed by the lower atmosphere or scattered back to space by
the atmospheric aerosols and molecules; as a result, only 80-85% of the
incoming radiation reaches the earth's surface. With average cloudiness,
only about 50% of the incoming radiation reaches the earth's surface, be-
cause of the additional interference of the clouds.
C. Albedo and Angle of Incidence
The portion of the incoming radiation reflected and scattered back to
space is the albedo. The albedo of clouds, snow, and ice-covered surfaces