Environmental Engineering Reference
In-Depth Information
2.5
Solar irradiance outside atmosphere
Solar irradiance at sea level
Spectral radiance of black body
at 5900 K
2.0
1.5
O
3
H
2
O
O
2
, H
2
O
1.0
H
2
O
0.5
H
2
O, CO
2
O
3
0
0
0.4
0.8
1.2 1.6
Wavelength (
2.0
2.4
2.8
3.2
m)
Figure 10.1
Solar spectrum at the top of the atmosphere (upper curve) and at sea level (lower curve).
Shaded areas indicate absorption of radiation by atmospheric molecules. The dashed curve is the radiance
spectrum of a black body heated to 5900 K, scaled to the solar irradiance curve at the top of atmosphere.
(Data from Valley, S. L., Ed., 1965.
Handbook of Geophysics.
Bedford: United States Air Force.)
radiation. On the UV side, oxygen and ozone in the stratosphere are the major absorbing gases. In
the visible portion, density fluctuations of atmospheric molecules scatter sunlight.
6
In the infrared,
polyatomic molecules present in the lower atmosphere (troposphere), such as H
2
O, CO
2
,O
3
,CH
4
,
N
2
O, and others, absorb solar radiation.
The earth radiates outward to space a spectrum commensurate to her surface temperature.
Figure 10.2 presents the earth's radiance (“
earth-shine
”) as observed by a spectro-photometer
housed on an artificial satellite looking toward the earth's surface. The solid curves in Figure 10.2
represent black-body radiances at various temperatures. The black-body radiance curve that best
represents the earth's radiance at the spot where the spectrum was taken is that for a temperature
of about 280-285 K.
The radiance curves span the far-infrared region of the electromagnetic spectrum, from about 5
to 50
µ
µ
m. But the earth's radiance curve has several deficits. These deficits
are the result of absorption, and emission at reduced levels, of the
outgoing
thermal radiation by
greenhouse gases.
m, peaking at about 18
10.2.2
Sun-Earth-Space Radiative Equilibrium
The sun-earth-space radiative equilibrium is depicted in Figure 10.3. The numbers on arrows
are average annual radiation received from the sun or emitted from the earth, in watts per square
meter. (The intensities in Figures 10.1 and 10.2 are instantaneous radiation fluxes from the sun and
6
Scattered sunlight causes the sky to be blue. Blue is the preferred scattered radiation of a pure atmosphere,
devoid of particles. With increasing particle concentration the scattered radiation becomes colorless (i.e.,
white).
