Environmental Engineering Reference
the condenser by steam ejectors (an energy loss), hydrogen sulfide, sulfur dioxide, and radon, all
of which are toxic to humans, must be safely vented.
A prodigious source of radiant energy, the sun emits electromagnetic radiation whose energy flux
per unit area, called irradiance, decreases as the square of the distance from the sun center. At the
mean sun-earth distance of 1.495E(8) km, the solar irradiance is 1367 W/m 2 . 12 The sun's light
is nearly, but not exactly, parallel; the sun's disc, viewed from the earth's mean orbit, subtends a
plane angle of 9.3E
steradian. It is this
energy stream from the sun that maintains the earth at a livable temperature, far above the cosmic
background temperature of 2.7 K that would exist if the sun's irradiance were zero.
Not all of this solar radiation reaches the earth's surface. Some is reflected from the earth's
atmospheric gases and clouds; some is absorbed by air molecules (principally oxygen, carbon
dioxide, water vapor, and ozone), clouds, and atmospheric dust; some is scattered by air molecules
and dust. As a consequence, only a fraction of the solar irradiance impinging on the earth's atmo-
sphere (called the extraterrestrial irradiance ) actually reaches ground level. Of that fraction, some
retains its solar direction, called beam irradiance, while the remainder, called diffuse irradiance,
has been scattered through large angles, approaching the ground from directions quite different
than that of the sun. 13 Of the sunlight reaching the ground, some is absorbed and the rest reflected
upward, with the latter undergoing scattering and absorption in the atmosphere on its way out to
extraterrestrial space. 14
The solar radiation approaching the earth comprises a wide band of wavelengths, but most
(94%) of the radiant energy lies between 0
31.0 minutes and a solid angle of 5.40E
m in wavelength. The distribution of the ex-
traterrestrial solar irradiance among its various wave lengths is shown as a dashed line in Figure 7.5.
The main components of the solar spectrum are the ultraviolet (
3 and 2
m), the visible ( 0
m). The energy content of these portions are 6%, 48%, and
46% of the total irradiance, respectively. The energy of an individual photon, the quantum packet
of electromagnetic radiation, is proportional to its frequency and hence inversely proportional to its
wavelength. Ultraviolet photons, having shorter wavelengths and hence higher individual energies
than visible or infrared photons, are potentially more damaging to living organisms, even though
their aggregate share of solar energy is much smaller.
m), and the infrared (
12 Because the earth's orbit around the sun is elliptical rather than circular, the solar irradiance varies by
1 W/m 2 during the year, being a maximum on December 26 and a minimum on July 1, when
the earth is closest and furthest from the sun, respectively.
13 Atmospheric molecules scatter blue light more than red light, making a clear sky uniformly blue and
14 A small portion of absorbed incoming sunlight causes photochemical reactions in the atmosphere, mainly
ozone formation in the stratosphere and smog formation in the lower troposphere. Also, some of the ground-
level incident radiation absorbed by plants results in photosynthesis. Nevertheless, the solar energy invested in
chemical change in the atmosphere and plants is tiny compared to the heating of the atmosphere, hydrosphere,
and geosphere by absorbed sunlight.