Environmental Engineering Reference
In-Depth Information
m, where the
earth's outgoing thermal radiation is prominent, can be seen as the deficits in Figure 10.2. Water
vapor absorbs at 5-7
The absorption bands of atmospheric molecules in the far IR, between 5 and 50
µ
µ
m and all wavelengths above 10
µ
m; CO
2
at 12-18
µ
m; CH
4
and N
2
Oat
7-8
m. Each absorption
band is composed of many individual absorption lines, due to vibrational-rotational transitions of
the molecule. At some wavelengths there is overlap of absorption by individual species. Water
vapor provides a quasi-continuum absorption (crowding of individual rotational lines) at the longer
wavelengths. The CO
2
band at 12-18
µ
m; O
3
at 9-10
µ
m; the major CFCs (CFCl
3
and CF
2
Cl
2
) at 10-12
µ
m is so strong that it is almost saturated at the center. Further
additions of CO
2
to the atmosphere will result in more absorption at the wings of the bands, but
little at the center. Thus, a doubling of the CO
2
concentration in the atmosphere will not necessarily
result in doubling of the earth's thermal radiation absorption, but only a fraction thereof. This is
analogous to a window that is painted black at the center, but shades of gray at the periphery.
Adding another coat of black on the entire window will not double its opaqueness. On the other
hand, CH
4
bands are not saturated at all; thus doubling of CH
4
concentrations will nearly double
CH
4
absorption. It is estimated that adding one molecule of CH
4
is as effective in absorbing the
far IR radiation as adding 20 molecules of CO
2
.
The absorption of energy at a particular wavelength by an individual molecule depends on line
intensity, line half-width, and lower state energy. The latter is a function of the temperature of the
surrounding atmosphere with which the molecule is in equilibrium. The atmosphere is modeled
by many layers, in each of which the absorbing gas concentration and temperature determine the
radiative transport away from and towards the earth, and from which the model predicts the surface
temperature and atmospheric temperature profile.
The earth's atmosphere has a complicated temperature structure. In the troposphere the tem-
perature decreases with altitude; in the stratosphere the temperature increases; in the mesosphere it
decreases; in the thermosphere it increases again with altitude. The GHG molecules can reduce the
outgoing terrestrial radiation only if they are at a colder temperature than the radiating earth. We
noted before that the earth's effective radiating temperature corresponds to a black-body tempera-
ture
T
E
µ
=
255 K. With an average surface temperature
T
S
=
288 K and an average temperature
gradient (lapse rate) in the troposphere of about
6 K / km, the apparent height (called the
mean
radiating height
), at which the temperature level of 255 K originates is 5.5 km. Because the effec-
tive radiating temperature of the earth will remain nearly at
T
E
−
255 K, addition of GHG to the
atmosphere will change the surface temperature
T
S
, the temperature profile of the atmosphere, and
the mean radiating height.
7
The sun-earth-space system remains in radiative equilibrium; what
changes with the addition of GHG to the atmosphere is the
redistribution
of energy, manifested by
a redistribution of temperature, between the earth's surface and her atmosphere. Increased GHG
concentrations have the effect that in the higher troposphere, where the GHG molecules are colder,
more outgoing terrestrial radiation is absorbed. That radiation is re-emitted in all directions. About
one-half is radiated downward to the earth's surface, raising
T
S
; the other half is radiated upward,
warming the even colder upper layers of the troposphere. On the other hand, the lower stratosphere
will be cooler because those layers receive less outgoing radiation from the surface, due to ab-
sorption of the intervening GHG. Taking the whole atmospheric temperature profile into account,
the mean radiating height will be relatively higher than at present. These changes are depicted in
Figure 10.4.
=
7
The radiative temperature of the earth can only change if the albedo changes [see equation (10.3)].
