Geoscience Reference
In-Depth Information
Stratospheric ozone allows little UV-C, some UV-B, and
most UV-A radiation to reach the troposphere. Ozone in
the background troposphere absorbs some of the UV-B
and -A not absorbed in the stratosphere. In polluted air,
additional absorbers of UV-B radiation include nitrated
aromatic gases and aerosol particle components, such
as black carbon, nitrated aromatics, PAHs, tar balls, and
soil-dust components (Section 7.1.3.1).
The major UV-A-absorbing gas is nitrogen dioxide
[NO 2 (g)]. Its mixing ratio in clean air is too low to affect
UV-A reaching the surface. In polluted air, the NO 2 (g)
mixing ratio is high only in the morning, when UV-
Aintensity is low. Other UV-A absorbers in polluted
air include the same aerosol particle components that
absorb UV-B radiation.
Gas and aerosol particle absorption is not the only
mechanism that reduces incident downward UV radia-
tion. Gas and aerosol particle backscattering, as well as
ground and cloud reflection, return some incident UV
radiation to space.
2500
Visible
Far UV
Near UV
2000
TOA
1500
Ground
1000
10 o N, 5 o W
August 3, 1990
500
Solar zenith angle 8.2 o
0
0.2
0.3
0.4
0.5
0.6
Wavelength (
ยต
m)
Figure 11.5. Downward solar radiation less than
0.65
minwavelength at the top of the atmosphere
and at the ground at a location near the Equator in
early August. The solar zenith angle is the angle of
the sun relative to a line perpendicular to the Earth's
surface.
ground. Thus, the air filters out all far-UV and UV-C
wavelengths before they reach the surface. Of the UV
that reaches the ground, about 9 percent is UV-B, and
the rest (91 percent) is UV-A. Of the total solar radiation
reaching the surface, about 5.2 percent is UV-A/-B, and
the rest (94.8 percent) is visible/solar IR.
Table 11.1 identifies the major absorbing components
responsible for reducing near- and far-UV radiation
between the TOA and the ground. Molecular nitro-
gen [N 2 (g)] absorbs far-UV wavelengths shorter than
100 nm (0.1
11.3. Chemistry of the Natural Ozone Layer
The chemistry of the natural stratospheric ozone layer
involves reactions among primarily oxygen-containing
compounds; however, the shape of the vertical profile
of the layer is affected by the chemistry of natural and
anthropogenic nitrogen- and hydrogen-containing com-
pounds as well. Next, the chemistry of the natural ozone
layer is discussed.
m) in the thermosphere and mesosphere,
and molecular oxygen [O 2 (g)] absorbs wavelengths
shorter than 250 nm in the thermosphere, mesosphere,
and stratosphere.
Ozone absorbs wavelengths shorter than 345 nm
(strongly below 310 nm and weakly from 310 to 345
nm). Ozone also absorbs weakly from 450 to 750 nm.
11.3.1. The Chapman Cycle
The photochemistry of the natural stratosphere is sim-
ilar to that of the background troposphere, except that
Table 11.1. Major absorbers of ultraviolet radiation in the atmosphere
Name of
Wavelengths
spectrum
(nm)
Dominant absorbers
Location of absorption
Far-UV
10-100
N 2 (g)
Thermosphere, mesosphere
10-250
O 2 (g)
Thermosphere, mesosphere, stratosphere
Near-UV
UV-C
250-290
O 3 (g)
Stratosphere
UV-B
290-345
O 3 (g)
Stratosphere, troposphere
Some particle components
Polluted troposphere
UV-A
320-380
NO 2 (g)
Polluted troposphere
Some particle components
Polluted troposphere
UV, ultraviolet.
 
 
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