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directly influenced by changes in solar radiative and energetic
particle inputs to this region.
All of the Sun's ultraviolet energy input at wavelengths between
about 150 and 300 nm is deposited in the Earth's middle atmosphere.
This energy plays an essential role in the chemistry, radiation,
and dynamics of the region. Figure 1.2 shows the altitude at which
solar radiation (from an overhead Sun) reaches 1/e of its original
intensity and gives an approximate measure of the penetration depth
for different wavelengths. The inversion in the atmospheric
temperature profile at about 15 km (Figure 1.2) that defines the
tropopause is a direct consequence of heating by solar UV energy
abosorbed by ozone in the middle atmosphere. The only significant
solar radiation of shorter wavelength reaching the middle
atmosphere is a small and highly sporadic contribution from X-rays,
and the strong H I Lyman line at 121.6 nm.
The ozone layer exists because of the interaction of solar UV
radiation with the constituents of the middle atmosphere.
Photodissociation of molecular oxygen by solar UV radiation at
wavelengths from 170 to 242 nm (in the O2 Schumann-Runge bands and Herzberg
continuum) is the chief source of atomic oxygen and hence ozone
production. Formed by combination of atomic and molecular oxygen,
ozone is in turn photodissociated, mainly by solar UV radiation at
wavelengths between 240 and 300 nm (in the strong O3 Hartley bands and continuum), but also
by longer wavelength visible solar radiation. Ozone's strong
absorption in the UV region of the spectrum serves the dual role of
heating the middle atmosphere and protecting the surface of the
Earth from damaging doses of ultraviolet radiation. Solar UV
radiation also creates other important trace constituents, such as
chlorine (Cl) and hydroxyl (OH), that participate in catalytic
reactions that destroy ozone. Current understanding of the
processes affecting ozone and other trace constituents of the
middle atmosphere has been reviewed in a number of reports (e.g.,
WMO, 1988), and is not repeated here.
Stratospheric ozone has received intensive study in recent
years, and is the subject of research under the Biogeochemical
Dynamics science element of the USGCRP. Ozone is known to be
influenced by human related sources such as chlorofluorocarbons
(CFCs), carbon dioxide (CO2), and
methane (CH4) and by natural
occurrences such as volcanoes and solar variability. As summarized
in Figure 3.1, definite changes in the total