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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 image 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



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