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A Hole in the Sky: Ozone Depletion - Earth's Protective Ozone Layer

oxygen atoms ground stratosphere

Ozone is a gas naturally present in Earth's atmosphere. Unlike regular oxygen, which contains two oxygen atoms (O2), ozone contains three oxygen atoms (O3). A molecule of regular oxygen can be converted to ozone by ultraviolet (UV) radiation, electrical discharge (such as from lightning), or complex chemical reactions. These processes split apart the two oxygen atoms, which are then free to bind with other loose oxygen atoms to form ozone.

Ozone exists in Earth's atmosphere at two levels—the troposphere and the stratosphere. (See Figure 3.1.) Tropospheric (or ground-level) ozone accounts for only a small portion of Earth's total ozone, but it is a potent air pollutant with serious health consequences. Ground-level ozone is the primary component in smog and is formed via complex chemical reactions involving emissions of industrial chemicals and through fossil fuel combustion. Ozone formation is intensified during hot weather, when more radiation reaches the ground. Smog retards crop and tree growth, impairs health, and limits visibility.

Approximately 90 percent of the Earth's ozone lies in the stratosphere at altitudes greater than about twenty miles. (See Figure 3.2.) Ozone molecules at this level protect life on Earth by absorbing UV radiation from the sun and preventing it from reaching the ground. The so-called ozone layer is actually a scattering of molecules constantly undergoing change from oxygen to ozone and back. Although most of the ozone changes back to oxygen, a small amount of ozone persists. As long as this natural process stays in balance, the overall ozone layer remains thick enough to do its job. The amount of ozone in the stratosphere varies greatly depending upon location, altitude, and temperature.

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