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Stratospheric and Tropospheric Ozone.
The usual (breathable) form of the oxygen molecule has two atoms (O2), but the ozone molecule has three (O3). The effects of atmospheric O3 vary depending on where in the atmosphere it is found.
O3 in the stratosphere is beneficial – this is the ozone layer that protects Earthlings from ultraviolet (UV) radiation. In this broad band of gas, extending through most of the stratosphere, UV radiation from the sun interacts with oxygen, forming and splitting molecules of O3, and these reactions absorb UV energy. This is one of the life-supporting properties of Earth's atmosphere: because oxygen molecules intercept UV radiation, they prevent much of the UV radiation from reaching Earth. This interception limits the background level of UV energy to one that life forms on Earth are able to tolerate. However, this beneficial layer of stratospheric O3 is damaged by pollution. Pollutants include gases such as CFCs which enter the air as a result of propellant sprays, nitrous oxide exhaust from supersonic transports, and oxides of sulphur and nitrogen produced by nuclear explosions. These gases reduce levels of ozone in the stratosphere, thus reducing the interception of UV.
While O3 in the stratosphere is beneficial, the increasing level of O3 in the troposphere is harmful. Tropospheric O3 is a serious pollutant. For example, because O3 is toxic to vegetation, O3 in the troposphere (which is near Earth's surface) can damage forests and crops, and reduce yields.
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