Although only a small percentage of the electromagnetic radiation that is emitted by the Sun isultraviolet (UV) radiation, the amount that is emitted would be enough to cause severe damage to most
forms of life on Earth were it all to reach the surface of the earth. Fortunately, all of the Sun’s
ultraviolet radiation does not reach the earth because of a layer of oxygen, called the ozone layer,
encircling
the earth in the stratosphere at an altitude of about 15 miles above the earth. The ozone layer
absorbs much of the Sun’s ultraviolet radiation and prevents it from reaching the earth.
Ozone is a form of oxygen in which each molecule consists of three atoms (O
3
) instead of thetwo atoms (O
2
) usually found in an oxygen molecule. Ozone forms in the stratosphere in a process thatis initiated by ultraviolet radiation from the Sun. UV radiation from the Sun splits oxygen moleculeswith two atoms into
free
oxygen atoms, and each of these unattached oxygen atoms then joins up withan oxygen molecule to form ozone. UV radiation is also capable of splitting up ozone molecules; thus,ozone is constantly forming, splitting, and reforming in the stratosphere. When UV radiation isabsorbed during the process of ozone formation and reformation,
it
is unable to reach Earth and causedamage there.
Recently, however, the ozone layer over parts of the earth has been diminishing. Chief amongthe culprits in the case of the disappearing ozone,
those
that are really responsible, are thechlorofluorocarbons (CFCs). CFCs meander up from Earth into the stratosphere, where they break down and release chlorine. The released chlorine reacts with ozone in the stratosphere to form chlorinemonoxide (ClO) and oxygen (O
2
). The chlorine then becomes free to go through the cycle over andover again. One chlorine atom can, in fact, destroy hundreds of thousands of ozone molecules in thisrepetitious cycle
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