Ozone layer

The ozone layer is a layer of the upper atmosphere lying about 20 to 25 km (12 to 15 mi) above the Earth's surface. It is so named because the unstable form of oxygen called ozone is concentrated in this layer. The ozone layer strongly absorbs ultraviolet radiation from the Sun. If this radiation reached the Earth's surface at unprotected levels, it would be deleterious to all forms of life. For example, it would raise the incidence of human skin cancers and cataracts, as well as reducing food production in general.

Disassociation and Recombination of Gases

The natural forces at work in this high-altitude layer maintain a mixture of gases that includes atomic oxygen, 0, ordinary oxygen, 0(2), and ozone, 0(3). The quantities of each form fluctuate diurnally and seasonally in
response to solar radiation, temperature changes, and other catalytic influences.The atoms of ordinary oxygen molecules become disassociated from each other and recombine with other molecules of ordinary oxygen to form ozone. This process consumes energy, which is supplied in nature by absorption of short-wave solar radiation and by discharges of lightning. Decomposition is hastened by a rise in temperature or interaction with a variety of substances.

Effect of Free Radicals

Within the ozone layer, the rate of formation of ozone equals the rate of its consumption. These rates have apparently stayed in general balance over a long period of geological time. Human activities, however, now appear to be affecting that balance through the introduction of certain destructive chemicals into the layer. The potential harm caused by the resulting loss of ozone has become a matter of international concern. The chemicals concerned are producers of FREE RADICALS, which are atoms and molecules with an unpaired electron. Free-radical chlorine is an example. When a chemical that can form free-radical chlorine is introduced into the ozone layer, the chlorine will react with the ozone in the following manner: O(3) + Cl --> OCl + O(2), and OCl + O(3) --> Cl +2O(2).

Decreases in ozone concentrations in the ozone layer were first detected by scientists in the late 1970's and have
since been confirmed. The most massive decreases have occurred over Antarctica, where ozone concentrations
temporarily decline each spring, resulting in an ozone "hole." Its size appears to be increasing, and similar
seasonal losses are apparently on the increase in the Arctic region as well.

The chemicals that have been linked most directly to this phenomenon are the chlorofluorocarbons, of CFCs, which are widely used as aerosol propellants, refrigerants, foaming agents for plastic packaging, and cleaning fluids. In 1987, in response, an international treaty called the Montreal Protocol on Substances that Deplete the Ozone Layer mandated a reduction in the use of CFCs. In 1989, 93 nations further agreed to phase out their production and to aid poorer nations in revising their technologies accordingly. The chemicals thus far devised as replacements are less versatile and more expensive than CFCs.