Ozone Depletion

Ozone is a relatively unstable form of molecular oxygen containing three oxygen atoms (03). Ozone near the earth's surface is the most noxious component of smog that is treated as an air pollutant in most industrial countries. The ozone layer in the stratosphere, however, shields the earth from ultraviolet radiation that is harmful to living organisms. This radiation comes from the sun, and too much of it can damage plant and animal cells, cause skin cancer and eye cataracts in humans, reduce crop yields, deplete marine fisheries, cause damage to materials of various kinds, and kill many smaller and more sensitive organisms.

Stratospheric ozone is found in a broad band, generally extending from about 15 to 35 km (9–22 miles) above the earth's surface. The amount and distribution of stratospheric ozone varies around the earth, but in general the layer of ozone is relatively thin when compared with the thickness of the stratosphere. Ozone is produced when upper-atmosphere oxygen molecules (02) are broken apart by ultraviolet light. Most of these freed oxygen atoms bond with ordinary oxygen molecules to form ozone. This ozone creation process is constantly at work producing more ozone.

However, ozone can also be destroyed by chemicals that react with it directly. One such destroyer was identified in 1974 by Molina and Rowland, two chemists at the University of California at Irvine, who theorized that chlorofluorocarbons (CFCs) could eventually drift up to the stratosphere to react chemically with ozone molecules in a destructive fashion. When CFCs reach the stratosphere, they are finally broken down by ultraviolet radiation to release chlorine atoms that act as a catalyst in a series of reactions that convert ozone into oxygen.

Because chlorine acts as a catalyst rather than as a reagent, a single molecule of chlorine can destroy thousands of ozone molecules before it eventually gets washed out of the atmosphere. CFCs can take as long as 6 to 8 years to reach the stratosphere to do their damage.

When first discovered, CFCs proved to be remarkable compounds with many uses. Since they were inert, they did not react with other chemicals with which they were mixed. They were neither toxic nor flammable at ground level. After their initial discovery, the number of CFC compounds grew quickly into the dozens and were used as a universal coolant; as a blowing agent in rigid insulation forms; as an aerosol propellant; as a solvent to remove glue, grease, and soldering residues from microchips and other electronic products; and as a component of foam packaging containers.

When Molina and Rowland first developed their theory, empirical validation was unavailable because of the difficulties involved in measuring actual levels of stratospheric ozone. No international action was taken to limit CFC usage until the discovery of the ozone hole over Antarctica. The British Antarctic survey conducted in 1983 found that concentrations of ozone in the stratosphere were dropping over Antarctica at a dramatic rate each austral spring to be replenished again by the end of the fall season. This discovery led to a $10 million scientific mission carried out by the United States under the combined sponsorship of NASA, the National Oceanic and Atmospheric Administration, and the Chemical Manufacturers Association to find out more about this phenomenon. By the spring of 1987, the average ozone concentration over the South Pole was discovered to be down about 50%, and in isolated spots it had actually disappeared entirely. Subsequent research showed that the ozone layer around the world was changing far more rapidly and in a different pattern than any model had predicted.

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