THE OZONE CRISIS AND CFCs

10 to 30 miles above the Earth, in the stratosphere, a thin shield of gas, the ozone layer, encircles the planet and protects it from the deadly UV-B radiation of the sun. This protective shield makes life on Earth possible. Ozone is naturally created when the ultraviolet radiation of the sun split oxygen (O2) molecules into single oxygen atoms, which then link with oxygen molecules to form ozone (O3). While ozone is constantly produced if all the ozone were compressed at earth's surface pressure, to would only be as two ply tissue paper, approximately an eigth of an inch. There is an average of about 3 ozone molecules for every ten million air molecule.

In the early 1970's scientists discovered that a class of chlorine containing industrial chemicals called halocarbons were harmful to the ozone layer. The most commonly used halocarbons are chlorofluorocarbons (CFCs). CFCs are stable, non-toxic and inflammable compounds. They have numerous industrial applications as refrigerants, foam blowing agents, electrical circuit-board solvents and aerosol propellants. Other prominently used ozone depleting substances include HCFCs, carbon tetrachloride, halons and the pesticide methyl bromide. Once CFCs are emitted, they have an atmospheric lifetime of 100-150 year. Because they are insoluable in water, they don't get washed out of the atmosphere by rain or snow. Consequently, eventhough CFC molecules are heavier than air, they get carried by winds and aircurrents and are distributed throughout the atmosphere,. Eventually they accumulate in the stratosphere.

As the CFCs move above the stratosphere, beyond the ozone layer, they get broken down by UV light, releasing the chlorine atom. The chlorine atom then combines with the highly unstable ozone molecule to form chlorine monoxide (ClO) and oxygen (O2). In a second chemical reaction, the chlorine monoxide combines with a single oxygen atom resulting in a free chlorine atom and an O2 molecule. The chlorine is then free to start the cycle over again. This chemical interaction not only destroys existing ozone molecules, but each time it prevents another ozone molecule from forming. One chlorine molecule can destroy up to 100,000 ozone molecules.