Understanding Ozone Depletion

The ozone layer absorbs almost 99 percent of the ultraviolet radiation that reaches the earth from the sun. It protects us from the effects of too much ultraviolet radiation, including sunburns, skin cancers, cataracts, and premature aging of the skin. Because of the ozone layer, most of us can enjoy the outdoors without head-to-toe protection.

How is ozone (O3) formed? Well, oxygen in the mesosphere — the part of the earth's atmosphere between the stratosphere and the thermosphere (the layer that extends to outer space) — is broken apart by ultraviolet radiation into highly reactive oxygen atoms. These oxygen atoms combine with oxygen molecules in the stratosphere to form ozone.

O2(g) + ultraviolet radiation --> 2 O(g)
O2(g) + O(g) --> O3(g)

As a society, humans release many gaseous chemicals into the atmosphere. Many of the gaseous chemicals rapidly decompose through reaction with each other, or they react with the water vapor in the atmosphere to form compounds, such as acids that fall to earth in the rain. Besides forming acid rain, some of these chemicals also form photochemical smog.

But these reactions occur rather quickly, and we can deal with them in a variety of ways, many of which are related to breaking the series of reactions that produce the pollutant by stopping the release of a critical chemical into the air.

Some classes of gaseous chemical compounds are rather inert (inactive and unreactive), so they remain with us for quite a while. Because these inert compounds stick around, they have a negative effect on the atmosphere. One such troublesome class of compounds are the chlorofluorocarbons, gaseous compounds composed of chlorine, fluorine, and carbon. These compounds are commonly called CFCs.

Because CFCs are relatively unreactive, they were extensively used in the past as refrigerants for such items as refrigerators and automobile air conditioners (Freon-12), foaming agents for plastics such as Styrofoam, and propellants for the aerosol cans of such consumer goods as hair spray and deodorants. As a result, they were released into the atmosphere in great quantities. Over the years, the CFCs have diffused into the stratosphere, and they're now doing damage to it.

How do CFCs hurt the ozone layer?

Although CFCs don't react much when they're close to earth — they're pretty inert — most scientists believe that they react with the ozone in the atmosphere and then harm the ozone layer in the stratosphere.

The reaction occurs in the following way:

1. A typical chlorofluorocarbon, CF2Cl2, reacts with ultraviolet radiation, and a highly reactive chlorine atom is formed.

CF2Cl2(g) + UV light --> CF2Cl(g) + Cl(g)

2. The reactive chlorine atom reacts with ozone in the stratosphere to produce oxygen gas molecules and chlorine oxide (ClO).

Cl(g) + O3(g) --> O2(g) + ClO(g)

This is the reaction that destroys the ozone layer. If things stopped here, the problems would actually be minimal.

3. The chlorine oxide (ClO) can then react with another oxygen atom in the stratosphere to produce an oxygen molecule and a chlorine atom; the newly created oxygen molecule and chlorine atom are now available to start the whole ozone-destroying process all over again.

CLO(g) + O(g) --> O2(g) + Cl(g)

So one CFC molecule can initiate a process that can destroy many molecules of ozone.

The problem of ozone depletion was identified in the 1970s. As a result, the governments of many industrialized nations began to require the reduction of the amount of CFCs and halons released into the atmosphere. (Halons, which contain bromine in addition to fluorine and chlorine, were commonly used as fire-extinguishing agents, especially in fire extinguishers used around computers.)

CFCs were banned for use as propellants in aerosol cans in many countries, and the CFCs used in the production of plastics and foams were recovered instead of released into the air. Laws were enacted to ensure that the CFCs and halons used as refrigerants were recovered during the recharging and repair of units. In 1991, Du Pont started producing refrigerants that weren't harmful to the ozone layer. And in 1996, the United States, along with 140 other countries, stopped producing chlorofluorocarbons altogether.

Unfortunately, though, these compounds are extremely stable. They'll remain in our atmosphere for many years. If the damage man has done to the ozone layer isn't too great, it may replenish itself (like new skin grows to replace sunburned skin). But it may well be several years before the ozone layer returns to its former composition.

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