with CFCs, he was referring to damage done to the ozone layer by this man-made chemical creation (Box 8.1). CFCs are classified as synthetic halocarbons, and by photolysis can be broken down to release fluorine and chlorine atoms. When liberated, a single chlorine atom can trigger a catalytic reaction and destroy thousands of ozone molecules, thus contributing to a breakdown of the Earth's protective ozone shield.

Ozone as a Pollutant

Ozone in the upper stratosphere is created largely by natural processes, and its presence serves a vital function. Lower down in the troposphere, however, ozone is classified as a pollutant, with adverse health effects. "In the Earth's atmosphere, the primary tendency of the chemistry is to oxidize any molecules emitted into the air," said McRae, who pinpointed ozone as one of the two primary species that mediate the process. Published work by McRae and Armistead G. Russell discusses some of the chemistry basic to models of air pollution and explains some of the primary phenomena (Box 8.2).

BOX 8.1. CHLOROFLUOROCARBONS

CFC molecules consist of either a single carbon atom or a pair of carbon atoms bonded to a fluorine and a chlorine atom, and have an atomic structure that confers on them a collection of beneficial properties. They are light in weight, neither flammable nor toxic, and are largely impervious to degradation by microorganisms or to reactions with other chemicals. Their applications are many: refrigerants in air conditioners and refrigerators, propellants for aerosol cans, foam blowing agents, and solvents for cleaning electronic components and other industrial products, to name only the most significant. But these same chemical qualities make them problematic, McRae explained. Since they do not react or degrade and are lighter in weight than nitrogen or oxygen, once they escape from their earthbound containers or have served their utilitarian function, they endure and migrate very slowly over a number of years or decades into the stratosphere.

It is upon their eventual arrival in the stratosphere that CFCs turn deadly—in McRae's terminology, reverse their positive sign. Again, photolysis occurs when radiant light energy reacts with valence electrons to dissociate atoms and break down chemical bonds.



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