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.