The answer is - chlorine.
When the CFC's are bellow the ozone layer they are protected by it from the ultraviolet light because the ozone absorbs big portion of it. But once the CFC's go above the ozone layer, things change immediately. They do not have the ozone protection anymore, and the CFC molecules are totally exposed to 'bombarding' from the Sun's ultraviolet energy, and than their molecules break up and release chlorine atoms. Chlorine atoms than react with the oxygen atoms in the ozone and form chlorine monoxide, which than leaves an ordinary oxygen molecule, thus damaging the ozone layer.