Cl - is not consumed in reactions 5 and 6 but simply keeps regenerating itself. Therefore, Cl - is called a catalyst. Each chlorine radical has the potential to destroy thousands of molecules of ozone and to consume thousands of singlet oxygen atoms as it cycles through the CCC over and over again. Concern over the potential implications of the CCC were sufficiently great that Canada, the U.S. and other industrialized nations banned the use of CFC's as an aerosol propellant in 1978. However, efforts to control other, far more significant, uses of CFC's were not successful at that time.
Nitrogen oxides form naturally in the atmosphere when lightning strikes a molecule of nitrogen gas.
Nitrogen oxides also come from combustion of fossil fuels in various types of engines. Two examples of NOX's are nitrogen monoxide, NO and nitrogen dioxide, NO 2 . As early as 1969, laboratory studies had indicated that NOX's could destroy ozone in a catalytic cycle similar to that of Cl - . CFC's had been accumulating in the atmosphere since the 1930's. Given the amounts of CFC's released and the potential for both the CCC and the NCC, calculations suggested that much more ozone should have been consumed than actually had been consumed.