Cl2, in the stratosphere, atomic (radical) chlorine is very destructive because it depletes the greatly needed ozone layer which protects the earth from ultraviolet radiation from the sun. in the Antarctic stratosphere, molecular chlorine along with nitric acid are formed by the reaction of hydrogen chloride and chlorine nitrate--both stratospheric chlorine reservoir species. This process occurs on polar stratospheric clouds which serve as the reaction sites. Once formed, Cl2 vaporizes into the surrounding air as nitric acid--also formed in that process--binds with the ice matrix. Cl2 is then photodissociated in sunlight (lambda <= 450 nm) into chlorine radicals. These chlorine radicals then catalytic destroy ozone.
[Graedel, T. E. and Crutzen, Paul J. Atmospheric Change, An Earth System Perspective. pgs 145-6. W. H. Freeman and Company, 1993.]
Source: Atmospheric Chemistry Glossary