Most importantly, the chlorine atoms so generated destroy ozone molecules in a catalytic cycle. In this cycle, a chlorine atom reacts with an ozone molecule, taking an oxygen atom with it (forming ClO) and leaving a normal oxygen molecule. A free oxygen atom then takes away the oxygen from the ClO, and the final result is an oxygen molecule and a chlorine atom, which then reinitiates the cycle. A single chlorine atom would keep on destroying ozone forever were it not for reactions that remove them from this cycle by forming reservoir species such as hydrochloric acid and chlorine nitrate
Beat me to it. The shorthand version is that chlorine is merely a catalyst for the destruction of ozone - the chlorine atom is not consumed or removed from the atmosphere during the process.