Particularly in Antarctica and to a lesser extent in the Arctic, frigid temperatures hasten the chlorine - catalytic cycles by removing nitrogen oxides, which strongly Interfere with them (Paradoxically, then, although the NOx gases can destroy ozone, their presence in the stratosphere often ameliorates chlorine -catalyzed ozone destruction.) Together with water, molecules, the NOx gases freeze to form particles making up what are known as polar stratospheric clouds. Worse yet, the cloud particles actually facilitate chemical reactions that release chlorine from compounds that do