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Observations of polar stratospheric clouds (PSCs) date back to the end of the 19th century. These opalescent clouds exist in the lower stratosphere, at altitudes between about 15 to 25 km. They are only found in the polar regions where stratospheric temperatures fall below about 200 K, cold enough to initiate their formation. Some PSCs are known to be liquid particles composed of supercooled ternary solutions of sulfuric acid, nitric acid, and water. Others are composed of nitric acid trihydrate which is the thermodynamically stable form of HNO3 and H2O in the polar stratosphere. Long thought to be essentially irrelevant for atmospheric chemistry it is now accepted that PSCs are an important ingredient in polar ozone destruction and the formation of the ozone hole. PSCs act as catalysts converting chlorine from the inert "reservoir" species ClONO2 and HCl to active species by heterogeneous chemical reactions. In addition, the polar stratosphere can be "denitrified" when HNO3 is permanently removed through sedimentation of large PSC particles. Denitrification removes gaseous nitric acid that could otherwise interrupt the catalytic ozone loss cycle by reforming the reservoir species ClONO2. Unfortunately, both denitrification and ozone loss predictions are uncertain because the composition and formation mechanisms of the particles responsible for denitrification and chlorine activation have still not been established.
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