| Summary: | Ozone hole formation over Antarctica during the southern hemisphere spring depends strongly on the presence of polar stratospheric clouds (PSCs) during the Antarctic winter, which in turn depend strongly on the temperature of the stratosphere1 . PSCs act as heterogeneous catalysts that mediate the conversion of chlorine reservoir species such as HCl and ClONO2 into active chlorine species such as ClO 2 . They also facilitate the removal of NOx by conversion into HNO3 and subsequent incorporation into PSC ice crystals 2 . The latter process is called denitrification, and is the main factor in the extent of ozone destruction, as NOx species convert active chlorine back into inactive forms. Permanent removal of HNO3 can occur if the ice crystals become heavy enough for sedimentation 2 . As the sun returns in spring, ClO is converted into Cl via photolysis and ozone destruction commences. As the sun’s activity increases the PSC ice crystals begin to melt and release HNO3, which is converted into NO2 via photolysis which converts the Cl species back to inactive forms 2 . As the atmosphere keeps on warming the polar vortex collapses, bringing ozone levels back to normal by November as ozone rich air from the tropics can now mix with the ozone depleted air 3 .
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