ClO is the dominant daytime form of reactive chlorine which destroys ozone. NASA ground and aircraft expeditions to Antarctica had previously observed enhanced abundances of ClO coincident with severe ozone depletion. It is now known that enhancement of ClO is triggered by chemistry occurring on the particulates of polar stratospheric clouds which form at low temperatures in the Antarctic winter lower stratosphere. MLS provided the first maps showing the full spatial extent of enhanced ClO and its coincidence with depleted ozone. This figure shows MLS maps of ClO and ozone on 15 September for 1992 and 1993. Enhanced ClO is indicated by red and darker colors, with red indicating a vertical column abundance of 1016 molecules per square meter. Purple/blue colors indicate ozone vertical abundances (above approximately 18 km) of less than approximately 200 Dobson Units. Note the correlation between enhanced ClO and depleted ozone. The white area polewards of 80 degrees latitude is where no measurements are made. See Waters, et al. Nature 362, 597-602 [1993] for more discussion of these data.