UARS Microwave Limb Sounder observations of denitrification and ozone lossin the 2000 Arctic late winter

The UARS Microwave Limb Sounder obtained measurements of CIO, HNO3, and O-3 inside the Arctic lower stratospheric vortex during two intervals in Febru ary and March 2000. The data show evidence of significant chemical processi ng in February, consistent with the exceptionally cold conditions that prev ailed earlier in the winter. Ozone at 465 K decreased by 0.04 +/- 0.01 ppmv /day in early February, implying chemical loss rates greater than those obs erved during the 1995-1996 winter, which was also unusually cold in the low er stratosphere. A persistent depression in the HNO3 abundances in late Mar ch, well after polar stratospheric cloud activity had ceased, suggests a mo derate degree (similar to 20%) of denitrification around the 465 K level at 70 degrees N and higher equivalent latitudes. This is the strongest eviden ce yet seen for the occurrence of denitrification in the Arctic over spatia l scales large enough to be detected in satellite measurements.