POLAR VORTEX CONDITIONS DURING THE 1995-96 ARCTIC WINTER - MILS CLO AND HNO3

Microwave Limb Sounder (MLS) measurements of lower stratospheric ClO a nd HNO3 during the 1995-96 Arctic winter are presented. The 1995-96 Ar ctic winter was both colder and more persistently cold than usual, lea ding to an enhancement in lower stratospheric ClO of greater magnitude , vertical extent, and duration than previously observed in the Arctic . Vortex concentrations of HNO3 in mid-December were large due to diab atic descent. Trajectory calculations indicate that localized severe d epletions of gas-phase HNO3 in mid-February and early March did not ar ise from entrainment of midlatitude air into the vortex and were there fore probably related to polar stratospheric cloud (PSC) formation. A strong correlation between temperature and gas-phase HNO3 was evident, consistent with recurring PSC condensation and evaporation cycles.