Polar vortex conditions during the 1995–96 Arctic Winter: MLS CLO and HNO3

Microwave Limb Sounder (MLS) measurements of lower stratospheric ClO and HNO 3 during the 1995–96 Arctic winter are presented. The 1995–96 Arctic winter was both colder and more persistently cold than usual, leading to an enhancement in lower stratospheric ClO of greater magnitude, vertical extent, and duration than previously observed in the Arctic. Vortex concentrations of HNO 3 in mid‐December were large due to diabatic descent. Trajectory calculations indicate that localized severe depletions of gas‐phase HNO 3 in mid‐February and early March did not arise from entrainment of midlatitude air into the vortex and were therefore probably related to polar stratospheric cloud (PSC) formation. A strong correlation between temperature and gas‐phase HNO 3 was evident, consistent with recurring PSC condensation and evaporation cycles.