POLAR OZONE AND AEROSOL MEASUREMENT (POAM) II STRATOSPHERIC NO2, 1993-1996

We describe, for the first time, measurements of stratospheric nitroge n dioxide (NO2) by the Polar Ozone and Aerosol Measurement (POAM II) i nstrument. Measurements span October 1993 through mid-November 1996 an d cover latitude ranges from 55 degrees to 72 degrees N and from 63 de grees to 88 degrees S. Comparisons with coincident satellite and space shuttle observations show good agreement and confirm the validity of POAM II measurements for scientific investigations. Overall seasonal v ariations in both hemispheres are qualitatively consistent with standa rd photochemistry. In the austral late winter/early spring of 1994, ho wever, anomalously high NO2 mixing ratios were observed above 22 km. W e conclude that these high NO2 levels resulted from downward transport of NOx-enhanced air from the mesosphere or thermosphere inside the po lar vortex. Enhanced NO2 mixing ratios in 1994 were factors of similar to 1.3 and 2.5 larger than at the corresponding times in 1995 and 199 6. We conclude that FOAM II observations of coincident, localized redu ctions in ozone of up to 40% were caused by the increased stratospheri c NOx via the standard catalytic NOx cycle.