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.