OBSERVATIONS OF HIGH-CONCENTRATIONS OF TOTAL REACTIVE NITROGEN (NOY) AND NITRIC-ACID (HNO3) IN THE LOWER ARCTIC STRATOSPHERE DURING THE STRATOSPHERE-TROPOSPHERE EXPERIMENT BY AIRCRAFT MEASUREMENTS (STREAM)II CAMPAIGN IN FEBRUARY 1995

Simultaneous in situ measurements of NOy, HNO3, O-3, N2O, and CO have been performed in the lower stratosphere during the Stratosphere-Tropo sphere Experiment by Aircraft Measurements (STREAM) II intensive winte r campaign in February 1995 from Kiruna airport (northern Sweden) with a Cessna Citation II twinjet aircraft up to a maximum altitude of 12. 8 km. The flights were coordinated with the Arctic Second European Str atospheric Arctic and Midlatitude Experiment (SESAME) winter campaign. Strongly elevated levels of total reactive nitrogen (NOy) and its mos t abundant contributing species, nitric acid (HNO3), with mixing ratio s up to 9 parts per billion by volume (ppbv), were observed during all flights at altitudes near 12 km. On average, the measured NO concentr ations exceed the expected levels by a factor of 2-3. Normal backgroun d NOy has been calculated from observed N2O mixing ratios using the NO y-N2O correlation reported for the undisturbed northern hemisphere. Th is indicates that subsidence of air in the vortex alone cannot explain these findings. We propose that the elevated NOy concentrations were caused by nitrification of the lower stratosphere associated with sedi mentation and evaporation of polar stratospheric cloud particles that carry down HNO3 from higher altitudes, that is, from altitudes up to a bout 25 km.