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 NO y , HNO 3 , O 3 , N 2 O, and CO have been performed in the lower stratosphere during the Stratosphere‐Troposphere Experiment by Aircraft Measurements (STREAM) II intensive winter 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 Stratospheric Arctic and Midlatitude Experiment (SESAME) winter campaign. Strongly elevated levels of total reactive nitrogen (NO y ) and its most abundant contributing species, nitric acid (HNO 3 ), with mixing ratios up to 9 parts per billion by volume (ppbv), were observed during all flights at altitudes near 12 km. On average, the measured NO y concentrations exceed the expected levels by a factor of 2–3. Normal background NO y has been calculated from observed N 2 O mixing ratios using the NO y ‐N 2 O correlation reported for the undisturbed northern hemisphere. This indicates that subsidence of air in the vortex alone cannot explain these findings. We propose that the elevated NO y concentrations were caused by nitrification of the lower stratosphere associated with sedimentation and evaporation of polar stratospheric cloud particles that carry down HNO 3 from higher altitudes, that is, from altitudes up to about 25 km.