Total reactive nitrogen (NOy) between 15 and 29 km was measured for the first time on board a balloon within the Arctic cold vortex. Observations of HNO3, aerosol, and ozone were made by instruments on the same balloon gondola which was launched from Esrange, Sweden (68¿N, 20¿E) on January 23, 1989. The NOy mixing ratio was observed to increase very rapidly from 6 ppbv at 18 km altitude to a maximum of 21 ppbv at 21 km, forming a sharp layer with a thickness of about 2 km. A minimum in the NOy mixing ratio of 5 ppbv was found at 27 km. The measured HNO3 profile shows broad similarities to that of NOy. This observation, together with the observed very low column amount of NO2, shows that NOx had been almost totally converted to HNO3, and that NOy was composed mainly of HNO3. The enhanced aerosol concentration between 19 and 22 km suggests that the maximum abundance of HNO3 trapped in the form of nitric acid trihydrate (NAT) was about 6 ppbv at 21 km. The sampled air parcels were highly supersaturated with respect to NAT. Although extensive denitrification throughout the stratosphere did not prevail, an indication of denitrification was found at altitudes of 27 and 22 km, and between 18 and 15 km. ¿ American Geophysical Union 1990