Total reactive nitrogen (NOy), nitrate aerosols, and ozone were measured between 12 and 30 km on board balloons launched from Esrange, near Kiruna, Sweden (68¿N, 20¿E), on January 18 and 31, 1990. A series of ozone measurements were performed using small balloons in addition to the measurements made simultaneously with NOy on the large gondola. On January 18, Kiruna was located inside the polar vortex, while it was outside on January 31. The NOy mixing ratio inside the vortex was 5¿1 parts per billion by volume (ppbv) at the altitudes between 20 and 22 km. This value is considerably smaller than the value of about 13 ppbv that is expected from gas phase chemistry, indicating that a degree of denitrification had occurred by mid-January in 1990. On the other hand, the NOy mixing ratio outside the vortex in the same altitude region ranged between 6 and 13 ppbv, suggesting less denitrification outside the vortex. The cause of the denitrification is interpreted in terms of the very cold stratospheric temperature that prevailed from December 1989 to January 1990. The mixing ratios of nitric acid in gas phase (Schlager et al., 1990) and particulate phase (Hofmann et al., 1990) were measured on January 31, but not on the same gondola. The HNO3/NOy ratio was close to unity in a polar stratospheric cloud but decreased to 0.75¿0.05 outside the cloud. Assuming this ratio, HNO3 has been found to be highly supersaturated over nitric acid trihydrate particles. ¿ American Geophysical Union 1992