1] We present measurements of unusually low mixing ratios of HNO 3 in the exceptionally cold Arctic vortex of late-January and early-February 2005. The measurements were obtained by the airborne submillimeter radiometer ASUR during the polar aura validation experiment (PAVE). The distribution of HNO 3 inside the vortex reaches minima below 4 ppbv around 22 km altitude and maxima above 13 ppbv around 16 km altitude, with a considerable spatial variability. We estimate a vortex averaged denitrification of 3.1 ± 0.8 ppbv around 20 km altitude, and slight renitrification below $15.5 km altitude. The observed HNO 3 deficit is largest ($6 ppbv) near the center of the vortex, where the air masses had experienced temperatures below the NAT formation threshold for 80– 100% of the previous 20 days according to back trajectories. This suggests that the main denitrification mechanism is based on sedimenting nitric acid trihydrate particles.