Dehydration of more than 0.5 ppmv water was observed between 18 and 19 km (&thgr;~450--465 K) at the edge of the Arctic polar vortex on February 1, 1996. More than half the reactive nitrogen (NOy) had also been removed, with layers of enhanced NOy at lower altitudes. Back trajectory calculations show that air parcels sampled inside the vortex had experienced temperatures as low as 188 K within the previous 12 days, consistent with a small amount of dehydration. The depth of the dehydrated layer (~1 km) and the fact that trajectories passed through the region of ice saturation in one day imply selective growth of a small fraction of particles to sizes large enough (>10 &mgr;m) to be irreversibly removed on this timescale. Over 25% of the Arctic vortex in a 20--30 K range of &thgr; is estimated to have been dehydrated in this event. ¿ 1998 American Geophysical Union