DEHYDRATION AND DENITRIFICATION IN THE ARCTIC POLAR VORTEX DURING THE1995-1996 WINTER

Dehydration of more than 0.5 ppmv water was observed between 18 and 19 km (theta similar to 450-465 K) at the edge of the Arctic polar vorte x on February 1, 1996. More than half the reactive nitrogen (NO,) had also been removed, with layers of enhanced NO, at lower altitudes. Bac k trajectory calculations show that air parcels sampled inside the vor tex had experienced temperatures as low as 188 K within the previous 1 2 days, consistent with a small amount of dehydration. The depth of th e dehydrated layer (similar to 1 km) and the fact that trajectories pa ssed through the region of ice saturation in one day imply selective g rowth of a small fraction of particles to sizes large enough (>10 mu m ) to be irreversibly removed on this timescale. Over 25% of the Arctic vortex in a 20-30 K range of theta is estimated to have been dehydrat ed in this event.