BALLOON OBSERVATIONS OF NITRIC-ACID AEROSOL FORMATION IN THE ARCTIC STRATOSPHERE .2. AEROSOL

Balloonborne aerosol measurements over the size range r = 0.01 to 10 μm on 31 January 1990 are used to determine the HNO3 vapor mass deposited in Arctic nitric acid trihydrate (NAT) clouds in the 21–24 km region. The clouds consisted mainly of small (r≈ 0.2 μm) particles and appeared to have been formed during recent cooling because the r ≥ 0.15 μm aerosol concentration in the coldest part of the cloud was nearly equal to the condensation nuclei concentration, suggesting a large vapor supersaturation. The maximum inferred HNO3 vapor mixing ratio which condensed in the cloud was about 4 ppbv, in good agreement with the observed depletion of HNO3 vapor in the same altitude region under similar temperature conditions 23 hours earlier. Copyright 1990 by the American Geophysical Union.