Balloonborne aerosol measurements over the size range r=0.01 to 10 &mgr;m on 31 January 1990 are used to determine the HNO3 vapor mass deposited in Arctic nitric acid trihydrate (NAY) clouds in the 21--24 km region. The clouds consisted mainly of small (r≈0.2 &mgr;m) particles and appeared to have been formed during recent cooling because the r≥0.15 &mgr;m 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. ¿ American Geophysical Union 1990