During the Arctic Airborne Stratospheric Expedition, simultaneous measurements of aerosol size distribution and NOy(HNO3+NO+NO2+2⋅N2O5) were made along ER-2 flight paths. The flow characteristics of the NOy instrument allow us to derive the condensed NOy amount (assumed to be HNO3) present during polar stratospheric cloud (PSC) events. Analysis of the January 24th flight indicates that this condensed HNO3 amount does not agree well with the aerosol volume if the observed PSCs are composed of solid nitric acid trihydrate (NAT), as is generally assumed. However, the composition agrees well with that predicted for liquid H2SO4/HNO3/H2O solution droplets using a new Aerosol Physical Chemistry Model (APCM). The agreement corresponds in detail to variations in temperature and humidity. The weight percentages of H2SO4, HNO3, and H2O derived from the measurements all correspond to those predicted for ternary, liquid solutions. ¿ American Geophysical Union 1994