ANALYSIS OF THE PHYSICAL STATE OF ONE ARCTIC POLAR STRATOSPHERIC CLOUD BASED ON OBSERVATIONS

During the Arctic Airborne Stratospheric Expedition, simultaneous meas urements of aerosol size distribution and NO(y) (HNO3 + NO + NO2 + 2.N 2O5) were made along ER-2 flight paths. The flow characteristics of th e NO(y) instrument allow us to derive the condensed NO(y) amount (assu med to be HNO3) present during polar stratospheric cloud (PSC) events. Analysis of the January 24th flight indicates that this condensed HNO 3 amount does not agree well with the aerosol volume if the observed P SCs are composed of solid nitric acid trihydrate (NAT), as is generall y assumed. However, the composition agrees well with that predicted fo r liquid H2SO4/HNO3/H2O solution droplets using a new Aerosol Physical Chemistry Model (APCM). The agreement corresponds in detail to variat ions in temperature and humidity. The weight percentages of H2SO4, HNO 3, and H2O derived from the measurements all correspond to those predi cted for ternary, liquid solutions.