SPECTROSCOPIC STUDIES OF MODEL POLAR STRATOSPHERIC CLOUD FILMS

Fourier transform infrared (FTIR) spectroscopy has been used to study nitric acid/ice films representative of type I polar stratospheric clouds (PSCs). These studies reveal that in addition to amorphous nitric acid/ice mixtures, there are three stable stoichiometric hydrates of nitric acid-nitric acid monohydrate (NAM), dihydrate (NAD) and trihydrate (NAT). Two distinct crystalline forms of the trihydrate were also observed. These two forms appear to differ in their concentration of crystalline defects, but not in their chemical composition. In addition to probing the composition of type I PSCs, we have also used FTIR spectroscopy to characterize laboratory surfaces on which measurements of heterogeneous reaction rates are performed. Our studies suggest that "water-rich NAT" is a two-phase system with separate ice and NAT crystalline regimes. Finally, we have used FTIR spectroscopy to determine the desorption kinetics for evaporation of model PSC films. Ice evaporation was found to follow zero-order desorption kinetics with a desorption barrier of 12+/-2 kcal/mol and a preexponential factor of 10(30.5+/-1.5) molec/cm2-s.