CHARACTERIZATION OF MODEL POLAR STRATOSPHERIC CLOUD FILMS USING FOURIER-TRANSFORM INFRARED-SPECTROSCOPY AND TEMPERATURE PROGRAMMED DESORPTION

This study combines Fourier transform infrared (FTIR) spectroscopy and temperature programmed desorption (TPD) measurements of nitric-acid/ice films representative of type I polar stratospheric clouds (PSCs). Using this combination of techniques, we are able to correlate the FTIR spectra with measurements of the film stoichiometry. The results confirm the assignments for amorphous nitric-acid/ice films and for crystalline nitric acid trihydrate (NAT), dihydrate, and monohydrate proposed by Ritzhaupt and Devlin (1991). In addition to these films, we observe a new high temperature nitric-acid/ice film which we attribute to a more stable structure of NAT with fewer defects. When low temperature crystalline NAT is heated, such as during TPD or slow annealing, the IR absorption spectrum irreversibly changes around -88-degrees to -75-degrees-C. Future IR absorption measurements of PSCs in the atmosphere should be compared with IR spectra of both amorphous and crystalline nitric acid/ice, including both forms of NAT.