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). We also observe two distinct crystalline forms of the trihydrate, which we denote (alpha) - and (beta) -NAT. 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 study the interaction of HCl with model PSC films. In this work we find that for HCl pressures in the range 10-5 - 10-7 Torr, HCl is taken up by ice at 155 K to form a thin layer of HCl (DOT) 6H2O. At 193 K, the uptake of HCl by ice was consistent with <EQ monolayer coverage. Uptake of HCl by (alpha) - and (beta) -NAT at 175 K was also consistent with <EQ monolayer coverage.

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