UV absorption spectra of H2O/HNO3 films

The ultraviolet (UV) absorption spectra were measured for amorphous and crystalline H2O/HNO3 films representative of polar stratospheric clouds (PSCs). Film compositions were determined using laser induced thermal desorption (LITD). Pure HNO3 films showed a broad absorption peak with a maximum absorbance at lambda approximate to 195 mn. The absorbances of amorphous H2O/HNO3 films were also broad and peaked at lambda=196-198 nm. The absorption cross sections increased slightly for more dilute films, although the band shape was nearly independent of the H2O content. This increase in cross section was consistent with a change in the absorbing species from molecular HNO3 to ionic NO3-. The major absorbing species in crystalline NAT, NAD, and NAM is also NO3- and these films all had very similar UV absorption spectre. Their absorption peaks were all at lambda approximate to 200 nm and the band shapes were nearly identical.