Variation of the infrared spectra of nitric acid hydrates with formation conditions: Impact on PSC identification

Despite years of effort, the composition of polar stratospheric clouds (PSC s) is still uncertain. Future studies may be able to use FTIR extinction me asurements to determine PSC composition. These studies require accurate opt ical constants for PSC materials. It has been suggested that accurate infra red (IR) optical constants for aerosols can only be derived from aerosol sp ectra. We have examined the IR spectra of nitric acid trihydrate (NAT) and nitric acid dihydrate (NAD) films and aerosols formed using a variety of ex perimental conditions. We conclude that several spectroscopically different types of NAD and NAT may exist. Alternatively, NAT could be birefringent a nd its spectra depend strongly on crystallographic orientation. The importa nt causes for the different types or orientations seem to be the formation conditions and temperature, rather than whether the material is a film or a erosol.