a) Polar Stratospheric Clouds (PSCs) play an important role in the development of the Antarctic ozone hole. The PSCs shift the relative amounts of active and inactive chlorine species. The PSCs induce surface chemistry reactions which produce large amounts of active chlorine gas, which then decrease the amount of ozone in the stratosphere. The CLAES (Cryogenic Limb Array Etalon Spectrometer) experiment on UARS measures many of the chemical species which are involved in stratospheric chemistry. CLAES also measures the global distribution of statosphere aerosol (due to the Mt. Pinatubo eruptions, and due to PSCs). The figure above shows the aerosol over Anatarctica on August 18, 1992 at 72 S latitude. The large amount of aerosol between pressure altitudes 30 and 50 hPa and 280 to 300 E.longitude shows the location of PSCs for this day. The CLAES observations, which map out the three dimensional structure of the PSCs, provide an important opportunity to study the growth and development of the PSCs.
b) In addition to the observation of PSCs, UARS instruments also measure the global distributions of nitric acid (HNO3) in its gas phase, temperature, and water vapor (H2O). As air parcels circulate around the south polar regions, they undergo temperature changes, and the HNO3 and H2O form PSCs. Current understanding has the HNO3 and H2O forming NAT (nitric acid trihydrate) PSC particles. The Figure above illustrates the CLAES observations of aerosol, HNO3 (from CLAES) and H2O (from the Microwave Limb Sounder-MLS) at 72 S latitude. The figure is suggestive of the interconversion process of H2O and HNO3 into PSC particles. As air parcels travel from 180 E longitude to 300 E longitude, high values of gaseous HNO3 and H2O decrease, are converted into aerosol, and the aerosol amount increases. Investigations are underway to study these interrelationships between gas and aerosol species. Calculations are being done to map out the trajectories which the air parcels have traveled to determine the time variations of the HNO3, H2O, temperature and aerosol amounts. These results will then be studied to confirm the current understanding of the manner in which PSCs are formed.