Satellite Observation of Polar Stratospheric Cloud Composition -Simultaneous Gas and Aerosol Retrieval

Polar Stratospheric Clouds (PSCs) have an important role in ozone destruction at polar regions through activation of chlorine reservoirs on their surfaces. In spite of their importance, however, PSC composition and phase are still open questions. Recently, some space-borne sensors measured PSCs with infrared spectrometers; measurements of infrared spectra provide direct information on PSC composition. Improved Limb Atmospheric Spectrometer (ILAS) has also observed infrared spectra from 6.2 to 11.7 micron in the polar stratosphere; the ILAS was an occultation sensor onboard the Advanced Earth Observing Satellite (ADEOS), a sun-synchronous, polar-orbit satellite launched in August 1996. ILAS has observed high latitudes (56.6?deg - 72.4?deg N and 64.0?deg - 87.9?deg S) 14 times daily from November 1996 through June 1997. Recently, Oshchepkov et al. [accepted to Applied Optics, 2005] developed a new method for simultaneous gas and aerosol retrievals. In the method, concentrations of gas species and volume density size distributions for several types of aerosol/PSCs are simultaneously retrieved. Based on the algorithm by Oshchepkov, Version 7 (V7) dataset was produced. In the V7-algorithm, six types of aerosols are considered with six bins of particle size, thus 36 parameters are resolved as unknown parameters in addition to gas concentrations. Liquid droplets are expressed as combination of sulfuric acid droplets and nitric acid droplets. Two types of Nitric Acid Trihydrate (NAT), ?alpha-NAT and ?beta-NAT, are considered with Nitric Acid Dihydrate (NAD) and water ice as solid particles. By using V7-dataset, all of the measured PSCs as well as background aerosols are categorized to specific compositions, and the ambient gas concentrations such as nitric acid and water vapor are also determined. Analysis of V7 data revealed that liquid binary solution droplets were observed at warm regions/altitudes reasonably. In January to March 1997 over the Northern Hemisphere, both NAT and nitric acid solution droplets were observed at cold regions/altitudes. The area of many PSC events was near the Scandinavian Peninsula in January. In early winter of 1997 in the Southern Hemisphere prominent PSC component was ?beta-NAT. Some ice events appeared in both hemispheres. However, those events are all at warmer temperatures than the ice frost temperature. Spectrum of ice is flat and difficult to distinguish from bias or the signals of other large particles. Careful interpretations are needed for the ice events. The gas species that are simultaneously observed with PSCs are also analyzed with tracer species like N2O to figure out chemical and physical processes like denitrification/ dehydration and activation/deactivation of chlorine species. The ILAS-II, successor to the ILAS on board ADEOS-II, carried out similar observations from December 2002 through October 2003. ILAS-II data are now under data processing with V7-like algorithm, so comparison of different winters will be possible in near future.