Freie Universita Berlin, Germany

Lidar measurements at 4 wavelengths and two polarizations were performed during the SESAME campaign in Sodankylii, Finland (67.37N, 26.65E). PSC's consisting of spherical (liquid) particles were observed. For this type of PSC we retrieved the aerosol size distribution and the refrac-tive index using the wavelength dependence of the particle scattering. The measured refractive index of 1.36 indicates a high water content of the PSC particles and we assume that this PSC consists of ternary solutions in contradiction to the NAT -hypothesis. On the other hand we detected layers of solid particles with very low mass densities of frozen background aerosols. Both types of aerosols can coexist within the same altitude region.

It is widely accepted that pure water cannot exist as a liquid below about -40°. Theoretical and laboratory studies confirm this behavior for pure water. Liquid droplets have been seldom observed in cirrus clouds down to about -50°C. The LIDAR technique can help to find out unusual supercooled clouds, when the depolarization technique is implemented: the presence of non-depolarizing layers in a cloud is indicative of a very special scattering media: scattering particles must have a symmetry axis oriented along the laser beam. This is possible either with spherical droplets or ice plates horizontally oriented. In this work, a -65°C cold, non-depolarizing cloud observed in Finland is studied, concluding that supercooled droplets are responsible for the absence of depolarization in most of the cloud. This is the coldest supercooled cirrus ever observed.

Polar Stratospheric Clouds and aerosols are here reported which do not clearly corresponding to present interpretations. They were monitored in the Antarctic and in the Arctic from 1989 to 1995 and therefore refer both to the pre-Pinatubo, the Pinatubo and after Pinatubo period. Such cases comprehend non depolarizing PSCs and warm depolarizing aerosols. Non Depolarizing PSCs have been observed over Dumont d'Urville, Antarctica during the POLE experiment (1989-today) and over Sodankyla during SESAME. For the case of Sodankyla particle size evaluation for non depolarizing PSCs were carried out with interesting results. An index of refraction has been used for Mie calculations which may well correspond to the one expected for a diluted ternary solution of H2O/H2SO4/HNO3

We present some preliminary results of MOANA (Measurements and modelling of Ozone and Aerosols in the Northern Atmosphere) multi-wavelength lidar system, which was operating at Sodankyla (SF) during SESAME. We observed various polar stratospheric cloud (PSC) episodes; in this work we report the measurements of January 12 (JI2) and January 19 (J19), 1995. The J12 PSC has a layered structure and two of the three particle layers show a depolarised backscattering signal. On the other hand the J19 PSC detected between 19 and 24km is likely composed of spherical (liquid) particles because no depolarization shows up. In the last case an analysis of the lidar backscattering at the different wavelengths allows to estimate the optical size distribution of the particles.