Wedekind, C.

Polar statospheric clouds (PSC) were observed with the milti-wavelengh lidar of the MOANA project during SESAME. The physical state, liquid or solid, of the cloud particles can be inferred from the lidar data. Using isentropic back-trajectories to obtain the thermal history of the sampled air masses, it is possible to reconcile most of the observations with current ideas on PSC formation and evolution. When the cloud particles were identified as liquid, changes in the size distributionof the droplets along the trajectory ewre calculated using micro-physical box model. Backscatter ratios ......

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.

Polar stratospheric clouds (PSC) play a major role in the process of Arctic and Antarctic ozone depletion due to heterogeneous chemical reactions responsible for chlorine acti-vation, and particle sedimentation redistributing nitrogen species in the stratosphere. Therefore the phase, size and the composition of PSCs should be known. PSC can be divided into PSC type I, observed at temperatures some degrees above the ice frostpoint, and PSC type II consisting of water ice particles occurring at temperatures below the frostpoint. PSC type I can be subdivided into aspherical (type Ia) and spherical (type Ib) particles. Measurements of gas phase HNO3 removal in presence of PSCs and labora¬tory studies led to the assumption that PSC type I consist of nitric acid trihydrate and theparticle shape depends on the cooling rate [1]. However the explanation ofPSC I based solely on the NAT-hypothesis can not explain a large amount of data [2], and other compositions like liquid supercooled ternary solu¬tions (STS) of H2O, HNO3 and H2SO 4 are discussed now [3]. Multiwavelength, 2-polarization lidar measurements give information about the size distribution, refractive index and physical state of the cloud particles.

Scattering ratios R processed from the same raw data at 353 and 532 nm by different lidar groups agree within about 10 % for high and within about 4 % for low aerosol loading. In the main layer aerosol backscatter coefficients agree within about 30 % for high and within about 40 % for low aerosol loading

Polar stratospheric clouds (PSC) play a major role in the process of Artic and Antartic ozone depletion due to the surface provided for heterogeneus chemical reactions and the removal of NO2 from the gas phase. Therefore the phase, size and composition of PSC's should be known. The microphysical structure of the PSC's depends on the actual temperature and the corresponding; airmass thermal history. At temperatures below the ice frostpoint, PSC's of ice particles (Type II) are observed, while PSC's seen at temperatures above the frostpoint are classified as PSC Type Ia (anisotropic particles) and PSC Ib (spherical particles). PSC I were believed to consist of nitric acid trihydrate (NA'r). NAT should be stable some degrees above the ice frostpoint with a particle shape depending on the cooling rate [Toon et al., 1990]. However, the explanation of PSC based solely on the NAT-hypothesis can not explain a large amount of data [Toon and Tolbert, 1995]. The spherical shape of PSC Ib can be explained with a liquid supercooled ternary solution (STS) consisting of H2O, HNO3 and H2SO4. Scenarios for the formation of frozen background aerosol (sulfuric acid tetrahydrate, SAT) are now investigated. The described variance in shape and size of the PSC can be sensed by multispectral 2-polarization lidar, measuring range resolved scattering properties of atmospheric aerosols. Here the lidar observations of PSC's during the SESAME campaign are compared to the critical formation temperatures of the different PSC types.

It is widely accepted that pure water cannot exist as liquid below about -40°C. Theoretical and laboratory studies confirm this behavior for pure water . Nevertheless, liquid droplets have been seldom observed in cirrus clouds down to -50°C. Miltiwaveleght depolarization LIDAR tecnique can help ti hunt usually cold supercooled clouds. The presence of non-depolarizing cloud layers is indicative of scattering with ylindrical symmetry, possible both with spherical droplets and with ice plates horizontally oriented. In this work, a -65°C cold, non- depolarizing cloud observed in Finland is analysed, concluding thath supercooled droplets are responsible for the absence of depolarization in most of the layer.

The physical condition of polar stratospheric aerosols is of great importance both for the modelling of surface chemistry reactions and for the understanding of particle production and evaporation in the polar vortex. The particles can be either liquid, supercooled liquids or solid material at different heights and temperatures. Since a solid particle can survive much longer when temperature rises above the freezing point, whereas liquid particles will evaporate quickly at temperatures above the condensation temperature, the knowledge of the physical state is an important parameter to estimate the contribution to heterogenous chemistry of the different aerosol types observed.