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Polar stratospheric clouds (PSC) were observed with the multi-wavelength lidar of the MOANA project (Modelling and Observations of Aerosols in the Northern Atmosphere) during SESAME (Second European Stratospheric Arctic and Mid-latitude Experiment). 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 thecloud particles were identified as liquid, changes in the size distribution of the droplets along the trajectory were calculated using a micro-physical box model. Backscatter ratios calculated from the sizedistributions are in broad agreement with the lidar data, giving confidence in current understanding of the evolution of ternary solution(H2SO4, HNO3 and H2O) droplets.Results from two soundings are shown which bear on the problemof the formation of solid particles. In the first, solid particles were detected. The air mass had cooled to the frost point 12 hours earlier. In the second no solid particles were detected although the airtemperature was below the nitric acid trihydrate existence point, and had decreased by 12K in the previous 14 hours. [ABSTRACT FROM AUTHOR]