Trajectory studies of polar stratospheric cloud lidar observations at Sodankyla (Finland) during SESAME: Comparison with box model results of particle evolution.

Polar stratospheric clouds (PSC) were observed with the multi-wavelength li dar of the MOANA project (Modelling and Observations of Aerosols in the Nor thern Atmosphere) during SESAME (Second European Stratospheric Arctic and M id-latitude Experiment). The physical state, liquid or solid, of the cloud particles can be inferred from the lidar data. Using isentropic back-trajec tories to obtain the thermal history of the sampled air masses, it is possi ble to reconcile most of the observations with current ideas on PSC formati on and evolution. When the cloud particles were identified as liquid, chang es in the size distribution of the droplets along the trajectory were calcu lated using a micro-physical box model. Backscatter ratios calculated from the size distributions are in broad agreement with the lidar data, giving c onfidence in current understanding of the evolution of ternary solution (H2 SO4, HNO3 and H2O) droplets. Results from two soundings an shown which bear on the problem of the format ion of solid particles. In the first, solid particles were detected. The ai r mass had cooled to the frost point 12 hours earlier. In the second no sol id particles were detected although the air temperature was below the nitri c acid trihydrate existence point, and had decreased by 12K in the previous 14 hours.