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The article presents the laser scattering and depolarisation  instrument SIMONE that is installed at the large aerosol and cloud  chamber facility AIDA of the Karlsruhe Institute of  Technology. SIMONE uses a 488 nm cw laser to probe simulated  atmospheric clouds by measuring the scattered light from the  1.8° and 178.2° directions. At 178.2°, the  scattered light is analysed for the linear polarisation state to  deduce the particle linear depolarisation ratio δ<sub>p</sub> which is  a common measurement parameter of atmospheric lidar  applications. The optical setup and the mathematical formalism of  the depolarisation detection concept are given. SIMONE  depolarisation measurements in spheroidal hematite aerosol and  supercooled liquid clouds are used to validate the instrument.SIMONE data from a series of AIDA ice nucleation experiments at  temperatures between 195 and 225 K were analysed in terms of the  impact of the ice particle microphysics on δ<sub>p</sub>. We  found strong depolarisation values of up to 0.4 in case of small  growing and sublimating ice particles with volume equivalent  diameters of only a few micrometers.Modelling runs with the T-matrix method showed that the measured  depolarisation ratios can be accurately reproduced assuming  spheroidal and cylindrical particles with a size distribution that  has been constrained by IR extinction spectroscopy. Based on the  T-matrix modelling runs, we demonstrate that in case of small ice  crystals the SIMONE depolarisation results are representative for  the lidar depolarisation ratio which is measured at exact  backscattering direction of 180°.The relevance of our results for the interpretation of recent lidar  observations in cirrus and contrails is discussed. In view of our  results, the high depolarisation ratios observed by the spaceborne  lidar CALIOP in the tropical upper troposphere might be a hint for  the presence of small (sublimating) ice particles in the outflows of  deep convective systems

atmospheric chemistry and physics

Influence of particle size and shape on the backscattering linear depolarisation ratio of small ice crystals – cloud chamber measurements in the context of contrail and cirrus microphysics