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Lidar was applied to identify atmospheric inhomogeneities by different scattering behavior of the aerosol particles. Aerosol is a general term, it includes also the aggregation to clouds. In the stratosphere there exist droplets of sulfuric acid (from volcanic eruptions), nitric acids and water vapor in mixed phases. In the polar region polar stratospheric (aerosol) clouds form during winter at temperatures around 195K. Usually the nucleation of nitric acid trihydrate (NAT) and nitric acids dihydrate (NAD) happens in a complicated mechanism. A lidar can identify the result of the nucleation process, not the gas phases. It can distinguish between droplets (as Mie particles) and crystal (frozen droplets of NAT and NAD) by polarization. Because the aerosols are driven with the wind and have a tendency to sediment, the orientation of the lidar to the aerosol particles is an important factor. Improvement of lidar measurements to distinguish between stratospheric aerosols and polar stratospheric clouds (i.e. by the given definition the frozen aggregation) was focused: 1) on the use of multiple wavelength lidar with a polarization channel, 2) on a theoretical study on the possibility to use multiple scattering as an additional discriminant and, 3) on scanning. Points 2 and 3 require a better detection and signal processing system. Statistical problems arise for the comparison of measurements, for example if one compares ground-based and airborne measurements of the same cloud. Airborne measurements can contribute to the problem because one can reduce the distance to the object and therefore the 1/R2-dependency leads to larger signals. Averaging with respect to ensemble statistics are covered in this report. It is accepted that ground-based lidar systems especially with a Raman channel measure with a high pulse repetition rate over a few minutes to get a signal which can be processed.
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