## 2 Materials and Methods
### Preprocessing of the Raw Lidar Data  
The laser emits linearly polarized light at 532 nm. The molecules and particles in the air may partly depolarize the reflected light. The main constituents of the atmosphere are oxygen and nitrogen molecules, which reflect mainly polarized light, with a small fraction of depolarized light. Particles with spherical symmetry (liquid droplets) do not depolarize the reflected light, while solid particles reflect both polarized and depolarized radiation. Thus, measuring two orthogonal polarizations of the reflected light provides information about the physical (and chemical) properties of the aerosol particles. A set of polarizing beam splitter cubes are used to separate two polarizations, one with a polarization parallel to the emitted laser beam and the second orthogonal to it. Here, we consider the preprocessing of the raw optical signals of the two polarizations (parallel and perpendicular). Both signals are attenuated by the molecules and particles in the air, and a correction is applied to compensate for this attenuation by using a simplified version of the [PERSON] method [13]  
Figure 6: The figure shows a schematic view of the remote control. that uses a fixed lidar ratio of 70 sr, which appears appropriate for PSCs. We tested this fixed lidar ratio on many lidar profiles and obtained satisfactory results compared with the method proposed by [PERSON] and [PERSON] [14]. [PERSON] and [PERSON] [15] report a lidar ratio of 55-60 sr for PSCs at 532 nm, while [PERSON] et al. [16] found a maximum value of 70 sr for the lidar ratio at 532 nm from their simulation for ice and NAT PSCs (run 86). The variable lidar ratio used in previous work [12], using the results of a number of model calculations for several particle size distributions [16] (runs 86 and 99) produced less satisfactory results, leading to a significant underestimation of the extinction. Unfortunately, the two recorded channels with orthogonal polarization do not fully represent the polarization of the lidar signal and suffer from crosstalk between the two polarization components. In particular, the perpendicular channel suffers from contributions that are not produced by the depolarization of molecules and aerosol. The imperfect polarization of the laser emission contributes to a small signal on the perpendicular channel but is estimated to be much smaller than 1%. Also, the imperfect behavior of the two polarizing beam splitter cubes introduces some crosstalk. Normally, the crosstalk of a polarizing beam splitter cube might be as large as 5%, but the combination of two cubes reduces this to less than 1%. Also, the nonperfect collimation of the optical return signal might contribute. A thorough discussion of the various factors that might contribute to crosstalk, as well as a method to calibrate the receiver with two polarizations, can be found in [17]. However, in the case of the lidar observatory at Concordia Station, the main contribution to the crosstalk is produced by the depolarization of the return signal by the optical viewport. This viewport, which separates the laboratory environment at room temperature (normally between 15 and 20 \({}^{\circ}\)C) from the outside air, with temperatures that might be as low as \(-70\)\({}^{\circ}\)C, consists of a triple glass construction, with three panes of glass held together in a glazing unit with two insulating air gaps of about 10 cm between two panes. This is necessary to provide sufficient thermal insulation and functions quite well, but apparently, it causes depolarization in the lidar signal which amounts to about 14%.