Polarimetric channels

The use of the lidar depolarization technique has proven to be a valuable tool for atmospheric sciences. Regarding aerosol characterization, the depolarization information has been widely used for aerosol typing when combined with other optical properties. In this sense, it can also be very useful in the retrieval of the atmospheric boundary layer (ABL) height since it allows to discriminate between the aerosol within this layer and different aerosol types coupled to the ABL height based on aerosol data. The depolarization data combined with the color ratio allow for discriminating among different kinds of aerosols and clouds, so depolarization information can be added to the set of parameters to be considered in aerosol classification.

Besides aerosol typing, the depolarization technique also provides relevant information for
the retrieval of aerosol microphysical properties. Due to the particle shape information associated
to lidar depolarization, the detection of non-spherical particles can be highly improved.

Two new depolarization measurement channels have been developed and implemented for the BarcelonaTech (UPC) 6-channel elastic/Raman lidar. The majority of the currently working systems use a single telescope and either a polarizing beam-splitter that separate the parallel and perpendicular polarization components of the light collected by the telescope or a non-polarizing beamsplitter in one of whose outputs a polarizer is inserted; these approaches present the issue of needing a very precise characterization of the crosstalk parameters of the beam-splitters. The main innovation of our system is the use of additional telescopes (in fact, telephoto lenses) to measure the cross-polarized return signal (and thus, the un-polarized component of this signal), without altering the original system. The light collected by the main telescope is coupled into the wavelength separation unit using an optical fiber bundle that produces a nearly total effective depolarization at its output, making the rest of the system insensitive to changes in polarization.

Figure 1 Volume depolarization measured during a Saharan dust event on February 23rd 2017.

Figure 2 Laser, main telescope and depolarization channels