Abstract
Abstract. Lidar depolarization measurements distinguish between spherical and non-spherical aerosol particles based on the change of the polarization state between the emitted and received signal. The particle shape information in combination with other aerosol optical properties allows the characterization of different aerosol types and the retrieval of aerosol particle microphysical properties. Regarding the microphysical inversions, the lidar depolarization technique is becoming a key method since particle shape information can be used by algorithms based on spheres and spheroids, optimizing the retrieval procedure. Thus, the identification of the depolarization error sources and the quantification of their effects are crucial. This work presents a new tool to assess the systematic error of the volume linear depolarization ratio (δ), combining the Stokes–Müller formalism and the complete sampling of the error space using the lidar model presented in Freudenthaler (2016a). This tool is applied to a synthetic lidar system and to several EARLINET lidars with depolarization capabilities at 355 or 532 nm. The lidar systems show relative errors of δ larger than 100 % for δ values around molecular linear depolarization ratios (∼ 0.004 and up to ∼ 10 % for δ = 0.45). However, one system shows only relative errors of 25 and 0.22 % for δ = 0.004 and δ = 0.45, respectively, and gives an example of how a proper identification and reduction of the main error sources can drastically reduce the systematic errors of δ. In this regard, we provide some indications of how to reduce the systematic errors.
Highlights
The lidar depolarization technique is very important for improving the retrieval of microphysical aerosol properties (e.g. Ansmann et al, 2011; Chaikovsky et al, 2002; Granados-Muñoz et al, 2014; Wagner et al, 2013; Samaras et al, 2015), becoming crucial for inversion algorithms based on modelling aerosol particles as spheres and spheroids
To quantify the systematic error of the linear depolarization ratio, the Polarimetric Lidar Simulator (PLS) is developed based on the matrix equations resulting from the theoretical framework given by Freudenthaler (2016a)
This work shows the numerical analysis of the polarizationrelated systematic errors of the linear depolarization ratio δ of one synthetic and seven EARLINET lidar systems, which all use one of the 90 calibration techniques described by Freudenthaler (2016a)
Summary
The possible effect of tilted scanning mirrors on depolarization measurements was highlighted by Bissonnette et al (2001) This functional block, formed by the telescope and dichroic beam splitters, leads the received signal to the photomultipliers and, in case of multiwavelength lidar, separates the received signal by wavelength. According to the Stokes–Müller formalism, the reflected (IR) and transmitted (IT ) signals can be obtained by multiplying the laser beam Stokes vector (I L) by the subsequent Müller matrices, which represent the different functional blocks, and the atmosphere F, which describes the scattering matrix for randomly oriented particles (van de Hulst, 1957; Mishchenko and Hovenier, 1995), by. To quantify the systematic error of the linear depolarization ratio, the Polarimetric Lidar Simulator (PLS) is developed based on the matrix equations resulting from the theoretical framework given by Freudenthaler (2016a) Calculation of the correction factors, GR, GT , HR, HT , and f (x1, . . .), is based on the assumed real parameter values x1, . . ., xn
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