Assessment of lidar depolarization uncertainty by means of a polarimetric lidar simulator

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 allow the characterization of different aerosol types and the retrieval of aerosol particle microphysical properties. Regarding the microphysical inversions, 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 lidar polarizing sensitivity and to estimate the systematic error of the volume linear depolarization ratio (δ), combining the Stokes–Müller formalism and the Monte Carlo technique. This tool is applied to a synthetic lidar system and to several EARLINET lidars with depolarization capabilities at 355 or 532 nm. The results evidence that the lidar polarization sensitivity can lead to δ relative errors larger than 100 %, being more probable its overestimation. The lidar systems show δ relative errors larger than 100 % for δ values around the molecular one (~0.004), decreasing up to ~10 % for δ = 0.45. However, among them, POLIS system shows the best behaviour with δ relative errors of 25 % and 0.22 % for δ = 0.004 and δ = 0.45, respectively, evidencing how a proper characterization of the lidar polarizing sensitivity can drastically reduce the δ systematic errors. In this regard, we provide some indications to reduce the lidar polarizing sensitivity and to improve its characterization.

An overview of the first decade of Polly<sup>NET</sup>: an emerging network of automated Raman-polarization lidars for continuous aerosol profiling

“…As a consequence, the networkoriented characterization of different aerosol types based on lidar ratio and Ångström exponent needs ancillary data (Schwarz, 2016). However, much progress has been made to incorporate polarization measurements within EARLINET Bravo-Aranda et al, 2016;Belegante et al, 2016) from which Polly NET has benefited as well so that this parameter will be available routinely in the nearfuture.…”

Experimental assessment of the lidar polarizing sensitivity

“…By comparing the calibration values obtained using the two calibrators, the investigator can assess the influence of the envisaged optical module on the depolarization products (the diattenuation parameter (Mattis et al, 2009)). Simulations performed by (Bravo-Aranda et al, 2015) shows that the effects of the diattenuation on depolarization products are highly significant. By using this method, we can correct the diattenuation effects for either the receiving optics (if the calibration modules are placed in front and after the receiving optics), the emitting 20 optics (if the calibration modules are placed in front of the emitting optics and in front of the receiving optics) or both.…”

“…We don't investigate the 15 effects of the emitter optics described by the Mueller matrix M E in this study, since at least two of the considered lidars send the laser radiation in the atmosphere without using any optic (MUSA and MULIS). For lidar systems that use emission optics to send the laser radiation in the atmosphere, further investigations are needed to fully characterize the effects of M E on the depolarization products (Bravo-Aranda et al, 2015). We also assume that the light emitted to the atmosphere, I E , is linearly polarised with an angle α with respect to the reference plane, which reduces Eq.…”

“…• as indicated by (Bravo-Aranda et al, 2015). Since the larger uncertainties are expected for D O and α, we neglect the effect of γ and β angles on these lidar systems.…”

EARLINET instrument intercomparison campaigns: overview on strategy and results

“…Currently (in 2015), there are 22 3 + 2 systems at 18 EARLINET stations, and their number is steadily growing. Many systems have polarization measurement capabilities in addition, i.e., the particle linear depolarization ratio is measured at least at one wavelength (Freudenthaler et al, 2009;Belegante et al, 2016;Bravo-Aranda et al, 2016). This quantity contains information about the presence of large, nonspherical particles and is an indispensable parameter for aerosol typing, in particular for the identification of mineral dust in the atmosphere.…”