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. The eVe dual-laser/dual-telescope lidar system is introduced here, focusingon the optical and mechanical parts of the system's emission and receiverunits. The compact design of the linear–circular emission unit along withthe linear–circular analyser in the receiver unit allows eVe tosimultaneously reproduce the operation of the ALADIN lidar on board Aeolusas well as to operate it as a traditional ground-based polarisation lidarsystem with linear emission. As such, the eVe lidar aims to provide (a) ground reference measurements for the validation of the Aeolus L2A aerosolproducts and (b) the conditions for which linear polarisation lidar systemscan be considered for Aeolus L2A validation, by identifying any possiblebiases arising from the different polarisation state in the emission betweenALADIN and these systems, and the detection of only the co-polar componentof the returned signal from ALADIN for the L2A products' retrieval. Inaddition, a brief description is given concerning the polarisationcalibration techniques that are applied in the system, as well as thedeveloped software for the analysis of the collected signals and theretrieval of the optical products. More specifically, the system's dualconfiguration enables the retrieval of the optical properties of particlebackscatter and extinction coefficients originating from the two differentpolarisation states of the emission and the linear and circular depolarisationratios, as well as the direct calculation of the Aeolus-like backscattercoefficient, i.e. the backscatter coefficient that Aeolus would measurefrom the ground. Two cases, one with slightly depolarising particles and onewith moderately depolarising particles, were selected from the firstconducted measurements of eVe in Athens in September 2020, in order todemonstrate the system's capabilities. In the slightly depolarising scene,the Aeolus-like backscatter coefficient agrees well with the actualbackscatter coefficient, which is also true when non-depolarising particlesare present. The agreement however fades out for strongly depolarisingscenes, where an underestimation of ∼18 % of the Aeoluslike backscatter coefficient is observed when moderately depolarisingparticles are probed.

The eVe reference polarisation lidar system for the calibration and validation of the Aeolus L2A product