Lidar measurements of aerosol depolarization ratio Xuan Wang 1 , Xiaomei Lu 2,3 , Libera Nasti 3 , Yiming Zhao 2,3 1 CRS Coherentia CNR-INFM, Napoli, Italy 2 School of Electronic Information Engineering, Beihang University, Beijing, China 3 CNISM and Dipartimento di Scienze Fisiche, Università degli Studi di Napoli “Federico II” , Napoli, Italy Lidar measurements of the aerosol depolarization ratio provide highly reliable informations to discriminate between spherical and non-spherical particles in the atmosphere and they can help to distinguish between liquid and solid phase aerosols. These measurements can be performed by using a linearly polarized laser source and a hardware configuration of the receiving system including two channels detecting simultaneously the backscattered light polarized in the parallel and orthogonal direction with respect to the laser beam. In these conditions the total depolarization ratio, i.e. the depolarization induced by both aerosols and atmospheric molecules, can be obtained if a calibration of the system is performed. In this paper three different calibration methods are described. For each method a sensitivity analysis is performed and the different contributions to the total error are evaluated by means of a numerical simulation. Results from numerical simulations allow comparing the performances of the three calibration methods in different atmospheric conditions. 1. Introduction Aerosols’ impact on atmospheric systems is complex and not perfectly well-known. A deeper interest in understanding aerosols’ role in climate balance and variable as well as their effect on human health is growing in these last years. Moreover, aerosol data from satellite and ground-based systems can help improve atmospheric models and in real-time observations. In this context Depolarization- sensitive Lidar can help distinguish between solid and liquid phase water and characterize the particle’s shape. The depolarization measurements can be performed by using a linearly polarized laser source and a hardware configuration of the receiving system including two channels detecting simultaneously the backscattered radiation in the parallel and orthogonal direction with respect to the laser beam [Schotland et al., 1971]. The total depolarization ratio, due both to molecular and aerosol contributions, is simply the calibrated ratio of the orthogonal signal to the parallel one. So, a key question to obtain high quality depolarization measurements is performing a good calibration of the Lidar system [Alvarez et al., 2006].