Analysis of lidar depolarization calibration procedure and application to the atmospheric aerosol characterization

Abstract

A Raman lidar system is used to monitor the aerosol depolarization features of the urban atmosphere at the Andalusian Centre for Environmental Research (CEAMA), in Granada, southeastern Spain. The lidar system was upgraded in 2010 to enable the application of the ±45° calibration method, which does not require any external optical device. We analyse the method and classify the atmospheric aerosol following the criteria based on depolarization. Backscatter coefficient, backscatter-related Angström exponent (å β), volume linear depolarization ratio (δv), and particle linear depolarization ratio (δp) profiles are studied in Saharan dust and biomass burning smoke events during the summer of 2010. The strength of these events was visualized in the aerosol optical depth (AOD) series obtained by Sun and star photometers operated at CEAMA. During the analysed events, the AOD at 440 nm ranged between 0.2 and 0.3, although the Angström exponent (å AOD) retrieved by the Sun photometer was considerably lower during the Saharan dust event (å AOD = 0.4 ± 0.1) than during the biomass burning event (å AOD = 1.4 ± 0.1). Regarding å β profiles, å β values were similar along the vertical profiles and comparable to å AOD values for each event. In contrast, the particle linear depolarization ratio (δp) at 532 nm showed an opposite behaviour to å β, changing along the vertical profiles. In fact, the aerosol layers located in the free troposphere showed mean values of δp of 0.13 ± 0.08 and 0.03 ± 0.01 in the Saharan dust and biomass burning events, respectively. These results show that the use of depolarization techniques enables an accurate aerosol typing and the understanding of the layer's composition in the atmosphere.