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 (δ^v), and particle linear depolarization ratio (δ^p) 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 (δ^p) 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 δ^p 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.

Acknowledgements

This work was supported by the Spanish Ministry of Science through grant CGL2007-28871-E/CLI and projects CGL2010-18782 and CSD2007-00067 and the Acciones Complementarias CGL2008-01330-E/CLI (Spanish and Portuguese Lidar Network); by the Autonomous Government of Andalusia through the projects P08-RNM-3568 and P10-RNM-6299; and by EARLINET-ASOS project (EU-CA.,025991,RICA). Thanks to NASA/University of Maryland. 2002, MODIS Hotspot/Active Fire Detections. MODIS Rapid Response Project, NASA/GSFC as producer and the University of Maryland, Fire Information for Resource Management System as distributor (http://maps.geog.umd.edu). We express gratitude to the NOAA Air Resources Laboratory and Naval Research Laboratory for the HYSPLIT transport and dispersion model and the NAAPs aerosol maps. We thank H. Lyamani and J. Gil Roca who revised the manuscript and helped with their suggestions to improve this work.

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