Characterization of aerosol backscatter-to-extinction ratio from multi-wavelength and multi-angular lidar profiles

The newly developed Nd:YAG portable 3-D-scanning lidar from the Universitat Politecnica de Catalunya (UPC, Technical University of Catalonia, Barcelona, Spain) was used to improve our knowledge of the aerosols properties in the Barcelona area where an important number of pollution and saharan dust events can be observed all year round in the atmosphere. The system simultaneously operated at the 1064-nm and 532-nm elastic wavelengths, and was used in its scanning mode from 15degrees to 70degrees from zenith with 5degrees steps. A variational method was used to invert the multiangular profiles and to retrieve the aerosols optical thickness and backscatter coefficient at each wavelength without making any assumption on the aerosol type. At the same time, the ratio of the backscatter profiles was used to retrieve the profile of the Angstrom coefficient in backscatter. The backscatter-to-extinction ratio could not be calculated directly but various values of this parameter were used in Klett method until backscatter coefficient profiles could match the one retrieved with the variational method (at least in some altitude regions). Very good agreement (differences less than 20 %) was observed in the 0.3-2.5 kin region with a value of 0.030 sr(-1), whereas no agreement could be achieved above where supposedly mixed aerosols were initially observed. The lidar profiles closer to the zenith at both wavelengths allowed to calculate a new Angstrom coefficient in backscatter that is compared to the one retrieved by the variational method. The comparison showed good agreement in the lower layers and thus validated the backscatter-to-extinction ratio profiles used in the Klett method. However, the difficulties encountered to invert the lidar signal above an altitude of 1.6 km show that non negligible inhomogeneities of the atmosphere were present in each line of sight, proving the dense and fast-moving aerosol load over the Barcelona basin.