Profiling of fine- and coarse-mode particles with LIRIC (LIdar/Radiometer Inversion Code)

Abstract. The paper investigates numerical procedures that allow determining the dependence on altitude of aerosol properties from multi wavelength elastic lidar signals. In particular, the potential of the LIdar/Radiometer Inversion Code (LIRIC) to retrieve the vertical profiles of fine and coarse-mode particles by combining 3-wavelength lidar measurements and collocated AERONET (AErosol RObotic NETwork) sun/sky photometer measurements is investigated. The used lidar signals are at 355, 532 and 1064 nm. Aerosol extinction coefficient (α_L), lidar ratio (LR_L), and Ångstrom exponent (Å_L) profiles from LIRIC are compared with the corresponding profiles (α, LR, and Å) retrieved from a Constrained Iterative Inversion (CII) procedure to investigate the LIRIC retrieval ability. Then, an aerosol classification framework which relies on the use of a graphical framework and on the combined analysis of the Ångstrom exponent (at the 355 and 1064 nm wavelength pair, Å(355, 1064)) and its spectral curvature (ΔÅ = Å(355, 532)–Å(532, 1064)) is used to investigate the ability of LIRIC to retrieve vertical profiles of fine and coarse-mode particles. The Å-ΔÅ aerosol classification framework allows estimating the dependence on altitude of the aerosol fine modal radius and of the fine mode contribution to the whole aerosol optical thickness, as discussed in Perrone et al. (2014). The application of LIRIC to three different aerosol scenarios dealing with aerosol properties dependent on altitude has revealed that the differences between α_L and α vary with the altitude and on average increase with the decrease of the lidar signal wavelength. It has also been found that the differences between Å_L and corresponding Å values vary with the altitude and the wavelength pair. The sensitivity of Ångstrom exponents to the aerosol size distribution which vary with the wavelength pair was responsible for these last results. The aerosol classification framework has revealed that the deviations between LIRIC and the corresponding CII-procedure retrieval products are due to the fact that LIRIC does not allow to the modal radius of fine mode particles to vary with the altitude. It is shown that this represents the main source of uncertainties in LIRIC results. The plot on the graphical framework of the Å-ΔÅ data points retrieved from the CII-procedure has indicated that the fine-mode-particle modal radius can vary with altitude when particles from different sources and/or from different advection routes contribute to the aerosol load. Analytical back trajectories combined with linear particle depolarization ratio profiles from lidar measurements at 355 nm and dust concentrations from the Barcelona Supercomputing Center-Dust REgional Atmospheric Model (BSC-DREAM) have been used to demonstrate the dependence on altitude of the aerosol properties.