Observed and simulated vertically-resolved optical properties of continental aerosols over South-East Italy: a closure study

A 2.5-year data set of Raman lidar measurements performed in Lecce within the European Aerosol Research Lidar Network and a numerical model are used to characterize continental aerosols over southeast Italy. Continental conditions are selected by means of back trajectories, limiting the data set to cases of advection from the European continent. The model is used to constrain the relevant aerosol microphysical parameters needed to reproduce the altitude-resolved optical properties measured by lidar, namely, aerosol extinction (α), backscatter (β), and lidar ratio (LR = α/β) at 351 nm. The observed variability of α, β, and LR is simulated by randomly varying, within appropriate ranges, all the microphysical parameters describing the aerosol composition and size distribution (assumed as bimodal). An optimal model configuration capable of reproducing within ±10% the mean optical properties retrieved by lidar, including their vertical pattern, is presented and discussed with sensitivity studies. Results indicate rather clean continental aerosols to characterize the site, with water-soluble fine-mode aerosols (dry modal radius of 0.05–0.10 μm) accounting for 98–99% of the total number of particles. The (dry) imaginary refractive index of fine-mode particles is extended up to 0.02 to better fit the lidar data. This suggests some mixing of water-soluble substances with more absorbing material. Coarse-mode particles (modal radius 0.4–0.5 μm) are shown to play a crucial role in determining the observed vertical profile of β and LR. Finally, an altitude-resolved relationship linking α to β is derived. This could be usefully employed in continental conditions to invert the lidar measurements when only the elastic signal is available (for example, in daylight).