Optical and microphysical properties of smoke over Cape Verde inferred from multiwavelength lidar measurements

Abstract

Lidar measurements of mixed dust/smoke plumes over the tropical Atlantic ocean were carried out during the winter campaign of SAMUM-2 at Cape Verde. Profiles of backscatter and extinction coefficients, lidar ratios, and Angstrom exponents related to pure biomass-burning aerosol from southern West Africa were extracted from these observations. Furthermore, these findings were used as input for an inversion algorithm to retrieve microphysical properties of pure smoke. Seven measurement days were found suitable for the procedure of aerosol-type separation and successive inversion of optical data that describe biomass-burning smoke. We inferred high smoke lidar ratios of 87 +/- 17 sr at 355 nm and 79 +/- 17 sr at 532 nm. Smoke lidar ratios and Angstrom exponents are higher compared to the ones for the dust/smoke mixture. These numbers indicate higher absorption and smaller sizes for pure smoke particles compared to the dust/smoke mixture. Inversion of the smoke data set results in mean effective radii of 0.22 +/- 0.08 mu m with individual results varying between 0.10 and 0.36 mu m. The single-scattering albedo for pure biomass-burning smoke was found to vary between 0.63 and 0.89 with a very low mean value of 0.75 +/- 0.07. This is in good agreement with findings of airborne in situ measurements which showed values of 0.77 +/- 0.03. Effective radii from the inversion were similar to the ones found for the fine mode of the in situ size distributions.