Continental Pollution in the Western Mediterranean Basin: Large Variability of the Aerosol Single Scattering Albedo and Influence on the Direct Shortwave Radiative Effect

Pollution aerosols strongly influence the composition of the Western Mediterranean basin, but at present little is known on their optical properties. We report in this study in situ observations of the single scattering albedo (ω) of pollution aerosol plumes measured over the Western Mediterranean basin during the TRAQA (TRansport and Air QuAlity) airborne campaign in summer 2012. Cases of pollution export from different source regions around the basin and at different altitudes between ~160 and 3500 m above sea level have been sampled during the flights. Data from this study show a large variability of ω, with values between 0.84–0.98 at 370 nm and 0.70–0.99 at 950 nm. The single scattering albedo generally decreases with the wavelength, with some exception associated to the mixing of pollution with sea spray over the sea surface. Lowest values of ω (0.84–0.70 between 370 and 950 nm) are measured in correspondence of a fresh plume possibly linked to ship emissions over the basin. The range of variability of ω observed in this study seems to be independent of the source region around the basin, as well as of the altitude and ageing time of the plumes. The observed variability of ω reflects in a large variability for the complex refractive index of pollution aerosols, which is estimated to span in the large range 1.41–1.75 and 0.002–0.068 for the real and the imaginary parts, respectively, between 370 and 950 nm. Radiative calculations in clear-sky conditions have been performed with the GAME radiative transfer model to test the sensitivity of the aerosol shortwave Direct Radiative Effect (DRE) to the variability of ω as observed in this study. Results from the calculations suggest up to a 50 % and 30 % change of the forcing efficiency (FE), i.e. the DRE per unit of optical depth, at the surface (−160÷−235 Wm−2 τ−1 at 60° solar zenith angle) and at the Top-Of-Atmosphere (−137÷−92 5 Wm−2 τ−1) for ω varying between its maximum and minimum value. This induces a change of up to an order of magnitude (+23÷+1435 Wm−2 τ−1) for the radiative effect within the atmosphere.