Articles | Volume 16, issue 16
Atmos. Chem. Phys., 16, 10591–10607, 2016

Special issue: CHemistry and AeRosols Mediterranean EXperiments (ChArMEx)...

Atmos. Chem. Phys., 16, 10591–10607, 2016

Research article 25 Aug 2016

Research article | 25 Aug 2016

Continental pollution in the Western Mediterranean basin: large variability of the aerosol single scattering albedo and influence on the direct shortwave radiative effect

Claudia Di Biagio1, Paola Formenti1, Lionel Doppler2, Cécile Gaimoz1, Noel Grand1, Gerard Ancellet3, Jean-Luc Attié4,5, Silvia Bucci6,7, Philippe Dubuisson8, Federico Fierli6, Marc Mallet5, and François Ravetta3 Claudia Di Biagio et al.
  • 1LISA, UMR CNRS 7583, Université Paris Est Créteil et Université Paris Diderot, Institut Pierre Simon Laplace, Créteil, France
  • 2Deutscher Wetterdienst, Meteorological Observatory Lindenberg, Lindenberg (Tauche), Germany
  • 3Sorbonne Universités, UPMC Univ. Paris 06, Université Versailles St-Quentin, CNRS/INSU, LATMOS-IPSL, Paris, France
  • 4Laboratoire d'Aérologie, University of Toulouse, UMR 5560 CNRS, Toulouse, France
  • 5CNRM UMR 3589, Météo-France/CNRS, Toulouse, France
  • 6Institute for Atmospheric Sciences and Climate of the National Research Council (ISAC-CNR), Rome, 00133, Italy
  • 7Sc. Dept. of Physics, Ferrara University, Ferrara, 44121, Italy
  • 8Laboratoire d'Optique Atmosphérique, Université Lille, Villeneuve d'Ascq, 59655 CEDEX, France

Abstract. 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 were 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 or dust particles over the sea surface. The 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 aging 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 1.41–1.77 and 0.002–0.097 for the real and the imaginary parts, respectively, between 370 and 950 nm.

Radiative calculations in clear-sky conditions were 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 W m−2τ−1 at 60° solar zenith angle) and at the Top-Of-Atmosphere (−137/−92 W m−2τ−1) for ω varying between its maximum and minimum value. This induces a change of up to an order of magnitude (+23/+143 W m−2τ−1) for the radiative effect within the atmosphere.

Short summary
Pollution aerosols strongly influence the composition of the Western Mediterranean, but at present little is known on their optical properties. Here, we report observations of pollution aerosols measured during the TRAQA airborne campaign in summer 2012. Data from this study indicate a large variability of the absorption for pollution particles. This variability strongly influences their direct radiative effect, with possible consequences on the hydrological cycle in this part of the basin.
Final-revised paper