Aerosol retrieval experiments in the ESA Aerosol_cci project by Holzer-Popp, T, de Leeuw, G, Griesfeller, J, Martynenko, D, Klueser, L, Bevan, S, Davies, W, Ducos, F, Deuze, J, Graigner, R, Heckel, A, von Hoyningen-Huene, W, Kolmonen, P, Litvinov, P, North, P, Poulsen, C, Ramon, D, Siddans, R, Sogacheva, L, Tanre, D, Thomas, G, Vountas, M, Descloitres, J, Griesfeller, J, Kinne, S