Evaluation of seven European aerosol optical depth retrieval algorithms for climate analysis by de Leeuw, G, Holzer-Popp, T, Bevan, S, Davies, W, Descloitres, J, Grainger, R, Griesfeller, J, Heckel, A, Kinne, S, Klüser, L, Kolmonen, P, Litvinov, P, Martynenko, D, North, P, Ovigneur, B, Pascal, N, Poulsen, C, Ramon, D, Schulz, M, Siddans, R, Sogacheva, L, Tanré, D, Thomas, G, Virtanen, T, von Hoyningen Huene, W, Vountas, M, Pinnock, S