Atmospheric chemistry: Measurements, mechanisms and models - General discussion by Cox, R, Crutzen, P, Scheer, V, George, C, Kerr, J, Atkinson, R, Zellner, R, Eberhard, J, Ravishankara, A, Horie, O, Doughty, A, Herrmann, H, Elias, H, Hynes, A, Plane, J, Rattigan, O, Heal, M, Nicholson, J, Ashfold, M, Wilson, S, Hancock, G, Wallington, T, Gershenzon, Y, Okumura, M, Davidovits, P