For aerosols this corresponds to particles larger than roughly 0.05–0.1 μm; smaller particles scatter light in the Rayleigh limit, and are therefore undistinguishable from atmospheric gases be-cause the concentrations of the latter are much higher. Hence the number of concentrations of the aerosol particles as observed by satellites is generally not repre-sentative of the actual atmospheric concentrations, and thus information on processes dependent on the number of aerosol particles (e.g. activation of cloud condensation nuclei or CCN) is difficult to obtain from satellites.