Atmospheric observations of the O4 absorption bands at 360.5, 380.2, 477.3, 532.2, 577.2 and 630.0 nm are presented for different atmospheric conditions (clear and cloudy skies) and viewing geometries (direct and zenith-scattered light observations). From the observations of direct moonlight it was possible to derive absolute O4 absorption cross sections for atmospheric conditions. We found that the relative shape of the observed absorption bands is similar to those of the O4 spectrum measured by Greenblatt et al. <1990> in the laboratory. However, in general (except for the absorption band at 380.2 nm), the O4 absorption cross sections derived in this study are larger by several percent compared to those of the other (mainly laboratory) observations. Using the observations of zenith-scattered light, we investigated the radiative transport through the atmosphere. Our observations under cloudy sky conditions confirmed that the light path enhancement due to multiple Mie scattering on cloud droplets is independent of wavelength. From the observations under clear-sky conditions we studied the influence of Mie scattering on aerosol. It was not possible to describe the selected clear-sky measurements by taking into account only Rayleigh scattering. We found that the comparison of the O4 measurements with model results for different sets of assumed aerosol extinctions provides a new, very sensitive tool to derive aerosol parameters from zenith sky ground-based measurements.