Observations of several atmospheric species can be performed by measuring the absorption of visible and near-ultraviolet light scattered from the zenith sky. The determination of vertical column abundances of molecules such as ozone and NO2 from such measurements is briefly reviewed. It is shown that the conversion of NO to NO2 can be of significance in the interpretation of measurements made near twilight. On the other hand, multiple scattering from the atmosphere or by clouds is likely to be a very small effect. ¿ American Geophysical Union 1987