Rotational Raman scattering and the ring effect in zenith‐sky spectra

The technique of zenith‐sky spectroscopy is widely used to measure the vertical columns of O3, NO2, OClO and BrO in the atmosphere. In this paper, a model to simulate the effect of rotational Raman scattering by O2 and N2 on zenith‐sky spectra is presented. The model is used to calculate the Raman‐scattering cross‐section for zenith‐sky measurements and this cross‐section is shown to correspond closely to the measured Ring cross‐section, supporting the case that Raman scattering is the major cause of the Ring effect. Raman scattering is also shown to reduce the depths of structured molecular absorptions in scattered light spectra, leading to a general underestimation of the slant columns of molecules measured by zenith‐sky spectroscopy which can be significant in some cases. This effect varies with solar zenith angle, so will affect particularly attempts to retrieve the vertical profile of an absorber from the variation of slant column with zenith angle. The calculated Ring cross‐section is used to infer the proportion of multiply‐scattered light which enters a zenith‐sky spectrometer at twilight, and thus to estimate the magnitude of the corresponding underestimation of measured slant columns.