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The "filling-in" (FI) of Fraunhofer lines, often referred to as the Ringeffect, was examined using measurements of near ultraviolet sunlightscattered from the zenith sky above Boulder, Colorado during July and August2005. The FI of the 344.1 nm Fe I line was directly determined by comparingdirect sun and cloud-free zenith sky spectra recorded on the same day. Theresults, obtained over solar zenith angles (SZA) from 20° to 70°,are compared to the predictions of a simple rotational Raman Scattering(RRS) spectral model. The measured FI was found to be up to 70% greaterthan that predicted by first-order molecular scattering with a much strongerSZA dependence. Simultaneously measured aerosol optical depths and MonteCarlo calculations show that the combination of aerosol scattering andsecond-order molecular scattering can account for these differences, andpotentially explain the contradictory SZA dependences in previouslypublished measurements of FI. These two scattering processes also introducea wavelength dependence to FI that complicates the fitting of diffusesunlight observations in differential optical absorption spectroscopy(DOAS). A simple correction to improve DOAS retrievals by removing thiswavelength dependence is described

On the variability of the Ring effect in the near ultraviolet: understanding the role of aerosols and multiple scattering