Observed polarization spectra of the cloud-free zenith sky are presented. The spectra, which cover the wavelength region from 350 to 400 nm, show spectral fine structure that coincides with Fraunhofer lines in the solar irradiance spectrum (the Ring effect). The observations show that the strength of the fine structure not only depends on the solar zenith angle but also on the atmospheric conditions, in particular the aerosol content. The spectral fine structure and its angular behavior as observed in the polarization spectra compare very well with simulations for a homogeneous, aerosol-containing model atmosphere, using a radiative transfer algorithm in which rotational Raman scattering is taken into account in the first- and second-order scattering processes. A description of the radiative transfer algorithm is given.