Spectral fine-structure in the polarisation of skylight

High-spectral resolution polarisation measurements of the clear daylight sky reveal an unexpected rich spectral fine-structure in the polarisation superimposed on the more familiar broad-band continuum. This polarisation spectrum shows the spectral fingerprints of scattering and absorption processes which is illustrated with radiative transfer calculations. In particular, radiative transfer calculations suggest the potential of high-spectral resolution polarimetry within absorption bands for remote sensing of aerosol vertical distribution, which is relevant to the Earth's radiation budget and ozone hole chemistry. In addition, the presence of spectral fine-structure in the polarisation of skylight also has relevance to radiance measurements performed with polarisation sensitive spectrometers, such as the space-based GOME instrument. change in the polarisation at 761 nm, corresponding to a molecular absorption band of oxygen, was observed. It was proposed by $tammes et at. [1994] that increasing oxygen absorption favours scattering of sunlight at higher altitudes in the atmosphere, and that this results in a change of the polarisation with wavelength reflecting the variation of scatterers with altitude. The studies by $tammes et at. and Preusker et at. were, however, limited to the wavelength region from 650 to 850 nm. In the present work we extended the wavelength region to the visible and ultra-violet o cover the range from 300 to 800 nm. As a result, much more spectral structure was observed which coincides with solar Fraunhofer lines and molecular absorption bands of ozone (O3), oxygen (O2) and water vapour (H2 O).