10 September 2005 Spectral measurements of the polarization of UV sky radiance
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Measurements of sky radiance have the potential to derive optical characteristics of tropospheric aerosols as aerosol optical depth, complex refraction index and aerosol size distribution. However, if the amount of aerosols is not very high, then the effect of polarization of diffuse sky radiance has to be considered, otherwise the derived aerosol parameters become very uncertain. The extension of these retrieval algorithms to the UV range provides additional information, but requires sophisticated radiative transfer models which account for polarization effects on molecular and aerosol scattering as well as for multiple scattering processes in the earth's atmosphere. Measurements of the degree of polarization in the UV and visible range under conditions with very low amounts of aerosols provide data for detailed model validation and for further model development. In 2004 measurements of polarization of diffuse sky radiance between 310 nm and 450 nm have been performed under cloudless conditions. Very low amounts of aerosols have been found in Lauder (New Zealand, 380 m above sea level). Scans of the sky in the vertical plane of the sun and in the horizontal plane through the sun with a 1.5° aperture were carried out with a polarization filter oriented at angles to the vertical between 0° and 135° every 45°. From these measurements, the Stokes-parameters for linear polarization are derived. The measurements at different solar zenith angles are compared with the results of radiative transfer modelling (libradtran), where no aerosols are considered. Reasonable agreement between measurements and modelling is found.
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Mario Blumthaler, Mario Blumthaler, Michael Schwarzmann, Michael Schwarzmann, } "Spectral measurements of the polarization of UV sky radiance", Proc. SPIE 5886, Ultraviolet Ground- and Space-based Measurements, Models, and Effects V, 588603 (10 September 2005); doi: 10.1117/12.618240; https://doi.org/10.1117/12.618240

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