Spectral measurements of vertically and horizontally polarized UV sky distribution

摘要

Theoretical and experimental investigations of the effects of atmospheric aerosols on diffuse sky radiance in the visible and infrared range of the solar spectrum show that the retrieval of physical aerosol properties from intensity measurements of diffuse skylight leads to non-unique solutions for aerosol optical depth, complex refraction index and aerosol size distribution. Additional photopolarimetric radiance measurements have shown to add valuable information to intensity data, thus allowing a more specific determination of aerosol parameters. The extension of these retrieval algorithms to the UV range provides additional information, but requires the development of 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. In order to provide a reference for these models, radiance measurements in the UV and visible range of the solar spectrum have been performed with a high resolution Bentham DTM300 double monochromator, equipped with a linear sheet polarizer. The measurements show strong differences between the directional distributions of horizontally and vertically polarized diffuse sky radiance of the upper hemisphere. Comparison between data taken at the Jungfraujoch (Switzerland, 3576 m a.s.l.) and in Thessaloniki (Greece, 20 m a.s.l.) under different atmospheric aerosol conditions reveals that aerosol Mie scattering effects horizontally and vertically polarized radiance in different ways, which confirms that photopolarimetric radiance data contain more information about aerosol properties than intensity measurements alone.