Measuring spectral and spatial variations of UVA and UVB sky radiance

Spectral sky radiance measurements have been obtained with a double monochromator using a 1.5 degrees field of view under varying atmospheric conditions at selected sites in Europe. Spatial variations of the intensity of the sky radiance up to a factor of 10 are observed in the UVA, which decrease to a factor of 2 in the UVB. A distinct minimum of the sky radiance is observed, the location depends on the solar zenith angle but not on wavelength. It is located at scattering angles between 90 degrees and 75 degrees for solar zenith angles between 80 degrees and 26 degrees. A comparison between measurements of the sky radiance and radiative transfer calculations with an aerosol free atmosphere shows a strong dependence of UVB sky radiance on aerosol particles in contrast to UVA sky radiance and possibly a dependence on tropospheric ozone as well.