A new actinic flux 4p-spectroradiometer: instrument design and application to clear sky and broken cloud conditions E. Eckstein1, D. Perner1, Ch. Brühl1, and T. Trautmann2 1Max-Planck-Institut für Chemie, Mainz, Germany 2Institut für Meteorologie, Universität Leipzig, Germany
Abstract. A new 4p-spectroradiometer was developed for measuring actinic flux especially under cloudy conditions based on a
fixed grating imaging spectrograph and a CCD-detector leading to a simultaneous measurement of the spectrum. The new
instrument incorporates a novel optical head with a 4p-field of view independent of angle of incidence. Comparisons
with the actinic flux spectroradiometer of the Institute of Atmospheric Chemistry of Forschungszentrum
Jülich showed a very good agreement within the limit of the uncertainties of the two instruments. Our spectroradiometer was applied to
investigate the effects of broken clouds on the actinic flux and photolysis frequencies on the ground during the
BERLIOZ campaign. Reductions as well as enhancements compared to the clear sky case were seen, both effects are larger
in the UV-A than the UV-B spectral region. Furthermore the new instrument was used for simultaneous measurements in different altitudes on a tower to study
the transmission and attenuation of actinic flux in low clouds. A correlation of attenuation with the simultaneously
measured liquid water content of the cloud was found.
Citation: Eckstein, E., Perner, D., Brühl, Ch., and Trautmann, T.: A new actinic flux 4p-spectroradiometer: instrument design and application to clear sky and broken cloud conditions, Atmos. Chem. Phys., 3, 1965-1979, doi:10.5194/acp-3-1965-2003, 2003.