Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 2 6 2002 10.5194/acpd-2-1939-2002 http://www.atmos-chem-phys-discuss.net/2/1939/2002/ http://www.atmos-chem-phys-discuss.net/2/1939/2002/acpd-2-1939-2002.html http://www.atmos-chem-phys-discuss.net/2/1939/2002/acpd-2-1939-2002.pdf 1939 1977 2002-11-11 A new actinic flux 4&pi;-spectroradiometer: Instrument design and application to clear sky and broken cloud conditions E. Eckstein D. Perner Ch. Brühl T. Trautmann Max-Planck-Institut für Chemie, Mainz, Germany Institut für Meteorologie, Universität Leipzig, Germany A new 4<font face="Symbol" >p</font>-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 4<font face="Symbol" >p</font>-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.<br> 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.