Intercomparison of solar UV direct irradiance spectral measurements at Izana in June 2005

Abstract

Spectral measurements of direct solar ultraviolet irradiance are very important for many applications in the field of atmospheric sciences. Despite its usefulness, few UV monitoring sites include such measurements in their regular observational programs. Standardization of measurement methodologies and calibration techniques is required in order to reach the quality standard of global irradiance measurements. This study presents preliminary results from an intercomparison campaign of seven UV spectroradiometers of different types that took place at the high altitude site of Izana (28.3°N, 16.5°W, 2367 m above sea level), in Tenerife, Canary Islands in June 2005. The campaign is focused primarily on spectral measurements of direct solar irradiance. Among the objectives is to improve the quality of direct solar irradiance spectral measurements, through instrumental modifications and standardization of calibration techniques, as well as to assess the significance of the differences in the field of view of the spectroradiometers with respect to aerosols and to solar zenith angle. Under the low aerosol conditions prevailing during this campaign, we aimed to establish the differences among the various instruments under "ideal" conditions. Moreover, continuous measurements under stable total ozone and aerosol optical depth will be used to determine the extraterrestrial solar flux, through the application of the Langley extrapolation method. A first comparison of sky radiance measurements of the zenith light and of various directions on the sky show effects of sensitivity to polarization of one type of instruments and the variability of the provisional radiance calibration of 4 instruments.