Quality assurance of broadband erythemal radiometers at the Extremadura UV Monitoring Network (Southwestern Spain)

This paper shows the necessity of adopting a proper quality assurance (QA) methodology in order to guarantee optimal measurements of broadband ultraviolet (UV) erythemal irradiance and describes the methods used to successfully operate the Extremadura UV Monitoring Network (Southwestern Spain). The quality assurance of the broadband radiometers follows two phases. Initially, the angular and spectral responses of these instruments are characterized at the INTA/ESAt Calibration Laboratory at El Arenosillo (Spain). The results obtained in the laboratory characterization show that the instruments present an excellent angular response very close to the ideal one, with cosine errors lower than 5%, depending on the sky radiance distribution and the solar zenith angle. It was also found that the spectral responses for this family of radiometers differ notably for longer UV wavelengths, emphasizing the need for the individual characterization of each instrument. Finally, the absolute calibration of the instruments is performed through an outdoor intercomparison with respect to a well-calibrated Brewer spectroradiometer which serves as the reference instrument. The stability of the calibration is addressed by comparing the results obtained at the calibration campaigns held in 2007 and 2009. The comparison shows notable differences between distinct instruments with values ranging from 2% up to 11%.