Correction of Angular Response Error in Brewer UV Irradiance Measurements

Ultraviolet spectral irradiance measured by spectroradiometers usually presents high deviations from the ideal angular response due to imperfections in the entrance optics. In this paper a methodology to correct the angular error in the global UV spectral measurements of a Brewer MKIII spectroradiometer under all weather conditions is presented. This methodology calculates the global correction factor as a function of three variables: the direct irradiance correction factor, the diffuse irradiance correction factor, and a factor depending on the direct-to-global irradiance ratio. This work contributes to better measuring the UV radiation by improving the parameterization of the clouds effects. Depending mainly on wavelength, solar zenith angle, and cloud optical thickness, the angular correction obtained ranges from 2% to 9%. The accuracy of this correction is limited by the uncertainties in the measured angular response and in the ratio of direct to global radiation. The original and the corrected Brewer measurements are compared with simultaneous values of the transportable Quality Assurance of Spectral Ultraviolet Measurements in Europe through the Development of a Transportable Unit (QASUME) reference spectroradiometer. A notable decrease (about a factor higher than 2) in the relative differences between the two instruments is obtained when Brewer-corrected measurements are considered.