A METHOD FOR CORRECTION OF COSINE ERRORS IN MEASUREMENTS OF SPECTRAL UV IRRADIANCE

One of the sources contributing to the overall uncertainty of spectral UV radiation measurements is the cosine error of the spectroradiomete r. It leads to measurement errors that depend on atmospheric condition s and on solar zenith angle, and thus time of the day and season. Thou gh the foreoptics of modern instruments are designed such as to minimi ze cosine errors, there remain deviations from the ideal cosine respon se. We have worked out a method to further reduce that remaining cosin e error in global spectral irradiance. This method was applied to spec tra of global UV radiation taken with a Brewer spectroradiometer. The only additional input data needed to apply the method of cosine correc tion to spectral irradiance data are concurrent broad-band UV-B radiat ion measurements of diffuse and global radiation recorded with filter UV instruments, which are used to estimate the optical thickness refer red to global UV radiation for the time when the spectral scan is take n. The method takes account of the variable conditions of cloudiness a nd turbidity. In the case of measurements taken with Brewer instrument No. 30, the cosine corrected global UV-B radiation was higher than th e measured irradiance by 9-20%, and even its daily totals turned out t o be higher than the uncorrected radiation by 13-18%. An estimate of t he uncertainty of +/-4 to +/-8% was derived from a theoretical approac h as well as from model calculations using a radiative transfer model. (C) 1997 Elsevier Science Ltd.