Uncertainty of measurements of spectral solar UV irradiance

Most investigations on the nature and effects of solar ultraviolet (UV) rad iation at the Earth's surface require measurements of high accuracy combine d with well-defined procedures to assess their quality. Here we present a g eneral evaluation of all relevant errors and uncertainties associated with measurements of spectral global irradiance in the UV. The uncertainties are quantified in terms of dependence of the characteristics of the spectrorad iometer, the uncertainty of calibration standards, the solar zenith angle, and atmospheric conditions. The methodologies and equations presented can b e applied to most spectroradiometers currently employed for UV research. Th e sources of error addressed include radiometric calibration, cosine error, spectral resolution, wavelength misalignment, stability, noise, stray ligh t, and timing errors. The practical application of the method is demonstrat ed by setting up a complete uncertainty table for the mobile spectroradiome ter of the Fraunhofer Institute for Atmospheric Environmental Research (IFU ). This instrument has successfully participated in several international i ntercomparisons of UV spectroradiometers. The expanded uncertainty (coverag e factor k = 2) for measurements of global spectral irradiance conducted wi th this instrument varies between 6.3% in the UVA and 12.7% at 300 nm and 6 0 degrees solar zenith angle. The expanded uncertainties in erythemally and DNA weighted irradiances are 6.1% and 6.6%, respectively. These expanded u ncertainties are comparable to uncertainties at the 2 sigma level in conven tional statistics. A substantial reduction of these uncertainties would req uire smaller uncertainties in the irradiance standards used to calibrate th e instrument. Though uncertainties caused by wavelength misalignment and no ise become prominent in the shortwave UVB, which is the most important spec tral range for UV trend detection, the results indicate that the accuracy o f the IFU radiometer is sufficient to detect long-term trends in UV arising from a 3% change in atmospheric ozone. The detection of trends caused by a 1% change in ozone may be beyond the capabilities of current instrumentati on.