Monte Carlo Evaluation of Uncertainties of UV Spectra Measured With Brewer Spectroradiometers

Precise spectral ultraviolet (UV) measurements are needed to ensure human protection as well as to support scientific research. Quantifying the uncertainty of the UV spectra recorded is crucial to evaluate the quality of the measurements which is needed, in turn, for the assessment of their reliability. However, for double-monochromator spectroradiometers, the analytical derivation of this uncertainty is a challenging task due to the difficulties involved in propagating individual uncertainties. Under these circumstances, a Monte Carlo simulation is a reliable alternative as it does not require the calculation of partial derivatives and considers both nonlinear effects and correlations in the data. In the present study, the uncertainty of the spectral UV irradiance measured by a Brewer MKIII spectrophotometer is evaluated using a Monte Carlo approach. This instrument belongs to the National Institute of Aerospace Technology and has successfully participated in several international campaigns, which ensures its precise calibration. The average expanded uncertainty (k = 2) of the global UV irradiance measured by this instrument varies between 10% at 300 nm and 7% at 363 nm. At shorter wavelengths, it increases sharply due to thermal and electronic noise as well as wavelength misalignment. The results indicate that a Brewer spectrophotometer is suitable for climatological studies and model validation. Nevertheless, a substantial reduction of these uncertainties might be required for accurately detecting long-term UV trends. Although the study focused on a Brewer spectrometer, the methodology used for the uncertainty analysis is general and can be adapted to most UV spectroradiometers. The solar ultraviolet (UV) spectrum covers the wavelength range from 100 to 400 nm. Although it represents less than 10% of the total radiation reaching the earth's surface, it is of great importance as it can be harmful to humans, aquatic, and terrestrial ecosystems. To study its effects, high-quality measurements are needed. To determine the quality of the irradiance recorded its uncertainty must be first evaluated. Most techniques used for uncertainty estimation rely on the calculation of partial derivatives, which are difficult to obtain for most instruments measuring UV irradiance. The reason for this is that the mathematical expression relating some uncertainty sources and the measured irradiance is complex, with unknown correlations between the different uncertainty sources. To solve this problem, the Monte Carlo technique was used instead of the analytical one. The Monte Carlo approach is easier to implement and needs no calculation of partial derivatives. The method was implemented for a Brewer spectrometer, one of the preferred devices to measure UV irradiance. The results indicate that the Brewer is suitable for model validation, but a substantial reduction of its uncertainty might be needed to accurately detect spectral UV trends. A Monte Carlo technique has been implemented to obtain the uncertainty of the spectral ultraviolet recorded by a Brewer MKIII spectrometerThe methodology used for the uncertainty propagation is general and can be adapted to most spectroradiometers used in ultraviolet (UV) researchBrewer spectrometers are suitable for model and satellite validation but might be limited in the determination of spectral UV trends