Comment on amt-2022-105

Abstract

Spectral measurements of UV irradiance are of great importance to ensure human health protection as well as to support scientific research. To perform these measurements, double monochromator scanning spectroradiometers are the preferred devices, thanks to their linearity and stray-light reduction. However, because of their high cost and demanding maintenance, CCD-array-based spectroradiometers are increasingly used for monitoring UV irradiance. Nevertheless, CCD-array spectroradiometers have specific limitations, such as a high detection threshold or stray-light contamination. To overcome these challenges, several manufacturers are striving to develop improved instrumentation. In particular, Gigahertz-Optik GmbH has developed the stray-light-reduced BTS2048-UV-S spectroradiometer series (from now on called BTS). In this study, the long-term performance of the BTS and its seasonal behavior, regarding global UV irradiance, has been assessed. To carry out the analysis, BTS’ irradiance measurements have been compared against measurements of the Brewer MK-III #150 scanning spectrophotometer during three campaigns. A total of 711 simultaneous spectra, measured under cloud-free conditions and covering a wide range of solar zenith angle (from 14° to 70°) and UV index (from 2.4 to 10.6), are used for the comparison. During the three measurement campaigns, the global UV spectral ratio BTS/Brewer was almost constant (at around 0.93) in the 300–360 nm region for solar zenith angles (SZAs) below 70°. Thus, the BTS calibration was stable during the whole period of study (∼1.5 years). Likewise, it showed no seasonal nor SZA significant dependence in this wavelength region. Regarding the UV index, a good correlation between the BTS and the Brewer #150 was found, i.e. the dynamic range of the BTS is comparable to that of the Brewer #150. These results confirm the quality of the long-term performance of the BTS array spectroradiometer to measure global UV irradiance.