Comparison of global UV spectral irradiance measurements between a BTS CCD-array and a Brewer spectroradiometer

Spectral measurements of UV irradiance are of great importance for protecting human health as well as for supporting 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 (hereafter "BTS"). In this study, the long-term performance of the BTS and its seasonal behavior, regarding global UV irradiance, was assessed. To carry out the analysis, BTS irradiance measurements were compared against measurements from 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 angles (SZAs; from 14 to 70 degrees) and UV indexes (from 2.4 to 10.6), were 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 305-360 nm region for SZAs below 70 degrees. Thus, the BTS calibration was stable during the whole period of study (similar to 1.5 years). Likewise, it showed no significant sea- sonal or SZA 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 in measuring global UV irradiance.