Abstract
Abstract. A Bentham DTMc300 spectroradiometer is deployed at the station of Aosta–Saint-Christophe, Italy, at the headquarters of the Regional Environmental Protection Agency (ARPA) and has been performing continuous high-quality spectral measurements of the solar ultraviolet (UV) irradiance since 2006. The measuring site is located in the north-western region of the Alps, on a large valley floor at the altitude of 570 m a.s.l., surrounded by mountains. It is very significant to have accurate measurements in such a sensitive environment, since the complex terrain and the strongly variable meteo-climatic conditions typical of the Alps induce large spatial and temporal variability in the surface levels of the solar UV irradiance. The spectroradiometer is also used as a reference of a regional UV network, with additional stations located at different altitudes (1640 and 3500 m a.s.l.) and environmental conditions (mountain and glacier). In the present study we discuss the procedures and the technical aspects which ensure the high quality of the measurements performed by the reference instrument and the procedures used to characterize the Bentham. The quality control and quality assurance (QA–QC) procedures are also discussed. We show that the good quality of the spectral measurements is further ensured by a strong traceability chain to the irradiance scale of the Physikalisch-Technische Bundesanstalt (PTB) and a strict calibration protocol. Recently, the spectral UV dataset of Aosta–Saint-Christophe has been re-evaluated and homogenized. The final spectra constitute one of the most accurate datasets globally. At wavelengths above 310 nm and for solar zenith angles below 75∘, the expanded (k=2) uncertainty in the final dataset decreases with time, from 7 % in 2006 to 4 % in the present. The present study not only serves as the reference document for any future use of the data, but also provides useful information for experiments and novel techniques which have been applied for the characterization of the instrument and the QA–QC of the spectral UV measurements. Furthermore, the study clearly shows that maintaining a strong traceability chain to a reference scale of spectral irradiance is critical for the good quality of the measurements. The studied spectral dataset is freely accessible at https://doi.org/10.5281/zenodo.4028907 (Fountoulakis et al., 2020b).
Highlights
Less than 10 % of the overall solar electromagnetic radiation that reaches the Earth’s surface is in the ultraviolet (UV) range, this particular band of the solar spectrum is very significant for life on Earth
Malignant melanoma (Moan et al, 2008) and cataracts (Taylor et al, 1988; Bourne et al, 2013) are common problems caused by the excessive exposure to solar UV radiation, while hypovitaminosis D (Juzeniene et al, 2011) is a common problem caused by the inadequate exposure to UV radiation
Significant progress has been achieved in the past 2 decades regarding the methods used for the instrument calibration and characterization and the quality assurance and quality control (QA–QC) procedures (Fountoulakis et al, 2016; Fountoulakis et al, 2017; Gröbner et al, 2010; Hülsen et al, 2016; Lakkala et al, 2018, 2008), which in conjunction with the improvement of the technical characteristics of the instruments (Gröbner, 2003; Pulli et al, 2013) allows spectral measurements with expanded uncertainty of ∼ 2 % for wavelengths above 310 nm (Hülsen et al, 2016)
Summary
Less than 10 % of the overall solar electromagnetic radiation that reaches the Earth’s surface is in the ultraviolet (UV) range, this particular band of the solar spectrum is very significant for life on Earth. The main sources of uncertainty in spectral UV measurements have been discussed in the studies of Bais (1997) and Bernhard and Seckmeyer (1999). Significant progress has been achieved in the past 2 decades regarding the methods used for the instrument calibration and characterization and the quality assurance and quality control (QA–QC) procedures (Fountoulakis et al, 2016; Fountoulakis et al, 2017; Gröbner et al, 2010; Hülsen et al, 2016; Lakkala et al, 2018, 2008), which in conjunction with the improvement of the technical characteristics of the instruments (Gröbner, 2003; Pulli et al, 2013) allows spectral measurements with expanded uncertainty of ∼ 2 % for wavelengths above 310 nm (Hülsen et al, 2016).
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