Search

During the 2019/2020 measurement campaign at Hohenpeißenberg (Germany) and Davos (Switzerland) we compared the well-established Dobson and Brewer spectrometers (single- and double-monochromator Brewer) with newer BTS array-spectroradiometer-based systems in terms of total ozone column (TOC) determination. The aim of this study is to validate the BTS performance in a longer-term TOC analysis over more than 1 year with seasonal and weather influences. Two different BTS setups have been used – a fibre-coupled entrance optic version by PMOD/WRC called Koherent and a diffusor optic version from Gigahertz Optik GmbH called BTS-Solar, which proved to be simpler in terms of calibration. The array-spectrometer-based BTS systems have been calibrated with traceability to NMI, and both versions of TOC retrieval algorithms are based on spectral measurements in the range of 305 to 350 nm instead of single-wavelength or wavelength pair measurements as per Brewer or Dobson. The two BTS-based systems, however, used fundamentally different retrieval algorithms for the TOC assessment, whereby the retrieval of the BTS-Solar turned out to achieve significantly smaller seasonal drifts. The intercomparison showed a difference of the BTS-Solar to Brewers of < 0.1 % with an expanded standard deviation (k=2) of < 1.5 % over the whole measurement campaign. Koherent showed a difference of 1.7 % with an expanded standard deviation (k=2) of 2.7 % mostly caused by a significant seasonal variation. To summarize, the BTS-Solar performed at the level of Brewers in the comparison in Hohenpeißenberg. The BTS-Solar showed very small dependence on the slant path column compared to the double-monochromator Brewer and performed better than the single-monochromator Brewer. Koherent showed a strong seasonal variation in Davos due to the sensitivity of its ozone retrieval algorithm to stratospheric temperature.

In the 2019/2020 measurement campaign at Hohenpeißenberg (Germany) and Davos (Switzerland) we compared the well-established Dobson and Brewer spectrometers (single and double monochromator Brewer) with newer BTS array spectroradiometer based systems in terms of total ozone column (TOC) determination. The aim of this study is to validate the BTS performance in a longer-term TOC analysis over more than one year with seasonal and weather influences. Two different BTS setups have been used. A fibre coupled entrance optic version by PMOD/WRC called Koherent and a diffusor optic which proved to be simpler in terms of calibration from Gigahertz-Optik GmbH called BTS Solar. The array-spectrometer based BTS systems have been traceable calibrated to National Metrology Institutes (NMI) and the used TOC retrieval algorithms are based on spectral measurements in the range of 305 nm and 350 nm instead of single wavelength measurements as for Brewer or Dobson. The two BTS based systems, however, used fundamentally different retrieval algorithms for the TOC assessment, whereby the retrieval of the BTS solar turned out to achieve significantly smaller seasonal drifts. The intercomparison showed a deviation of the BTS Solar to Brewers of < 0.1 % with an expanded standard deviation of < 1.5 % within the whole measurement campaign. Koherent showed a deviation of 1.7 % with an expanded standard deviation of 2.7 % mostly given by a significant seasonal drift. Resulting, the BTS Solar performance is comparable to Brewers at the comparison in Hohenpeißenberg. The slant path slope is in-between double monochromator and single monochromator Brewer. Koherent shows a strong seasonal variation in Davos due to the sensitivity of its ozone retrieval algorithm to stratospheric temperature similar to the Dobson results.

Solar spectral irradiance measured by two recently developed array spectroradiometers (called UV-BTS and VIS-BTS) are compared to the results of a scanning double monochromator system which is certified as a travelling reference instrument by the Network for the detection of atmospheric composition change (NDACC) and fulfils the specifications of S-2 UV instruments of the world meteorological organization (WMO). The comparison took place between 15 and 18 May 2017 at the Institute of Meteorology and Climatology of the University of Hanover (IMuK) between 4:00 and 17:00UTC. The UV-BTS array spectroradiometer is equipped with special hardware to significantly reduce internal stray light which has been the limiting factor of many array spectroradiometers in the past. It covers a wavelength range of 200 nm-430 nm. The VIS-BTS covers a wider spectral range from 280 nm up to 1050 nm, and stray light reduction is achieved by mathematical methods. For the evaluation, wavelength integrated quantities and spectral global irradiance are compared. The deviation for UV index measured by the UV-BTS, is within ±1% for solar zenith angles (SZA) below 70° and increased to a maximum of ±3% for SZA between 70° and 85° when synchronisation between measurements was possible. The deviation of global spectral irradiance is smaller ±2.5% in the spectral range from 300 nm to 420 nm (evaluated for SZA < 70°). The VIS-BTS achieved the same deviation for blue light hazard as the UV-BTS for the UV index. The evaluations of global spectral irradiance data of the VIS-BTS show a deviation smaller than ±2% in the spectral range from 365 nm to 900 nm (evaluated for SZA < 70°). Below 365 nm, the deviation rises up to ±7% at 305 nm due to remaining stray light. The agreement within the limited time of the intercomparison is considered to be satisfactory for a number of applications and provides a good basis for further investigations. © 2018 IOP Publishing Ltd.

A compact array spectroradiometer technology that enables precise and robust measurements of UV spectral irradiance is presented. The spectroradiometer design allows various applications such as measurement of high-power UV lamps, risk assessment of sources and measurement of solar irradiance. We show that this technology can perform precise total ozone column (TOC) retrieval. The internal stray light, which is often the limiting factor for measurements in the UV spectral range and increases the uncertainty for TOC analysis significantly, is physically reduced so that no other stray light 15 reduction methods, such as mathematical corrections, are necessary. The instrument and its elaborate components have been extensively characterised at the Physikalisch-Technische Bundesanstalt (PTB) in Germany. During an international total ozone measurement intercomparison at the Izaña observatory in Tenerife, the high-quality applicability of the instrument was verified with measurements of the direct solar irradiance and subsequent TOC evaluations based on the spectral data between 12 and 30 September 2016. The results showed deviations of less than 1.5 % to most other instruments in most 20 situations not exceeding 3 % compared to established TOC measurement systems such as Dobson or Brewer.

Technologies to determine spectral sky radiance distributions have evolved in recent years and have enabled new applications in remote sensing, for sky radiance measurements, in biological/diagnostic applications and luminance measurements. Most classical spectral imaging radiance technologies are based on mechanical and/or spectral scans. However, these methods require scanning time in which the spectral radiance distribution might change. To overcome this limitation, different so-called snapshot spectral imaging technologies have been developed that enable spectral and spatial non-scanning measurements. We present a new setup based on a facet mirror that is already used in imaging slicing spectrometers. By duplicating the input image instead of slicing it and using a specially designed entrance slit, we are able to select nearly 200 (14 × 14) channels within the field of view (FOV) for detecting spectral radiance in different directions. In addition, a megapixel image of the FOV is captured by an additional RGB camera. This image can be mapped onto the snapshot spectral image. In this paper, the mechanical setup, technical design considerations and first measurement results of a prototype are presented. For a proof of concept, the device is radiometrically calibrated and a 10 mm × 10 mm test pattern measured within a spectral range of 380 nm–800 nm with an optical bandwidth of 10 nm (full width at half maximum or FWHM). To show its potential in the UV spectral region, zenith sky radiance measurements in the UV of a clear sky were performed. Hence, the prototype was equipped with an entrance optic with a FOV of 0.5° and modified to obtain a radiometrically calibrated spectral range of 280 nm–470 nm with a FWHM of 3 nm. The measurement results have been compared to modeled data processed by UVSPEC, which showed deviations of less than 30%. This is far from being ideal, but an acceptable result with respect to available state-of-the-art intercomparisons.