The INFO3RS project aims to investigate the future evolution of total column ozone (TCO) in Switzerland. In order to supplement the existing measurements from Dobson and Brewer instruments, a novel system, KOHERENT, was developed to ensure traceable TCO measurements in Davos. INFO3RS is funded by the MeteoSwiss GAW-CH programme.
The general objective of the INFO3RS project, funded by the Global Atmosphere Watch Programme Switzerland (GAW-CH), can be summarised with the following question: “How and when can we detect the recovery of the ozone layer over Switzerland?” “How” is related to the ozone observation techniques with ground- and space-borne instruments, such as Dobsons, Brewers and the SBUV satellite. In the framework of the INFO3RS project, PMOD/WRC developed a new cost effective and simple system, KOHERENT (Figure 1), based on an array spectroradiometer to retrieve TCO from a full spectrum in the UV band (300 – 350 nm). Contrary to Dobsons and Brewers, which use four wavelengths for the TCO measurement, the array spectroradiometer allows ozone to be retrieved from wavelengths of the full solar spectrum, in order to reduce the uncertainty of the ozone retrieval. Furthermore, the Dobson and Brewer networks are calibrated with their respective reference instruments, which require regular travel to Dobson or Brewer intercomparisons. In contrast, the KOHERENT system can be characterised and calibrated with laboratory facilities at PMOD/WRC on a regular schedule to ensure fully traceable TCO measurements.
Figure 1. KOHERENT telescope on the right of the solar tracker.
During the first year of INFO3RS, the BTS-2048-UV-S-F array spectroradiometer from Gigahertz-Optik (Zuber et al., 2018), with optimised stray-light reduction using bandpass filters, was purchased and thoroughly tested at PMOD/WRC. An operational data acquisition software based on Python was developed in-house to ensure reliable TCO measurements. A new fore-optic telescope was assembled according to an optimised design with commercial optical parts and connected to an optical fibre light-guide on the BTS array spectroradiometer. The entire system is housed in a temperature stabilised box, which allows TCO observations to be performed on the PMOD/WRC measurement platform during all weather conditions in summer and winter.
Total column ozone is retrieved in near-realtime by a data post-processing chain including: a) spectral calibration, b) wavelength shift correction with the MatSHIC algorithm, and c) non-linear least-squares minimisation using the new QASUMEFTS reference extraterrestrial solar spectrum (Gröbner et al., 2017). The results show that TCO retrievals by KOHERENT agree to within ~1% of the Brewer TCO data in October 2018 (Figure 2), based on an in-situ spectral calibration with the QASUME reference spectroradiometer. In December 2018, after laboratory characterisation and calibration, the TCO observations showed a larger bias of up to 2%. This effect may be attributed to the very low temperatures in winter or some straylight effects during calibration. The long-term performance and potential limitations of the new KOHERENT system will be further investigated during the next three years of the INFO3RS project.
Figure 2. Intercomparison of TCO from the fully traceable KOHERENT system and all Brewers in Davos and Arosa. The deviation to Brewer #163 is less than 1% for all instruments (first number in the legend) including the PMOD/WRC reference instrument, QASUME.