Total column ozone retrieval from a novel array spectroradiometer
Authors: Egli Luca , Gröbner Julian , Schill Herbert , Maillard Barras Eliane 
Published: June 2023

Abstract. This study presents a new total column ozone (TCO) retrieval from the
Koherent system, developed at the Physikalisch-Meteorologisches Observatorium Davos, World Radiation
Center (PMOD/WRC). The instrument is based on a small,
cost-effective, robust, low-maintenance and state-of-the-art-technology
array spectroradiometer. It consists of a BTS-2048-UV-S-F array
spectroradiometer from Gigahertz-Optik GmbH, coupled with an optical fibre
to a lens-based telescope mounted on a sun tracker for measuring direct UV
irradiance in the ultraviolet wavelength band between 305 to 345 nm. Two different algorithms are developed for retrieving TCO from these
spectral measurements: (1) TCO retrieved by a least-squares-fit algorithm
(LSF) and (2) a custom-double-ratio (CDR) technique using four specifically
selected wavelengths from the spectral measurements. The double-ratio
technique is analogous to the retrieval algorithm applied for the Dobson instruments and
the Brewer instruments but is adopted here for TCO retrieval with Koherent. The instrument
was calibrated in two different ways: (a) absolute calibration of the spectra
using the portable reference for ultraviolet radiation QASUME for the LSF
retrieval and (b) relative calibration of the extraterrestrial constant (ETC)
of the CDR retrieval by minimising the slope between air mass and the
relative differences of TCO from QASUME and Koherent. This adjustment of the
ETC allows the instrument to be calibrated with standard TCO reference
instruments during calibration campaigns, such as a double-monochromator
Brewer. A 2-year comparison in Davos, Switzerland, between Koherent and the Brewer
156 (double monochromator) shows that TCO derived from Koherent and the
Brewer 156 agree, on average, over the entire period within 0.7 % for all
retrievals in terms of offset. The performance in terms of slant path
depends on the selected retrieval and the applied corrections. The stray-light-corrected LSF retrieval exhibits a smaller slant path dependency than
the CDR retrieval and performs almost as well as a double-monochromator
system. The slant path dependency of the CDR is comparable to the slant path
dependency of a single Brewer monochromator. The combination of both
retrievals leads to performance with an offset close to zero compared to
Brewer 156, a seasonal amplitude of the relative difference of 0.08 % and
a slant path dependency of maximum 1.64 %, which is similar to other
standard TCO instruments such as single Brewer or Dobson. Applying the double-ratio technique by selecting the wavelengths and slit
functions from Brewer and Dobson, respectively, allows for the determination of
the effective ozone temperature with an uncertainty of 3 K in terms of daily averages. With the
improved TCO retrieval, Koherent serves as a new low-maintenance instrument
which could also be used to monitor TCO at remote sites. The TCO retrieval
presented here may be applied to other array-based spectroradiometers,
providing direct spectral measurements in the ultraviolet wavelength band.