Total Column Ozone Retrieval from Novel Array Spectroradiometer
Abstract. This study presents total column ozone (TCO) retrieval from a new system, called Koherent, developed at 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 fiber to a lens-based telescope mounted on a sun tracker for measuring direct UV irradiance in the ultraviolet wavelength band between 305 nm to 345 nm.
Two different algorithms are developed for retrieving TCO from these spectral measurements: 1) TCO retrieved by a minimal 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 and the Brewer but adopted and optimized 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 minimizing 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 two-year comparison in Davos, Switzerland, between Koherent and the Brewer 156 (double monochromator) shows that TCO derived from Koherent and the Brewer 156 agree in average over the entire period within less than 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 for 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 as 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, allow to determine the effective ozone temperature within 3 K on daily averages. With the improved TCO retrieval, Koherent serves as a new low maintenance instrument to operationally monitor TCO at remote sites. The presented TCO retrieval may be applied to other array based spectroradiometers providing direct spectral measurements in the ultraviolet.
Luca Egli et al.
Luca Egli et al.
Luca Egli et al.
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The manuscript "Total Column Ozone Retrieval from Novel Array Spectroradiometer" published by Egli et al., presents a study on the use of a relatively new array spectroradiometer for ground-based measurements of total ozone column in the atmosphere. The new array spectroradiometer has the potential to provide more accurate and precise measurements compared to traditional methods such as those using Dobson instruments, grating spectrophotometers, etc. The study also provides a comparison of total column ozone retrievals between the proposed method and established methods. This helps to demonstrate the potential of the new system and the associated retrieval technique.
One of the strengths of this study is that it presents an approach for measuring total column ozone that is new, fast and automated, while utilizing available, easy to acquire software packages and hardware. The study also leverages on the established instrumentation and expertise at PMOD for standard calibrations.
However, I believe that this manuscript would still benefit from a chapter on error analysis containing a detailed error budget. I understand that some of the aspects of the methodology have already been done elsewhere, nevertheless it would be useful and important to include such a chapter. For example, I would be interested in knowing the signal to noise ratios of the spectra, typical wavelength shifts, how the uncertainty in the LSQ retrieval is calculated, and so on, without much digging through literature.
In conclusion, this manuscript fits well within the scope of AMT. Therefore, I recommend its publication after addressing the general comments and some of the comments and corrections below.
Technical Corrections and Suggestions:
P.1, Line 24: “within less than 0.7%” --> within 0.7%
P.2, Line 45: “In the Dobson instruments, prisms are selecting ...” --> In the Dobson instruments, prisms are used to select ...
P2, Line 46: “Most of the Dobsons are manually operated and require therefore …” --> Most of the Dobsons are manually operated and therefore require …
P2, Line 52: “contrary to single ...” --> in contrast to single …
P2, Line 52 and 69: “suffer from stray light …” --> suffer from the effects of stray light …
P2, Line 53: “The Brewers were formed to a network of automatic stations, which required few ...” --> The Brewers were used to form a network of automated stations, which required less …
P2, Line 53: “best consistency …” --> greatest consistency
P2, Line 60: “irradiance ratio at the top of the atmosphere” : irradiance ratio of what?
P2, Line 76: “Similarly as Pandora …” --> Similar to the Pandora …
P2, Line 83: “Contrary to the …” --> In contrast to the …
P2, Line 83: “fiber coupled” --> fiber-coupled
P9, Line 363: “clears sky” --> clear sky
P9, Line 369: ”The two-years ...” --> The two-year …
Fig. 1. Caption: “… spectra on morning of 15 September …” --> spectra on the morning of 15 September