| 10 Dec 2025
Characterisation of spectroscopic properties of DOAS instruments using high-resolution solar spectra
Abstract. The characterisation of spectroscopic properties of DOAS instruments is important for accurate trace gas retrievals. In this study, we investigate and extend existing methods for the determination of spectroscopic properties using high-resolution solar spectra (also known as Kurucz fit, KF, approaches). We apply these methods to long-term zenith sky DOAS measurements in Kiruna (northern Sweden). This unique data set allows to study the performance and precision of such fitting procedures under different environmental and observational conditions. Also, the effect of the change of the detector from a photodiode array to a modern CCD is investigated. One key finding of our study is that the so-called Ring effect (caused by rotational Raman scattering) leads to a systematic broadening (by typically about 10 %) of the width of the instrument spectral response function (ISRF) derived from a KF compared to the true ISRF derived from atomic line lamp measurements. Especially for measurements of trace gases located close to the ground this broadening can lead to errors of the trace gas results if a KF-derived ISRF is used for the preparation of trace gas reference spectra and Ring spectra. Here it is important that the strength of the Ring effect can strongly change due to clouds, in particular in the presence of optically thick clouds. Measurements in the presence of optically thick clouds should thus not be chosen for the application of the KF. Another specific finding for the Kiruna measurements (also relevant for other high latitude stations) is that the Ring effect changes systematically with season because of the changing surface albedo (caused by snow cover in the winter). From KF, different instrument properties can be obtained. We give specific recommendations for different KF variants for the determination of the ISRF, intensity offsets (e.g. caused by spectrograph straylight), or the wavelength dependence of the light throughput of the instrument. We also show that a strong wavelength dependence of the light throughput (e.g. caused by the Fabry–Pérot etalon effect) can lead to wrong trace gas results. This finding might also be relevant for other instruments affected by strong Fabry–Pérot etalon effects, or containing other optical elements with strong wavelength-dependent light throughputs. Finally, we introduce a method to correct such a wavelength-dependent light throughput using the results of a modified KF.
Competing interests: Steffen beirle, Ulrich Platt, and Thomas Wagner are members of the editorial board of Atmospheric Measurement Techniques
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The paper "Characterisation of spectroscopic properties of DOAS instruments using high-resolution solar spectra” by Thomas Wagner et al., presents a comprehensive investigation of spectroscopic properties of DOAS instruments using a Kurucz-fit approach applied to a long-term zenith-sky dataset from Kiruna, Sweden, spanning 30 years. The study investigates the impact of a detector change from a photodiode array to a CCD, demonstrates that the Ring effect leads to a systematic broadening of the retrieved ISRF, with a strong dependence on cloud conditions and seasonal changes in surface albedo. The authors provide also recommendations for the determination of the ISRF, intensity offsets and the wavelength dependence of the light throughput of the instrument. Continuous monitoring of the instrument properties is very important, particularly for long-term trend analyses, where instrumental effects can otherwise introduce biases.
The paper is well written, clearly structured and easy to follow, and its scientific content fits the scope of AMT. Below are my review, comments and remarks.
General comments:
(1) The authors state that the spectrometer of the instrument is maintained at 30±0.1°C (P.33, L.700). Nevertheless, could small temperature instabilities still contribute, at least partly, to the observed variability of retrieved instrumental parameters such as the FWHM? While the manuscript suggests that the dominant seasonal variability is driven by the Ring effect, an investigation of the diurnal variability of the retrieved instrument parameters during clear-sky days might help to assess the magnitude of (if any) remaining temperature-related contributions.
(2) Can the authors comment to what extent their main conclusions and recommendations apply to lower-latitude sites with weaker but still variable surface albedo and to other viewing geometries (e.g. MAX-DOAS)?
(3) In an ideal spectrometer the spectral line shape is determined by the grating, slit, and optics, while the detector merely samples the spectrum. The authors demonstrate that the older PDA detector is affected by the Fabry–Pérot etalon effect, which alters the sampled spectrum and influences the effective ISRF. Is this the only detector-related parameter that is different between the PDA and CCD sensor used in the study that may affect the ISRF?
(4) The authors conclude that, in order to obtain reasonable and spectrally consistent results of the ISRF FWHM, neither an intensity offset nor a Ring spectrum should be included in the KF. I think the authors should comment on whether this is expected to be an instrument-dependent conclusion and/or and if the same behavior is expected for different spectral regions (e.g. in the visible)
Specific comments:
P.3, L.71: The Fabry–Pérot etalon effect is first introduced at this point. While a short description of the effect is given in Sect. 4.2 (L.411-412), I think such a description is more appropriate here.
P.4, Fig. 1: What do IRF 1 and IRF 2 represent? The small hut on the roof and the room inside the institute, respectively?
P.5, L.117-120: Can the authors comment on how the KF performs at lower SZAs? Are similar results expected?
P.5, L.137-138: Is an ozone absorption cross section included in the fit as stated in L.110? Please clarify
P.7, L.154-157: Can the authors comment on why doesn’t the inclusion of a Ring spectrum and/or of an intensity offset improve the results, especially in the UV range? Is this an instrument-dependent observation?
P.8, Fig. 3b: Are there any missing data in panel b) between 2003 and 2007? Or is this due to a visualization reason?
P.13, Fig. 5: The simulated RSP (panel a) should either become differential RSP, relative to 80 deg. SZA, or for the measured RSPs (panels b and c) the ylabels should be dRSP
P. 20, L.309-400: Are there any lamp measurements to confirm the increase of the FWHM?
Technical corrections:
P.3, L.61: “ist” -> “is”
P.4, L.104: “ERS” -> “ESR”
P.4, L.109: “software QDOAS” -> “QDOAS software”
P.6, L.149: “Finally, also the effect…”. -> “Finally, the effect…”
P.7, L.160: “Mio spectra” -> “million spectra”
P.8, Fig. 3 caption: Color assignments are wrong. They should probably be magenta, cyan, orange and black. Please revise. Same applies for Fig. 14 and Fig. A4
P.9, L.206: “prpared” -> “prepared”
P.9, L.207: “teh” -> “the”
P.9, L.216-217: “According to Wikipedia (2025), snow cover generally lasts from late September to mid-May”. This sentence may be omitted since afterwards, the snow depth is given in Fig. 4a
P.10, Table 1 caption: “Also shown are the settings from Alliwell et al., (2002)” -> “The settings from Alliwell et al., (2002) are also shown/included”
P.11, Fig. 4: The x label names (month names) should be given in English. Same applies for Fig. 13. Also in panel d, the marker color of the legend for 341-348 nm is wrong
P.12, L.241: “Kuruzc” -> “Kurucz”
P.12, L.252: “…are shown (black dots)” -> “filled markers”?
P.12, L.253: “Fig, 5a” -> “Fig. 5a”
P.12, L.255 “(blue dots)” -> “(small dots/markers)”?
P.15, Fig. 7: A ylabel should be given
P.15, Fig. 7 caption and in all other places: “ISFR” -> “ISRF”
P.16, L.311-312: “the better choice” -> “the best choice” or “a better choice”
P.16, L.315: the word “however” is not necessary
P.16, Table 2 caption and in all other places: “super Gaussian” -> “super-Gaussian”
P. 20, L.408 “charateristics” -> “characteristics”
P. 21, L. 437: “Appendix 3” -> “Appendix A3”
P. 33, L.698 and 700: “spectromter” -> “spectrometer”
P. 33, Fig. A1 caption: Either include a) and b) texts in the two panels or replace with “left”-“right”. Also replace “the visible spectrometer” with “the spectrometer operating in the visible range” or something similar.
P. 41, L.797: “of scattered” is a duplicate