| 05 Jan 2026
Ground-based total ozone column measurements in the Huggins and Chappuis bands using Direct-Sun DOAS observations
Abstract. Accurate routine monitoring of the Total Ozone Column (TOC) is essential for understanding ozone temporal variability, assessing long-term trends and supporting satellite validation. In this work, we present TOC retrievals in both ultraviolet (Huggins bands) and visible (Chappuis bands) spectral regions using direct-sun Differential Optical Absorption Spectroscopy (DS-DOAS). We use the Delta UV–VIS DOAS system, recently designed and operated at the Laboratory of Atmospheric Physics in Thessaloniki, Greece. A dedicated retrieval algorithm was developed that includes the calibration of a measured reference DS spectrum using the Langley plot and Bootstrap Estimation approaches. The analysis suggests that TOCs derived from the visible channel for the first time in Thessaloniki are highly consistent with those from the ultraviolet channel, with a median difference of –0.44 % and Pearson's correlation coefficient R = 0.97. The Delta TOC retrievals were compared with two collocated instruments, Brewer and Pandora, yielding very good agreement in both spectral regions (R > 0.98), with median biases –0.18 % and –0.63 % for the VIS and 0.08 % and –0.32 % for the UV channel compared to the Brewer and Pandora, respectively. The seasonal and diurnal variabilities of total ozone were captured consistently from all three instruments, confirming the robustness of the retrievals. Enhanced aerosol loads, such as those observed during an extreme wildfire event, introduced significant deviations in the VIS TOC comparisons with the reference instruments, while the comparisons in the UV remained largely unaffected. The findings of this study confirm the capability of Delta to provide accurate and consistent TOC retrievals in both UV and VIS spectral bands. The successful exploitation of the Chappuis bands extends the applicability of DS-DOAS to conditions where UV sensitivity is limited, such as at high solar zenith angles, thereby extending the continuity of ozone monitoring from ground-based systems and providing a complementary approach to traditional UV-based TOC retrievals.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Measurement Techniques.
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Review of the manuscript “Ground-based total ozone column measurements in the Huggins and Chappuis bands using Direct-Sun DOAS observations” by Karagkiozidis et al.
The manuscript describes measurements of total column ozone using a DOAS system from direct solar irradiance measurements in Thessaloniki, Greece. The retrieved ozone values are validated by comparison to two collocated instruments, a Brewer spectrophotometer and a Pandora system. The retrievals are performed both in the UV and VIS spectral regions and show consistent results with the collocated reference instruments, with the exception of the VIS retrievals during high aerosol contamination.
The manuscript is well written, the structure is clear and the results and conclusions follow from the discussions.
The figures are mostly informative and useful for the understanding of the arguments. Possibly the need for the SCD figures along with the corresponding TOC scatter plots are somewhat redundant, but I see the point of showing that the magnitude of ozone absorption has no systematic impact on the retrieval.
A point that needs to be clarified is the concept of “I0-correction” which is used without definition on line 266 and Table 1. While it may be familiar to the DOAS community, it is not a common term to the wider community.
The paper could also highlight the advantage of the TOC retrieval in the Chappuis band of not being sensitive to the stratospheric temperature in contrast to the Huggins band retrievals, which are an issue for TOC retrievals in the UV by some instruments (e.g. Dobson and Pandora, see for example publications by Gröbner et al., 2021 amt-14-3319-2021 and Xiaoyi et al., 2016 amt-9-5747-2016).
In that respect, in Section 3.2 where the ozone layer temperature is discussed, I wonder how the tropospheric contribution of ozone could impact the retrieval due to its significantly different temperature than the stratospheric component?
In section 3.2, two methods are discussed for the TOC retrieval. As briefly mentioned in the conclusion, one could also attempt a third method which would consist in using a reference top of the atmospheric reference solar spectrum, and retrieve the TOC from calibrated spectral measurements, as in Egli et al., 2022, amt-15-1917-2022. The advantage of this method would be that the reference spectrum obviously does not contain any residual ozone, and the method does not rely on zero-airmass extrapolations which require exceptionally stable conditions to produce reliable results, usually only found at high altitude, low latitude stations.
In section 4.4 on the AOD impact on the ozone retrieval in the VIS, AMF is used as a possible influencing factor. I am not sure if that argument is valid, since the AOD is predominantly in the low troposphere, where there is no ozone, so any path enhancement due to aerosol scattering would only have an effect due to the tropospheric ozone in that layer. Did the authors consider that?