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 (UV, Huggins bands) and visible (VIS, Chappuis bands) spectral regions using direct sun Differential Optical Absorption Spectroscopy (DS-DOAS). We use the research-grade UV-VIS DOAS system Delta, 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 spectrum using the Langley plot and the Minimum-Amount Langley-Extrapolation 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 of −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 TOC 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 or at high-latitude locations, thereby extending the continuity of ozone monitoring from ground-based systems and providing a complementary approach to traditional UV-based TOC retrievals.