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the Creative Commons Attribution 4.0 License.
| 03 Feb 2026
UV/Vis Stratospheric Air Mass Factors considering photochemistry at two Antarctic stations
Abstract. The molecules NO2, O3, OClO and BrO play a major role in the photochemistry of stratospheric ozone, notably in the formation of the springtime Antarctic ozone hole. For this reason, these species have been monitored by Differential Optical Absorption Spectroscopy (DOAS) instrumentation for many decades. In order to transform DOAS Slant Column Densities (SCDs) into Vertical Column Densities (VCDs), independent of the viewing geometry, the Air Mass Factors (AMFs) relating these quantities are needed. Ground-based stratospheric trace gas measurements are performed in zenith-viewing geometry at twilight, around and beyond 90° solar zenith angle (SZA). At those solar angles, the Earth’s sphericity and the rapid changes in photochemical parameters (e.g., photolysis rate coefficients) affect the calculation of the AMFs, particularly for photochemically active species such as NO2, OClO and BrO. This study presents a methodology to infer AMFs that account for sphericity and photochemical effects. We estimate stratospheric AMFs of NO2, O3, OClO and BrO for Belgrano and Marambio Antarctic stations using the MYSTIC [Mayer, 2009; Emde et al., 2010] Radiative Transfer Model (RTM). The photochemical changes taking place during twilight are considered using a photochemical box-model based on the SLIMCAT chemistry transport model [Chipperfield et al., 1999, 2006]. Vertical profile concentrations obtained by this model are averaged over the optical paths and used as an input for the MYSTIC RTM. The robustness of the proposed methodology is tested against measurements of NO2, O3, OClO and BrO SCDs obtained at Marambio and Belgrano. A good agreement is observed between modelled and measured values of NO2, O3 and OClO SCDs. For BrO bigger differences are obtained but they have been attributed to the tropospheric BrO contribution that has not been included in the model. Our results show that monthly averaged AMFs can be considered as a good approximation for O3 and BrO, but more temporally resolved sampling is recommended for NO2 and especially OClO during July. This work shows the large impact of photochemistry for both the magnitude and also the SZA dependency of the evolution of the AMFs during twilight.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Measurement Techniques.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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RC1: 'Comment on egusphere-2026-17', Anonymous Referee #1, 17 Mar 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-17/egusphere-2026-17-RC1-supplement.pdfCitation: https://doi.org/
10.5194/egusphere-2026-17-RC1 -
RC2: 'Comment on egusphere-2026-17', Anonymous Referee #2, 20 Mar 2026
The paper of Gomez-Martin et al. adresses the calculation of Air Mass Factors (AMFs) for ground-based zenith DOAS observations at twilight for two Antarctic stations. The subject is complicated in particular due to the need to account for the earth sphericity and photochemical variations of several of the measured species. The authors use the models SLIMCAT (CTM) to derive effective concentration profiles along the light path, and MYSTIC (RTM) to calculate corresponding AMFs. They then compare these calculations with DOAS measurements of NO2, O3, BrO, and OClO, finding a good agreement except for BrO, which is explained by tropospheric contributions not considered in the model.
This study is important for the community, the method is new and robust and the results consistent so I have only minor comments on the content. However some work could be useful to improve the presentation before publication.
In several sections (2.2 and especially 4.1) the text should be split in paragraphs to make it digestible. For instance, when they introduce a new figure, it is normal to start a new paragraph. Same for a new line of reasoning.
Carefully using a LLM could improve the text readability as several sentences are hard to read, with for instance the subject too far away from the verb. Exemples of complicated sentence which could be improved in my opinion
L. 248 'To check the validity of these approximations, SCDs of the four target species obtained using AURA in 2-D mode and
considering the photochemical model concentration profiles (different for each SZA), have been compared with the SCDs
obtained with AURA in 1-D mode, using a single averaged concentration profile (eq. 12)'L. 165 Thus, the concentration of a given gas in a given atmospheric layer encountered
by the sunlight reaching an instrument at the surface, for SZA = ( 𝑐, ), is actually an average of the concentrations
corresponding to all values of the SZAs encountered by all the beams passing through this layer along their optical paths (see
Fig. 3).Regarding the figures: the texts are often too small, especially in the legend. It s partly due to the fact that they are loaded. Some of the figures could be split in two or more, e.g. figure 7 (we dont see the value for ozone AMF relative difference due to the scale defined by OCLO). In Figure 8, some material could go the supplement. Do we need all the months in the main text?
Figure 5: There is nothing to see for 80° because the average profile perfectly matches the model. So showing 80° is not that useful. It would be interesting on the other hand to plot the ratio of the maximum average concentration with respect to the model, to see at which SZA the averaging starts to matter.
AMF calculation: it would be useful for readability to have a table for the inputs and algorithms used in p. 15.
L 428 'Figure 10' seems to relate to figure 11. I missed where the true figure 10 was presented in the text.
Specific comments:
L.54 Since 2012, the ground-based DOAS community working on stratospheric research [e.g. Adams et al., 2012]. If it s the DOAS community, there should be one more than one paper to refer to?
L 77 reads contradictory with L 81. Can you please explain or reformulate?
L. 77 'To our knowledge, the only publication showing OClO AMFs for SZA up to 95º is Pinardi et al. [2023].'... ''
L. 81 'Finally, in Kühl et al. [2004], OClO AMFs are estimated with the AMFTRAN radiative transfer model [Marquard et al., 2000] for different Gaussian vertical profiles and for SZAs between 40 and 95.'
Figure 1: SLIMCAT should be in the figure?
L 130: I think a_l is the box-AMF? but it s not explained in the text, could you add that in the text?Equation (2) to (6) Do I understand correctly that this calculation is for one SZA at the station? If so, it would be pedagogical to explicitly state that VCDtotal_ph depends on this SZA.
L.151-152: 'In addition to gas phase chemistry ...' -> I assume you do not use this PSC module? If not, please indicate or remove this sentence, if yes, please give more details
L.162: 'The concentration ... representative for ... 2.8° -> In Chipperfield (2006), I read that the model grid is variable. So it s the model runs that you have that were calculated at 2.8°, not the model itself. If this it true, can you please reformulate?
l.172 takes chemistry into account -> photochemistry would be clearer
L.177: 'single scattering is already a good approximation' -> can you be quantitative on that?
Figure 4 y label should be 'scattering altitude' for clarity
L.303 'daily or even more resolved sampling is recommended for OClO'-> what would it mean, more resolved than daily? Two AMFs for the two twilights a day?
L.460 'source of error for all modelled SCDs could be the estimation of the reference ' -> measured instead of modelled?
Citation: https://doi.org/10.5194/egusphere-2026-17-RC2
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