the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Mar 2026
TCOM-CFC11 and TCOM-CFC12: A Gap-Free, Observationally Constrained Global Dataset of Stratospheric CFC-11 and CFC-12 Profiles (v2.0)
Abstract. Understanding the long-term trends of ozone-depleting substances (ODSs), particularly CFC-11 (CFCl3) and CFC-12 (CF2Cl2), is essential for evaluating the effectiveness of the Montreal Protocol. However, reliably estimating these trends is complicated by the inherent sparse spatial and temporal coverage of high-quality stratospheric observations, such as those from the Atmospheric Chemistry Experiment–Fourier Transform Spectrometer (ACE-FTS). To address this limitation, we have developed an innovative machine learning methodology to combine the strengths of sparse ACE-FTS observations with the continuous output of the TOMCAT global Chemical Transport Model (CTM).
We use XGBoost regression to constrain the TOMCAT tracers against co-located ACE-FTS measurements, thereby creating the TCOM (TOMCAT CTM and occultation-measurement-based) stratospheric profile datasets for CFC-11 and CFC-12. The resulting TCOM datasets described here (version 2.0) provide continuous, gap-free, global daily vertical profiles from 2000 to 2024. A comprehensive evaluation confirms the method’s effectiveness, showing the corrected TCOM data clustering significantly closer to the observations than the CTM and successfully removing a systematic low bias present in TOMCAT-simulated CFC concentrations. Furthermore, interpretable machine learning analysis reveals that the XGBoost model primarily functions as a "transport corrector", with dynamical features (like Age-of-Air, temperature, long-lived-tracers) being highly influential. This suggests that the dominant source of bias in the baseline TOMCAT simulation relates to its simulation of stratospheric circulation. These TCOM datasets are publicly available at https://doi.org/10.5281/zenodo.18145730 (Dhomse, 2026a) and https://doi.org/10.5281/zenodo.18147392 (Dhomse, 2026b), and will provide a valuable, observationally-constrained benchmark for refining chemical models and reducing uncertainties in ODS trend analyses.
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Status: final response (author comments only)
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RC1: 'Comment on essd-2026-3', Chris Boone, 13 Apr 2026
- AC1: 'Reply on RC1', Sandip Dhomse, 07 Jul 2026
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RC2: 'Comment on essd-2026-3', Anonymous Referee #2, 20 Apr 2026
The manuscript provides an overview of two newly developed CFC-11 and 12 products, which combine the TOMCAT chemistry transport model capabilities with solar occultation observations to obtain a gap-free data set over the last two decades.
The manuscript is well-written, the methodology is clearly exposed, and the results are overall logically presented. A couple of sections need some reworking to improve the description of the results, and some points need to be clarified, particularly Section 5.4.
I recommend the publication with minor comments, which are discussed below.
Minor comments
- I found the mention of Version 1 of the presented algorithm in the ACE-FTS section out of place. I would move this to the end of Sect. 3, where “Version 1” is already mentioned. In other parts of the paper version 1 is also discussed: was this version developed in Dohmse and Chipperfield (2023), or was it an internal processing? Can you give some more references/introduction to this versioning in Sect. 3?
- One point in the explanation of the XGBoost training is not clear to me: the ML is said to be trained at each latitude band and altitude independently. However, from line 210, it seems that the CFC concentration time series is derived for all TOMCAT latitude grid points at the same time. Can you clarify this point?
- Sect. 5.1: it seems to me that the three bullet points are explaining the reasons for the performance differences between tropics and mid latitudes, and not the contradictory results (small RMSE and small R2 values at the same time) found in the tropics, as stated at line 226. Can this point be better discussed, maybe in 5.1.1?
- Sect. 5.2: I found confusing the description of SHAP values for Figs. 5 and 6. For example, at line ~316 the authors say that high AoA and low CFC-11 concentrations are associated with positive SHAP values, but it seems to be the opposite case from Fig. 5: red AoA and blue CFC-11 dots correspond to negative SHAP values. Can you please clarify this point that seems contradictory?
- Sect 5.4: the description of the results in this section is not very clear. I see the difficulty in condensing several time series at different latitudes in the same plot, but it is hard to identify in Figs. 9 and 10 features that are described in the text. For example, I can clearly spot a negative TOMCAT bias only in the tropics (0° lat) at both 50 hPa and 100 hPa. At mid-latitudes, TOMCAT seems to be in good agreement with observations at 50 hPa and with TCOM, whereas TCOM introduces positive/negative corrections at 100 hPa in the N/S hemisphere, respectively. This is quite different from what is written in the text at lines 374-378. Another example at line ~391: I can hardly see any bias removed in the 32.5° N/S panels at 50 hPa, for either CFC-11 or CFC-12. Again, at line ~398, I don’t see how TCOM is successfully tracking the MIPAS data in the polar regions in Fig. 9, as the orange line is very different from the green dots. It is not always clear if the sentences are referring to CFC-11 or CFC-12. I suggest revising the whole description of these figures.
- Can you mention in the conclusion the future of this data set without ACE-FTS observations? And possibly in the introduction, if any upcoming limb mission is going to have the capabilities of measuring these molecules?
Typos and technical corrections
L56: for February →from February;
L85: add space between “the” and “SCISAT”;
~L135: please add that you show some results in Figs. S1 and S2;
L153: “...that are small”→ “...are small”;
L190: The sentence “These updates have improved…” would fit better at the end of 4.2 section;
L235: stratospheres → stratosphere;
L257: “...in XGBoost indicating that the discrepancies…” → “...in XGBoost, the discrepancies…”;
Figs. 7 and 8: why are the color bars moving upwards with the subplots?
Caption of Fig. 9 : “e.g.” → “from”;
L376: I would delete “observed”;
L421: I would remove that the datasets “will be publicly released in Dec 2025”.
Citation: https://doi.org/10.5194/essd-2026-3-RC2 - AC2: 'Reply on RC2', Sandip Dhomse, 07 Jul 2026
Data sets
TCOM-CFC11 : TOMCAT CTM and Occultation Mesurement based CFC-11 profile data set Sandip S. Dhomse https://zenodo.org/records/18145730
TCOM-CFC12 : TOMCAT CTM and Occultation measurement-based CFC12 profile data set Sandip S. Dhomse https://zenodo.org/records/18147392
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- 1
This article describes a new model dataset for CFC-11 and CFC-12 where TOMCAT model outputs have been empirically nudged to achieve agreement with ACE-FTS measurements for the two molecules, providing global, gap-free information that is consistent with observational data. Most of the observational constraints appear to be compensation for transport effects, such as the descent of air within the winter polar vortex. It looks like a good dataset and should be useful to the community. The dataset is reasonably well described, although there is an unexplained asymmetry in the Tropics for the differences from the TOMCAT model (discussed more later) that would be interesting to explore. It implies an inconsistency between the TOMCAT model and the ACE-FTS observations, but it doesn’t necessarily need to be understood to use the data (the whole point of the empirical observational constraint is to correct for differences whether or not the source of the differences are known). I would say an explanation for the asymmetry is not required in the current paper, but it might be something worth understanding in the future.
>Line 278: Beyond the primary TOMCAT tracer feature, the remaining features show subtle, altitude-dependent differences
Both molecules show similar altitude dependences on Age of air and measurement date, which seems heartening. Higher altitude (> 20 km) CFC-12 has a strong dependence on tracers (N2O and CFC-113) that isn’t reflected in the CFC-11 results. I assume that is a way to track atmospheric descent within the winter polar vortex for CFC-12, but I wondered why something similar wasn’t seen for CFC-11. Perhaps the slightly enhanced dependence on pV above 20 km is sufficient to account for atmospheric descent with the CFC-11 data?
>Line 351: At 10 km, a band of sustained positive difference is visible across the tropical and mid-latitude regions (50◦S to 50◦N) throughout the entire 2000–2024 period for both CFCs.
Looking at Figures 7 and 8, the bulk of the difference is not symmetric about the equator; it is larger in the Northern Hemisphere than in the Southern Hemisphere for both molecules. This suggests an asymmetry about the equator in the Tropics in one of the two sets that is not present in the other. It is most easily seen in the 15 km plots. I suspect it is somehow associated with an asymmetry seen in ACE comparisons to results from ground-based measurements of CFCs, HCFCs, and HFCs: Schmidt et al., “Trends in atmospheric composition between 2004-2023 using version 5 ACE-FTS data,” JQSRT, 325, 109088 (2024). For these molecules, ACE results generally agreed well with measurements from Southern Hemisphere ground-based stations, but a 2.25-year time lag was required to get good agreement with results from Northern Hemisphere ground-based stations.
So, is the asymmetry in the Tropics that drives the differences for the Northern Hemisphere Tropics found in the model or in the ACE-FTS data? That might provide a clue as to the source of the difference.
Minor issues:
>Line 52: For example, the longest time series (March 2004 to present)
>Line 86: The instrument has been operational since April 2004
There are two contradictory indications for the start time of ACE-FTS measurements. The instrument has been operational since late February 2004.
>Line 106: the ACE-FTS Data archive: https://uwaterloo.ca/atmospheric-chemistry-experiment
The link provided is for the general website, not the data archive. One could explore the website and figure out where to find the data, or you could provide a more direct link to the archive: https://databace.scisat.ca
>Figures 5 and 6: the vertical lines on the TCOM data are somewhat difficult to see.
>Supplementary figures
Figure S2 is supposed to be a plot for CFC-12 but instead appears to be a repeat of the CFC-11 plot in Figure S1 (judging by the titles and the altitude ranges, and the plots also look identical). Similarly, Figure S4 looks identical to Figure S3 (including the title).