| 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.
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).