the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Aug 2025
Atmospheric lifetime of sulfur hexafluoride (SF6) and five other trace gases in the BASCOE model driven by three reanalyses
Abstract. In this work, sulfur hexafluoride (SF6), which is often used as a tracer for stratospheric transport due to its inertness in the stratosphere and nearly linear growth rate in the troposphere, is included in the chemistry transport model (CTM) of the Belgian Assimilation System for Chemical ObsErvations (BASCOE). Sink and recovery reactions for this species are implemented in the model, which has a top in the mesosphere at 0.01 hPa. The simulated SF6 distributions are compared with MIPAS and ACE-FTS observations and the global atmospheric lifetime is computed from CTM runs driven by three recent meteorological reanalyses: ERA5, MERRA2 and JRA-3Q. The results show that BASCOE SF6 profiles are generally within 10 % of the satellite observations below 10 hPa, although discrepancies increase at higher altitudes. The global atmospheric lifetime is used as an additional diagnostic for the implementation of the chemistry in the mesosphere, where satellite measurements are not available. The derived SF6 lifetimes are 2646 years with ERA5, 1909 years with MERRA2 and 2147 years with JRA-3Q, in accordance with recent literature. Due to the large spread of published lifetimes for SF6, the study is extended to N2O, CH4, CFC-11, CFC-12 and HCFC-22, to validate the SF6 results. The lifetimes for these species are in agreement with previously reported values, and their spread between simulations is smaller compared to SF6. This analysis highlights the sensitivity of SF6 to the input reanalysis data sets and thus to differences in dynamics.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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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Status: open (until 17 Oct 2025)
- RC1: 'Comment on egusphere-2025-3597', Anonymous Referee #1, 10 Sep 2025 reply
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RC2: 'Comment on egusphere-2025-3597', Anonymous Referee #2, 01 Oct 2025
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This manuscript presents the implementation of SF6, an important species to indirectly help constrain stratospheric circulation speeds, into the BASCOE model. An important result is the dependency of the model-based lifetime estimate of any trace gas on the meteorological reanalysis used. A lot of work, both technical and scientific, appears to have gone into the development of the model and its setups. This is very commendable. At the same time, the manuscript is somewhat light on the discussion of the results and their placing in a wider context, particular when it comes to the interpretation of differences with observations and other studies. Section 5 lacks both depth and detail in its current state, particularly also with regard to the actual uncertainties of the lifetimes derived. Once these issues, as well as the more detailed ones listed below, have been addressed, the manuscript could be published in ACP.
l16-17 Consider revising this statement to make it less absolute. Not all studies of middle atmospheric transport have been "motivated by the existence of the stratospheric ozone layer".
l17-19 Transport in the mesosphere (which is part of the middle atmosphere) is not dominated by the stratospheric BDC, nor is isentropic transport in the LMS. Consider revising this statement accordingly.
l24-27 It seems a bit of a stretch to use publications that are 16-24 years old for a claim on what climate models predict.
l28-29 Recommend modifying this to "Changes in AoA at a given location in the stratosphere
thus reflect changes in circulation speed" or similar.l30-31 Two-way mixing is not correctly represented here (see, e.g., Garny et al., 2014)
l31 Three incomplete references.
l31-32 This is a bit confusing. Why are four references required for a definition of AoA?
l34-35 SF6 has its weaknesses, too. A more balanced discussion is recommendable here.
l38 Climate studies?
l39-40 It would be useful to have a bit more detail on the nature and scale of these discrepancies.
l42-44 Please add references for the two satellite instruments and the three reanalyses.
l70-85 The definition of IMK/IAA is missing. Also, it is not clear (here, in the table, and throughout the manuscript) what is meant by bias. Is this the actual accuracy of the instruments, and if yes, against which "ground truth"? Or would perhaps "difference" be the more appropriate term?
l82 Looking this up in Saunders et al. 2025, ACE SF6 in V5.2 is problematic. Has this issue been resolved in V5.3?
l105 According to Ploeger et al., ACP, 2021, ERA-5 also has a representation of the BDC that is likely too slow.
l115 This sentence appears to suggest that Burkholder et al., 2015 is the most recent version, which is not the case.
l128 Why were the rate constants taken from a modelling paper and not from kinetic studies (such as Burkholder et al. 2015)?
l134 Please explain why electrons from the ionosphere are relevant for mesospheric chemistry.
Table 2 Consider adding "in the mesosphere" to the caption.
Figure 1 Longitude should be E or W, not N? Also, according to the literature (e.g., Ravishankara et al., 1993 or Morris et al., 1995), the electron energy plays an important role for the relative importance of the individual SF6 reactions. How was this taken into account here?
l157-158 This statement does not seem to be correct for the ERA-5 vertical resolution.
l175 "Spatial grid" instead of "space"?
l199-202 A similar constraint applies to the oceanic lifetime of N2O. Considering this may affect some of the later results and discussion.
l238-240 This is a repetition from section 4.
l245 Figure 9 represents an apples-and-oranges comparison as some of the lifetimes in there are equilibrium lifetimes and some are not. Given the susbtantial growth rates of the mole fractions of some of the species considered here (especially SF6), the equilibrium lifetime will be significantly different from the instantaneous one (see, e.g., SPARC, 2013). Not considering this is a major shortcoming of this work.
l246-248 This is a very short summary of the state of the art of SF6 lifetime estimates. It does not take into account (or discuss) the wide range of approaches (satellite and in situ observations as well as model-based approaches) and their strengths and weaknesses, nor does it consider the advances that have been made since 1993.
Figure 7 Why is the scale on the y-axis so large? It results in most of the figure showing empty space.
l268 The more recent Prather et al., ACP, 2023 might be a better reference here.
l275 Consider rewording to clarify whether the mesospheric SAO is dependent on gravity or gravity waves.
Table 4 The uncertainties listed here are, as far as I understand, purely based on the variability of the respective lifetimes over roughly a decade. This should be made clear in the caption. Given that the manuscript already contains a comparison with observational data, the derivation of actual lifetime uncertainties (e.g., based on the observed differences) would be highly desirable. As this would undoubtedly create a lot of work, an alternative way forward might be the honest discussion of the probable scale of such uncertainties, so as not to mislead the readership.
l285-287 What uncertainty in the lifetime would a +-10% difference in the volume mixing ratio translate into? Also, given the differences between the two satellite data sets, it is not clear whether the word "bias" is appropriate here (as it assumes that at least one of the observational data sets is correct, whereas in reality it could be neither).
l290-293 This is not a balanced assessment of the state of the literature. Several of the lifetime estimates shown in Figure 9 do not agree with the results of this study. This should be mentioned and the potential reasons discussed, including the proper uncertainties of the lifetimes derived here.
l292-293 All other trace gases for which lifetimes were derived here, are much shorter-lived than SF6 and have their dominant sink regions far below the mesosphere. Their ability to confirm the validity of the SF6 lifetime derivation is therefore severely limited, and this should be made clear.
Citation: https://doi.org/10.5194/egusphere-2025-3597-RC2
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This paper describes the results of an update to the BASCOE model to include detailed SF6 chemistry in the mesosphere. The lifetime of SF6 as well as of five other trace gases are calculated from the model output and they all generally agree well with previous lifetime estimates. This suggests that the SF6 chemistry implemented in BASCOE is at least sufficient to represent the main loss processes of SF6 and that it is then primarily the model transport that will determine the SF6 lifetime. The SF6 lifetime is shown to vary considerably with different reanalysis data sets driving the model but the lifetimes still agree within uncertainties. The accurate representation of SF6 chemistry in BASCOE is an important advancement in assessing model transport compared to observations.
The paper is well written and the results are described clearly. The topic is appropriate for ACP and so I suggest publication with consideration of the minor comments listed below.
Minor comments:
Line 24: I would suggest replacing ‘General’ with ‘Chemistry-‘.
Table 1: The entries in this table are somewhat confusing and repetitive. It doesn’t seem like you need the fifth column with the Data Versions since they’re all the same. Maybe just state the versions in the table header. Also, the ‘Agreement’ and ‘Notes and References’ columns sometimes overlap in their content and frequently include the subjective term ‘Good’ that isn’t necessarily helpful. The ‘Compared instruments’ and Data versions’ columns seem to have conflicting versions. For instance, for SF6 the ACE V2.2 and V5.3 are listed. I would suggest trimming this table down to the basic information and make sure it isn’t repetitive or conflicting.
Line 131 ‘parameterized’ is misspelled
Fig. 7: The y-axis scales here seem much too large, it’s difficult to see any features. Maybe that’s the point but it seems like you could at least go to +/-30%. It also might be helpful to indicate the instrument differences for each species by dashed lines for instance.