| 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.
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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.