| 01 Jun 2026
Estimates of carbonyl sulfide and methane stratospheric lifetimes based on AirCore profiles
Abstract. Stratospheric loss is a major sink for both carbonyl sulfide (COS) and methane (CH4), but their stratospheric lifetimes and sinks remain poorly constrained because high-resolution observations of their vertical distributions in the lower stratosphere are sparse. Here, we estimate the mean stratospheric lifetime and sink of COS and CH4 from their correlations with N2O using two distinct methods applied to AirCore vertical profile measurements from three Northern Hemisphere summer campaigns. From these profiles, we derive a COS stratospheric lifetime of 69–90 years, corresponding to a sink of 30–41 GgS yr-1. For CH4, we find a stratospheric lifetime of 149–168 years, corresponding to a sink of 23–26 TgC yr-1. These values are in good agreement with previous estimates (39–76 years for COS, 152–160 years for CH4) and with estimates based on ACE-FTS observations (75–76 years for COS, 146–172 years for CH4). As has been noted previously, we also find a decline in the COS tropospheric burden between 2016 and 2020 in our AirCore samples, in contrast to the continued growth of CH4 and N2O. In addition, we found that tracer-tracer correlations vary among flights, and even within the same campaign, pointing to variability in lower-stratospheric composition. Although this variability may reflect differences in stratospheric transport, its origin remains unclear and requires further investigation. These results provide observational constraints on the stratospheric budgets of COS and CH4 and help refine their representation in atmospheric chemistry and transport models.
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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The study by Alessandro Zanchetta and coauthors deduces stratospheric lifetimes from vertical profiles of carbonyl sulfide and methane from AirCore measurements and ACE-FTS satellite observations. The analysis is sound and rather comprehensive, comparing slightly variant methodologies of estimating lifetimes and pointing out caveats with respect to, e.g., regional sampling and observation altitude. The manuscript is generally well presented and well written and fits into the scope of ACP.
One major aspect that, in my opinion, should be improved is the length of the paper and the amount of information presented. Specifically, I wonder if all 13 figures and 11 tables are needed. In that context, please consider the following:
Besides the number of figures and tables, there is quite a bit of redundancy in the test as well. Essentially the same numbers for COS and CH4 lifetime and stratospheric sink estimates from the two different methods (Plumb and Ko, 1992 vs. Volk et al., 1997) and for polar vs. mid latitudes are given in a similar way in Section 3 (lines 297 – 310 and 339 - 347), Section 4 (lines 380 - 382 + Section 4.1 and Section 4.2) and Section 5 (lines 506 - 516). While this information does fit into the flow in each individual Section, it may be worth avoiding some of this redundancy by cross referencing.
Note that these suggestions are not meant to push the paper below a certain page or word limit because shorter is always better. But I had the impression that the take-home messages of the study are somewhat hidden underneath the wealth on information shown.
Besides the general recommendations to make the paper more concise, I have a few specific suggestions and technical corrections:
Introduction:
The introduction almost exclusively focuses on COS, while in the title and the rest of the paper, CH4 is treated in parallel. I suggest adding one or two paragraphs on CH4 in the introduction as well.
Lines 39/40: an explicit reference for the COS tropospheric lifetime would be appropriate. And for COS being able to reach the stratosphere, a more original reference such as Crutzen (1976) would be more appropriate than the ones cited.
Lines 43 – 45: Again, the chosen references appear a bit arbitrary. For example, Vernier et al. (2011) don’t explicitly discuss the COS contribution but rather the effects of cumulative sulfate injection from volcanic eruptions. I suggest the Kremser et al. (2016) review paper, where the tropospheric budget and stratospheric sink of COS is discussed in detail and relevant literature is cited.
Methods:
Lines 100 – 105: It would be nice to show a comparison between the actual AirCore and ACE-FTS profiles. It’s not essential to actually show this in the paper, so such a figure could go into the supplemental material.
Line 114: I tend to agree that this is probably a fair assumption, but maybe you can add one sentence to explain why it is justified for the gases considered.
Line 132 – 134: I find this explanation a bit confusing. Did you use the Prather et al. estimate of 116 years for the full period, or did you apply a 2.1 % per year correction, even though you state that the trend is not significant? Please clarify!
Lines 163 – 170: In my opinion, limiting the analysis to reasonably low N2O mole fractions so that a tropospheric influence is minimized is important. This is a fundamental difference to the study by Karu et al., who analyzed only data with N2O > 317 ppb due to the altitude limitation. This may explain why that study compares least well to your analysis and other COS lifetime estimates in the literature, and it is worth highlighting that here or in one of the later sections.
Results:
Lines 287 – 288: Either here or in Section 2.3, please add the actual number of ACE-FTS profiles that were averaged.
Line 329: For CH4, why are the uncertainties for satellite data as large as for campaigns, different than for COS?
Discussion:
Lines 393 – 397: I think that the statement that Karu et al. (2023) “introduced” another point of uncertainty is misleading here. They did not introduce this but, due to the very limited altitude range of commercial aircraft, had to use data with a very limited altitude (and consequentially N2O, see my comment above) range. In my opinion, this warrants more discussion and explains why their lifetime estimate differs from your and other studies.
Lines 404 – 405 and Table 10: I suggest including stratospheric sink estimates from the important modeling studies by Brühl et al., (2012) of 35 Gg S/yr and Sheng et al. (2015) of 40.7 Gg S/yr. These papers do not explicitly state stratospheric lifetimes, but that also goes for some of the cited papers (e.g. Weisenstein, 1997).
Lines 439 – 440: Please discuss this in more detail (cf. my previous comments on the comparison with the Karu et al. estimate)!
Technical corrections:
Line 206: it should be “advocate” instead of “advocates”
Line 233: Please replace the term “evident criticalities” (e.g. by “atypical behavior” of “distinctive behavior”)!
Line 252: This reference is meant to be Figures 5 – 6 and 8 – 9, correct?
Line 290: Figures and tables should be referenced in order of appearance. So please refer to Table 5 prior to Table 6 or swap the numbering of these tables!
Lines 326 – 327: There seems to be a grammar issue. I suggest: “The application of the Plumb and Ko (1992) method leads to an average slope at mid latitudes of 0.2537 ± 0.0022, while at polar latitudes it is 0.2381 ± 0.0063.”
Line 492: It should be “applied in this study”.