Review of the revised manuscript by Martinsson et al.
I thank the authors for their response to the reviewers and for the revised version of the manuscript. The revised manuscript contains some useful additions and clarifications, however I find that several of my main concerns have not been satisfactorily addressed. Some revisions are useful, but they remain rather limited relative to the issues raised in the first review. In its present form, the manuscript still does not provide a sufficiently critical and well-contextualized synthesis for ACP. I therefore cannot recommend publication in the present form and suggest that further substantial revision is needed.
Major remaining comments
1. Structure and logical organization
My previous comment on the structure of the paper has not really been addressed. The authors state in their response that the structure is logical to them, but the revised manuscript still has the same fundamental problem: Sect. 4 continues to mix presentation of additional results, interpretation, and discussion. In particular, Sects. 4.1–4.3 still contain substantial new analysis and quantitative results, while the more critical comparison with other datasets remains concentrated in Sect. 4.4.
I still think that the manuscript would benefit from a clearer separation between results, methodological interpretation, and broader discussion. If the authors prefer to keep the current structure, they should at least make the logic of the section organization more explicit and avoid presenting major new quantitative results in sections labelled as discussion.
2. Spatio-seasonal coverage of nighttime CALIOP data
The authors have now clarified that the missing nighttime CALIOP data at high latitudes were linearly extrapolated to 80° latitude and state that the uncertainty is minor because the affected region represents a small fraction of the Earth’s surface. This clarification is useful, but it is not sufficient.
The issue is not only the contribution of the polar regions to the global surface area. High-latitude stratospheric aerosol perturbations, especially those related to wildfire smoke intrusions, can be spatially localized but optically significant. Therefore, the statement that the uncertainty is minor should be supported by at least a simple sensitivity test, for example by comparing global AOD estimates with and without the extrapolated high-latitude regions, or by showing the maximum possible contribution of the extrapolated domain during major high-latitude events. Alternatively, the figures should explicitly mark where data are extrapolated rather than observed.
As it stands, the extrapolation procedure is described but not quantitatively justified.
3. Definition and interpretation of background aerosol
The revised manuscript is clearer than the original in stating that the applied method extracts the minimum aerosol loading observed during the CALIOP period and that this operational background may include weak wildfire influence, ATAL contributions, and extratropical tropopause exchange. This partly addresses my previous comment.
However, the manuscript should be more consistent in the way it presents this quantity. It should not be described too strongly as “the” stratospheric background aerosol without reminding the reader that it is an operational minimum derived from the CALIOP period. The CALIOP era was not free from perturbations, and the authors themselves acknowledge that weak wildfire effects and ATAL contributions are included. This is acceptable, but the distinction between an operational minimum, a climatological background, and a physically unperturbed background should remain clear throughout the manuscript, including the abstract and conclusions.
I also suggest avoiding the term “minimum concentration” where the method is based on backscattering. Unless an actual aerosol concentration is derived, “minimum backscattering” or “minimum aerosol loading” would be more appropriate.
4. Interpretation of the seasonal background signal
The revised discussion of the seasonal LMS variability is improved. The authors now mention the role of Brewer–Dobson circulation seasonality, weakening of the subtropical jet, tropospheric influence on the LMS, ATAL, small wildfires, and seasonal changes in aerosol composition. This is a useful addition.
Nevertheless, the manuscript still does not fully disentangle layer-volume effects from real aerosol variability. Since the seasonal signal in the extratropical LMS is said to coincide with the seasonal variation of LMS volume, the authors should be cautious in interpreting Fig. 4 as evidence of aerosol source or transport variability. I recommend explicitly stating that part of the seasonal cycle may reflect the changing geometry and mass of the considered layers, not only changes in aerosol abundance or sources.
5. Radiative forcing estimates
This remains one of my main concerns. The authors now state that the Schmidt et al. relationship is based on volcanic sulfate aerosol and is not designed for absorbing wildfire aerosol. However, this is only a minimal caveat and does not resolve the issue.
The manuscript still reports RF values as quantitative results, including in the abstract and conclusions. The approach still does not account for the vertical distribution of extinction, the latitude of the perturbation, aerosol absorption, or the different radiative behavior of wildfire smoke compared with volcanic sulfate aerosol. The statement that volcanic sulfate aerosol is the dominant event type during the period is not sufficient, because some of the most important perturbations discussed in the paper include wildfire smoke, especially the 2019/2020 ANYSO event.
I maintain my previous recommendation: either remove the RF estimates or clearly present them as purely illustrative order-of-magnitude values, with a much more explicit uncertainty discussion. The abstract should not present these numbers as a robust estimate of “climate impact” unless the limitations are clearly stated.
6. Balance of the CALIOP versus solar occultation discussion
The comparison with solar occultation and GloSSAC remains interesting, but the tone is still somewhat one-sided. The manuscript argues that lidar measurements should be given a more prominent role in stratospheric aerosol climatologies, but this conclusion should be balanced by a clearer discussion of CALIOP-specific uncertainties: lidar ratio assumptions, multiple scattering, cloud contamination near the tropopause, sampling limitations, and the fact that the background lidar ratio cannot be estimated with the method used here.
The discussion would be stronger if the authors framed this section less as an argument in favor of CALIOP and more as a balanced assessment of the complementarity and unresolved discrepancies between nadir lidar and limb/occultation techniques.
Remaining specific comments
• l.284–289: Sect. 3.2 still relies heavily on Figs. S1–S6 in the Supplement. I understand the authors’ concern that six full-page figures would interrupt the manuscript flow, but the present section uses these figures for central parts of the scientific interpretation. The authors could consider moving one synthetic version of these figures to the main text or reducing the detailed discussion of features that are only visible in the Supplement.
• l.302–304: The clarification of “overshooting plumes” is only partly satisfactory. The phrase “overshooting plumes reaching above the main effluents” remains awkward and should be rephrased more clearly, for example as “plumes injected above the main aerosol layer”.
• Fig. 3 / l.311–315: The caption is clearer than before, but the plotted quantity remains unusual: “Global AOD contribution per degree of latitude.” Since this is not the standard way SAOD is presented, the normalization should be explained more explicitly, either in the caption or in the Methods. The figure should also clearly distinguish missing/extrapolated values from physically low values.
• l.320–323: The sentence on the Australian wildfires remains confusing: “rapidly (half-life 10 days) lost 90% of the aerosol…” A half-life of 10 days does not mean that 90% is lost in 10 days. Please rephrase and clarify whether the 90% loss refers to a specific period, altitude range, or aerosol component.
• l.330–331: “The LMS, the last stop passage…” is still awkward and colloquial. Please rephrase more formally.
• l.293–296 and l.538–540: The discussion of the Hunga aerosol remains underdeveloped. The manuscript repeatedly links the event to “intense volcanism–sea interaction,” but it does not explain how this inferred composition affects either the lidar-ratio behaviour shown in Fig. 2l or the resulting AOD interpretation. If the authors are not prepared to discuss this link, they should avoid implying one.
• l.460–467: The statement that the transport pathway is supported by Fig. 6 should be softened. Fig. 6 may be consistent with the expected Brewer–Dobson transport pathways, but it does not by itself demonstrate them. The same applies to the claim that CALIOP can constrain modelling efforts “by reconstructing the CALIOP observations in Fig. 6”; this should be made more specific. What model quantities would be constrained: source altitude, vertical aerosol distribution, transport pathways, or aerosol formation rates?
• l.481–485: The explanation of enhanced lower-stratospheric aerosol load by compression during downwelling remains too categorical. Tropospheric sources and exchange processes are now mentioned elsewhere, but the statement here should be phrased more cautiously and acknowledge that these processes may also contribute to the LMS aerosol load.
• l.503–529: The practical significance of the lidar-ratio correction remains unclear. The manuscript states that AOD corrections were applied when the effective lidar ratio deviated by more than 5%, but then concludes that the overall change in AOD due to the correction was minor. This should be explicitly discussed in relation to the stated motivation of making CALIOP extinction quantitative.
• l.553–569: The RF estimate still needs stronger caveating. The manuscript now states that the Schmidt et al. relationship is not designed for absorbing wildfire aerosol, but the RF values are still presented as quantitative results. These estimates should either be clearly described as illustrative/order-of-magnitude values or accompanied by a more complete uncertainty discussion.
• l.661–665: The discussion of Brewer–Dobson transport in the Conclusions should be phrased carefully. Some of this material reflects established stratospheric transport knowledge rather than a conclusion newly demonstrated by this study. The authors should distinguish more clearly between results obtained here and background interpretation. |
Review of "A global view of the stratospheric background, volcanic and wildfire aerosol in the CALIOP era (2006 – 2023)" by Martinsson et al.
This paper deals with the stratospheric aerosol loading, both the background and perturbations from the volcanoes and wildfire events observed during the mission lifetime of spaceborne lidar CALIOP on-board the CALIPSO satellite. It attempts to delineate the background aerosol in nine different parts of the stratosphere on the assumption that the stratospheric background levels can exist at different levels and time. After subtracting this background, the authors discuss the strong perturbations from several volcanic events and wildfire events, for which they derive the lidar ratio and obtain the AOD. They highlight the importance of space lidars like CALIOP to characterize the stratospheric aerosol burden, in relation to the solar occultation and limb scatter instruments. The paper is within the scope of ACP and generally well written. However, in several places the information is generally well known, in particular the overall impacts of the volcanic and wildfire events in recent years. I am a little unsure of the motivations for this work, particularly because a new level 3 CALIOP stratospheric aerosol product with several updates is currently available--perhaps the authors could better emphasize the new information from this study. In any case, the paper presents an independent assessment of CALIOP stratospheric measurements and I recommend publication of the manuscript. I have a few comments to improve the paper in no particular order.
1. The authors have used version 4.51 level 1 CALIOP backscatter measurements for their study. Why not use the latest version 5 data product, although the results presented here should not change much. Also, the version 2.0 level 3 stratospheric aerosol product as available in the CALIPSO database incorporates the schemes to address the low energy laser shots which have been impacting the measurements after 2017 during which time several of the strong volcanic and wildfire events discussed in this paper occurred. It is not clear to me if the authors addressed the low energy shots in some way. I wonder how their background component compares with the official product.
2. line 34--can the seasonally present and anthropogenically sourced ATAL be considered as background aerosol?
3. line 41--suggest adding more recent references from the works of Fromm and co-authors.
4. line 66--suggest adding SAGE II/III references. Also please spell out the acronym GloSSAC and OSIRIS in line 72.
5. line 101-- Add "respectively" after " ...300m"
6. Line 104.--Add MERRA 2 reference
7. The methods section (2.1) needs to be expanded a little bit. For instance, in line 109 the authors mention the depolarization of the signal, without first mentioning the measurements in the perpendicular channel. Please add a few sentences on the perpendicular channel at 532 nm and 1064 channel in CALIOP measurements. What was the threshold used for cloud depolarization? Since the authors do not attempt any validation, it is not clear as to how much clouds might impact their lowermost stratosphere results.
8. Line 158 and later--what is meant by "beside"--is it the top of the layer?
9. Figure 5--May be point out that the vertical scales for the different regions are different--it's a little difficult to read all 9 panels in this plot.
10. Line 380 "the order 100 days"--please rephrase
11. I think the authors should discuss the fidelity of their lidar ratio estimates presented, which impact the AOD calculations crucially. I didn't find any comparison with estimates by other authors e.g. Prata et al. (2017) gave 69 sr for PuyeHue Cordon Caulle, much more than ~55 sr shown in Figure 7a. The lidar ratios for the Australian fires of January 2020 were ~ 100 sr as retrieved by a Raman lidar in Punta Arenas (Ohneiser et al., 2020, https://doi.org/10.5194/acp-20-8003-2020), again much higher than estimated here (need to define Fi20 in Figure 2). Any clues as to why the Raikoke lidar ratio is distinctly lower (~45 sr) than others? In particular the latter might be relevant to the recent debate about presence of smoke in Raikoke plumes (Ohneiser et al., 2021, https://doi.org/10.5194/acp-21-15783-2021 etc.). Are some of these differences coming from the multiple scattering factor? Figure 7a shows the Effective Lidar Ratios whereas Figure 7d shows Lidar Ratios--may be I am missing something here.
12. Line 479: "unverified assumption of a lidar ratio of 50 sr"-- in a recent paper in JGR, Deshler and Kalnajs, 2026, https://doi.org/10.1029/2025JD045262, from decades long OPC measurements provide a single value for the stratospheric aerosol lidar ratio of 49.9 sr at 532 nm.
13. Add unit of extinction coefficient to the color bars in Figure S4-S6.