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the Creative Commons Attribution 4.0 License.
| 26 Feb 2026
A global view of the stratospheric background, volcanic and wildfire aerosol in the CALIOP era (2006–2023)
Abstract. This study deals with the stratospheric aerosol during the 17 years of lidar measurements with CALIOP. To obtain extinction from the backscattering measurements, we estimated the lidar ratios of the main aerosol injections into the stratosphere. The stratospheric background is estimated by making a subdivision of the stratosphere into nine parts, spanned by three latitude and altitude intervals, reaching background conditions individually at different times. The extracted background shows excellent agreement with solar occultation measurements in the volcanically quiescent period 1998–2000. Our results show that 70 % of the background aerosol in the deep Brewer-Dobson (dBD) branch is formed above 19 km altitude, indicating strong influence of carbonyl sulfide on the stratospheric background aerosol. The stratosphere was clearly affected by 15 volcanic eruptions and 5 wildfires. Their combined aerosol load affected Southern extratropics, tropics and Northern extratropics almost equally, and the altitude distribution shows that the shallow Brewer-Dobson branch was most affected (43 %) followed by the dBD (31 %) and lowermost stratosphere (26 %). The most important events in order of maximum AOD were the Hunga Ha’apai eruption (2022), Australian wildfires (2019-20) and the eruptions of Raikoke (2019), Sarychev (2009) and Nabro (2011). These events induced strong variability in the stratospheric aerosol optical depth (AOD), causing highly variable climate impact in the period studied with yearly average global effective radiative forcing ranging from -0.14 W/m2 at background conditions to -0.4 W/m2. CALIOP provided important data for stratospheric aerosol climatologies during its 17 years of operation.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-1059', Anonymous Referee #1, 15 Mar 2026
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CC1: 'Comment on egusphere-2026-1059', Albert Ansmann, 25 Mar 2026
I studied the manuscript with large interest. To my opinion, it provides a very good overview of all the perturbations of the stratospheric aerosol layer around the globe during the last almost 20 years. And the paper comes on the market to the right time, now, in this era with increasing wildfire activities, and now, after completion of the CALIPSO mission.
However, during reading many questions came up, especially concerning the obtained column-integrated lidar ratio. Reviewer #1 mentioned already that more information and more comparison with the literature is needed here. So, I came to the conclusion, I should contribute to the discussion by writing my personal list of comments and recommendations.
Detailed list of comments:
Line 32: We observe stratospheric aerosol with a powerful lidar at Leipzig since 1995 and other groups (e.g., Sakai et al., JGR, 2025, https://doi.org/10.1029/2024JD041329, and Trickl et al., 2013, https://doi.org/10.5194/acp-13-5205-201323, Trickl et al,, 2024, https://doi.org/10.5194/acp-24-1997-2024) also do that. The stratosphere contains always some kind of background from the tropopause upward. But mention an aerosol layer above 20 km. Is the aerosol below 20 km then the result of downward mixing and sedimentation of aerosols from above 20 km? Should be clarified in the text.
Line 41: Here, one could provide more wildfire-related references. We in Leipzig have strong focus on wildfire smoke since the Canadian Fire Event in August 2017: Baars et al., ACP, 2019, Haarig et al., ACP 2018, Ohneiser et al., ACP, 2020, 2021, 2022). There are further papers of Khaykin et al. (GRL, 2018, and Comm. Earth & Environ., 2020, https://doi.org/10.1038/s43247-020-00022-5) that could be cited.
Line 54: Regarding radiation aspects there are many papers (e.g., Hirsch and Kohren, Science, 371, 1269–1274, 2021; Heinold et al, ACP, 2022) that contributed to the field.
Lines 89-93: Raikoke was stronger than Sarychev! SO2 emissions were as high as 1.5 Mt SO2 (Raikoke) and 1.2 Mt SO2 (Sarychev). The literature shows maximum AOD values of 0.02 in the case of Sarychev and 0.025 (at 500-550nm) in the case of Raikoke.
Lines 126-139: Our comparison with CALIOP observations (Ansmann et al., ACP 2023, MOSAiC paper) indicated a bias of 532nm AOD of 0.03 (CALIOP AOD values were too low by 0.03). This is also mentioned in one of the CALIOP papers cited in our paper. CALIOP does not detected very weak backscatter …adding up to an AOD bias of 0.03.
Lines 155-160: I checked the Martinsson et al. 2022 paper (already in 2022). I could imagine that multiple scattering may influence the retrieval. Furthermore, the reference values ‘behind’ the detected layer (close to the tropopause) are set to values varying around zero. This may too low! There is always some aerosol so that the backscatter is not zero, but larger. Then, the lidar ratio is the column-integrated lidar ratio. That means it is the backscatter-weighted mean lidar ratio of a given layer and not simply the layer mean value. The largest backscatter controls the lidar ratio result. Please have look into: Ansmann, Ground-truth aerosol lidar observations: can the Klett solutions obtained from ground and space be equal for the same aerosol case?, Appl. Opt, 2006, https://doi.org/10.1364/AO.45.003367 .
Line 172 and everywhere in the text: the lidar ratio should be given in sr, not in Sr (check wiki steradian).
Lines 209-232, and Figure 2: These values in the figure should be compared with available literature values. For smoke, there are several papers: Haarig et al, ACP, 2018, Ohneiser et al. 2020, 2022, Hu et al., ACP, 2019, https://doi.org/10.5194/acp-19-1173-2019. Haarig and Ohneiser observed much higher lidar ratios by using the classical Raman lidar method to obtain the lidar ratios. Hu et al. used a method similar to the Klett method to derive the stratospheric lidar ratios if I remember correctly. Their values appear to be too low.
For volcanic sulfate aerosol, one may check the computations of Jaeger and Deshler, GRL, 2003, doi:10.1029/2003GL017189. The lidar ratio should be low, below 30sr at 532nm, during the first month of the eruption. Our recent observations during the first half year of 2025 , about one year after the Ruang volcanic eruptions, yielded 50-60sr at 532 nm.
The Raikoko observations with CALIOP are not easy to interpret. We found that Siberian smoke as well as Raikoke aerosol polluted the stratosphere at least at latitudes greater than 65°N (Ansmann et al, JGR, 2024, comment paper, Ohneiser et al., ACP, 2021).
Herrore-Anta et al., ACP 2026 and Zhong et al , Nature comm., 2026, https://doi.org/10.1038/s41467-026-69728-y also conclude that smoke cannot be ignored. Kloss et al., ACP, 2021, did not know about the Siberian smoke and thus did not consider that in their analysis.
Lines 269-270: I disagree that the perturbations rapidly faded! Baars et al., ACP, 2019, measured the wildfire smoke in the stratosphere over Europe even 6 months after the Canadian fire event in August 2017, and Ohneiser et al., ACP, 2022, measured a two-year long decay phase of the Australian fire perturbation in the stratosphere over southern Chile in 2020 and 2021. Chemical processing was probably not a big issue, because smoke particles are glassy at low temperatures in a dry atmosphere and probably chemically rather inactive.
Line 283: 2009 instead of 2006?
Lines 292-294: So, if that is true how could we observe the smoke for more than 6 months (Canadian smoke) and 2 years (Australian smoke). Maybe, because the smoke particles are glassy at stratospheric conditions? The initial stratospheric smoke peaks cannot be used to describe the decay phases. These are signs of individual pyroCb events. The coherent decay phase starts about 2-3 weeks after the strong injection events.
Section 4 and Figures 4 and 5: I strongly recommend to compare your time series (the integrated column backscatter values) with the ones presented by Sakai et al., JGR, 2025 and Trickl et al., 2013, 2024.
Figure 4: I estimate that the full stratospheric column background AOD is about 0.004 at 532 nm (assuming a lidar ratio of 50sr).
Then the Raikoko 532 nm AOD in the NH is roughly 0.01 – 0.004? when combining Figure 5 and 4 and assuming a lidar ratio of 50sr. This is quite low!
And the CALIOP bias is about 0.03 as we pointed out in Ansmann et al. 2023?
Please clarify, many readers will have problems with these numbers. Please compare your findings with values presented by Sakai et al. (2025). Agreement with long term observations of Sakai et al. are very helpful in these discussions. Ground-based systems should be able to correctly measure the stratospheric column-integrated backscatter coefficient and therefore provide good estimates for the 532 nm AOD (even when simply assuming 50 sr for the lidar ratio).
Lines 470-480, and Figure 8: In Fig. 8, we have 532 nm AODs up to 0.01 and in the text we have AOD numbers of 0.06 (line 475), 0.05 (line 476) and 0.04 (line 477). Should that be 0.006, 0.005, 0.004? But all these values are quite low, to my opinion…?
Citation: https://doi.org/10.5194/egusphere-2026-1059-CC1 -
RC2: 'Comment on egusphere-2026-1059', Anonymous Referee #2, 07 Apr 2026
Review of manuscript by Martinsson et al., “A global view of the stratospheric background, volcanic and 2 wildfire aerosol in the CALIOP era (2006 – 2023)”.
The manuscript by Bengt Martinsson and coauthors presents observations of stratospheric aerosol by CALIOP instrument over 17 years. The study identifies stratospheric aerosol perturbations generated by volcanic eruptions and wildfire outbreaks, analyses the seasonal variation of background aerosol abundance and discusses the validity of CALIOP-derived stratospheric AOD in consideration of the estimated lidar ratio of aerosols produced by various perturbation events.
The study is built on the methodology previously presented by Friberg et al. (2018) and Martinsson et al. (2022), uses the same approach and targets the same scientific questions, while presenting the updated post-2019 CALIOP observation record. The new aspects of this study – with respect to the previous publications on the topic by this team – are the consideration of variable lidar ratios and the analysis of background stratospheric aerosol variability. In addition, the paper discusses the validity of CALIOP-derived AOD by referring to the solar occultation data from the literature and provides an estimate of radiative forcing using a simple relation between AOD and RF. The conclusion of the study points out the advantages of satellite lidar technique and the need to sort out the discrepancies between active and passive aerosol remote sensing.
While the stratospheric aerosol variability is a topical research subject, whereas CALIOP observation record is obviously of great value for stratospheric aerosol science, I find that the scientific novelty of the manuscript remains limited. At the same time, the scientific value of some of the new results is reduced by several shortcomings discussed below. In general, the manuscript currently reads as an incremental extension of earlier work by this group of authors rather than a sufficiently mature synthesis for ACP. I therefore recommend substantial revision before the manuscript can be reconsidered.
Major/general remarks.
- Structure and logical organization of the manuscript
The repartition of the presented material between Results and Discussion is disproportional. In practice, the Discussion section (Sect. 4) is not much different from the Results section, as Sects. 4.1–4.3 continue to describe and extend the results shown in Figs. 4–7, whereas the actual critical discussion is largely restricted to Sect. 4.4, where the CALIOP-derived AOD is compared with solar occultation-based records and GloSSAC. The authors should reconsider the organization of the paper and the naming of sections in order to improve the logical flow and better distinguish presentation of results from their discussion.
2. Discussion and literature review.
While the Discussion section takes up the most of the manuscript, it finally lacks a critical review of the results presented in the context of available literature on the topic of stratospheric aerosols. I got an impression that the authors strongly prefer auto-citations whilst largely ignoring the relevant literature.
Related to this, while the Discussion section takes up a substantial part of the manuscript, it still lacks a sufficiently critical review of the results in the context of the broader literature on stratospheric aerosols. The discussion relies heavily on the authors’ earlier work and could engage more directly with relevant literature, especially in relation to the definition of background aerosol (Sect. 4.1), the interpretation of the CALIOP–solar occultation discrepancies (Sect. 4.4), and the implications for climatological datasets such as GloSSAC. As it stands, the manuscript gives too little space to alternative interpretations and external context
3. Spatio-seasonal coverage of CALIOP nighttime data.
Another important issue concerns the spatio-seasonal coverage of the nighttime CALIOP data. According to Sect. 2.1, only nighttime CALIOP measurements are used in the general evaluation, following Friberg et al. (2018) and Martinsson et al. (2022). However, nighttime CALIOP observations do not provide continuous coverage in the polar day regions around the Summer solstice, the nighttime coverage is limited to about 60° latitude (see Fig. 4 in Friberg et al., 2018). In this light, the latitude–time presentation in Fig. 3 requires clarification, because it appears to show continuous coverage up to +/- 82° regardless of season. The authors should explain how the missing data in the polar-day regions were treated, whether any interpolation or averaging procedure was applied, and what implications this has for the discussion of aerosol variability at high latitudes, which are frequently affected by wildfire smoke intrusions.
4. Factors of background aerosol variability.
I also have concerns regarding the treatment of background aerosol variability. First, although Sect. 2.2 and Sect. 4.1 provide an operational procedure for extracting the background aerosol, the physical definition of “background” remains insufficiently clear. In Sect. 4.1, the authors themselves note that stratospheric background aerosol is “not a well-defined concept,” and contrast their approach with the definitions used by Solomon et al. (2011) and Kremser et al. (2016). In the present manuscript, the background level is established from the cleanest 8-day periods in nine latitude–altitude regions (Sect. 2.2; Fig. 4), but it is not fully clear to what extent this estimate can be interpreted as a true background state rather than as the minimum aerosol loading observed within an already perturbed CALIOP era. This is particularly important now that the stratospheric influence of wildfires has become more frequent. The authors should clarify whether recurring wildfire-related injections, including weaker or secondary summertime smoke emissions, are conceptually part of the background or excluded from it.
Second, I am not convinced that the seasonal variation of the background aerosol burden shown in Fig. 4 can be interpreted straightforwardly in terms of aerosol transport and source processes. In Sect. 4.1, the authors themselves note that the seasonal cycle in the extratropical LMS closely follows the seasonal variation of LMS air volume, citing Appenzeller et al. (1996). This suggests that a substantial part of the observed variability may simply reflect geometrical or layer-volume effects, rather than a true seasonal modulation of aerosol abundance. I therefore encourage the authors to discuss this point much more carefully and to distinguish more clearly between changes in aerosol amount and changes induced by the seasonal structure of the layers themselves. In addition, the interpretation should better account for processes that are likely to affect background aerosol seasonality, such as uplift of pollution aerosol in the Asian monsoon anticyclone / ATAL region (Vernier et al., 2015), tropical upwelling, and the convective transport of relatively clean air across the tropical tropopause. Without a more careful discussion of these competing factors, the physical interpretation of Fig. 4, and more generally its relevance for understanding stratospheric aerosol processes, remains unconvincing.
5. Radiative forcing estimates
Finally, I find the radiative forcing estimates to be of rather limited scientific significance in their current form. The manuscript converts stratospheric AOD into effective radiative forcing using the simple relationship derived by Schmidt et al. (2018) for volcanic aerosol. However, this approach does not account for the vertical distribution of extinction, the latitude of the aerosol perturbation, or differences in aerosol absorptive properties. These limitations are particularly important when wildfire aerosol is considered, because the relation derived by Schmidt et al. for volcanic aerosol cannot be assumed to apply directly to carbonaceous smoke aerosol. Given the very large uncertainty associated with such a simplified estimate, and the lack of a sufficiently critical discussion of these limitations, I believe that the RF values (which are among the main conclusions in this study) should either be accompanied by a much more comprehensive uncertainty discussion or removed from the paper.
Specific remarks
Fig. 2 would benefit from explicit event labels within each panel rather than relying solely on the caption. It would also be useful to indicate, where available, lidar ratio estimates from the literature for the same events to facilitate comparison with previous studies and discuss the differences.
Sect. 3.2: An important part of this section (p.10) relies on Figs. S1–S6 in the Supplement. The authors should reconsider whether some of this material is essential to the scientific argument and should therefore be moved to the main manuscript; otherwise, the discussion in Sect. 3.2 should rely less heavily on supplementary figures.
l.252. Please clarify what is meant by “overshooting plumes” in the context of the Raikoke eruption.
Fig. 3. What parameter is reported here: integrated backscatter coefficient or SAOD?
Fig. 3. Please ensure that fill values are visually distinguishable from physically low values. In addition, as noted above, the treatment of missing nighttime CALIOP coverage in polar-day regions should be explained.
l.269. “rapidly faded”: as can be inferred from Fig. 3, the ANYSO perturbation persisted for more than one year.
l.277. “the last stop”: too colloquial, consider rewording
l.289. The relation between the volcanism-sea interaction for Hunga eruption and lidar ratios reported in Fig. 2l should be explained, in particular how the inferred particle composition associated with volcanism–sea interaction is expected to affect the lidar ratio.
l.372-373. I do not follow how the transport pathway in the dBD branch can be inferred directly from Fig. 6a. Please clarify the argument or refer to the specific feature of the figure that supports this interpretation.
l.392-393. Please clarify how exactly the vertical resolution of CALIOP data can be used to constrain modeling efforts.
l.406-407. I am not convinced by the interpretation of higher aerosol abundance in the LMS by “compressing” process. Could it rather be due to tropospheric sources?
Fig. 8. The thin grey line in Fig. 8a is barely visible and appears to differ only marginally from the corrected AOD time series. Since Sect. 4.2 concludes that the overall impact of the lidar-ratio correction is minor, the authors may wish to comment more explicitly on the practical significance of this correction for the long-term CALIOP record.
l.538-539. Which version of GloSSAC is referred to here?
- 571-574. I am not sure that the basic description of stratospheric transport by the BDC should be presented as a conclusion of the present study.
Citation: https://doi.org/10.5194/egusphere-2026-1059-RC2 -
RC3: 'Comment on egusphere-2026-1059', Anonymous Referee #3, 08 Apr 2026
General comments:
This paper provides an overview of CALIOP observations of aerosols in the stratosphere covering the entire CALIPSO mission period from 2006 to 2023. The paper is in principle of interest to the stratospheric community and should eventually be published in my opinion. There are, however, many little inconsistencies or unclear statements that should be corrected (see specific comments below). Two of the comments are more general and are mentioned here:
- The terminology used throughout the paper is often not very precise: “aerosol backscatter” is often used. In my opinion this is a very vague term and the correct terms probably are “backscattering coefficient” or “volume backscattering coefficient”? Are the units correct, i.e. only “1/sr2?. Perhaps different terms are used in the CALIOP community. It would in any case be good to briefly define the quantity at the beginning. Perhaps I’m wrong, then I’m happy to be informed.
- The rescaling of the presented AOD values to the contribution of each degree latitude is misleading in my opinion. Each atmospheric column (or latitude band) certainly has an AOD value that can be calculated by vertically integrating the extinction coefficient profile. The global mean AOD should be determined by averaging these values (simply put). Your way to depict the results makes it unnecessarily more complicated to compare with other studies. Please consider change the way the results are depicted.
Specific comments:
Line 32: „an aerosol layer above 20 km altitude”
At mid and high latitudes the Junge layer is below 20 km.
Line 52: “inducing a global 1-year average effective radiative forcing of – 0.24 W/m2 .. “
Is there an error estimate for this forcing?
Line 60: “On the other hand, SO2-poor eruptions … is” -> “.. are”
Line 83: “is divided into nine altitude and latitude parts”
In total or nine for each?
Line 87: “We find that the aerosol backscattering on average exceeded the background by 55% in the 17 years studied”
Considering all (i.e. averaged over all) altitude and latitude ranges?
Line 91: “and 2019 (Raikoke eruption).”
Perhaps the Ulawun eruption also contributed? Ulawun (tropics) and Raikoke (NH mid-latitudes) both erupted in June 2019.
Line 112: “and were first converted to extinction by the standard effective lidar ratio S = 50 Sr ..”
It would be good to mention what particle size distribution (or at least what effective radius) this corresponds to. Particle size is quite variable in the stratosphere (with altitude, latitude and time).
Line 127: “the lowermost stratosphere (LMS, from the tropopause to the 380 K isentrope,”
Is the topopause always below the 380 K isentrope?
Line 135: “To estimate the background conditions, the averages of the three years with the lowest backscattering measurements of each 8-day period were formed.”
Not fully clear what was done here. Is the background an average over 3 years or an average over selected 8-day periods in these three years? This should be explained precisely.
Line 140: “The extracted lowest 8-day values”
8-day values over 3 years?
Line 140 following: The results of the background subtraction (including the background) are shown in Fig. 5, right? Please refer to this Figure here.
Line 157: “In that method a target value in scattering ratio (R) obtained beside the studied aerosol layer (RT) is reached below the layer in an iterative procedure that results in an estimate of the effective lidar ratio, while correcting for attenuation of the backscattered signal.”
I have several comments/questions about this sentence. First, I don’t understand its meaning, particularly of “.. a target value in scattering ratio (R) obtained beside the studied aerosol layer (RT) is reached below the layer”. What does “beside” mean here? How is “layer” defined? Is it the entire aerosol layer or one of the three vertical layers you define? I think some essential information is missing for the reader to understand this sentence. It would also be good to describe the approach to determine the lidar ratio in a bit more detail here (a few sentences). If I understand this correctly the approach provides a height averaged lidar ratio? The ratio is highly variable with altitude. What are the effects of neglecting the altitude variation?
I think it should be mentioned explicitly that the lidar ratios are height averaged (in a probably non-trivial way).
Figure 2: What latitudes are analysed here? They are probably different for the different eruptions. What about the variation of the lidar ratio over longer periods? It usually takes about a month for the volcanic aerosol layer to "develop fully", in the sense that the SAOD reaches its maximum value.
Line 244: “formed large quantities of stratospheric aerosol (Martinsson et al., 2025).”
There are many early studies to back up this statement.
Caption, Figure 3 (& several other Figure captions): “Aerosol scattering” is not precise enough. Please name the shown physical quantity correctly.
Same caption (also applicable to several other captions): “Color scale: Global AOD contribution per degree of latitude, i.e. the sum over latitude is the total AOD at any given time”
This is an unusual way to present results and in my opinion it is misleading. Every latitude bin has its own SAOD based on the vertical integral of the extinction coefficients over the corresponding altitude range. I suggest presenting the actual SAOD values, not rescaled ones. Their average then gives the global mean value. Your values are difficult to compare to other studies.
Line 277: “The LMS .. are affected” -> “the LMS .. is affected”
Line 285: “because of the time required to transform sulfur dioxide to sulfate,”
It's both, the conversion to sulfate and the microphysics, e.g. growth of existing or newly formed particles.
Line 288: “The aerosol of the latter eruption mainly consisted of volcanic ash (Vernier et al., 2013) and the former by aerosol from volcanism – sea interaction (Martinsson et al., 2025).”
The latter part of the sentence doesn't fit to the first part from a linguistic point of view.
Line 290: “These eruptions are thus less influenced by delay in aerosol formation from chemical transformation”
The main component of the Hunga aerosol layer in the stratosphere is also sulfate, right?
Line 296: “Background aerosol backscattering”
Please use the correct technical term for this quantity.
Often the term “backscattering” is used throughout the paper. Please check all incidences and correct them.
Line 329: “using the average of the three lowest backscattering values”
What about years with strong volcanic perturbations? Are these years also used?
Line 331: “Seven of the nine layers ..”
I had to read this sentence several times to understand it. Perhaps you can add a first introductory sentence to this paragraph to set the stage.
Line 362: “The stratospheric background aerosol is often thought of as a layer located above 20 km altitude.”
This is not generally assumed for high latitudes.
Line 412: “The lidar ratios of the individual measurements are shown in Figure 2.”
What altitude & latitude is shown in Fig. 2? What about the altitude variation in lidar ratio?
Line 463: “The intense volcanism – sea interaction of the Hunga Ha’apai eruption in the beginning of 2022 (Martinsson et al., 2025)”
Again, there are dozens of earlier papers on this eruption.
Line 482: “resulting in a variability range of 0.010 around the average of 0.0031”
Not really “around”, as this would also include negative values.
Line 482: “Making use of previous estimates of the relation between radiative forcing and stratospheric AOD”
Please mention how the conversion was done, i.e. what the conversion factor was.
Line 491: “Comparisons of CALIOP lidar-based results with solar occultation (SAGE III/ISS) show discrepancy at mid- and high latitudes”
Discrepancies in which parameters and how large are the differences? Please describe briefly.
Line 500 following: for the AOD comparisons the reader needs to know the wavelengths of the data sets used (CALIOP and SAGE II/III). Or have the AODs been converted to the same wavelength? Also in this case the wavelength should be given.
Lie 519: “2 – 3 months after the Canada/USA fire (Table 1) similar deviations were found at high altitudes as in Kar et al. (2019),”
Please describe these discrepancies briefly semi-quantitatively. Right now, the reader cannot really interpret the statements.
Line 565: “Seven of the nine layers each contain 11- 15% of the entire background aerosol.”
With respect to mass? This is a difficult statement and I'm not sure it is correct.
Supplement: I think a bit of introductory text would be appropriate at the beginning of the supplement.
Captions: “Aerosol scattering” is quite vague, as mentioned above. Also, the depiction of AOD contribution per degree latitude is difficult to interpret and to compare to other studies.
Caption Fig. S7: “The sum of the nine AOD curves displayed is the average AOD from the tropopause to 35 km altitude in the latitude range -80 to 80°.”
It is of course OK to sum up the three AODs for each latitude range to yield the total AOD for this latitude range, but the rescaling with latitude bins is not good in my opinion. How is this done in this case anyway? The tropical latitude range is narrower than the two other ranges?
Citation: https://doi.org/10.5194/egusphere-2026-1059-RC3
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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.