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Investigating a Persistent Stratospheric Aerosol Layer Observed over Southern Europe during 2019

Remote Sens. 2023, 15(22), 5394; https://doi.org/10.3390/rs15225394

by Kalliopi Artemis Voudouri^1,2,*

, Konstantinos Michailidis², Maria-Elissavet Koukouli²

, Samuel Rémy³, Antje Inness⁴, Ghassan Taha^5,6

, Georgia Peletidou²

, Nikolaos Siomos⁷, Dimitrios Balis²

and Mark Parrington⁴

Reviewer 1: Anonymous

Reviewer 2: Anonymous

Remote Sens. 2023, 15(22), 5394; https://doi.org/10.3390/rs15225394

Submission received: 15 September 2023 / Revised: 3 November 2023 / Accepted: 12 November 2023 / Published: 17 November 2023

(This article belongs to the Special Issue Understanding the Roles of Aerosols and Clouds in Environment, Meteorology and Climate with Advanced Lidar Remote Sensing Techniques)

Round 1

Reviewer 1 Report (Previous Reviewer 3)

Comments and Suggestions for Authors

Review of article: “Investigating a persistent stratospheric aerosol layer observed over Southern Europe during 2019” by Kalliopi Artemis Voudouri et al., submission #2640733 (version 2) in Remote Sensing (September 2023)

I. General remarks

1. Content of the article

This article presents an observational study of a stratospheric aerosol layer that persisted during five months over Greece, based mainly on lidar measurements (from a ground-based station and space-borne) and supported by other passive space-borne and model/reanalyzes observations. The organization of the article is standard with an introduction, a section presenting the instrumentation along with the satellite data and modelling products used in the study, a results’ section and a conclusion. The results’ section discusses the temporal evolution and statistical properties of the layer mainly in terms of aerosol optical thickness (AOD), altitude and geometric depth. In this section, the layer is also attributed to the Raikoke volcano eruption using aerosol typing from the CALIPSO space-borne lidar and model outputs from stratospheric reanalyzes that assimilate stratospheric SO₂ space-borne observations. Finally, the results’ section includes a case study to compare the different observation more in detail.

Compared to the first version of the article, the corrections have been limited, consisting mainly in precisions here and there, and a some additional figures about the infrared AOD and the angstrom coefficient. My main concerns about the first version of this article were that the scientific purpose of the study was not clear to me, and that I found the article to be too descriptive, with an accumulation of observational and modelling data without a strong coherence. The authors answered me that the article’s objective was not to provide a metrology of the aerosol optical properties in the layer, nor to compare quantitatively the different measurement techniques, but to show how synergies in the different instruments and models can be used to monitor the plume.

Firstly, I think that, to answer the article’s objective, the conclusion should not only be a summary listing observational results from the different observations and models. It should also include some conclusions about the advantages and limitations of the different datasets, and how combining them allows to obtain a better characterization of the aerosol layer (maybe, a separate discussion section should be included, dedicated to emphasize instrumental complementarities).

Secondly, in the article body, the text should emphasize the complementarities and explain clearly what each instrument or model is bringing to the study, not just present a suite of observations and model outputs. It can be small additions, e.g. in Section 3.5, it could be saying that the same layer heights are retrieved using active remote sensing (CALIPSO lidar), that is more precise but more localized, and passive remote sensing (OMS-LP), that offer a better spatial coverage but lower vertical resolution. Without this type of simple coherence-making sentences, the paper gives an impression of piling up observations without proper aim, especially as there is no discussion section to expose explicitly the instrumental complementarities.

Thirdly, not comparing quantitatively the different measurement techniques is a thing, but not commenting at all on differences that are clearly visible on the figures is another thing (e.g. the lower stratospheric AOD values from CALIPSO compared to THELYSIS in Section 3.4). This type of omissions also gives me the impression of piling up the observations without clear aim or coherence. Even if I clearly see from your answers that it is not the case, the way the paper is written does not do justice to your work.

Similarly, when a reviewer asks for additional information about the retrieval process (e.g. the tropopause height in THELYSIS inversion) or the simulations presented (e.g. the modified CAMS system used in Figure 3) or a peculiar feature appearing on a figure (e.g. the secondary peak on THELYSIS AOD distribution), even if it is not requested explicitly to add this information in the article body, the authors should consider adding it. If a reviewer wondered about a point, then other readers are likely to do so, so that answering only in the document destined to the reviewer who asked the question is a bit light. This would avoid wasting time in a second review. This wrongly gives the impression that the corrections were done at the minimum.

Another point is the choice of the datasets included, or not, in the article. Some of my remarks were answered by something like “we tried this, we did have a look at this dataset, but it was not conclusive to answer the question”. In my opinion, this could enrich the conclusion, or a discussion section, because it is as useful to explain what does not work than what does work, and why. Especially, if the purpose of the paper is to show observations / model complementarities, explaining why you chose a peculiar dataset and discarded another one would be useful to the reader who has to perform a similar study. It would also avoid that the reader wonders why you did not explore other tracks to answer the open questions: you did, but chose not to mention it…

2. Form

A better care in formulating the sentences would have been appreciated, as some the text that was added is gibberish. Please proofread your text for clarity, if possible have a native speaker check it for English language.

I know the final typesetting is the work of the publisher, but providing a clean enough document to the reviewers is of the authors responsibility. The images inserted in the middle of the text in Section 3.5 do not make it easy to review the manuscript. Providing another set of pixelated images for Figure 4 while stating this point was improved does not look very careful.

Also, when you decide not to follow a reviewer’s suggestion (e.g. the suggested change of subsections in section 3), it would be good to explain why, and try to figure out what could be done to provide an alternative answer (in this case, maybe adding transitions phrases at the end of the sections could help following your reasoning).

3. Overall rating

As said above, my opinion on the first version of the article was that it remained too descriptive, and that the objectives of the study were not clear, so that the result were a bit weak for a research article. The authors have better explained their objective in the “answer to reviewer” document, but they did not implement changes in the manuscript on the objectives, nor on the “too descriptive” points. In fact, I have the impression that only minor changes were made in the manuscript, to answer the specific remarks, while a more thorough work of editing would have been needed to better exploit the data presented in regards with the objective. Consequently, I am still not convinced and still recommend major revision. These remarks may seem harsh, but I really think that what lacks to have a good paper is only a better discussion of the results, to actually bring an answer the objective.

II. Specific remarks

Abstract

Maybe the objectives should also be better exposed here. There is no mention of combining different instruments or so, which gives the impression you intend to perform a full characterization.

Section 1: Introduction

I found the exposition of the objectives provided in the answer to my review (the very first sentence) clearer than the one that is in the article, so I suggest rephrasing lines 86 to 90 to insist on the combined observations aspect.

Section 2: Instrumentation and data products

Line 126. “The Layer-base altitude…” should be “The layer base altitude…” (remove dash and capital).

Lines 127-129. “… defined as that level at which there is first an increase in signal above the clear background level and of a magnitude equal to 3 times the standard deviation of the background fluctuations.”. This is gibberish; the first version was clearer in describing the process, and better written; only the part from “whilst the top” (line 125) needed to be changed (starting by the new sentence “It is further required…”.

Also, I think you need to precise here that you apply this layer detection algorithm starting from the tropopause height, that you retrieved from radiosondes. This way, you would include in the text the information about tropopause height retrieval that I requested (if I asked the question, other readers will likely wonder too).

Line 131-133. I think the sentence can be simplified into “The layer top (or effective top) is detected following the same process, but moving downward in altitude from the maximum range.”

Line 145-146. “The SNR thresholds are applied following the methodology described in [38]. The SNR is selected above 3, since…” This can be simplified in “A SNR threshold of 3 is applied, following the methodology described in Voudouri et al. [38], since…” (/!\ missing author name for the citation).

Line 147-148. “(above the selected threshold, boundary layer estimation found independent from SNR).” There is no verb in that sentence + what does the boundary layer does here? Please rephrase.

Line 152-153. “The methodology has already been applied in cirrus clouds retrievals [39]. In the study of Giannakaki al., the transmittance…” The transition is not fluid when using number citations, the middle sentence can be removed writing simply “Giannakaki al. [39] used the transmittance…”

Line 166. A-Train, not C-Train.

Line 175-177. “CALIPSO measured products are the attenuated backscatter coefficients at 532 and 1064nm and, the volume linear depolarization ratio at 532nm and the retrieved product is the particle linear depolarization ratio at 532nm”. “measured products” is awkward, “observables” would sound better + the “retrieved product” addition in the middle of the sentence does not make sense.

Line 192. “described in [44]” missing author name for the citation.

Lines 225 to 232. I think you should give in this paragraph the dates covered by the simulation you use in Figure 3, to avoid questions about why you show only a limited period in Figure 3.

Section 3: Results & Discussion

If you do not want to follow my suggestion to re-organize the subsections in this part, please find a title for section 3.2 that is more specific and allows to differentiate it from section 3.3 (e.g. “Identification of the stratospheric layer origin using CALIOP aerosol typing”)

Section 3.1: The temporal evolution of the stratospheric layer

Lines 311-315. “The attenuated scattering ratios at 1064nm i.e., the measured 1064 nm attenuated backscatter profiles divided by profiles of molecular attenuated backscatter coefficients (obtained from radiosondes) and calibrated via normalization to the molecular attenuated backscatter profile at 8km, shows that…”. The insertion between comas is too long for the sentence to remain understandable. Please split in two sentences, something like “Figure 1 present the attenuated scattering ratio… It shows that…” + the coma should be before the “i.e.”

Lines 324-326. The legend should mention what the dotted black line represents (tropopause height, I guess) and the altitude unit should be added. The color bar legend should be “1064 nm attenuated scattering ratio” for clarity. And the title “attenuated signal” is confusing too; it can be removed if the information about the instrument and what is plotted is correctly given in the legend.

Lines 327. The technical details about normalization and the definition of the scattering ratio do not need to be repeated if given in the main text, but the instrument name should be in the legend, not only in the title of the plot (as I already said in the first review). The legend could simply start by “Time-height cross-section of the 1064 nm attenuated scattering ratio retrieved from THELYSIS observations between June and November 2019”.

Line 357, Figure 3. Could you add a white edge to your black triangle and diamond to make them more visible on the dark blue background?

Line 373. You could remind here that the later evolution of the plume was not modelled in this experiment.

Section 3.2: Identification of the origin of the stratospheric layer

Line 375. As said above, the title of the section should mention CALIOP aerosol typing.

Line 386. “.. as stratospheric aerosol layers, of aerosol subtype 10. i.e., sulphate aerosol, due to…” The first coma should be removed and the “aerosol” word does not need to be repeated. The second coma should be placed before the “i.e.”, not after (and the point should be removed).

Line 390, Figure 4. The resolution of the new figure does not seem to have improved; I still cannot read the latitude/longitude labels on the x-axes. Maybe the original image has a better resolution and the problem arises when exporting the document to PDF, in which case please check the exporting options before submitting the next version (the so-called “web quality” should be avoided). If you use JPEG images, they should have a minimal resolution of 300dpi. PNG format gives usually better results.

Figure 5. The authors provided an additional figure in the “answer to reviewer” document (a map of CALIPSO tracks with a stratospheric layer). I think this figure could be included in the supplementary material to show that none of the CALIPSO tracks were actually closer from THELYSIS than ~40km. This is an argument not to discuss a direct comparison over the whole study period.

Section 3.3: Identification of the origin of the stratospheric layer CAMS ICBG simulations of the stratospheric layer

This section’s title is gibberish; it should be something like: “Identification of the stratospheric layer origin using CAMS ICBG simulations”.

Line 404. “The smoke particles could possibly originated from…” Remove the “d” or rephrase.

Line 424. “…plumes originated from sulfates…” should be “plumes classified as sulfate” or something like this. Sulfate is not a particle source.

Line 425. “between July – September 2019” for me it should be either “from June to September” or “during the July-September period”.

Line 431. “Wang et al., [63]” coma to remove

Line 433. “Wells et al., [51]” same remark

Line 455. “provided by [65]” missing author name for in-text citation.

Section 3.4: Geometrical and optical aerosol properties from ground and space-based measurements

Line 482-483, Figure 8. If these are THELYSIS observations, then why do you indicate that there are symbols corresponding to CALIPPSO daytime/nighttime overpasses?

Line 495, Figure 9. I asked about the origin of the secondary peak in AOD in my comments on the first version of the article; the authors provided a satisfying explanation in the “answer to reviewer” document, but it should also be included it in the paper (if I wondered about it, other reader will likely do the same).

Line 528. The fact that CALIPSO stratospheric AOD values are significantly smaller than the THELYSIS ones should be commented in the text. This way it gives the impression of piling up observations without connecting them, which is a pity for a paper which aim is about crossing different types of observations.

Section 3.5: Correlative stratospheric measurements on July 25, 2019

Line 552. “In the following section” you are already in, so it is the current section.

Lines 565-575. It is a mess with images in the middle of the text. Please use the “text and label only” when adding a reference in the text.

Lines 576-577. No need to copy here the legend of the figure. It can be just “Figure 14 (right) shows the geometric boundaries of the stratospheric sulfate aerosol layer detected by CALIPSO…” The labels (c) and (d) should be used instead of (right).

Line 581. “km⁻¹.sr⁻¹” there should be a dot between the two units.

Line 625, Table 3. Writing “AOD IR (nm)” looks as if the AOD is in nanometers. I would suggest “AOD IR (wavelength in nm)” and something similar for the visible AOD. Could you remind in the legend why there are two values of base and top height for ICBG?

Section 4: Conclusions

The paragraph from line 676 to line 680 contains conclusions, while the paragraph from line 667 to 675 contains perspectives: the order is usually the reverse.

The paragraph from line 676 to line 680 is the only attempt to actually answer the objectives of the article, about the complementarities between the different observations and models. However, this paragraph is very general, it does not contain any specific conclusions from your study. In my opinion, if you really want to answer your objective, you have to conclude showing how combining the different datasets bring more insight than separate observations, how the weaknesses in the different measurement techniques can be overcome by combining them etc.

References

[63] Wang et al. On the journal web page, it says: “Status: this preprint was under review for the journal AMT but the revision was not accepted.” So the reference should be updated.

Comments on the Quality of English Language

See section I.2 of the review.

Author Response

I. General remarks

1. Content of the article

2. Form

3. Overall rating

We would like to thank the Reviewer #1 for his/her fruitful comments that led to the improvement of the manuscript. In the revised version the reviewer’s comments and suggestions have been considered, by improving the discussion of the abstract, the introduction and the conclusion section and by including some omitted parts from the previous review. Below we report the changes included in the revised manuscript as a response to the comments of the reviewer.

II. Specific remarks

Abstract

Maybe the objectives should also be better exposed here. There is no mention of combining different instruments or so, which gives the impression you intend to perform a full characterization.

Answer: We thank the reviewer for pointing out the importance of mentioning the combined information of the different instrumentation used in this study. Thus, the following sentence is included in the revised version of the manuscript: “The main aim of the paper is to present an overview of this atmospheric feature and to identify any temporal changes in the aerosol properties that would signify substantial changes in the composition of this long-lasting stratospheric plume over Thessaloniki. This aim is further enhanced by emphasizing the importance of the combined information based on active ground- and space- borne lidars, passive remote sensing, and models during the complex stratospheric aerosol conditions as those encountered during 2019.”

Section 1: Introduction

Answer: The objectives of the paper are now described as follows: “The overall purpose of the paper is three-ford: (a) to present an overview of the signatures of the long-lasting stratospheric plume over Thessaloniki, (b) to identify any temporal changes in the aerosol properties of this persistent atmospheric layer hence indicating compositional changes and (c) to emphasize the importance of the combined information based on active ground- and space-borne lidars, passive remote sensing, and models towards a trustworthy and complementary global stratospheric aerosol monitoring, given the complex stratospheric aerosol conditions as those encountered during 2019.”

Section 2: Instrumentation and data products

Line 126. “The Layer-base altitude…” should be “The layer base altitude…” (remove dash and capital).

Answer: The change is made as the reviewer suggested.

Answer: The sentence is altered as follows: “A SNR threshold of 3 is applied, following the methodology described in Voudouri et al. [38], since the lidar signal is strongly attenuated at higher altitude levels and the noisy parts of the signal should be rejected. The layer detection algorithm is searching the layer base above the tropopause height that was defined from radiosondes that are released at Thessaloniki airport, twice a day.”

Answer: The following sentence is added in the revised version of the manuscript: “The layer detection algorithm starts above the tropopause height that was defined from radiosondes that are released at Thessaloniki airport, twice a day.”

Line 131-133. I think the sentence can be simplified into “The layer top (or effective top) is detected following the same process, but moving downward in altitude from the maximum range.”

Answer: The change is made as the reviewer suggested.

Line 147-148. “(above the selected threshold, boundary layer estimation found independent from SNR).” There is no verb in that sentence + what does the boundary layer does here? Please rephrase.

Answer: We thank the reviewer for pointing this out. he sentence is deleted in the revised version of the manuscript. reads: the cloud boundary estimation is found independent from SNR.

Answer: The change is made as the reviewer suggested.

Line 166. A-Train, not C-Train.

Answer: The change is made as the reviewer suggested.

Answer: The sentence now reads: “The CALIPSO main observables are the attenuated backscatter coefficients at 532 and 1064nm and the volume linear depolarization ratio at 532nm, while the main output is the particle linear depolarization ratio at 532nm”.

Line 192. “described in [44]” missing author name for the citation.

Answer: Author’s name is added as the reviewer suggested.

Lines 225 to 232. I think you should give in this paragraph the dates covered by the simulation you use in Figure 3, to avoid questions about why you show only a limited period in Figure 3.

Answer: The following sentence is added in the revised version of the manuscript: “and in the Mediterranean area in the weeks following the eruption, i.e., from 22 of June to 29 of September 2019.”

Section 3: Results & Discussion

Answer: The title of the section 3.2 now reads: “Identification of the stratospheric layer origin using CALIOP aerosol typing”.

Section 3.1: The temporal evolution of the stratospheric layer

Answer: The sentence now reads: “Figure 1 presents the attenuated scattering ratios at 1064nm, i.e., the measured 1064 nm attenuated backscatter profiles divided by profiles of molecular attenuated backscatter coefficients (obtained from radiosondes) and calibrated via normalization to the molecular attenuated backscatter profile at 8km. It shows that the stratospheric layers first appeared on 11 July 2019, with a significant month-to-month variability in the plume heights.”

Answer: Figure 1 is updated in the revised version of the manuscript, following the reviewer’s suggestions.

Answer: The legend now reads: “Time-height cross-section of the 1064 nm attenuated scattering ratio retrieved from THELISYS observations between June and November 2019. Cases have been placed one next to the other for visualization purposes. The black dashed line represents the tropopause altitude derived using radiosondes.”

Line 357, Figure 3. Could you add a white edge to your black triangle and diamond to make them more visible on the dark blue background?

Answer: Figure 3 is changed as the reviewer suggested.

Line 373. You could remind here that the later evolution of the plume was not modelled in this experiment.

Answer: The sentence now reads: “This figure illustrates how SO2 from the eruption was transported around the NH during the month after the eruption till the end of September.”

Section 3.2: Identification of the origin of the stratospheric layer

Line 375. As said above, the title of the section should mention CALIOP aerosol typing.

Answer: We thank the reviewer for bringing this issue to our attention. The title of the section is changed to: “Identification of the stratospheric layer origin using CALIOP aerosol typing”.

Answer: The sentence is changed as suggested.

Answer: We really thank the reviewer for caring for the figure’s quality. We will tackle this issue with the copy editor in due course.

Answer: We thank the reviewer for pointing this out. The Figure with the CALIPSO tracks with a stratospheric layer is added in the supplementary material.

Section 3.3: Identification of the origin of the stratospheric layer CAMS ICBG simulations of the stratospheric layer

This section’s title is gibberish; it should be something like: “Identification of the stratospheric layer origin using CAMS ICBG simulations”.

Answer: The change is made as the reviewer suggested.

Line 404. “The smoke particles could possibly originated from…” Remove the “d” or rephrase.

Answer: The change is made as the reviewer suggested.

Line 424. “…plumes originated from sulfates…” should be “plumes classified as sulfate” or something like this. Sulfate is not a particle source.

Answer: The change is made as the reviewer suggested.

Line 425. “between July – September 2019” for me it should be either “from June to September” or “during the July-September period”.

Answer: The sentence now reads: “from June to September”.

Line 431. “Wang et al., [63]” coma to remove

Answer: The change is made as the reviewer suggested.

Line 433. “Wells et al., [51]” same remark

Answer: The change is made as the reviewer suggested.

Line 455. “provided by [65]” missing author name for in-text citation.

Answer: The change is made as the reviewer suggested.

Section 3.4: Geometrical and optical aerosol properties from ground and space-based measurements

Line 482-483, Figure 8. If these are THELYSIS observations, then why do you indicate that there are symbols corresponding to CALIPPSO daytime/nighttime overpasses?

Answer: We thank the reviewer for this comment. Figure 8 presents only THELISYS retrievals. It was a typo that corrected in the revised version of the manuscript.

Answer: We thank the reviewer for pointing out this omission. The following sentence is added in the revised version of the manuscript: “The secondary peak observed over Thessaloniki denoting the existence of highly absorbing aerosols is deemed to be a local effect. It could be possibly linked to smoke contamination from local sources.”

Answer: We thank the reviewer for bringing this to our attention. The following sentence is added in the revised version of the manuscript “The higher AOD values at 1064nm observed from THELISYS lidar compared to CALIPSO retrievals could be attributed to the absorptivity and contamination of localized smoke particles.”

Section 3.5: Correlative stratospheric measurements on July 25, 2019

Line 552. “In the following section” you are already in, so it is the current section.

Answer: The sentence now reads: “In the current section”.

Lines 565-575. It is a mess with images in the middle of the text. Please use the “text and label only” when adding a reference in the text.

Answer: This issue falls under the journal’s format which we followed by using the MPDI-provided template. Any lingering issues will be dealt with the copy editor in due course.

Answer: The change is made as the reviewer suggested.

Line 581. “km−1.sr−1” there should be a dot between the two units.

Answer: The change is made as the reviewer suggested.

Answer: The change is made as the reviewer suggested. The following sentence is added in the revised version of the manuscript: “with the extinction profile at 550nm showing a weak signal between 10.7 and 12.38km and a stronger well-defined peak from 14.77 to 16.13km.”

Section 4: Conclusions

The paragraph from line 676 to line 680 contains conclusions, while the paragraph from line 667 to 675 contains perspectives: the order is usually the reverse.

Answer: We thank the reviewer for pointing this out. A paragraph is added in the discussion part as follows: “All sensors with different detection limits captured the temporal and height variability of the observed layer above Thessaloniki during the complex stratospheric aerosol conditions of 2019. In short, the ground-based system monitored the stratospheric layer with high temporal sampling and high vertical resolution denoting an increased thickness till the end of 2019, whilst the correlative CALIPSO and OMPS-LP retrievals, having different temporal coverage, enhanced the spatial sampling by capturing the stratospheric features after the eruption. On top of that, the model simulations suggested the presence of volcanic particles in the stratosphere and the CALIPSO typing scheme identified the plume’s origin and geometrical and optical properties.”

References

[63] Wang et al. On the journal web page, it says: “Status: this preprint was under review for the journal AMT but the revision was not accepted.” So the reference should be updated.

Answer: We thank the reviewer for pointing out this. We thought preprints with DOI can be referenced, however, as this reference is not published, we decided to remove it in the revised version of the manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report (New Reviewer)

Comments and Suggestions for Authors

The paper by Voudouri et al. investigates stratospheric aerosol layers of primarily volcanic origin in Southeastern Europe by means of a multi-data approach. It focuses mainly on ground- and space-based active sensors that provide information on the vertical dimension. The combination of different sensors along with model simulations offers a complete picture of the stratospheric layers and an important insight into their geometrical and optical properties.

The paper is well within the scope of the journal. The methodology and analysis are thorough and well documented. I suggest minor revisions before publication.

In the following, specific and technical comments are given for consideration.

Specific Comments:

§2.2.1: How are the clouds cleared?

Ln134-136: Did you apply only the transmittance approach? Please, clarify.

Ln137: Does this mean that all cases were cloud-free?

Ln253-262: Is this paragraph needed?

Fig1: This figure is a bit confusing to me. I have some suggestions that could help. You could focus above 7-8 km. You can try to insert gaps when obs. are not available. Try to unsaturate the color map. Also, is this figure cloud-cleared?

Fig2: Can you add an inset graph with the grid?

§3.2: How do you identify the origin of the layers with the CALIPSO observations? Maybe you should consider a different title for this section.

Ln392-395: Can you expand on this. What is the difference of smoke and sulfate particles for CALIPSO? Is it safe to assume the coexistence of smoke and sulfate particles? I think backward trajectory analysis, maybe HYSPLIT simulations, can assist on this idea.

Fig5: Why not combine the two figures in one?

Ln407: How do you identify the origin of the layers with OMPS-LP retirevals? Can you expand?

Ln450-452: With which figure are you comparing the lidar data?

Fig9 & 10: Why not present together these two figures?

Fig11: I do not think that this figure adds much to the discussion.

Technical Comments:

Ln27: Change "space-born" with "space-borne" and everywhere else.

Ln160: Change "C-Train" with "A-Train".

Ln187: Please rephrase.

Tab2 (CAMS LHexp): Remove "Illustrate".

Ln307: Remove "strong".

§3.3: Check the wording of this title.

Comments on the Quality of English Language

No issues were detected apart from some minor technical comments that I provided.

Author Response

The paper is well within the scope of the journal. The methodology and analysis are thorough and well documented. I suggest minor revisions before publication.

In the following, specific and technical comments are given for consideration.

We would like to thank the Reviewer #2 for his/her fruitful comments that led to the improvement of the manuscript. In the revised version the reviewer’s comments and suggestions have been considered.

Specific Comments:

2.2.1: How are the clouds cleared?

Answer: Ice crystal clouds and high-level clouds were screened out from our operational algorithm, following the methodology applied in Voudouri et al., 2020. The wavelet covariance transformation is applied in the 1064nm signal.

Ln134-136: Did you apply only the transmittance approach? Please, clarify.

Answer: Yes, we apply only the transmittance approach, as stated in the following sentence: “In our study, the transmittance approach is applied to the lidar signals at 1064nm, as the infrared wavelength magnifies the differences in the vertical distribution of the aerosol load, resulting in layers that are easily identified.”

Ln137: Does this mean that all cases were cloud-free?

Answer: Figure 1 presents all THELISYS measurements performed between June and December 2019. However, the stratospheric layer AOD values are retrieved from measurements that were cloud free.

Ln253-262: Is this paragraph needed?

Answer: We decided to keep this paragraph, as it gives a short introduction to the section 2.5. We did separate it into two shorter ones, for clarify.

Answer: The Figure 1 is updated, along with its legend. It is now focused above the molecular region. The figure is depicting also cloud layers which were cloud cleared in the next step, following the procedure described in Voudouri et al., 2020. We decided not to insert gaps when measurements were not available, as lidar measured following the EARLINET schedule and during dust and fires events.

Fig2: Can you add an inset graph with the grid?

Answer: We kindly ask the reviewer to explain more about the graph that is asked to be added.

3.2: How do you identify the origin of the layers with the CALIPSO observations? Maybe you should consider a different title for this section.

Answer: The title for this section is changed to: “Identification of the stratospheric layer origin using CALIOP aerosol typing”.

Answer: The higher depolarization values is an indicator for the existence of smoke particles and its discrimination from sulfate particles in the stratosphere as shown in the study of Ansmann et al., 2021. However, smoke layers are identified from CALIPSO beyond and to the North of Thessaloniki at lower heights.

We thank the reviewer for this suggestion for the HYSPLIT analysis. It is however within the future aims of the team to include HYSPLIT simulations when investigating such interesting aerosol features over Europe.

Fig5: Why not combine the two figures in one?

Answer: We decided to present separately the two figures for better depicting the latitudinal detection of the two types showing that the CALIPSO observed smoke particles congregate further to the north of Thessaloniki [~40°N].

Ln407: How do you identify the origin of the layers with OMPS-LP retrievals? Can you expand?

Answer: The paragraph is about the volcanic aerosol properties, not the origin of the layers with OMPS-LP retrievals, which would be beyond the scope of this study.

Ln450-452: With which figure are you comparing the lidar data?

Answer: Figure 8 contains only THELISYS retrievals. There was a typo that corrected in the revised version of the manuscript.

Fig9 & 10: Why not present together these two figures?

Answer: As directly comparing the retrievals from the different monitoring techniques used in our work is out of scope of this study, we discuss separately the retrievals from THELISYS and CALIPSO.

Fig11: I do not think that this figure adds much to the discussion.

Answer: We thank the reviewer for pointing this out. We added some discussion in the revised version related to the Figure 11.

Technical Comments:

Ln27: Change "space-born" with "space-borne" and everywhere else.

Answer: We replaced as suggested by the reviewer.

Ln160: Change "C-Train" with "A-Train".

Answer: Changed as suggested.

Ln187: Please rephrase.

Answer: The sentence now reads: Additionally, the tropopause height and temperature are considered in the classification.

Tab2 (CAMS LHexp): Remove "Illustrate".

Answer: Changed as suggested.

Ln307: Remove "strong".

Answer: Removed as suggested.

3.3: Check the wording of this title.

Answer: The title is changed as: “Identification of the stratospheric layer origin using CAMS ICBG simulations”

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report (Previous Reviewer 3)

Comments and Suggestions for Authors

Review of article:

“Investigating a persistent stratospheric aerosol layer observed
over Southern Europe during 2019”
by Kalliopi Artemis Voudouri et al.,

submission #2640733 (version 3) in Remote Sensing (October 2023)

I. General remarks

1. Content of the article

This article presents an observational study of a stratospheric aerosol layer that persisted during five months over Greece, based mainly on lidar measurements (from a ground-based station and space-borne) and supported by other passive space-borne and model/reanalyzes observations. The organization of the article is standard. The results’ section discusses the temporal evolution and statistical properties of the layer optical properties, altitude and geometric depth. In this section, the layer is also attributed to the Raikoke volcano eruption using aerosol typing from the CALIPSO space-borne lidar and model outputs from stratospheric reanalyzes that assimilate stratospheric SO₂ space-borne observations. Finally, the results’ section includes a case study to compare the different observation more in detail.

Compared to the second version of the article, the weaknesses in the presentation of the study have been corrected, i.e. the objectives are now clearly presented in the abstract and introduction, and the conclusions explicitly refer to these objectives. The specific corrections I requested have been implemented.

2. Form

The sentences added to the article still need some English corrections and some rephrasing for clarity, but this does not bear on the content.

3. Overall rating

This article is now at the minor correction stage. Only the few language issue still need to be fixed.

II. Specific remarks

Section 1: Introduction

Lines 87-91. The displacement operated in this sentence is not relevant, as the resulting sentence does not make sense (“this permanent during the second part of 2019, but not always stable, aerosol layer”). Here is my suggestion, cutting the sentence in two for more readability: “In this contribution, we discuss the complex stratospheric aerosol conditions that took place above Thessaloniki during the second part of 2019, with simultaneously occurring volcanic and smoke layers. These events are depicted combining different ground and space-based sensing platforms, with the aim of identifying the main source for this long-lasting, but not always stable, stratospheric aerosol layer.”

Line 92. Typo: “three-ford” should be “threefold” or “three-fold”.

Line 94. “compositional” should be “composition”, it does not have the same meaning.

Lines 93-94. I do not think “hence” is properly used here (what comes after “hence” should be the consequence of what comes before). My suggestion: “to identify any temporal changes in the aerosol properties indicative of composition changes in the layer” (no need here to mention that the layer was persistent, as it said just before it is long-lasting).

Section 2: Instrumentation and data products

Lines 140-142. “defined as that level at which there is first an increase in signal above the clear background level and of a magnitude equal to 3 times the standard deviation of the background fluctuations” this sentence is not very good English. My suggestion: “defined as the lowest altitude where the signal increases above the clear background level by a magnitude of at least 3 times the standard deviation of the background fluctuations”

Line 168. “’in a good agreement”, the a should be removed.

Lines 278-279. “their expected input to this study” the word “expected” would fit in a project proposal, but is strange when presenting results. My suggestion: simply write “their input/contribution to this study” or make a phrase like “why they were selected for this study”

Section 3: Results & Discussion

Line 298-300. I think the first sentence of the paragraph should be removed; it just exposes what is generally expected of a results and discussion section, but does not contain any specific information about the section’s structure.

Section 3.1: The temporal evolution of the stratospheric layer

Lines 331-332. “the molecular attenuated backscatter profile value at 8km” If the backscatter is taken at a given altitude, it is not a profile anymore, but a value.

Line 333. “11 July 2019” the authors should decide if their just write the day number or use the “th” exponent (I think both are possible nowadays), but this should be consistent along the paper (the other dates are written as 11^th July 2019).

Line 344, Figure 1. The title is strangely cut in the middle: “THELISYS time-height cross-section of the 1064nm”, the 1064nm what? Given the fact that all the information is already in the legend and color bar title, I think the title can be removed completely or replaced by something more generic such as “THELISYS stratospheric profiles”

Line 371. “As to the stratospheric content…”; “as to” is not proper English. It should be “Regarding/concerning the stratospheric content…”

Section 3.2: Identification of the origin of the stratospheric layer

Line 411. I think you have had plenty of time to improve the quality of Figure 4, please provide a non-pixelated copy for the next article version.

Lines 451-454. I think the sentence would more readable this way (in any case, the verbs should be at the past tense): “Using model simulations, the OMPS-LP extinction measurements et 869nm and the Angstrom exponent, also for the Raikoke eruption, Wells et al. [51] showed that including ash in the model simulation provided better agreement with the measurements, indicating that the observed volcanic particles were mostly sulfate mixed with ash.”

Section 3.5: Correlative stratospheric measurements on July 25, 2019

Line 590. The sentence should end by a reference to Figure 2 of the supplementary material (that should be renamed Figure S2 to avoid confusion). About this figure, there is no use reproducing Figure 13; Figure S2 should contain only the orbit track. It is possible to color the points along the track following the stratospheric AOD to make it more obvious where the layer was observed.

Line 593. Typo: “analysed” should be “analyzed”.

Lines 593-595. I think this sentence about the location of CALIPSO overpasses should explicitly refer to Figure 1 of the supplementary (that should be renamed Figure S1 for clarity).

Line 602. Typo, text to remove: “Figure 14 (right). Figure 14 (b and d) shows…”

Lines 603-604. No need to repeat the figure legend in the text. All the following can be removed: “where blue and red lines represent the base and top heights of the detected aerosol”.

Lines 604 to 608. The past tense should be used when describing the results: line 604 “were found”, line 605 “showed”, line 607 “exhibited”, line 608 “was found”.

Line 609. Choose between plural and singular: “at height ranges” or at a height range of”

Lines 613-621. To avoid repeating the track date in Figure 14 legend, it could be rephrased as: “Aerosol layer properties observed by CALISPO on 25^th July 2019 during the daytime (a, b) and night-time (c, d) overpasses across Greece: aerosol subtyping (a, c) and layer geometrical boundaries (b, d), with top and base altitudes represented as red and blue dots, respectively. Figures adapted from https://www-calipso.larc.nasa.gov/products/lidar/browse_images/std_v4_index.php, last access: 31 August 2023. The red vertical line denotes the closest track point location to Thessaloniki lidar station.” Same remark about the date format, with or without the “th”, that should be homogeneous throughout the paper.

Line 652. Again, same remark about the date format, with or without the “th”.

Section 4: Conclusions

Line 668. “different detection limits” I think the proper word here is “limitations”

Line 710. “will strongly indicate the specific nature of the particles” the word “strongly” is not the proper word here. I think this should be rephrased into something like: “will indicate more specifically the particles’ nature” or “will allow to determine the particles’ nature with more confidence”.

Appendix

Figure A1 should be in the supplementary material, there is no point in putting one additional figure in Appendix and two in the supplementary material, as figures in the supplementary can (and should) also be referred to in the text, provided they are named S1, S2 etc. The reference in line 523 should be changed accordingly.

References

Line 882. Remove “Retrieved Jan 29 2023 from”

Supplementary material

As said above, the supplementary figures should be renamed S1 and S2 to avoid confusion with those in the main paper.

Figure S1. “The red dot indicates the geographical location of the study area” a dot does not show an area. It should be: “The red dot indicates the location of the THELISYS ground-based lidar”

Figure S2. The blue cross is actually red. As said above, Figure 13 should not be repeated here.

Comments on the Quality of English Language

English corrections are still needed, in the sentences that have been added to the paper in this version. See the specific comments.

Author Response

General remarks

We thank again the reviewer for such diligent attention which led to us greatly improving our manuscript.

Section 1: Introduction

We thank the reviewer for this comment. Updated in the revised version as suggested.

Line 92. Typo: “three-ford” should be “threefold” or “three-fold”.

Updated as suggested.

Line 94. “compositional” should be “composition”, it does not have the same meaning.

Updated as suggested.

Section 2: Instrumentation and data products

Altered as suggested to “A SNR threshold, defined as the lowest altitude where the signal increases above the clear background level by a magnitude of at least 3 times the standard deviation of the background fluctuations [38], since the lidar signal is strongly attenuated at higher altitude levels and the noisy parts of the signal should be rejected.”

Line 168. “’in a good agreement”, the a should be removed.

Updated as suggested.

Section 3: Results & Discussion

Updated as suggested.

Section 3.1: The temporal evolution of the stratospheric layer

Lines 331-332. “the molecular attenuated backscatter profile value at 8km” If the backscatter is taken at a given altitude, it is not a profile anymore, but a value.

The sentence is rewritten as follows: “the molecular attenuated backscatter profile value at 8km.”

Dates were homogenized with the exponent format.

Updated as suggested.

Line 371. “As to the stratospheric content…”; “as to” is not proper English. It should be “Regarding/concerning the stratospheric content…”

Updated as suggested.

Section 3.2: Identification of the origin of the stratospheric layer

Line 411. I think you have had plenty of time to improve the quality of Figure 4, please provide a non-pixelated copy for the next article version.

We thank the reviewer for pointing this out. The original images are in PNG format (better quality), and we have checked the right exporting options (higher quality) from word to pdf format. However, the figure has the right resolution only after accepting the changes in the revised version of the manuscript. Thus, the pictures quality will be high in the final pdf.

Updated as suggested.

Section 3.5: Correlative stratospheric measurements on July 25, 2019

The figure is updated as suggested.

Line 593. Typo: “analysed” should be “analyzed”.

Updated as suggested.

Lines 593-595. I think this sentence about the location of CALIPSO overpasses should explicitly refer to Figure 1 of the supplementary (that should be renamed Figure S1 for clarity).

Updated as suggested.

Line 602. Typo, text to remove: “Figure 14 (right). Figure 14 (b and d) shows…”

Updated correctly.

Lines 603-604. No need to repeat the figure legend in the text. All the following can be removed: “where blue and red lines represent the base and top heights of the detected aerosol”.

Updated as suggested.

Lines 604 to 608. The past tense should be used when describing the results: line 604 “were found”, line 605 “showed”, line 607 “exhibited”, line 608 “was found”.

Updated as suggested.

Line 609. Choose between plural and singular: “at height ranges” or at a height range of”

Updated as suggested.

Lines 613-621. To avoid repeating the track date in Figure 14 legend, it could be rephrased as: “Aerosol layer properties observed by CALISPO on 25th July 2019 during the daytime (a, b) and night-time (c, d) overpasses across Greece: aerosol subtyping (a, c) and layer geometrical boundaries (b, d), with top and base altitudes represented as red and blue dots, respectively. Figures adapted from https://www-calipso.larc.nasa.gov/products/lidar/browse_images/std_v4_index.php, last access: 31 August 2023. The red vertical line denotes the closest track point location to Thessaloniki lidar station.” Same remark about the date format, with or without the “th”, that should be homogeneous throughout the paper.

Updated as suggested.

Line 652. Again, same remark about the date format, with or without the “th”.

Updated as suggested.

Section 4: Conclusions

Line 668. “different detection limits” I think the proper word here is “limitations”

Updated as suggested.

Appendix

Updated as suggested. Figure A1 is moved to the supplementary material (as Figure S2) and referred accordingly in the revised version of the manuscript.

References

Line 882. Remove “Retrieved Jan 29 2023 from”

Updated as suggested.

Supplementary material

As said above, the supplementary figures should be renamed S1 and S2 to avoid confusion with those in the main paper.

Figures in the supplementary file are updated as suggested.

Updated as suggested.

Figure S2. The blue cross is actually red. As said above, Figure 13 should not be repeated here.

Updated as suggested.

Author Response File: Author Response.docx

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The main purpose of this paper is to provide an overview of the characteristics of atmospheric characteristics during the 2019 measurement period, which indicate changes in the composition of long-term stratospheric plumes over Thessaloniki. This article provides a very detailed analysis of the stratospheric aerosol layer over southern Europe in 2019 using ground-based, satellite observation data, and reanalysis data, which has a very positive impact on our understanding of the physical properties of the aerosol layer. This article mainly focuses on describing phenomena, and I have no scientific doubts about this article. Overall, the logical structure of the article is reasonable and has clear language expression. I believe this article can be directly published.

Reviewer 2 Report

Comments and Suggestions for Authors

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Reviewer 3 Report

Comments and Suggestions for Authors

Review of article: “Investigating a persistent stratospheric aerosol layer observed over Southern Europe during 2019” by Kalliopi Artemis Voudouri et al., submission #2419662, version 1, in Remote Sensing (May 2023)

I. General remarks

1. Content of the article

This article presents an observational study of a stratospheric aerosol layer that persisted during five months over Greece, based mainly on lidar measurements (from a ground-based station and space-borne) and supported by other passive space-borne and reanalyzes observations. The organization of the article is standard with an introduction, a section presenting the instrumentation along with the satellite data and modelling products used in the study, a results’ section and a conclusion. The results’ section discusses the temporal evolution and statistical properties of the layer in terms of aerosol optical thickness (AOD), altitude and geometric depth. In this section, the layer is also attributed to the Raikoke volcano eruption using aerosol typing from the CALIPSO space-borne lidar and model outputs from stratospheric reanalyzes that assimilate stratospheric SO₂ space-borne observations. Finally, the results’ section includes a case study to compare the different observation in detail.

My main concern about this article is that the scientific purpose(s) that motivated the study is(are) not clear to me, nor what it really brings to the community. If this article were oriented as a study aiming to determine the volcanic aerosol optical properties and their variability, then I would expect more on this side (e.g. lidar AOD values provided at 532 and 1064 nm for both instruments, distributions of extinction-to-backscatter ratio values…). If the idea was to show the impact of the volcanic layer contamination by wildfire smoke aerosols, then I would expect model or observational data to identify precisely during which periods a smoke plume was present above Greece (e.g. CO space-borne observations, back-trajectories…). This would allow to discriminate pure volcanic and mixed layer observations and compare the optical properties of the layers in both types of cases.

However, the possibility that wildfire smoke aerosols contaminated the volcanic layer is only supported by two references given in the introduction, while the CALIPSO data presented do not allow to discriminate the pure/mixed cases, and the effect of the mixing on the aerosol optical properties is not assessed. This is my second concern about this study.

2. Form

I think that Section 3 (results) would be clearer if all the layer observations (temporal evolution and statistical properties) were presented in a row (currently, they are in Sections 3.1 and 3.4). Then, all the clues to identify the layer content should be put together (currently, they are distributed in Sections 3.2 and 3.3). This would give something like:

3.1 Long-term observations of the stratospheric layer

3.1.1 Temporal evolution of the layer

3.1.2 Geometrical and optical aerosol properties of the layer

3.2 Origin of the stratospheric layer

3.2.1 Identification from satellite observations

3.2.2 Identification from models / reanalyzes

3.3 Case study

Several figure and table legends are not precise enough, as they do not say which instrument or model was used to produce them. Sometimes, the information is present in the graph title, but this is not convenient, as one normally reads the image details only after reading the legend below. In a general way, figure legends should be self-consistent, so that someone peering through the paper by looking only at the figures could understand them without reading the main text.

Some acronyms are not properly defined, particularly in Section 2.4, please refer to the guidelines for authors: “Acronyms/Abbreviations/Initialisms should be defined the first time they appear in each of three sections: the abstract; the main text; the first figure or table. When defined for the first time, the acronym/abbreviation/initialism should be added in parentheses after the written-out form.”

Numbers and their corresponding units are supposed to be separated by a space. Use an unbreakable space to avoid line break or horizontal stretching in justified text (CTRL+SHIFT+SPACE in MS Office Word, ~ symbol in LaTeX). Dates in text format do not include a coma between the day/month and the year (e.g. “25 June 2019”, but not “25 June, 2019”). Sulfur dioxide is noted SO2 in the text and SO₂ in the tables, please harmonize.

1. Overall rating

To summarize, this article presents a large and interesting collection of data, but remains too descriptive in my opinion. This would be a good conference paper, but it is still a bit weak for a research article. The authors should decide on what scientific objective they want to pursue, and push the analysis as far as possible in this direction. Therefore, I recommend major revision.

I. Specific remarks

Abstract

Line 34. “contributed”, past would be better.

Section 1: Introduction

Line 46. One of the references cited dates back from 1989; are there some more recent references about the impact of stratospheric aerosols on climate?

Lines 61-62. “as well as satellite-borne instrumentation [20, 21, 22]-and space-born spectrometers [23]”. It is strange to separate spectrometers from the rest of the instrumentation; maybe rephrase as “as well as satellite-borne instrumentation, including spectrometers [20, 21, 22, 23]”. If not, missing space after the first reference list.

Line 92 and 95. “Sect. 2”: I would not use the abbreviation in the main text of the sentence.

Section 2: Instrumentation and data products

Line 122. Typo: “Platt et al., [34]”, coma to remove.

Line 122-125. “The base of the plume is defined as the height where the increase in the system’s signal level equals two times the standard deviation of the background, whilst the top is determined by calculating the standard deviation of noise above cloud top and moving downward in altitude from maximum range.” I guess that, for the top, there is a similar threshold of two times the standard deviation of the noise but the sentence is not clear on this point: could you precise?

Line 137. English: “in the case whence lidar signals” should be something like “assuming lidar signals” or “in the cases for which”.

Lines 162-163. “VFM products are classified into characteristic classes [41], namely into clear air, cloud, aerosol, stratospheric, surface, subsurface, totally attenuated or invalid feature types.” For people who are not using this product, it is not clear. It is not the products that are classified but the observation points that are attributed with classes. Also, if I understand correctly, the classes are overlapping as aerosols can be stratospheric or not. Please rephrase this sentence for better clarity.

Line 167. Where do the tropopause height and temperature used in CALIPSO data processing come from? The legend of Figure 2 mentions the “NASA Global Modelling and Assimilation Office near-real time atmospheric analyses” for OMPS-LP data. Was the same height used for CALIPSO and for the THELYSIS data processing? This information should be in the main text as the tropopause height will influence greatly the computation of the stratospheric AOD.

Line 195. “5-day forecast”, please avoid abbreviation.

Line 207. Acronym “LH” not previously defined, I guess it stands for “layer height”.

Line 209. Acronym “NH” not previously defined, I guess it stands for “northern hemisphere”.

Line 215. Acronym “ICBG” not defined.

Lines 215-216. Acronyms “IFS”, “GLOMAP”, “BASCOE” are defined afterwards. To avoid this, I suggest rephrase the beginning of the sentence as “The ICBG results from the coupling of two integrated forecast systems (IFS):” and then use a bullet list.

Lines 234-236. “an overview of the observational and modelling geophysical parameters derived products of the in tandem usage of the different instrumentation”, this part of the sentence is not clear, please rephrase.

Table 1. Does “Monday-Thursday” means Monday and Thursday or Monday to Thursday?

Table 2. “Layer top &base” missing space after the &. Acronym “D.U.” not defined.

Section 3: Results & Discussion

Line 256. “considerable amounts of [missing word] and ash”.

Section 3.1: The temporal evolution of the stratospheric layer

The initial “the” in the section title should be removed.

Line 282-283. “divided with the molecular profiles at 8km”, talking of a profile at a given altitude seems antonymic. Is it “divided with the molecular backscatter at 8km”?

Line 283 and 284-285. Typo: “11 July, 2019” and “1st of August, 2019” coma to remove in the date.

Figure 1. The legend should mention that these are THELYSIS observations, and that the width of each case has been adjusted to keep a regular time scale on the graph abscissa.

Figure 2. “at 997 nm”, unit is missing. “Top panel is…” → “The top panel is…”

Line 303. “around Thessaloniki for between July and December”, word for to remove.

Figure 3. Why is the SO₂ plume spread illustrated only for the first 4 weeks? What does CAMS show for the rest of the period? This does not necessary need to add subfigures, if they all show a full northern hemisphere spread of the plume, but the matter should be discussed in the text. Also, for this type of figure, consider making a video out of the series of maps; this could be added as supplementary material and would allow illustrating the full period. Under Unix this can be done very simply using the ffmpeg command.

Section 3.2: Identification of the origin of the stratospheric layer

Figure 4. The resolution of the image is not sufficient to read the axes legends when zooming in. Please improve the resolution of the image. Same for Figure 13.

Line 364. Typo: “Ansmann et al., [60]”, coma to remove.

Figure 5 (left). The legend indicates this figure aggregates data from July to September but does not explain how. As this is aerosol typing data, it is obviously not a simple average. What happens if both types of layers (smoke and ash) are detected at the same height/latitude, but at different dates: which one will prevail? I suggest plotting, separately for each aerosol type, the number of detections cumulated over the 3-month period, as a function of height/latitude.

Figure 5 (right). The pie chart is not really useful, giving the fraction in the text is enough.

I have more general remark about this section: the article mentions the possibility of smoke from arctic fires contaminating the volcanic plume, so it would have been interesting to find another satellite product allowing to identify smoke plumes (e.g. using CO as a tracer). Another possibility would have been to cross back-trajectories (e.g. from HYSPLIT) with a satellite fire product (e.g. MODIS active fire product, available from https://modis-fire.umd.edu/). At least, try something to identify during which periods exactly smoke was present over Thessaloniki.

Section 3.3: Identification of the origin of the stratospheric layer CAMS ICBG simulations of the stratospheric layer

I think this sub-section should be grouped with the previous one, possibly using sub-subsections.

Line 389. The use of “whence” is correct here, but this word is archaic and not widely known. I suggest replacing by “at the time when”.

Figure 6. Typo: “(bottom.)” the dot should be out of the parentheses. Adding the tropopause height line on this figure would be nice.

Line 396. “with [in blue] and without [in black]”, the color legend does not need to be reminded in the main text.

Figure 7. I think this figure should be integrated as a subfigure of Figure 6, as it proceeds from the same data source.

Section 3.4: Geometrical and optical aerosol properties from ground and space-based measurements

I think the understanding would be easier if this section were to follow directly Section 3.1. I suggest grouping them into a single one, with two sub-subsections.

Line 411. Typo: “with an (infrared) AOD”, no need for the parentheses here + missing “an”.

Line 411-412. Typo: “possible” should be “possibly”.

Line 412. I would expect a few words to compare the stratospheric AOD from the lidar and the model, that was presented just before. The higher values observed on the lidar between mid-August and early October could support your hypothesis on smoke contamination.

Line 423. “in the altitude range”, missing “the”.

Line 425. English: “found to be equal to”, missing verb.

Line 426. “of (infrared) AODs being”, no need for parentheses.

Figure 9. The legend should mention that these are THELYSIS observations (the reader should not need to go to the plot title to get this piece of information).

Line 442-443. “AOD (at 532 nm) column between 1 July – 30 September 2019”, the word column is redundant with AOD + the em-dash should be replaced by a “and” if using “between”.

Figure 10 should be put in supplementary as it does not bring interesting elements to the discussion.

Figure 11. The legend should mention that these are CALIPSO observations (the reader should not need to go to the plot title to get this piece of information).

Figures 9 and 11 should be gathered in a single figure, that would allow to present side by side the THELYSIS and CALIPSO results. Any comment on why the secondary peak on stratospheric AOD (between 0.07 and 0.10) is not visible on CALIPSO data?

I have more general questions and remarks about this subsection:

− To ease the comparison with THELYSIS, why not compute the stratospheric AOD using the 1064 nm channel of CALIPSO?

− Why not include OMPS-LP in the stratospheric AOD comparison? Could you not derive some layer altitude information from OMPS-LP, although the vertical resolution is of course, much coarser?

− Why not compute the layer-average depolarization from CALIPSO and THELYSIS and compare both instruments also based on this property, in addition to the stratospheric AOD and layer boundaries’ altitude?

− Why present only statistical distributions and not point-to-point (scatter plots) of CALIPSO vs THELYSIS? This would allow, for instance, to discuss the accuracy of CALIPSO’s layer boundaries retrieval algorithm.

− Why is the extinction-to-backscatter ratio values retrieved from the Raman inversion of night-time THELYSIS data not presented? Having values compatible with other observations of sulfate aerosols would also be an indication of the plume composition.

Section 3.5: Correlative stratospheric measurements on July 25, 2019

Subsection title (line 454-455): please remove the coma in the date.

I suggest adding to the supplementary material the CAMS map for the day of the case study and a map showing the orbit track of CALISPO and OMPS-LP on this day. This would help discuss CALIOP and OMPS-LP observations presented on Figures 12 and 13.

Figure 12. The tropopause height drops abruptly when entering the plume (around 40°N), this raises questions about how the aerosol layer influences the determinations of the tropopause height (from which source, the legend does not say). Does the radiative effect of the aerosol layer change the temperature profile enough to lower the retrieved tropopause height by a few km? This might seem out of the scope of the paper, but the tropopause height is used to compute the stratospheric AOD, and it is important to know if the plume is indeed contained in the stratosphere. Are there other observations that could help checking the tropopause height, e.g. radiosonde profiles?

Line 473. “Similarly, OMPS-LP observations”, similarly to what? If this is just a transition word, please chose another one.

Figure 13 and 14. It is not straightforward to correlate the layers visible on Figure 13 with the layer boundaries represented on Figure 14 (in particular, for the nighttime overpass, for which the abscissa direction is reverted between the two figures). Therefore, I suggest to gather these two figures, and replot the layer boundaries using the same abscissa as for the backscatter and aerosol typing plots. The new Figure would then have 6 panels, e.g. with the left (right) column being the daytime (nighttime) overpass plots, and the three lines being the backscatter, the aerosol typing and the layer boundaries plots. For comparison purposes, it would also be interesting to add the THELYSIS points of the layer boundaries’ plot, at the closest track point location.

Table 3. How are the CALIPSO, ICBG and OMPS-LP values computed? For CALIPSO, are they averages along the track for the layer highlighted by the dashed ellipses on Figures 13 and 14? For OMPS-LP, is it like in Figure 2, i.e. an average over an area? Would it not be possible to compute the AOD at 1064 from CALIPSO measurements?

Table 3 (continued). The wavelengths should be reminded in the table legend, while the line names would be “AOD infrared” and “AOD green”. The unit for the altitudes (km) should be put preferentially in the line name to avoid repetition. Typo: the value 15.4±1.71 km should be 15.40±1.71 km, with the same precision of the average and standard deviation values.

Section 4: Conclusions

Line 541. “are” should be “was” to be coherent with the rest of the text.

Line 545. Repeated words “was found”.

The conclusion ends abruptly after the list of findings. Conclusions usually end with a discussion about the limitations of the study and how it could / will be further improved. Are there no perspectives to this study?

References

Ref. 8. Missing volume/issue and page number.

Ref. 12. Please add the full author list.

Ref. 16. Missing page number and DOI.

Ref. 24. “Atmos. Chem. Phys. 21, 2021, 5597–5604” should likely be ”Atmos. Chem. Phys., 2021, 21, 5597–5604”. Please check.

Ref. 28. Same as ref. 24: the volume number seems to be stacked at the end of the journal name.

Ref. 33, 34, 35, 36 and 37. Missing DOI.

Ref. 51. Missing volumes/issue and page number.

Comments on the Quality of English Language

The English is overall good, modulo the few typos that exist in all article. I only advise removing the few archaic words like "whence" that are not commonly known.

Article Menu

Investigating a Persistent Stratospheric Aerosol Layer Observed over Southern Europe during 2019

I. General remarks

1. Content of the article

2. Form

3. Overall rating

II. Specific remarks

Abstract

Section 1: Introduction

Section 2: Instrumentation and data products

Section 3: Results & Discussion

Section 3.1: The temporal evolution of the stratospheric layer

Section 3.2: Identification of the origin of the stratospheric layer

Section 3.3: Identification of the origin of the stratospheric layer CAMS ICBG simulations of the stratospheric layer

Section 3.4: Geometrical and optical aerosol properties from ground and space-based measurements

Section 3.5: Correlative stratospheric measurements on July 25, 2019

Section 4: Conclusions

References

I. General remarks

1. Content of the article

2. Form

3. Overall rating

II. Specific remarks

Abstract

Section 1: Introduction

Section 2: Instrumentation and data products

Section 3: Results & Discussion

Section 3.1: The temporal evolution of the stratospheric layer

Section 3.2: Identification of the origin of the stratospheric layer

Section 3.3: Identification of the origin of the stratospheric layer CAMS ICBG simulations of the stratospheric layer

Section 3.4: Geometrical and optical aerosol properties from ground and space-based measurements

Section 3.5: Correlative stratospheric measurements on July 25, 2019

Section 4: Conclusions

References

I. General remarks

1. Content of the article

2. Form

3. Overall rating

II. Specific remarks

Section 1: Introduction

Section 2: Instrumentation and data products

Section 3: Results & Discussion

Section 3.1: The temporal evolution of the stratospheric layer

Section 3.2: Identification of the origin of the stratospheric layer

Section 3.5: Correlative stratospheric measurements on July 25, 2019

Section 4: Conclusions

Appendix

References

Supplementary material

I. General remarks

1. Content of the article

2. Form

1. Overall rating

I. Specific remarks

Abstract

Section 1: Introduction

Section 2: Instrumentation and data products

Section 3: Results & Discussion

Section 3.1: The temporal evolution of the stratospheric layer

Section 3.2: Identification of the origin of the stratospheric layer

Section 3.3: Identification of the origin of the stratospheric layer CAMS ICBG simulations of the stratospheric layer

Section 3.4: Geometrical and optical aerosol properties from ground and space-based measurements

Section 3.5: Correlative stratospheric measurements on July 25, 2019

Section 4: Conclusions

References

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