Optical Properties of North Atlantic Aerosols Through a compact dual-wavelength depolarization Lidar Observations

González, Yenny; Sánchez-Barrero, María F.; Popovici, Ioana; Barreto, África; Victori, Stephane; Welton, Ellsworth J.; García, Rosa D.; Sicilia, Pablo G.; Almansa, Fernando A.; Torres, Carlos; Goloub, Philippe

doi:https://doi.org/10.5194/egusphere-2024-2727

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https://doi.org/10.5194/egusphere-2024-2727

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19 Nov 2024

| 19 Nov 2024

Optical Properties of North Atlantic Aerosols Through a compact dual-wavelength depolarization Lidar Observations

Yenny González, María F. Sánchez-Barrero, Ioana Popovici, África Barreto, Stephane Victori, Ellsworth J. Welton, Rosa D. García, Pablo G. Sicilia, Fernando A. Almansa, Carlos Torres, and Philippe Goloub

Abstract. We present a characterization of the optical properties of different aerosol types based on data collected using a compact dual-wavelength depolarization elastic lidar (532 and 808 nm, CIMEL CE376). We evaluate the vertical distribution and temporal evolution of a variety of aerosols observed in the subtropical North Atlantic region, covering from Saharan dust outbreaks and volcanic aerosols to fresh and aged wildfires, observed between August 2021 and August 2023, using a modified two-wavelength Klett inversion method to derive the aerosol backscatter and extinction coefficients from CE376 lidar observations. We assessed the performance of the CE376 lidar during an intercomparison campaign with an MPL-4B lidar (MPLNET) while collocated at the facilities of the Izaña Atmospheric Research Centre (Canary Islands, Spain). Both instruments depicted the vertical aerosol structure similarly. The main differences were attributed to errors arising from the determination of the overlap function and the depolarization calibration in each instrument and the larger effect of the solar background on the CE376 system during daylight. Absolute differences in the volume depolarization ratio (δ^v) were 0.3 % reduced to 0.2 % when only nighttime data was used. The measurements of particle linear depolarization (δ^P), extinction Ångström exponent (EAE (532/808)) and attenuated colour ratio (ACR (808-532)), provided by the combination of the two channels of the CE376, allow us to describe the composition and size of the studied aerosols. Fresh Saharan dust particles were the largest particles found in this study with non-spherical morphology and traveling in a well-mixed layer, exhibiting the lowest EAE (532/808) and highest ACR (808-532) and δ^P (532) > 0.15. The smaller particles with quasi homogeneous morphology were attributed to sulphate aerosol from the initial stage of the Cumbre Vieja volcano eruption and aged Canadian wildfires traveling across the Atlantic, showing the lowest δ^P (< 0.08) and the highest EAE (532/808) (>1). Middle values of these retrieved parameters were associated with the heterogenous mixture of ash, soot, and charred vegetation from fresh local forest wildfires. The retrieved properties demonstrate the excellent performance of the CE376 micro-lidar as a tool for continuous monitoring and characterisation of the temporal and vertical distribution of atmospheric aerosols.

Received: 30 Aug 2024 – Discussion started: 19 Nov 2024

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RC1: 'Comment on egusphere-2024-2727', Anonymous Referee #1, 09 Dec 2024

The authors present a comparison study of two lidar instruments (one well established and one rather novel, upgraded one) together with an interesting, however not highly novel, set of measurements and products (optical properties from the rather novel instrument) which can be incorporated in aerosol-typing, -monitoring, and -modelling efforts. Therefore, the manuscript is suitable for publication in AMT and I suggest the publication after minor revisions after addressing the points listed below.
Major comments
Line 1–2: I suggest to modify the title of the publication slightly. First, I would avoid capitalization. Second, I would also avoid “North Atlantic aerosol” but write “different aerosol types observed at the North Atlantic” or similar.
Line 189 and following: You retrieved (columnar) lidar ratios iteratively by fitting your (whole/or parts of) lidar profile to AERONET AOD, which is a sound method. However, I recommend to be cautious or even to refrain from calling the lidar ratios “measured” or “observed” (e.g., Line 575, Line 606). In Line 321 (and some more), you call it “effective LR retrieved”, which I believe is more appropriate. I recommend to do it in this way consistently throughout the manuscript.
In Line 191, you say that daylight impairs 808 nm detection above 2 km a.g.l., and that you solve this by forward Klett. How about 532 nm? Is it totally free of this impairment? Or does it just occur at higher altitudes as well (which would less impact your retrieval)? Also, in Line 193, concerning the estimated AOD at 808 nm, I think this needs to be explained more clearly, or a reference to Sanchez-Barrero et al. (2024) has to be given (like in Line 380–381).
Line 224–233: This whole section rather belongs to the uncertainty discussion (Line 171 following).
Line 231–232: The sentence “The convergence of the AOD leads then to the uncertainties of the LR.” is not clear enough to me. How does this work? Or is this explained in detail in Sanchez-Barrero et al. (2024)? Then add that reference and a sentence stating that there as well.
Line 269 (and others): Do the reported ±-ranges include uncertainties of the individual instruments? What is the reference? I guess, the MPL? So, the values of the CIMEL are on average all smaller?
In general, when speaking about Cumbre Vieja sulphate, it would be valuable to cite Gebauer et al. (2024, https://doi.org/10.5194/acp-24-5047-2024), especially with regards to lidar ratios (of 66.9 ± 10.1, 60.2 ± 9.2 and 30.8 ± 8.7 sr at 355, 532 and 1064 nm) reported by them, which were directly measured by Raman lidar.
Minor comments:
Line 25–26: I would avoid to report volume depolarization ratios in %, but rather in floating point numbers. This then also avoids confusion about absolute and relative differences.
Line 55: “cloud altitude”, I suggest to write cloud base height.
After Line 95, a short paragraph summarizing the content of the manuscript could be added to guide the reader about a little bit. Something like “In Sect. xyz, we describe this and that. The results are shown in Sect. xyz”.
Line 100–102: Is the laser at 808 nm also eye-safe? And Line 655–656: would it be still eye-safe after that upgrade?
Line 176: “AOD<0.03”. From lidar or photometer? At which wavelength?
Line 204: The software iAAMS, is it open source? Is it part of the CE376 CIMEL micro-lidar package? Can it be cited?
Also Lines 655–656: “Upcoming developments on the CE376 CIMEL micro-lidar aim to upgrade both lasers, the visible laser (532 nm) to a more powerful one, and the near-infrared laser (808nm) to reduce its sensitivity to solar background during daylight.” This sentence is not well comprehensible (due to imho wrong comas and position of parts of the sentence). The sensitivity to solar background during daylight is not a property of the laser but the detectors/field-of-view. Of course, stronger lasers help in detecting signals also in daylight contaminated periods of the day. Please, change the sentence.
In most figures: Is there really a.g.l. and not a.s.l. meant? I believe the latter. I recommend to call the y-axis always the same (Z, but better altitude or even height) with always the same meaning (a.s.l. or a.g.l.) and unit (m or km (not Km)).
Minor technical and spelling comments:
Generally, write “Fig.” when you cite a figure in a sentence. Only use “Figure” at the very beginning of a sentence. The same is true for Section (Sect.) and Equation (Eq.).
Generally, try to use a single, consistent date format (everywhere, in text, figures and captions). I suggest as an example: “15 December 1965” or “December 15, 1965”. In Line 493 for example, it is suddenly different. Furthermore, when you give a timestamp, always add UTC (I believe it is UTC everywhere).
Generally, use – (--) hyphen for intervals. Either consistently use spaces before and after plus-minus signs or not, for example 5~$\pm$~6 (as mostly in the manuscript) or only 5$\pm$6 (as in Sect. 5).
Figure 1: The sentence in the caption “Reference: Google (2023) Montaña de Izaña. Imágenes ©2023 CNES / Airbus, 252 GRAFCAN, Maxar Technologies [accessed on January 31, 2024]” is not a particular help if one wants to find this, even though it is much information/text. Better provide a clear link and add “adapted”.
Also in Fig. 1, the sentence “Yellow dashed bars in (a)-(e) highlight the time frames selected to calculate the averaged profiles described in Figure 2.“ has to be placed before f) and g) are described.
Line 10: In “F-59000”, delete “F-“, as you also do not add it in Line 8 (“Paris”).
Line 12: Add USA in the end, as you added France and Spain as well above.
Line 13: What is TRAGSATEC? Is it an abbreviation? Write it out.
Line 67: “Laboratoire d’optique atmosphérique” with capital letters like in Line 10. This whole sentence concerning software and the joint laboratory could be rephrased or moved to some other place, where it fits better (maybe along iAAMS in Line 204?).
Line 72–73 before unit hPa add non-line-breaking space (~ or \,).
Line 86: You use MPLNET before you introduce it (in Line 207–208).
Line 146–147: EARLINET abbreviation is not given. Last access date of url is not given. Furthermore, I suggest to cite Pappalardo et al. (2014, https://doi.org/10.5194/amt-7-2389-2014) in this context.
Line 147–148: “RCS (λ,r)” definetly should not line-break.
Line 160: I suggest to call it “Δ90° method” instead of “±45° method”, and also cite Freudenthaler et al. (2016). Or like Papetta et al. (2024, https://doi.org/10.5194/amt-17-1721-2024) who managed it this way: “The depolarization calibration suggested by the manufacturer follows the Δ(±45°) method described in Freudenthaler et al. (2009), later renamed to Δ(90°) (Freudenthaler, 2016).”
Line 178 and 228: “kilometer” instead of “kilometre”.
Line 238–239: add non-line-breaking space between wavelength (532, 808) and unit (nm).
Line 241: MODIS instead of Modis, add last access date to url, or better not to give url in text at all, but only in the caption of Fig. 1.
Line 253: and last access date to url.
Line 269: “21h- 6h” what is this? Local time? UTC? I would write “21:00-06:00 UTC” or even better night-time.
Line 270-271: non-line-breaking space before km.
Line 293: Call it everywhere the same “extinction Ångström exponent”.
Line 299: “In this section,” comma.
Line 311–312: The abbreviation SAL does not need to be explained again (it is already explained in Line 306). So just use SAL there.
Line 318, 326, 333, 378, 390, 397, 518, 524, 529, 535: I do not understand the underscoring. Avoid it.
Line 320: 808 non-line-breaking space nm.
Line 329–330: non-line-breaking space before unit.
Line 333: Do not call it “the arrival of a new episode” but rather “arrival of a new dust layer”, “a new dust episode occurred”, or something like that.
Line 336–337: The unit has to be non-line-breaking as well (-1).
Line 358–359: non-line-breaking space before unit.
Line 370: last access date to url.
Line 367: “Image” or “Photo” instead of “Imagen”.
Line 385–386: no space between number and %, or \, which Copernicus uses, I believe.
Figure 5h and 6h. UTC? Date format?
Line 400–401: non-line-breaking spaces before unit.
Line 406: “slight” instead of “slightly”.
Line 440–441: non-line-breaking spaces before unit.
Line 441–442: non-line-breaking spaces before figure number.
Line 444: (non-line-breaking) spaces before units.
Line 476: non-line-breaking spaces before unit
Line 490, 491, 550, 551: Use μm instead of micron.
Line 551,552: Omit \cdot in units, as it is also not used in Mm^-1 sr^-1 for example.
Line 575–576: non-line-breaking spaces before unit.
Line 586: μm instead of um.
Line 629: “(0-24h)”, again, I would avoid that, just write the “full-day dataset” or something similar.
Line 656: non-line-breaking spaces before unit.
Line 657: For GRASP, a citation has to be added.
References:
Generally, use either standard abbreviations (as is in the end standard at Copernicus) or full journal names.
Generally, use a consistent doi format. Standard is with https://.
Generally, between initials of the first names, there needs to be a space (at Copernicus).
Line 710: last access dates have to be given.
Line 758: dois do not need a last access date.
Line 759: Cuevas et al. (2021): Would be nice to have an url (with last access date): https://www.aemet.es/documentos/es/conocermas/recursos_en_linea/publicaciones_y_estudios/publicaciones/GAW_Report_No_259/GAW_Report_No_259.pdf
Line 803: García et al. (2014): Pages are wrong (besides strange doi format). It should be 179–194. Furthermore, there should be a colon (:) after “station” in the title.
Line 806–809: Gasteiger and Freudenthaler (2014) is somehow mixed up with some chapter of IPCC.
Line 843: Kusmierczyk-Michulec et al. (2002) has three authors, not only one.
Line 867–871: Milford et al. (2023): There are strange semicolons in the author list. Furthermore, ISSN is not needed in a journal and the year (2023) usually does not need to be given twice.
Line 899–900: In Rodríguez et al. (2015), not “Löpez-Solano” but “López-Solano” and in the title not “North Africa dipol” but “North African dipol”.
Line 928–937: All three Tesche et al. (2009a,b,2011) citations have strange format (a missing year, “63:4”, “vol.”).

Citation: https://doi.org/10.5194/egusphere-2024-2727-RC1
RC2:
'Comment on egusphere-2024-2727', Anonymous Referee #2, 25 Dec 2024
The paper presents a comparative performance assessment of two collocated lidar systems in various atmospheric aerosol loading scenarios over the Izaña site in the North Atlantic. Although both systems were described in previous papers, the added value brought by this work is considered relevant and suitable for publication in AMT after the authors address the following minor revisions:

General comments:
The structure of the paper could be improved for a better understanding by the readers of the scope of the paper. A clearer structure, in line with the twofold motivation presented, with minimal effort from the authors’ part, could be something like:

Introduction

Instrumentation and methodology

Results and discussions
Intercomparison between the CE376 and MPL-4B

Distribution of aerosol properties derived from CE376
Saharan dust

Volcanic

Aerosol classification

Conclusions

Bibliography

Please have wording and phrasing checked by a native English speaker.

Please check all references for the correct citation form.

Particular comments:

Lines 1-2: Rephrase the title: introduce the idea that there are 2 lidar systems in discussion; the use of “North Atlantic aerosols” might induce to readers that it’s a new class of aerosols, which is not the case; “…a compact dual-wavelength depolarization Lidar Observations”, either you delete “a” or you delete the plural from” Observations”

Line 13: Please detail the abbreviation TRAGSATEC, provide address, etc.

Line 22: Consider using “while the systems were collocated”

Lines 31-32: When discussing fine mode, consider separating the values for Cumbre Vieja and Canadian wildfires

Line 34: Given the limitations discussed in the paper, consider replacing “excellent performance” with “suitability”

Line 42: Consider using “Ground-based remote sensing instruments…”

Line 80: Detail “These events”. What events are you referring to?

Line 86: You already used the CE376 abbreviation above. Introduce it when first used.

Lines 88-91: Mention also the use of the CE-318T photometer

Lines 92-95: This is a bit redundant placed here, you stated this at lines 39-47. Consider removing this paragraph.

Lines 97-204: Consider including in this section a paragraph discussing advantages and disadvantages of the lidar systems used in this paper compared to high-power lidars.

Line 97: Consider adding the CE-318T in the section title, given the fact that you discuss it extensively at lines 117-126.

Lines 102-103: Is the 808nm also eye-safe?

Lines 109-110: Add a citation for the technical data of the electronic cards, if available.

Line 146: Describe the acronym EARLINET (first used here).

Line 161-163: Why did you not perform more frequent +/- 45 polarization calibrations?

Line 176: AOD<0.03 @ what wavelength?

Lines 191-193: Please elaborate on the solar background limitation during daytime, and why it’s different between the 532nm and 808nm channels.

Line 206: Is this a proper validation process? “Intercomparison of the aerosol products of the CE376 and MPL-4B” would be more suitable.

Line 241: Rephrase “the grade of impact” with “spatial extent of the Saharan dust layer(s) over…”

Line 241: Replace “Modis” with “MODIS VIS channel”

Line 242: “The additional information given by the 808 nm channel of the CE376 is later discussed in more detail in section 4.1.” Either expand a bit here or remove this sentence.

Line 244: Increase the size of the photometer data and legend at e). It can be hard to read. This is valid for all subsequent similar plots.

Lines 252-253: Rephrase “the grade of impact” with “spatial extent of the Saharan dust layer(s) over…”; Replace “Modis” with “MODIS VIS channel”.

Line 289: Consider replacing “catching the impact of” with “depicting”;

Line 312: for dust - oil smoke mixes have a look at: https://doi.org/10.5194/acp-22-5071-2022

Lines 406-408: Please rephrase. The wording can be confusig. Avoid using “likely”, “slightly”. If you decide to keep it, it shoule be: “This is in slight contrast”

Line 417, Line 428, Line 431: As mentioned above, consider replacing “Impact” when describing aerosol optical properties.

Lines 456-457: For consistency, mention MODIS VIS channel as above.

Line 575: Add “retrieved” before LR.

Lines 596-597: Please rephrase: “In contrast, better constrained lidar ratios using sun photometers, alone, still not good for aerosol typing when using alone”.
Citation: https://doi.org/10.5194/egusphere-2024-2727-RC2

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Short summary

We characterize the optical properties of various aerosols using a compact dual-wavelength depolarization lidar (CIMEL CE376) at 532 and 808 nm. Through a modified two-wavelength Klett inversion method, we assess the vertical distribution and temporal evolution of Saharan dust, volcanic aerosols, and wildfire smoke in the subtropical North Atlantic from August 2021 to August 2023. The study confirms the CE376 lidar's effectiveness in monitoring and characterizing atmospheric aerosols over time.


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