the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 May 2026
Optical properties and global distribution of the Hunga aerosols 2022 observed by Aeolus and atmospheric lidars: new insights into the vertical sedimentation of stratospheric sulfate plumes
Abstract. Stratospheric aerosol plumes from the 2022 Hunga eruption were observed by the first-ever ultraviolet (UV) 355 nm Doppler and High Spectral Resolution Lidar (HSRL) on board the European Space Agency’s Aeolus satellite. Independent measurement of extinction and co-polarized backscatter coefficients for particles are shown, capturing the Hunga plumes up to ≈ 28 km in altitude. Global map of L2A product are produced for latitudes band up to [35° S–10° N]. They are analysed with sulphur dioxide (SO2) concentration and sulfate aerosol (SA) optical depth (OD). A plume composed of optically thick patches with high SA OD above 0.025 is captured above 26 km in altitude. It exhibits high UV signal extinction up to 350 Mm⁻¹, scattering ratio (SR) up to 40, local optical depth (LOD) above 0.2, and lidar ratio (LR) above 100 sr. These SA long-lived patches are observed drifting south and transported west. Two branches separate by mid February 2022: a southern tale at ≈ 25 to 27 km in altitude around latitudes [30° S–15° S] and a northern tale at ≈ 23 to 25 km in altitude around latitudes [15° S–10° N]. The LR and LOD measured by Aeolus for the ageing branches reveal lower values below 80 sr and 0.04 respectively. A short-lived plume with low SA OD and high SO2 concentration is observed at lower altitudes ≈ 18 to 22 km with less strong UV scattering properties, and appears to disaggregate quickly.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-2575', Anonymous Referee #1, 19 Jul 2026
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RC2: 'Comment on egusphere-2026-2575', Anonymous Referee #2, 08 Aug 2026
This study is overall well written and clear. It demonstrates the value of ALADIN for sensing and optically characterizing sizable stratospheric aerosol plumes. The work is technically solid and publication-worthy. The manuscript is, however, rather technical in nature. It often focuses more on how the observations were made than on why the observed properties matter scientifically. For that reason, I would argue that the manuscript does not fully meet the ACP standard in terms of scientific novelty. It brings only limited new findings on the Hunga aerosol plume and mainly corroborates results already established in the literature. Conversely, the study would fit well as a measurement report, since it primarily describes original observations, but the scientific narrative leaves the reader wanting a stronger interpretive framework.
Major comments
- Scientific framing and novelty: The paper is technically strong, but the overarching scientific question is still underdeveloped. The introduction should more clearly state what new understanding the study adds about the Hunga plume beyond an instrument demonstration.
- Interpretation and discussion: Several sections describe interesting observational patterns, but the discussion often stops before explaining why those patterns arise. The paper would benefit from a more explicit physical interpretation of altitude-dependent optical properties, plume branching, and sedimentation.
- Reference choices and terminology: Some references could be better aligned with the specific point being made, especially for eruption timing, injection altitude, lidar ratio interpretation, and particle size estimates. This is mostly in the introduction. A few terms also need tightening for accuracy and consistency.
Detailed comments
Abstract line 4: The plume descended from the initial injection, but the sentence wording makes it ambiguous. Please clarify.
Abstract line 6: Measurements
Intro general comment: The introduction is heavily instrument-centric. Please foreground the science question earlier: why does the Hunga plume matter, what is the open knowledge gap, and what specific new insight is gained from Aeolus beyond a technical demonstration? As written, the section reads more like an instrument overview than an ACP-style science introduction.
Lines 22-23: Clarify the range-bin description, maybe: “24 altitude bins ranging from 250 m to 2 km”.
Line 29: shape and size are comprised within microphysical properties - LR also depends on the particle chemical composition, please clarify.
Line 30: Reference: Muller et al 2007 https://doi.org/10.1029%2F2006JD008292 is also a good reference to underline the historical use of LR for aerosol typing.
Line 32: Choice of reference could be better - Khaykin et al 2022 maybe
Lines 32-33: It also erupted on the 13, but indeed the 15th eruption is the one of stratospheric magnitude. HTHH is the name of the couple of islands atop the Hunga volcano, partly obliterated during the eruption.
Line 34-35: Proud et al 2022 (https://www.science.org/doi/10.1126/science.abo4076) and Carr et al 2022 (https://onlinelibrary.wiley.com/doi/abs/10.1029/2022GL098131) are better suited as reference for the injection altitude. The one referenced are mostly on the radiative impact of the perturbation.
Line 47: Referencing size - maybe better to cite Asher et al. 2023 (https://www.pnas.org/doi/10.1073/pnas.2219547120)
Line 61: Knepp et al did not measured effective radius. It's inferred from SAGE-III extinctions.
Line 82: please define “co-polar”
Line 91: restrictive or conservative?
Line 106: ESA?
Lines 105-111: The RBS paragraph is technically important but would read more smoothly if the sentence explaining the Tonga-band purpose were moved earlier. That would help the reader understand why the vertical sampling changes before the details of the setting are listed.
Figure 1 caption: It is quite hard to see the OPAR location with a blue marker on a blue background.
Line 125: The wording can be improved.
Line 128: Here and later, the use of “cross” is confusing. “Compare” seems more appropriate.
Figure 2 caption: “cross of 2D”, what is a 2D profile?
Line 137: Please address the altitude discrepancy between Aeolus and the ground-based lidars. It would help the reader to know whether the difference is mainly due to geolocation offset, temporal averaging, horizontal sampling, or a genuine vertical structure in the plume.
Line 153: transported westward.
Line 163: “at lower altitudes”.
Line 164: “Hunga aerosol optical properties”
Lines 166-167: Wording a bit redundant.
Line 176: “the world map”. Also – “Global map”, the map is not global.
Line 178: white or transparent?
Line 183: values of SR
Line 184: These values are roughly of the same order of magnitude – relative to the background they are significant and there is still something to be said about the observed range.
Line 188: different altitude
Line 206: the red box crosses LR isolines down to 40 sr. Please revise or clarify.
Figure 5 caption: It is a scatterplot rather than a histogram, because no counts are accumulated. Please rename it accordingly (there and in the text, as well as in all the other mislabeled figures in appendix).
Line 227: Tales --> tails
Line 231-232: Interesting to note but could you provide/discuss some possible explanation?
Line 240: “coarse mode” can be loosely defined can you precise?
Figure 6 caption: The markers all look to be of the same size, aren’t they?
Lines 272-274: The discussion is strongest when it stays close to the observations, but it should go one step further and explain why the plume shows the observed altitude-dependent LR, LOD, and sedimentation behavior. Where you speculate about particle growth or water vapour, either support the claim with data or clearly label it as a hypothesis.
Line 285: “The decrease towards/ down to lower values”.
Line 288: Nucleation will yield much smaller aerosol - not sensed by lidar. Processes of condensation and coagulation must be at play to produce the observed particle size.
Line 291: Did see much about the role of water vapour. Is it a possible future analysis you are referring to?
Lines 293-294: The wind product was never used here. Not sure it is relevant to mention it now.
Citation: https://doi.org/10.5194/egusphere-2026-2575-RC2
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Review of Trapon et al., 2026 - Optical properties and global distribution of the Hunga aerosols 2022 observed by Aeolus and atmospheric lidars: new insights into the vertical sedimentation of stratospheric sulfate plumes
Summary
This well written and detailed study by Trapon et al., describes the stratospheric sulfate plumes generated by the Hunga-Tonga volcanic eruption in January 2022 as observed, on a global scale, by the Aladin lidar onboard the ESA Aeolus satellite. Based on the optical properties derived from the lidar measurements, the authors analyse the zonal and meridional evolution of the aerosol plumes over a 6-week period stating 10 days after the eruption, highlighting diffusion, sedimentation and disaggregation processes of the plumes. They also provide indications on the microphysical properties of the particles composing the plumes.
The authors follow a rigorous path in their study, what strongly supports their findings and conclusions. Tracking of the stratospheric aerosols with single overpasses is first demonstrated and validated by comparing the Aladin observations with those performed by ground-based lidars (in La Réunion) and another space-borne lidar (CALIOP). Weekly maps of the aerosol optical properties are then produced and compared to quantities yielded from passive instruments onboard the Metop satellite (sulphate aerosols and SO4). Each step of the analysis is precisely detailed.
This paper is of importance for several reasons. From lidar measurement perspective, it shows that the Aladin UV HSL lidar instrument could characterise particles in the stratosphere, in particular of sub-micron size, and thus even if the instrument was originally designed for wind measurements. It allowed developing and testing algorithms and data processing tools, which are certainly useful for the present EarthCARE mission with the ATLID lidar onboard. Still from sub-micron size particle measurement perspective, Aladin bridges a gap between CALIOP and ATLID, an advantage for the continuity of lidar measurements from space. This manuscript also confirms several hypotheses previously made in other studies (based on in-situ or passive instruments) on the Hunga plumes evolution or on the size and growth process of the sulfate particles at different ages.
The referee enjoyed reading the manuscript paper and following its argumentation (although if quite dense sometime) and strongly support its publication in this proposed journal, which is adapted to the subject.
Specific comments
1) The manuscript deals with optical properties of particles in the atmosphere. The main referee’s remark concerns the precision of the measurement products, or, said in other words, the uncertainties on the reported optical values.
The uncertainty aspect is tackled once, in lines 89-90 for the MLEsub products. The particle extinction coefficient αpart is said to by QC flagged valid if, among other, its standard deviations is below 1.0 × 10−2 m−1. Similarly, βpart is flagged valid if, among other, its standard deviations is below 1.0 × 10−3 m−1sr−1. Reported values for αpart and βpart in Table 1, are (order of magnitude) at the level of 100 Mm-1 and 5 Mm-1sr-1, respectively, that is, 1.0 × 10−4 m−1 and 5.0 × 10−6 m−1sr−1. It would say that the error (standard deviation) is in both cases much larger than the reported value. Can the authors explain the referee where his/her thinking is wrong ? Are the indicated MLEsub standard deviation upper values given for one single laser shot ? Should they be understood in another way ? Completing the text accordingly is recommended, to avoid other readers to be in the same situation as the referee.
2) Section 4 (line 271). Where, in Section 3 on the results, can a SR of 40 be read or found? In particular in Figure 6, the color scale goes up to 14, what prevents from evidencing such a value. Can the authors explain a bit more the source of this value ? (The referee apologies if he/she missed something.) This value is claimed to be a “characteristic” property of the early SA patch and is cited in the abstract - and therefore, prone to propagate further in the literature. Consequently, it should be well supported. (Related extinction coefficient and LR values can be read easily from Figure A3.) The same remark applies to the LOD value of 0.2.
Other specific comments
3) Line 80: It would be worth remembering to the reader that, due to instrument design, only co-polarized signals are measured with ALADIN - particularly in view of Section 3.1.2, where ALADIN and CALIPSO results are compared.
4) Line 140. “[...] signal accumulation was done between ≈ 02:00 and ≈ 15:00 UTC”. Not clear. Shall it be understood that the (night-time, according to Baron et al, 2023) ground-based lidar measurement started on 25 Jan. 15:00 UTC and finished on 26 Jan. 02:00 UTC ? Please, explain more clearly.
5) Line 269. As shown in Fig. A1b,d, a two-week Earth circumnavigation was observed. Are there evidences that such a phenomena was observed several times ? Shoudn’t the word “every” be replaced by “in” ?
Technical comments