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the Creative Commons Attribution 4.0 License.
| 04 Jan 2026
Balloon Observations Suggesting Sea Salt Injection into the Stratosphere from Hunga Tonga-Hunga Ha'apai
Abstract. The explosive eruption of Hunga Tonga-Hunga Ha'apai (HTHH) in January 2022 marked a historic event as one of the largest volcanic explosions in the past 140 years. Unlike typical volcanic eruptions, which primarily inject sulfur dioxide (SO₂), HTHH introduced a massive plume of marine water vapor up to an altitude of ~57 km, reaching the mesosphere.
In this study, we use balloon-borne measurements to investigate the optical, microphysical and chemical properties of the HTHH aerosol plume eight months after the eruption. The peak concentration of the Hunga plume located between 20.5–23 km was near 8–9 #/cm3 for aerosol diameter greater than 0.3µm and Scattering Ratio at 940 nm near 4–5. Our balloon-based sampling and ion chromatographic analysis revealed the presence of key ions such as Na⁺, K⁺, NH₄⁺, Ca²⁺, Cl⁻, and traces of SO₄²⁻ in the samples collected in the lower part of the HTHH plume. These findings suggest a substantial contribution of marine aerosols to the stratospheric aerosol burden.
The results suggest that the interaction between volcanic ash, water vapor, and marine aerosols led to unique chemical processes, which significantly influenced the composition and behavior of the stratospheric aerosol layer.
Competing interests: At least one of the co-authors is a member of the editorial board of ACP.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-6226', Anonymous Referee #1, 23 Feb 2026
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AC1: 'Reply on RC1', Hazel Vernier, 02 Mar 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2025-6226/egusphere-2025-6226-AC1-supplement.pdf
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AC1: 'Reply on RC1', Hazel Vernier, 02 Mar 2026
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RC2: 'Comment on egusphere-2025-6226', Anonymous Referee #2, 20 Apr 2026
Review of “Balloon Observations Suggesting Sea Salt Injection into the Stratosphere from Hunga Tonga-Hunga Ha’apai” by Vernier et al. for publication in Atmospheric Chemistry and Physics.
The paper presents data from a balloon campaign in Brazil in August 2022, about 7 months following the eruption of the Hunga volcano in the south Pacific. Plume material from the eruption is observed in the balloon dataset consistent with ground-based lidar and satellite observations. Number concentrations and scattering ratios are observed on three flights, while separately filters are collected on a single flight. The high altitude volcanic plume is observed on all 4 flights. Some heterogeneity in the plume characteristics is noted, but in the main the plume is observed at the same altitude (roughly 17 - 27 km). Filter samples are captured on August 22, 2022 on a flight that tops out at ~21km altitude, well into the main part of the plume as observed on previous days. Controls on the filters allow analysis over three distinct altitude ranges. Filters are recovered and subjected to offline analysis including ion chromatography. The main result articulated in the paper is that there is compelling evidence for marine aerosol in the body of the volcanic plume. This is plausible because of the large amount of sea water that was lifted to the lower stratosphere by the eruption.
This is a challenging manuscript to evaluate, and I don’t find it suitable in its present form but I think it can be improved.
First, a lot of the microphysical information about the POPC and COBALD measurements is buried in supplemental material, but this seems pretty important to interpretation of the later chemical results. Line 246-247 and section S1 discuss that a size correction is needed for the POPC measurements, although it is not explained how this done (there is a reference to a manuscript in preparation). I’m unclear on how the size thresholds for the POPC measurements are supposed to be determined, though as Figure S1 shows I can understand that a given amount of material would have a different scattering depending on its composition. In Section S2 the POPC extinction is computed for different assumptions of the particle properties, with best agreement between the balloon observations and SAGE III/ISS obtained by assuming the refractive index of sea salt. This anticipates the later conclusion of the paper. The improved consistency between POPC and COBALD measurements is interesting, but I’m unclear on whether this is related simply to the change in refractive index assumed or also to a size correction (does the size distribution assumed in the calculation change from the first to second to third row in Figure S2?). There is also a reference in the S2 text to Table 3 that should be I think to Table S1. Finally, the particle size distribution in Figure S3 needs more information provided with it. Since you show the equation, what are the final parameters for the curve fit in S3? By a little guesswork I computed a number median diameter D_pg = 0.36 microns and width sigma_g = 1.25 that gave me a curve like Figure S3. From these parameters I infer an effective radius of 0.2 microns, which is half the value quoted from Selitto et al. 2024 on line 598 of 0.4 microns. I’m not sure where the discrepancy is, or if my guessed parameters are wrong, but this seems important to be explicit about. In short, please better integrate the microphysical measurements into the main body of the text.
Second, to the main conclusion of the paper, let me just start with the fact that there are a whole lot of hypotheses advanced for the plume composition from a single profile. I appreciate that the authors are not over-selling the result in the abstract, but the implications seem to be that there is almost no extant sulfate aerosol in the observed plume and that the composition seems to be dominated by marine aerosol and sulfate coated ash. This has profound implications for the modeling and remote sensing of the volcanic plume. To confounding factors there is only a single sentence on the sample handling, lines 298-299, and a hint in section S3 of losses on the filters. I am unsure if there is any possibility for sulfate to have evaporated or otherwise been lost on the filters. Further explanation is needed. And to bring it full circle, can the microphysical properties be better integrated with this compositional analysis to strengthen the conclusion?
Minor comments:
Line 79: suggest “…sulfate aerosols and caused…” as more correct
Line 154: “lower” should be “larger”, shouldn’t it? Entirely different meaning.
Line 201: Figure 2 shows zonal mean CALIOP products, right? I don’t think this is stated explicitly, so I was confused when later you said data didn’t cover Brazil.
Line 269: What is “Orbitrap”?
Line 299: What does “positioned into a dry to minimize” mean? I feel like some words are missing here.
Line 390: Please put altitudes on panels for Figure 5.
Line 402: I think you mean 18 - 21 km altitude here.
Line 430: Should be Table 6 and not Table 4.
Line 458: You mean “for >0.01 um” and not “ford”?
Line 461: What is “oonic”?
Citation: https://doi.org/10.5194/egusphere-2025-6226-RC2 -
AC2: 'Reply on RC2', Hazel Vernier, 30 Apr 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2025-6226/egusphere-2025-6226-AC2-supplement.pdf
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AC2: 'Reply on RC2', Hazel Vernier, 30 Apr 2026
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This paper presents some interesting in situ observations following the Hunga-Tonga eruption, but the presentation is poor and sometimes misleading. Explanations are unclear, unnecessary figures are shown, useful figures are in the supplement or not shown, references are not in the bibliography, assumptions are mis-stated as theories. Fixing these and other problems to convert it to a useful contribution is possible, but will take a significant effort to clear up and strengthen the presentation. The authors may also want to consider a reference to Martinsson, B. G., Friberg, J., & Sporre, M. K. (2025). Comments follow by line number.
48 Use standard MKS units and notation, e.g. 8-9 cm-3
71 Where does a VEI of 6 for Hunga-Tonga eruption come from? Needs a reference. The Global volcanism network states 5 and others have rated it lower.
Table 1. What is the meaning of the column labeled injection altitude. Is this the top of the injection layer, as suggested by the Hunga-Tonga entry, or the height of the primary injection layer for the most material as suggested by maybe Nabro and Raoikoke? If it is the peak, then it is clearly wrong for Pinatubo. Also what are the references for these height measurements?
130 The last 35 years? This would extend back to 1990, barely including Pinatubo and certainly not El Chichon.
144-151 This paragraph needs work as it is currently confusing. SAOD from GLoSCAC does not represent sulfate sSAOD. GLoSSAC cannot distinguish the components of SAOD. So the second half of the first sentence does not make sense. GLoSSAC cannot be used in the way suggested.
149 What is semi-annual sulfate? Better to state exactly what Fig. 1b) is showing, that is: SO2 injection compared to SAOD averaged over the 6 months following each volcanic eruption. It is not clear why 6 months is chosen. It would be better to use a quantity more reflective of each volcanoes character and so2 injection amount, such as a similar e-folding time, e.g. when SAOD reaches 1/e of its initial value. This might provide a more consistent way to compare the volcanoes.
222 Is there a reference for the impact of the SAA on CALIOP measurements?
237 POPC is not a good acronym for this new instrument. There is another instrument with a very similar acronym POPS (portable optical particle spectrometer). Thus is seems unfortunate to use an acronym that will soon conflate these two different instruments. Profiling also is not unique to the POPC. All OPCs designed for stratospheric measurements are profiling instruments.
247 Corrected to what?
Fig. s1. The text claims the instrument is sensitive to particles 0.3 – 10 µm, yet the figure only extends to 2 µm. It would be helpful to also show the instrument’s response to the PSL calibration aerosol here, that is the laboratory measurements confirming the instrument response including their error bars.
But exactly what does Fig. s1 show? The text for Fig. s1 includes the following, “… Figure S1 shows the scattering efficiency for individual particles derived from Mie calculations (Hagan et al., 2022) for PSL, sulfate (70%H2SO4/30%H20) (Knepp et al., 2024), and sea salt (Bi et al., 2018) by…” So Hagan, Knepp, Bi did the Mie calculations for the instrument or are they supplying the index of refraction for these compositions of aerosol? The text should be clear, and there are no entries for Knepp, Hagan, or Bi et al., in the reference list. The title for Fig. s1 is POPC Theoretical Response. This implies it is the counter response function, but then the ordinate should have units of volts, or photon energy, or something similar. Instead no units are provided and the caption states that it is “mean scattering efficiency.” Thus it seems that this figure has nothing to do with POPC, as this would be the same without any consideration of an OPC. If it is the counter response function then it should be so stated and the appropriate labels added to the graph along with the results of the measurements with PSL.
252-253 What is the backscatter ratio measurement? Is this the ratio of backscattered light to the emitted light? How does the backscatter ratio compare to the scattering ratio?
255 Such concentrations are usually written 8-9 cm-3.
259-261 Why is Fig. s2 in the supplementary material? It should be in the paper.
In Fig. s2, there is no reason to show the POPC measurements with no correction from PSL for index of refraction. Everyone knows there is no PSL in the stratosphere and all OPCs have to be corrected to the aerosol expected in the stratosphere. The first row of Fig. s2 just misleads the reader, including this one.
Fig. 3 Why are these profiles interesting? More interesting would be to show the POPC estimates of the COBALD scattering ratios, which would also illustrate the same point, that is that both instruments clearly detect the Hunga-Tonga plume.
299 “Positioned into a dry to …” Something missing here.
In general there should be more explanation of how the various filter samples were kept sealed from outside contamination both before and after exposure. It would seem difficult to keep them at the pressure of their exposure, but maybe that’s the case? Or were they somehow bathed in dry nitrogen before and after exposure to not have to deal with the pressure differentials? This needs to be clarified.
330 “…Filters from positions P 1-P 5 were extracted and analyzed by ion chromatography. Each extract …” What is the extract and how does that differ from the filters on which the extract is deposited? Most readers aren’t going to be so familiar with ion chromatography.
423 …This helps identify non-marine sources or processing. … What does or processing mean?
4.1 “Theoretical considerations …” Why theoretical? Aren’t the authors presenting measurements, albeit based on some assumptions?
463-466 If a size distribution has been fit to the OPC data, why isn’t that used to directly calculate the aerosol volume along the flight path? Then the mass is given by V*d. Isn’t that what is done? The authors are not converting number concentration to mass concentration. They are using the number concentration to calculate volume by an integration. If that is the case then, aside from the uncertainty of the density, isn’t the mass measured? And if so why is it referred to as theoretical?
In fact what the authors are doing, which is reasonable, is to assume that the stratospheric aerosol size distribution they measured from 8/07-12 would be similar to the distribution that existed on the day of the flight. It’s not a theory, it is an assumption. And if this is really what the authors are doing, why don’t they plot in Fig. 6a a vertical profile of the aerosol mass concentration, rather than number concentration which has already been done? They could do it for all three days, and take the average for the mass to be used for the flight on 8/16. The step by step approach is detailed in the supplementary material, but the authors could eliminate a lot of confusion by a clearer explanation here and by adding some of the supplementary material to the text.
Fig. 6b) Why are masses calculated for d > 0.3 and > 0.45 µm? What is the pore size of the filters, or what is the minimum size of particle that the filters will capture? This should determine where the OPC size distribution should be integrated from to get the comparable volume. How is the right hand axis, arrived at? The label is mass, but the OPC only provides mass/volume sampled. So what volume is used to obtain mass? Then shouldn’t the axis be labelled accordingly?
475-476 “The theoretical mass derived from POPC data is near 42–43 ng for the first two flights and 27 ng …” Again why theoretical? There is nothing theoretical about integrating a size distribution or assuming a density. What do the numbers 42, 43, 27 represent? Is this the total mass that would have been collected by the filter in the altitude range of 18-21 km, and was calculated by using the OPC mass concentration times the total volume to which the filter P4 was exposed? This is what it should be but it is extremely unclear what was done.
476 Is 513 ng the total mass obtained from washing the filter? Is an ion mass different than the total mass? A much clearer explanation of how the OPC size distributions were used, and a profile of the mass concentration derived from the OPC measurements would go a long way in understanding how to compare these numbers. At present this comparison is a black box to the readers, and 513 ng is over 10 times the other masses listed.
481-590 The text here describes, briefly, a dozen chemical scenarios to explain the observations that were made. These scenarios are beyond my expertise, and need to be reviewed by a qualified chemist to substantiate their reasonableness.
543-544 What are the discrepancies with ACE-FTS? This is explained in the supplementary material, but should be more clearly included here, along with how it is resolved.
561 “(Finlayson-Pitts & Pitts, 2000) …” Neither of these references are in the bibliography.
589-590 Are these discrepancies shown or discussed somewhere?
Martinsson, B. G., Friberg, J., & Sporre, M. K. (2025). Stratospheric aerosol formed by intense volcanism–sea interaction during the 2022 Hunga Ha’apai eruption. Atmospheric Chemistry and Physics, 25(18), 10677–10690. https://doi.org/10.5194/acp-25-10677-2025