Review of Taquet et al.,: “New insights into the 2021 La Palma eruption degassing processes from direct-sun spectroscopic measurements.”

General comments

There are a few things which I find amazing about this study.

First, the authors are able to derive not only SO₂, HCl and HF from their solar-occultation FTIR measurements but also CO and CO₂. This is a major advance because unlike the other volcanic gas species listed, CO₂ has a very high (>400 ppm) background concentration, making the volcanic contribution over the large path length of solar occultation measurement (the entire atmosphere) too low to resolve prior to the latest generation of portable FTIR used here.

Second the authors provide measurements of the gas compositions over the entire duration of the eruption which is a beautiful dataset.

Third the authors performed measurements at two sites, one close to the eruption on La Palma and one 140 km away on Tenerife.     

I would encourage the authors to publish the code they used to analyse the FTIR spectra as they have made significant modifications in their retrieval strategy compared to the openly available code. I also encourage the authors to upload all the spectra they used on an open platform (this might be a journal requirement anyway)..      

I also encourage the authors to look for OCS in their FTIR spectra. Retrieving OCS is probably possible given the authors were able to retrieve volcanic CO. If you can retrieve OCS then you will have two redox species (e.g., https://www.nature.com/articles/s41561-018-0194-5) and may be able to tell a lot more about magmatic evolution during your observation period (see: https://comptes-rendus.academie-sciences.fr/geoscience/articles/10.5802/crgeos.158/).   

Specific comments

Please add “e.g.,” in front of all citations which are examples.

Line 51: “…CO2 and H2O are among the deepest exsolved gas species, followed by SO2 and halogens in sub-surface.” This is not entirely/always true I suggest taking out this sentence.

Line 59-64: Overly vague statements.

Line 107: “…Lava evolved from a’a’ to fluid basaltic flows with changing composition.” You mean basanitic flows no?

Line 170: “…Base map was obtained from © Google Earth (©Google).” Add the original sources of the satellite data.

Figure 2: A photograph of the real setup would be better here that the Schematic (or in addition to).

Line 398: “We also report Cl, F and S contents in tephra glasses that were measured during the analyses published in Gonzalez-Garcia et al. (2023).” I don’t understand this, if these are already published you are not reporting them here. Or where these analysed for major elements but not S, Cl, Fl ? Please explain.

Figure 3: Maybe add the data from Asensio-Ramos et al., (2025) too.

Figure 3: You could have HF/SO2 and HF/CO2 plotted in their own panel to make the figure a bit clearer.

Figure 4 and CO/CO2: You say the data from the FUE and IZO observation cites are similar but it rather looks like the CO/CO2 ratios measured at IZO tend to be lower than the ones measured at FUE. This may be evidence of oxidation of the gas plume during transport.

The comparison between SO2 flux and TADR is interesting, I would suggest citing this article which found the same thing during the Fagradalsfjall eruption: https://www.sciencedirect.com/science/article/pii/S0377027324000568

Line 745: The difference in S content of the glass may between your data and previous publications may be related to the type of sample (flow vs tephra) used in each study. Please specify if these are all from the same type of samples.

Line 753: You say that sulfide droplets are “…absent from the matrix.” But then you show a picture of Sulfide droplets in the matrix glass (Figure B1). This is confusing please rephrase and improve the explanations. Do you see any sulfide inclusions in minerals? Can you plot the S content of Melt inclusions versus FeO to see when the melt reached sulfide saturation?

Figure B2: Please specify the sample types (tephra vs flow).

Figure D1: The data from Asensio-Ramos et al. (2025) could be added to your Figure 4 also.

Congratulations on a beautiful study!

Yours sincerely,

Prof. Yves Moussallam

The work of N. Taquet et al., “New insights into the 2021 La Palma eruption degassing processes from direct-sun spectroscopic measurements” present remote sensing and in-situ measurements over the entire period of the La Palma eruption, which took place in autumn 2021.  Without doubt this is a wonderful data set, my congratulation – it is really an exceptional degassing data set of the La Palma eruption and will for sure be useful also to others for instance for more in depth modelling of this eruption. It also shows the strength and opportunities of such standardised measurement network instruments (like NDACC) outside their daily business. The authors give a large detailed description of the methods and show the potential of continuously working instruments even in far distances of volcanic emission sources and the strength of combining different technologies.

However, it is an atmospheric chemistry and physics journal so I would have liked to see a more extensive discussion about the impact of that volcanic emission on the local and regional (or even global) scale. You nicely determined the masses etc. but only compare them very shortly by mentioning few other sources, but what effect, impact have those gases measured here on the atmosphere, what changes on the chemistry or physic might they cause, what are the implications ... some back on the envelop calculation or at least consideration on the impact on pH in aerosol, clouds, on lifetimes of other atmospheric species, or if you assume the try oxidation for SO2 to sulphate can you estimate that amount of OH necessary from your data? … you even have particle and sulphate measurement please use those and please extend a bit more on the data interpretation. There is a much longer discussion interpretation of the data in a volcanological context than in an atmospheric one, which gives the impression that another journal would have been more adapted, or let’s say a different community of readers could profit more of it (?) However, for some reason you decided for ACP, great journal and yes also your data fit, it is just the discussion and interpretation which remains a bit poor in the context of this journal topics.

There are several smaller comments I just added in the pdf, including typos, small changes or suggestions and few questions to make the text clearer for all readers. Please take those into account by revising your manuscript.

Some question, arguments, which are in my opinion a little more important, I like to point out here and would like to see answers on it before publishing. The suggestion and questions are in the order they appear in the manuscript.

• Figure 1 should be extended also viewing the various measurement locations at La Palma which are mentioned in the text and which are part of discussions about the comparability, etc. so maybe realised by another inset. Also in Figure 1, the wind velocities might be displayed always with the same colour for the same height. (page 3)
• Instrumental descriptions the FTS instruments are nicely referenced and specified, the instrument used for the UV spectroscopy (“the DOAS”) a little less – could you please add which fibre you used (400 micrometres, mono?) and what about the slit wide? Regarding the software could you add a reference or specify a bit, I made some suggestion in the manuscript, but certainly I’m not 100% sure (page 5, line 203ff)
• For your MultiGAS measurements could you be a bit more specific how large was the time lag your determined between your sensors, and what about your smoothing parameters you mentioned. (line 255 ff)
• Regarding the background correction for CO2 and CO – you show some nice examples for CO2 but none for CO – could you please add one in the supplement A, Figure A3
• The analysis details of the DOAS instruments could be added to table 2 (ground based and satellite)
• Could you please add some details for the satellite based SO2 fluxes – e.g. the distance you chose for putting the traverse or did you use multiple distances from the source and then calculating a mean? where did you get the plume height from? (line 372 ff)
• Figure 3 (page 11), the ratio HCl/CO2 should be probable circles instead of rectangles?
• Figure 4 (page 12), please add here also the literature data to the plot you mention in the text and adapt the style of the Figure consistent with Figure 3. Please also confirm are that background corrected ratios for CO and CO2?
• You correlate TADR and SO2 by excluding more than 1/3 of the data, what was the criteria to exclude data – if I overlooked it, sorry but I haven’t seen an explanation for the choice of which data to include and which to leave out rather than randomly taken into account just data that fit?
• CO2 emission is constant during the eruption and you also write that it is independent from volcanic emissions (you even say in your summary and conclusion: “CO2 emissions  are  decoupled  from  the  volcanic  emissions” (which confused me a bit) so do we have also before and after the eruption such high CO2 emissions? If yes, please report that, if not please explain better why we have them when they are independent from volcanic emissions
• The total emission estimates are done by a Monte Carlo approach – if you would simply integrate the surface under the curve over the time for each molecule how large would be the difference to your current results. Wouldn’t this be much more straight forward? What is the advantage of your method? (line 521)
• It is true that there are not that many CO measurements of volcanic emission, but it is not true that only Wardell et al presented a CO measurement during an eruption. I mean those authors measured at Erebus which has a continuous lava lake, but also the measurements at Nyiragongo or Erta Ale were taken during Lava lake measurements, or also the more recent measurements at Iceland from Scott et al were taken during an eruption. Please correct. (line 559 ff)
• Figure 6B please add units on the axis’s, interesting that there is a higher conversion at the beginning of the eruption – can you explain that? (page 17)
• Why do you think your first estimate of 1,93±0,21 Mt of SO2 degassed during the eruption is in disagreement with the Tropomi estimate of 1,81±0,18 Mt SO2 – I would say it agrees perfectly within the given errors. (line 747 ff)
• It is not really clear to me if your emitted CO2 volume fits roughly to your estimated pressure decrease, lava volume, etc? The conclusion of 5.3.2 is not clear to me, please could you rephrase and make it more clear.
• 3.3 you could not determine the F and Cl emission from the petrological data if I understood right? Because the difference of MI and Glass content is too small, could you please state if this is a general issue? Or are the values in you MI and glasses are particularly small? Thanks.
• Appendix D – Honestly this is very confusing appendix for me – I don’t agree that your CO/CO2 data for the two station match very well and at the same time disagree with the one from Asensio-Ramos. Looking up the Figure D1 it seems that IZO and earlier data match better than IZO and FUE.

Please comment on this. Further on, please improve, discuss at all the data you are showing on the lower part of the same Figure in context with your data. Do you want to show that a mismatch might be larger areas are covered – more fires? Or what do you like to show here?

I hope the suggestions are useful for you, please don’t hesitate if you have any questions

Nicole Bobrowski