Satellite measurements of plumes from the 2021 eruption of La Soufri&egrave;re, St Vincent

Taylor, Isabelle A.; Grainger, Roy G.; Prata, Andrew T.; Proud, Simon R.; Mather, Tamsin A.; Pyle, David M.

doi:https://doi.org/10.5194/acp-2022-772

Preprints

https://doi.org/10.5194/acp-2022-772

Preprints

17 Nov 2022

| 17 Nov 2022

Status: a revised version of this preprint is currently under review for the journal ACP.

Satellite measurements of plumes from the 2021 eruption of La Soufrière, St Vincent

Isabelle A. Taylor, Roy G. Grainger, Andrew T. Prata, Simon R. Proud, Tamsin A. Mather, and David M. Pyle

Abstract. Satellite instruments play a valuable role in detecting, monitoring and characterising emissions of ash and gas into the atmosphere during volcanic eruptions. Plumes of ash and sulfur dioxide (SO₂) from the April 2021 eruption of La Soufrière volcano on St Vincent in the Eastern Caribbean were observed by a multiple satellite instruments. This study looks at these plumes with two satellite instruments: the Advanced Baseline Imager (ABI) on the Geostationary Operational Environmental Satellite (GOES), and the Infrared Atmospheric Sounding Interferometer (IASI) on the MetOp platforms. Using true and false colour images, and brightness temperature difference images produced from the ABI data, a minimum of 32 eruptive events were identified. The ABI images were used to determine the approximate start and end times and character of each event. In this way the eruption has been divided into four phases: (1) an initial explosive event, (2) a sustained event lasting over nine hours, (3) a pulsatory phase with 23 explosive events in a 54 hour period and (4) a waning sequence of explosive events. The IASI instrument was used to study the dispersion of SO₂ from this eruption. The results showed a highly complex structure to the plume, in terms of the column amounts and height, which is likely linked to the multiple explosive events. The SO₂ is shown to have largely been emitted between 13 and 19 km. This was primarily in the upper troposphere and around the height of the tropopause, but with some emission into the stratosphere. The SO₂ was transported around the globe with parts of the plume reaching as far as 45° S and 45° N. The largest SO₂ atmospheric burden measured with IASI was 0.31±0.09 Tg, recorded on the 13 April 2021 (descending orbits). The SO₂ masses were converted into fluxes. The SO₂ flux was shown to peak on 10 April and then shown to decrease over time. By summing the IASI SO₂ flux results, it is estimated that a total of 0.57±0.44 Tg of SO₂ was emitted to the atmosphere. However, due to the limitations associated with the retrieval this should be considered a minimum estimate of the total mass of SO₂ emitted. An average e-folding time of 7.09±5.70 days was computed based on the IASI SO₂ results: similar to other tropical eruptions of this magnitude. There are a number of similarities between the 1979 and 2021 eruptions at La Soufrière. For example, both eruptions consisted of a series of explosive events with varied heights including some emission into the stratosphere. The similarities between the 1979 and 2021 highlight the importance of studying these eruptions to be prepared for future activity.

Received: 11 Nov 2022 – Discussion started: 17 Nov 2022

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Isabelle A. Taylor et al.

Status: final response (author comments only)

RC1:
'Comment on acp-2022-772', Anonymous Referee #1, 11 Dec 2022

The manuscript "Satellite measurements of plumes from the 2021 eruption of La Soufrière, St Vincent" discusses the relatively large, partially stratospheric recent eruption of La Soufrière volcano, based on satellite observations - basically IASI and ABI - and notably reconstructs the eruption timing and the emitted SO2 mass. The authors even attempt at making connections of the different eruption phases with the geochemical/volcanological state of the volcano, in a truly multidisciplinary effort. In my opinion, the manuscript is exceptionnally well organised and written. It was really a pleasure to read it. I only have very minor comments, listed in the following.

My best regards.

Minor comments:

1) L36: Is there an acronym for SMS-1?

2) L55-56: maybe a mention to the temporal resolution of satellite observations is needed here, in particular for the high-temporal resolution of geostationary instruments.

3) L99: RTTOV must be introduced and defined earlier in the text

4) L102-103: "column amounts exceeding 0 DU", you mean "positive column amounts"? And also "positive heights"?

5) L122-123: not clear what do you mean here "Note that...analysis."

6) Eq. 1: why not putting this in its integral shape, i.e. as an exponential decrease? And why not using another symbol for the e-folding time - lambda may be confusing in this context, and taken as a wavelength - ?

7) Is Fig. 2 more pertinent in the "Results" section than here in the "Data" section?

8) Maybe this information might be more visible in a Table.

9) L259-260: please add references for the possible reasons of differences between ground based and satellite observations.

10) Fig. 1, 7 and 8: please consider to annotate the ash and SO2 amounts color bars in linear scales and not log10

11) As the emitted SO2 mass for the recent Hunga Tonga-Hunga Hapa'ai eruption was initially estimated at 0.4 Tg (https://www.frontiersin.org/articles/10.3389/feart.2022.976962/full), this is maybe an interesting event to compare with your mass estimations for La Soufrière. Please note that the impact of the Hunga Tonga-Hunga Hapa'ai eruption on the stratospheric aerosol layer and climate was much larger than what expected for such a relatively small estimations of the SO2 emissions (https://www.nature.com/articles/s43247-022-00618-z). This discrepancy of estimated emissions and the impacts is also pertinent in your discussion, in my opinion.

12) L320-321: with which satellite instrument the SO2 mass of 0.4 Tg was estilmated by the GVP?

Citation: https://doi.org/10.5194/acp-2022-772-RC1
- AC1: 'Reply on RC1', Isabelle Taylor, 06 Apr 2023
  
  We thank the referee for their helpful comments. Please see our responses in the attached document. Note that responses to both RCs are contained in this PDF
  
  Citation: https://doi.org/10.5194/acp-2022-772-AC1
RC2:
'Comment on acp-2022-772', Anonymous Referee #2, 02 Jan 2023

Review of Taylor et al. manuscript submitted for publication in ACP, "Satellite measurements of plumes

from the 2021 eruption of La Soufriere, St Vincent"

This manuscript presents an analysis of satellite measurements to quantify a chronology of the series of

large-magnitude explosive eruptions from the La Soufriere volcano on St. Vincent island (13.3N, 61.2W)

that emitted SO2 and ash into the tropical upper troposphere and lower stratosphere during a two-week

period of April 2021.

The analysis is based primarily on two satellite instruments, the ABI sensor on the geostationery GOES-W

satellite, and the IASI polar-orbiting satellite. The high temporal resolution dataset from the

ABI geostationery data enables to identify the specific timing of each explosive eruption, the La Soufriere

case known to have had a pulsed phase of repeating large-magnitude explosions, generating a series of

volcanic plumes on the days after the initial highly explosive eruption of 9th April 2021.

The manuscript then analyses IASI SO2 measurements, both in terms of the SO2 column burden and also

for also spatio-temporal variations in plume altitude and ash-presence detection occurrences as the

multi-plume volcanic cloud disperses in the weeks after the 1st large-magnitude explosive event.

In combining analysis of the geostationery ABI ash signatures and plume heights, with also the SO2

datasets from the polar-orbiting IASI measurements, through such an explosive tropical eruption, the

manuscript represents a substantial endeavour, with clearly novel scientific findings. The study will

be of particular interest to the stratospheric/volcanic atmospheric science community, and is certainly

within scope for publication in ACP.

The manuscript is generally quite well written, with the Figures in good shape, and represent a substantial

analysis. However, whereas section 3.4 and section 4 of the results included a good degree of scientific

interpretation, the text within sections 3.2 and 3.3 remained at rather a basic level, and needs to be

improved to highlight the atmospheric processes involve before the manuscript is ready for publication.

There needs also to be a strengthening of the overarching science narrative, and for this to be reflected

in a revised title, over and above the basic "analysis of satellite measurements for the La Soufriere eruption".

The changes required are all quite minor in nature however, and I have listed below the series of specific

revisions required, before the manuscript will then be suitable for publication in ACP.

Considering the science interpretation in section 3.2, the specific revisions here are to better identify

the relevance of the sudden change in wind regime seen in Figure 4a, re: some parts of the plume being

dispersed to the west (easterly) whilst the majority of the plume is dispersed towards the east (westerly).

Figure 4a very nicely shows both the mean and standard deviation of the wind direction, then clearly

identifying the 8-16km altitudes consistently had easterly winds, with westerly winds only occurring at

higher altitudes, or near the surface. Figure 5 then represents a further analysis of the zonal and

meridional wind components at three distinct time-periods, labelled eruptions 1, 2 and 3.

The corresponding text in the submitted manuscript (lines 218 to 235) currently does not (yet) explain

the significance of the variations mentioned, presenting only each variation in the Figure without a

coherent narrative of the main findings here, in relation to the plume's dispersion from these 3 main

explosive phases of the eruption. It may be that the text can only hint at the interpretation at this

stage, and that the themes of the interpretation can then be picked up later within the SO2 analysis

in Figures 7, 8 and 9, but the text here (lines 218 to 235) should already be introducing the clear

vertical shift in wind regime revealing then an important variation relevant for the dispersion of the

plume. Currently the text has only rather abstract terms such as "supporting a tropospheric solution",

when it should make a clearer statement such as the westerly winds only being present in the upper

altitudes or so. Similarly, the section 3.3 text only discusses the actual variations in the Figure

without communicating the basic processes involved, in this case there the progressing oxidation of

the volcanic SO2 during downstream transport of the plume, with any removal from the stratosphere on

the timescales considered.

In both cases (sections 3.2 and 3.3), there is a notable absence of citations of studies analysing

these studies from other highly explosive eruptions reaching the stratosphere, with section 3.2

needing to mention also the well-known dominant mode of variability in the tropical stratosphere,

the quasi-biennial oscillation (Reed et al., 1961; Baldwin et al., 2001) and its role in the

dispersion of volcanic clouds in the stratosphere (e.g. Trepte and Hitchmann, 1992; Trepte et al., 1993;

Langford et al., 1995). Similarly, re: section 3.3., there should be citations of some studies that

have analysed the daily progression of stratospheric SO2 burden within other tropical volcanic SO2

clouds, e.g. Guo et al. (2004) for Pinatubo, Zhu et al. (2020) for 2014 Kelud and Zhu et al. (2022)

for 2022 Hunga-Tonga. There should also be mention of the recent recognition of the potential for

ash particles to accelerate the SO2 conversion via heterogeneous oxidation on the surface of the

ash particles (Zhu et al., 2020).

That said however, this manuscript represents a very good analysis, and will be valuable addition

to the published literature, both in relation to the specific episodes within the April 2021

La Soufriere eruption, and more generally on the SO2 variations and ash presence within tropical

volcanic plumes in the stratosphere. And whereas the list below of specific revisions is quite long,

there are all minor changes, and overall I am recommending "publish after minor revisions".

The Abstract was rather poorly worded, and 9 of the first 10 of the specific revisions focus on

improving the Abstract. I also found the sentence structure seemed to have very short sentences,

then unsuited for the scientific interpretation required within an ACP article. Specifically,

I wonder whether the autuor may have used a grammar-checker, and amended their original sentence

structure into some recommended series of short sentences, which then no longer communicate the

depth of explanation in the original sentence structure? In any case, I flag up here that several

of the specific revisions are where the manuscript had very short sentence that didn't seem to

communicate any content. And then I wonder if previously the wording did communicate a coherent

structure, but this was lost in some shortening of sentences via an auomated grammar-checking

software or similar?

List of specific recommended changes (required before publication):

------------------------------------

1) Title -- I'm suggesting here the authors consider amending the title of the manuscript, with

"Satellite measurements of plumes from the 2021 eruption of La Soufriere" not really communicating

the science focus of the study. I think the information in Tables 2 and 3 could potentially be called

an "eruption chronology" or so, in relation to the timing of the individual explosive events the

analysis identifies. I think including the duration of the series of explosive events explicitly

(14 days) could aleo help the reader appreciate the multiple large-magnitude explosive events.

And a specific suggestion is to change the title to "A 14-day explosive eruption chronology

for the April 2021 eruption of La Soufriere, St Vincent". I leave it to the authors to decide whether

to change the title, but some re-wording, certainly to include the month "April" with the suggest

to also include some better specifics within the title.

2) Abstract, line 3 -- Re-word "were observed by a multiple satellite instruments."

Although the Abstract can include some introductory sentence, the current Abstract

has the first two sentences as the context for the study, and suggest to merge the 2nd and 3rd

sentences into 1 sentence focused on the specific science focus of the manuscript.

As per my general comments above, I think the analysis of the geostationary satellite measurements

in relation to defining an eruption chronology represents a valuable and novel aspect of the study.

My suggestion here would be to incorporate this specific outcome from the 1st part of the study into

a revised wording of the current 2nd and 3rd sentences. Maybe have the new 2nd sentence begin

"This study analyses geostationery sallite measurements for a 14-day chronology of the

series of explosive eruptions of the La Soufriere volcano in April 2021." or similar.

To save words, the location of La Soufriere (St Vincent in the Eastern Caribbean) does not need

to be communicated in the Abstract, with this given within the manuscript text.

With then the 3rd sentence focusing on the way the study combines these then with SO2 measurements

from the IASI sensor on the polar-orbiting MetOp satellites.

Within this revised 3rd sentence, please re-phrase "looks at these plumes" to more scientific wording.

I'd advise to keep the acronyms as acronyms here, with these being fine to be explained in the

main text of the article. It's currently very clunky with multiple brackets within this early

part of the Abstract.

3) Abstract, lines 5-7 --> Re-word this sentence to have the main "object" of the sentence at

the start of the sentence, this being the 32 eruptive events that are defined, with the

methodological aspects coming after the specifics -- it just makes the sentence easier to read.

Also, suggest to change the word "character" instead to "characteristics", as I think this is

what is meant, in relation to this referring to the content within the "Notes" column of Table 2.

4) Abstract, line 7 --> Change "The ABI images were used..." instead to "The high temporal resolution

BTD images from the geostationary ABI measurements were used..". Suggest to add "in this 14 day period"

after "32 eruptive events" and replace "eruptive events" with "large-magnitude explosive events".

(The acronym BTD can be introduced in the preceding sentence.)

5) Abstract, line 8 --> Re-word "In this way the eruption has been divided into four phases..."

instead to "From this analysis, we define four distinct phases of the 14-day eruption", each consisting

of multiple events" or similar. ("In this way" is not scientific enough and good to be clear this is

within the 14 days (9th to 22nd April). Also good to introduce the terms "events" and "phases").

6) Abstract, lines 9-12 --> There are 3 short sentences here which would seem better re-worded into

a single sentence, the 2nd sentence clearly continuing from the content communicated in the first

very short sentence, and the 3rd very short noting the altitude range. As per my general comments above,

this is an example where the writing in very short sentences seems to me to lose the ability to communicate

the scientifically-related aspects. I wonder if this change is a result of using a grammar checker that

has advised to split the text in this way? The current 1st sentence communicates only methodological aspects

and the order in which these 3 aspects are communicated in a revised sentence should also be changed.

In my opinion it is best to begin with the main object of the sentence rather than the methodological aspects.

My specific sugestion then is to have a merged sentences communicating all 3 aspects within a wording such as

"Analysis of the IASI SO2 measurements shows the dispersing La Soufriere SO2 cloud had a highly complex

structure, the multiple explosive events generating several plume enhancements in the altitude range 13 to 19km",

or something similar.

7) Abstract, lines 12-14 --> Again there are 2 sentences here in strangely worded very-short sentences.

The wording seems to be compartmentalising each individual aspect into a separate sentence, with no

over-arching structure to communicate the science behind these related aspects. As per my general comments

above, a key aspect here is to mention the fact that whereas the majority of the plume dispersed eastwards,

part of the plume dispersed towards the west, and this bi-directional plume transport must be reflecting

the wind structure shown in Figure 4a. Depending on how the authors re-word the section 3.2 content, the

main finding in that regard should be stated within this re-wording of these 2 sentences of the Abstract.

I'd suggest to potentially word as "We show that most of the SO2 was transported eastwards (westerly flow),

but a proportion of the plume was transported westward (easterly), reflecting the higher-altitude wind regime

in the lower stratosphere (or similar). This comment relates to the specific comments xx and yy to improve

the interpretation within the section 3.2 results text.

8) Abstract, lines 14-16 --> Yet again, there are 3 sentences here, that should be joined together into one

sentence, to give the information in the 2nd and 3rd sentence as a continuation of the same sentence.

It makes no sense to separate this information in this way. In particular the middle sentence reports very

little information, and the words here should refer to the temporally varying emissions flux (aligned to

the main "emission chronology" narrative for the manuscript. Suggest to continue with the 2nd sentence

wording changed to ", with derived emissions fluxes highest on 10th April, phase 3 of the eruption..."

with then the last part, "... with later explosive events of leser magnitude and injection height, and

generally decreasing". Or similar wording to this.

9) Abstract, lines 16-18 --> The wording should also be improved here, with the start of the sentence

again better communicating the main information rather than the method. The current wording "By summing

the IASI SO2 flux results, it is estimated that" should be reduced, to "We derive a total emissions flux

for the 0.57 +/- 0.44 Tg of SO2, ...." The reader will be aware this is derived from the IASI SO2, it

doesn't need to be stated again, and only serves to detracts from communicating the main result to again

state the specific sensor analysed.

10) Abstract, lines 19-22 -- This is a nice last part of the Abstract to compare back to the 1979 eruption,

but again the wording of this last part of the Abstract would be better in a flowing sentence than in

the compartmentalised into 3 short sentences. Suggest also to re-word "be prepared for future activity"

more specific wording.

11) Introduction, line 25 -- This sentence needs to be re-worded. Suggest ot insert "The" at the start

of the sentence, and replace "located" with a comma, inserting also a comma after "the Caribbean".

Suggest to delete "including the slow extrusion of a lava dome", to instead stay focused on the

science topic of the manuscript, and re-word "having been in a lower level state of eruption" instead

to "having been a series of low-explosivity events since late December" or similar. The "in the Caribbean"

could be deleted since this information is given in the sentence after.

12) Introduction, line 38 -- change "detected" to "measured" (the SAGE instrument did more than detect

the enhancement, it measured the magnitude of the enhancements). Also change "following" to "from".

Similarly, change "were detected by" to "were observed by".

13) Introduction, lines 49 to 51 -- The word "satellite is used twice in this sentence, and suggest

to change the first instance from "Advances in satellite technology" instead to "remote sensing technology",

also reducing the current text "in the last four decades mean that in 2021 multiple aspects" instead to

"in the four decades since the 1979 eruption, multiple aspects" and changing "the volcano's activity"

instead to "the 2021 activity". Replacing "were studied with satellite instruments" to

"were monitored from multiple satellite instruments".

14) Introduction, line 53 to 56 -- Again the split into two sentences detracts from the science

communication here. Suggest to improve the wording by changing "plume properties. This is essential

for assessing the potential hazard of volcanic plumes to aircraft" instead to "plume properties,

essential for assessing potential hazard to aviation", and also change "providing estimates of the

eruption parameters" to "enabling to provide more sophisticated information on eruption source

parameters". Please add "Marshall et al. (2018)" in addition to Aubry et al. (2021) re: eruption

source parameters.

15) Introduction lines 57 -- state which (or how many -- 2 or 3?) of the MetOp satellites (-A, -B or -C)

the IASI data included in the analysis. It's good that the word "satellites" is plural but, also since

the better to state specifically the number of these -- two or three? And maybe keep the text general

here, replacing "on the MetOp satellites" to "on two polar orbiting satellites".

16) Introduction line 58 -- change "to study the" to "to analyse the" -- the paper is presenting an analysis

of the measurements, more than simply studying the eruption.

17) Introduction line 68 -- insert comma between "pixels" and "each with" and delete "at nadir" after "square".

18) Section 2.1.1, lines 75-78 -- These 2 sentences should be merged to just 1, and the wording reduced,

with the words "have been developed to obtain information about other volcanic gas species including"

changed to "have been developed also for". Changing ". Additional work has been done on the retrieval

of sulfate" instead to worded ", and for sulfate", then enabling this to continue in the same sentence.

Insert "the infra-red method means that" before "there is no break in coverage", and then reduce

"associated with the loss of solar radiation at night and during high-latitude winters" instead to

"at night or during high-latitude winters".

19) Section 2.1.1, line 79 -- Hyphenate "near global" to "near-global" and add comma after "twice a day".

20) Section 2.1.2, lines 80-81 -- I'd suggest to delete this sentence but if the authors prefer this

to remain that is OK.

21) Section 2, line 85 -- Change "two methods have been employed for studying the" instead to

"two methods have been used to analyse" -- again, the methods are doing more than "studying" the

SO2 plumes, and the word "employed" seems strange in this contex.

22) Section 2.1.2, line 86-87 -- Insert "column" after "elevated quantities of" --> it's the

elevation of the column SO2 the method detects (at least to my understanding), and that's

an important caveat to communicate there. The very short sentence beginning "Full details of"

should be included simply as a continuation of that sentence, changing ". Full details of

this method can be found in Walker et al. (2011, 2012)." instead to ", see Walker et al.

(2011, 2012)."

23) Section 2.1.2, line 88 -- Improve this sentence by deleting "which is able to", then

changing "quantify information about the plume, including..." instead to

", quantifying additional information about the plume, including..."

24) Section 2.1.2, lines 89-90 -- Change "In this study it has been applied.." (the first sentence

of this paragraph already starts "In this study", and better to re-word to "The method is applied.."

Again, merge these two very short sentences into 1 sentence, changing ". The retrieval has been run

in much the say way as is described in Carboni (2012, 2016, 2019)" instead to

", (see Carboni et al., 2012, 2016, 2019)." Since the differences are explained in the text,

there is no need to state "in much the same way as" -- the reader may initially think it's

the same method, but will read the next sentence, there communicating the slight differences

in the methodology (without that needing to be stated explicitly).

25) Section 2.1.2, line 99 -- Change "of the RTTOV (the forward model used) pressure levels"

to "of the pressure levels in the RTTOV forward model".

26) Section 2.1.2, lines 100-101 -- Delete this sentence -- it refers to future work not applied

in this study.

27) Section 2.1.2, lines 102-104 -- This sentnece concerns information too specific to the coding

implementation of the algorithm. Whilst in an ESSD or GMD manuscript this level of information

might be able to be incorporated, for an ACP manuscript this is too specific to the implementation

of the methods. Please delete as it does not provide information on the scientific methods, only

the implementation of these at the coding level.

28) Section 2.1.2, lines 111-112 -- Again there are two very short sentences here, which seem

better as one long-ish sentence. Please change "the iterative SO2 retrieval. This analysis

demonstrated that ash with..." instead to "the iterative SO2 retrieval, demonstrating that

ash with...". Also change "can significantly effect" to "can significantly affect".

29) Section 2.1.2, line 116 -- "This retrieval is run at three pressure levels (400, 600 and

800 hPa) to obtain three estimates of the optical depth". This sentence needs to be re-worded,

or explained by the authors. A reader will assume the "optical depth" must be for the column,

rather than specific to any given altitude, and this wording here is confusing.

If the method is analysing three different threshold altitudes (e.g. for altitudes above

some minimum-altitude pressure level) then change "at three pressure levels" instead

using three different threshold pressure levels" to make that clear. But if it is actually

assessing an optical depth only for a particular shallow-layer of the atmosphere at these

different pressures, then the wording "optical depth" needs to be changed to

"aerosol extinction" or "optical depth contribution" or similar (to ensure the reader

is aware it's chcking for ash at these levels. Please always include "ash" wherever

"optical depth" is stated here (assuming I'm correct that this is the optical depth of

the ash particles --> i.e. "ash optical depth" or "ash optical depth contribution".

30) Section 2.1.2, Caption to Figure 1 -- My understanding is that this is essentially a

"daily composite" image, summing up the total ash AOD detected from the set of

IASI ash-detections during the 24-hour period 00:00UT to 23:59UT on 10th April 2021.

Assuming that's correct, please change the start of the caption from "Ash optical depth"

instead to "Daily composite images of ash optical depth".

31) Section 2.1.2, line 119 -- Please clarify the time-interval for the two images in

Figure 1. My understanding from the text is that both are sampling differently from

within the same time-interval, with this presumably being the 24-hour period from

00:00UT to 23:59UT on 10th April 2021? Assuming that's the case, then please change

"an example of the" instead to "an example daily composite image of the", and add

", 12 to 36 hours after the first explosive eruption at 12:41 UT on the previous day."

32) Section 2.1.2, line 119-121 -- The two sentences beginning "The retrievals show" and

"It is therefore likely" are communicating a related point, and this is best explained

simply within 1 sentence, also being clear this follows from the analysis from

Carboni et al. (2012) explained in the preceding paragraph. The 3rd sentence of this

paragraph is essentially repeating the information given in the caption to Table 1,

and then that sentence can be deleted, simply citing the Table 1 within the re-worded

merged sentence of the currently first and 2nd sentences. Suggest then to combine

the first two sentences of the para, re-wording from "volcano. It is therefore likely..."

instead to "volcano, analysis of the percentage ash-detections (Table 1) showing

the 10th April retrieved SO2 column amounts must be affected by ash."

33) Section 2.1.2, lines 125-126 -- Re-word "gives some information" to better communicate

the basis of this. Suggest to change this to "gives a strong quantitative indication of",

and then change "impact of ash" to "substantial impact of ash" and "retrieved results"

instead to "retrieved SO2 burden".

34) Section 2.1.3, lines 130-136 -- The use of the symbol lamda here for e-folding time

is confusing. For first-order loss rate equations, the term lambda should be used

for the loss rate (i.e. the term with units of "per unit time") -- a lambda symbol

would be better, the greek letter l being short-hand for "loss-rate". For the e-folding

timescale, the tau symbol would be much more appropriate, the greek letter t then

being shorthand for "timescale". Please change all instances of lamda in this section

of text instead to tau.

35) Section 2.1.3, line 133 -- Insert "average" between "of the" and "e-folding time"

and insert "over a given period" at the end of the sentence.

36) Section 2.1.3, lines 133-134 -- It's not correct to say that approach "neglects

the variable flux of SO2 emitted from the volcano". The approach derives an average

over the period there has been that variable flux of SO2 emitted from the volcano.

It's better to make a positive point here, that the optimal estimation scheme

enables to derive time-varying information. Suggest then to re-word the current

"this approach neglects the variable flux of SO2 emitted from the volcano" instead to

"for an eruption with multiple significant SO2 emissions episodes, a time-varying

SO2 emission flux can be derived."

37) Section 2.1.3, lines 134-135 -- The current wording "considers that the total

SO2 value is effected by both the e-folding time and variable SO2 flux" needs to

be improved, to better communicate the processes involved here. I think the text

is indicating that SO2 oxidation is acting to reduce the SO2, whereas the emission

is acting to increase the SO2 -- i.e. there are both localised source terms from

the emission, and also a first-order loss term (i.e. sink tends broadly to scale

with the concentration in the air, with also the concentration of the oxidant).

And then in that sense the spelling "effected" is correct (rather than affected).

But when the authors say "e-folding time" I think they mean the loss due to

oxidation of the SO2, right? In which case, please replace "e-folding time" with

"oxidation sink". Similarly, the wording is better to state "the multiple SO2

emissions from large-magnitude explosions in the 14-day period" or similar.

The word "value" there I think means the total SO2 burden, right?

Or do they mean the emissions flux at each explosion? Assuming it's the former,

suggest then to re-word to "is effected by both the oxidation sink and the

multiple SO2 emissions from large-magnitude explosions in the 14-day period".

Or something more specific such as this.

38) Section 2.1.3, lines 139 -- Insert "emissions" before "flux" (after

"estimates of the"), and insert "for the entire period" at the end of

the sentence.

39) Section 2.1.3, lines 139-140 -- The two sentences beginning "It was noted..."

and "Subsequently, an independent..." should be merged (again they are making

related points about the same issue, and then explaining in 1 sentence is better).

The wording of the start of the 1st sentence should be improved, and I'm suggesting

to make this more general from the current "It was noted that in thie case the.."

instead to "In some cases, the...", then changing "was strongly influenced by"

instead to the general tense "can be strongly influenced by". And then have

the wording of the continuation (former 2nd sentence), re-wording from

"the a priori value. Subsequently, an independent estimate..." instead to

"the a priori value, and for this case an independent estimate...".

40) Section 2.1.3, line 143 -- Re-word "approach to getting flux and e-folding

time for the 9 to 22 April 2021" instead to "approach, to generate a time-varying

emissions flux for the 14-day emission period (9th to 22nd April 2021)."

41) Section 2.1.3, line 144 -- Insert "average" before "e-folding time of".

42) Section 2.1.3, line 145 -- Delete "The event start and end times obtained

from the ABI data were used to inform this." That information is already

communicated in the "as identified with the ABI data" in the preceding sentence.

43) Section 2.1.3, lines 148-151 -- Yet again, there are 3 short sentences here

that relate to the same issue, again better communicated in one carefully worded

long-ish sentence. In fact the 3rd of the sentences here is simply noting the

way the lower-bound estimate was derived, and this can be deleted, having the

re-worded sentence refer both upper-bound and lower-bound estimates.

The term quadrature will be unfamiliar to some readers, and the first required

edit here is to change "summed in quadrature and then multiplied by delta-t"

instead to "summed in quadrature (i.e. a sum multiplying each event-flux by

delta-t)". And then the continuation to the follow-on sentence re-worded

from "delta-t. An alternative estimates of the maximum total erupted mass value"

instead to "delta-t), with upper- and lower-bound total emissions estimates

obtained by summing the corresponding +/- standard deviation individual SO2

emissions at each time (see Figure 2)." or similar.

44) Section 2.2.1, line 155 -- insert comma before "launched in 2016".

45) Section 2.2.1, lines 155-158 -- These 2 sentences can easily be worded into

1 sentence, which much better communicates the issue here (rather than 2

compartmentalised short-sentences with two parts of the same issue).

This is easily done by changing "the Caribbean. In addition, it covers the.."

instead to "the Caribbean, extending across the...".

Change also "of the Atlantic Ocean" to the more specific "of the North

Atlantic ocean".

46) Section 2.2.1, lines 159-160 -- Change "Seven channels between 7.3 and

13.3 microns mean that the instrument.." instead to "Having seven channels

between 7.3 and 13.3 microns means that the instrument..."

47) Section 2.2.1, lines 164-165 -- This short sentence here refers to the

GOES team enacting a higher-temporal resolution "mesoscale" data-stream

focused in the region around the La Soufiere volcano. Presumably this

decision was made by a steering group for the GOES operating procedures,

in response to the substantial eruption having taken place. This would be

a good opportunity to cite some further information on the basis of this.

Is it possible to cite a report or paper that explains some background to

there being this flexibility to enact localised high temporal-resolution

data-flows as a response to the eruption? Or is there a webpage on the

GOES/ABI website that explains this? A slight extension to this sentence

would enable readers to appreciate the forward-planning discussions that

enabled this to happen.

48) Section 2.2.1, line 166 -- change "event" to "events".

49) Section 2.2.1, line 168 -- re-word "have been used for the identification

of the start and end times of the eruptive events" to a more succinct

wording such as "have been used to identify the start and end times...."

50) Section 2.2.1, line 170 -- insert comma before "it has not been".

51) Section 2.2.1, line 170 -- change "Instead it has been used to..."

instead to "Instead, the ABI data has been used to..."

52) Section 2.2.2, lines 173-174 -- This sentence needs to be re-worded

to make clear to the reader whether these RGB images are generated

operationally within the GOES team, or if this is something specific the

authors have done for this analysis. My understanding is that these

ash RGB images are already available to view, based on an established

methodology from the peer-reviewed literature. Please improve these

initial sentences to refer to the published studies that have established

this methodology, and mention whether the images are already supplied

from the GOES team (and the extent of the analysis carried out by the

manuscript authorship team).

53) Section 2.2.2, line 188 -- "inference of the character of the event"

needs to be clearer what is meant by "character". I think you mean the

information included in the "Notes" column of Table 2, and briefly

note (in brackets after the "character") what you mean here.

54) Section 2.2.2, line 191 -- Change "It is also possible that lower

level eruptive activity may not be identifiable" to be clearer here.

Firstly, by "lower level eruptive activity" I think you mean explosive

events of lesser magnitude -- i.e. "lower-magnitude explosive events"

or so. Secondly, you've writen "possible" but I think "likely" seems

more consistent with what is meant here? Please re-word accordingly.

55) Section 2.2.2, line 192 -- insert commas after "Note that" and

"full disc" to improve the grammar of the sentence on this line.

56) Section 2.2.2, lines 193-194 -- Re-word "the rough measurement start

and end times". The subsequent wording indicates the method here is

quite precise, so I'd suggest simply to delete the word "rough".

Similarly delete "roughly", as again this seems relatively precise.

I realise the aim here is to communicate that, with the frequency

of the data within 10-minute intervals, the calculation is more

approximate than during the minute time-resolution "mesoscale" data-flow.

But that can be communicated in the follow-on sentence (see 57 below).

The follow-on sentence beginning "This is estimated to be" makes for

very poor sentence construction, and again merging the 2nd very short

sentence to slighly extend the first sentence would seem to improve

the readability of the text. Suggest then to change "end times. This

is estimated to be roughly 243 seconds (4 minutes and 3 seconds)

from the start time" instead to "end times, roughly 243 seconds from

the start time". It is not necessary to give the minutes and seconds

translation of this, the SI unit for time is seconds, and all readers

will be familiar with converting seconds to minutes.

57) Section 2.2.2, line 194 -- insert "approximate" before "times for..".

58) Section 2.2.2, line 195 -- change "where the measurement start

time has been used" instead to "where the 1-minute time resolution

ensures the timing is highly accurate." or something like this.

59) Section 2.2.2, line 198 -- Improve the wording "This method will

now be referred to as the..." instead to "We refer to this as the...".

60) Section 2.2.2, lines 202-203 -- "The box size should remove any

effect due to parallax". Re-word to state the size of the box,

and insert "significant" between "any" and "error".

61) Section 2.2.2, lines 205 to 217 -- There needs to be citations

given for most of these caveats, where this effect is explained

in further detail. Also, re: the minimum BT method -- please can

the authors clarify whether this method still works for large

magnitude explosive events that penetrate deep into the stratosphere?

I am not so familiar with the literature here, but I note the

discussion of this issue within Woods and Self (1992), in relation

specifically to very large magnitude eruptions penetrating the

stratosphere.

62) Section 2.2.2, line 230 -- re-word "compared against the plume

vector components to assign a degree of confidence". Firstly, I don't

know what is meant here by "plume vector components". You mean the

horizontal velocity components from the motion of the plume?

Please clarify what is meant here.

Also, re-word instead to "compared to the plume vector components

(i.e. horizontal plume motion from successive geostationary images),

and a degree of confidence assigned accordingly". Please clarify

re: the plume vector components, and make amendments to the suggested

re-wording accordingly.

63) Section 2.2.2 lines 233-234 -- Improve this very short final sentence

of this paragraph -- re-word to "the different sampling periods assessing

different portions of the plume" or similar.

64) Section 3.1, lines 254-255 -- The sentences beginning "This is seen..."

and "This corresponds well..." should be re-worded to 1 sentence, and this

can be done easily, changing "9 April. This corresponds" instead to

"9 April, and corresponds well", the resulting merged sentence being

easier to read. Correct typo also with the bracket in the wrong place in

the current wording "with the 08:41 (LT 12:41 UTC)..." --> correct this to

"with the 08:41 LT (12:41 UTC)..."

65) Section 3.1, lines 255-256 -- Correct "Relatively short lived lasting..."

to "Relatively short-lived, lasting..."

66) Section 3.1, line 260 -- Correct "second lower altitude" instead to

"second lower-altitude", and add comma before "travelling to the west".

67) Section 3.1, line 263-264 -- Correct "the plume direction and the wind

profile does not help" to "the plume direction and wind profile do not help".

68) Section 3.1, line 279 -- Insert "distinct" after "further seven".

69) Section 3.2, lines 294 to 308 -- It is thie section of the text that

requires improvement to refer to the QBO (see general points above).

In particular the text on lines 295 to 298 requires improvement, as it

does not give any scientific interpretation of the initial plumes being

transported entirely eastwards (westerly flow) whereas the later daily

composite images for 13th to 17th April indicate a portion of the plume

is being transported westward. There needs to be a better link between

the wind direction profile Figure 4a and what is seen here in the

Figure 7 maps. Specifically, whether the westward transport is indicating

transport at a different level than the initial plumes on the 9th to 11th?

Figure 8 shows the plume heights for the 10th to 11th April, and the

altitudes for the later eruptions are given in Table 3 with the

tropospheric and stratospheric "solutions". But the text needs to be

clearer what this means in relation to the dispersion of the column

seen in the Figure 7 maps. Is the mapping indicative of the upper

tropospheric portion of plumes is transported in a different direction

to the stratospheric plumes, as suggested by the Figure 4a?

Mention of the QBO and the suddent shift in wind direction into

the lower stratosphere should be mentioned here.

70) Section 3.3, lines 310-311 -- Again, these first 2 sentences of the

paragraph are both very short, and I do not understand why the text has

been structured in this way. Please change "Fig. 9c. The mass is computed

for the following region..." to "Fig. 9c, computed for the region...",

and delete the 2nd "region" at the end of the revised single sentence.

71) Section 3.3, line 311 -- Delete "which" after "in Peru)" and

change "at the same time" to "at this time", also changing "and whose

plumes entered this box are also be.." to "and any SO2 from these

eruptions entering the region will also be..".

72) Section 3.3, line 313 -- change "small plumes" to "small plumes, and

at lower altitude" (assuming that is the case).

73) Section 3.3, lines 317-320 -- this segment of text is where there needs

to be a better scientific interpretation of the results, in relation to the

processes occurring as the multiple explosive-emission SO2 plume disperses.

A specific suggestion is to have the sentence "The fact that the total SO2.."

start a new paragraph, and expand this sentence to refer to the oxidation

of the SO2, and the studies mentioned in the general comments for the studies

I mentioned interpreting the progressing SO2 burden within volcanic clouds

large-magnitude explosive eruptions (Pinatubo, Kelut, Hunga-Tonga).

There should be mention of the potential accelerated SO2 oxidation from

reactions on the surface of ash particles (heterogeneous chemistry),

referring to the Zhu et al. (2020) study.

74) Section 4, line 415 -- Improve the sentence beginning "This is similar

to this study which reports..." -- probably this can be joined with the

preceding sentence for a more coherent and readable statement.

75) Section 5, line 431 -- Re-word "emitted large plumes of ash and SO2

into the atmosphere" to better state the science narrative of the study

re: the study generating an eruption chronology across the several large

magnitude explosive events in the initial days, through to a full 14-day

dataset, comprising 4 distinct eruption phases. A specific suggestion

would be to expand "emitted large plumes" instead to "comprised several

large-magnitude explosive events, each generating tropopause-penetrating

plumes of volcanic SO2 and ash", with later phases continuing to emit

SO2 into the upper troposphere up to xx days after the initial explosion".

Or similar wording to this.

References

----------

Baldwin (2001): "The quasi-biennial oscillation", Reviews of Geophysics,

vol. 38, no. 2, pp. 179-229. https://doi.org/10.1029/1999RG000073

Guo et al. (2004): "Re-evaluation of SO2 release of the 15 June 1991

Pinatubo eruption using ultraviolet and infrared satellite sensors",

Geochemistry Geophysics Geosystems, vol. 5, no. 4, 31 pages,

https://doi.org/10.1029/2003GC000654

Langford et al. (1995): "Transport of the Pinatubo volcanic aerosol

to a northern midlatitude site", J. Geophys. Res., vol. 100, no. D5,

pp. 9007-9016, https://doi.org/10.1029/95JD00384.

Marshall et al. (2018): "Exploring how eruption source parameters

affect volcanic radiative forcing using Statistical emulation",

J. Geophys Res. Atmos., vol. 124. https://doi.org/10.1029/2018JD028675

Reed et al. (1961): "Evidence of a downward-propagating annual wind

reversal in the equatorial stratosphere" J. Geophys. Res., vol. 66,

no. 3, pp. 813-818, https://doi.org/10.1029/JZ066i003p00813.

Trepte and Hitchmann (1992): "Tropical stratospheric circulation

deduced from satellite aerosol data", Nature, vol. 355, pp. 626-628,

https://doi.org/10.1038/355626a0

Trepte et al. (1993): "The poleward dispersal of Mount Pinatubo

volcanic aerosol" J. Geophys. Res., vol. 98, no. D10, pp. 18,563-18,573,

https://doi.org/10.1029/93JD01362

Woods and Self (1992): "Thermal disequilibrium at the top of volcanic

clouds and its effect on estimates of the column height

Nature, vol. 355, pp. 628-630, https://doi.org/10.1038/355628a0

Zhu et al. (2020): "Persisting volcanic ash particles impact

stratospheric SO2 lifetime and aerosol optical properties",

Nature Communications, https://doi.org/10.1038/s41467-020-18352-5

Zhu et al. (2022): "2022 Hunga-Tonga eruption: stratospheric aerosol

evolution in a water-rich plume" Communications Earth and Environment,

https://doi.org/10.1038/s43247-022-00580-w

Citation: https://doi.org/10.5194/acp-2022-772-RC2
- RC3: 'Reply on RC2', Anonymous Referee #2, 02 Jan 2023
  
  I just uploaded this review comment, and the paragraph justification seemed all
  
  to be fine in the "Preview" page.
  
  After clicking to submit, I was then very surprised to see the formatting to have been
  
  disrupted with line-breaks in the online-viewable version, and with formatting across
  
  the lines not retaining the paragraph structure that had appeared fine from the
  
  Preview page of the reviewer system.
  I am therefore attaching here a zip file containing Word and pdf versions of the
  
  review I submitted.
  
  I would be grateful if the Handling Editor could refer to the Executive editors to
  
  establish any required change in procedures such that reviewers can preview
  
  the way their review will appear after clicking submit.
  
  To my understanding, currently this is not possible, and only the text-window
  
  preview is possible. Options to retain the formatting within the text-window
  
  preview would not seem a reasonable option with a future progression of
  
  the electronic system for handling reviewer submissions.
  
  Citation: https://doi.org/10.5194/acp-2022-772-RC3
- AC2: 'Reply on RC2', Isabelle Taylor, 06 Apr 2023
  
  We thank the referee for their helpful comments. Please see our responses in the attached document. Note that responses to both RCs are contained in this PDF
  
  Citation: https://doi.org/10.5194/acp-2022-772-AC2

Isabelle A. Taylor et al.

Supplement

https://doi.org/10.5194/acp-2022-772-supplement

Data sets

IASI SO2 La Soufriere data Isabelle A. Taylor; Roy G. Grainger, Andrew T. Prata, Simon R. Proud, Tamsin A. Mather, David M. Pyle http://eodg.atm.ox.ac.uk/IASI/la_soufriere/

Video supplement

Animations of ABI results for La Soufriere Isabelle A. Taylor; Roy G. Grainger, Andrew T. Prata, Simon R. Proud, Tamsin A. Mather, David M. Pyle http://eodg.atm.ox.ac.uk/ABI/la_soufriere/

Isabelle A. Taylor et al.

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Short summary

This study looks at sulfur dioxide (SO₂) and ash emissions from the 2021 eruption of La Soufrière on St Vinent. Using satellite data, 32 eruptive events were identified. Satellite data was used to track SO₂ as it was transported around the globe. The majority of SO₂ was emitted into the upper troposphere and lower stratosphere. Similarities with the 1979 eruption of La Soufrière highlight the value of studying these eruptions to be better prepared for future eruptions.


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