From the journal Environmental Science: Atmospheres Peer review history

26-Jul-2021

Dear Dr Perron:

Manuscript ID: EA-ART-07-2021-000054
TITLE: Atmospheric inputs of volcanic Fe around Heard and McDonald Islands, Southern Indian Ocean.

Thank you for your submission to Environmental Science: Atmospheres, published by the Royal Society of Chemistry. I sent your manuscript to reviewers and I have now received their reports which are copied below.

Both reviewers pointed out some minor issues with the manuscript, mostly related to clarity. After careful evaluation of your manuscript and the reviewers’ reports, I will be pleased to accept your manuscript for publication after minor revisions.

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************

Reviewer 1

Comments on the originality, importance, impact and reliability of the science
Volcanic Fe have been studied for many years, but the details of the mechanisms to enhance Fe solubility in volcanic ashes are mostly unquantified and require further research. The authors captured the aerosol samples influenced by volcanic emissions. They found enhanced aerosol Fe solubility in the volcanic ashes, compared to mineral dust. This may contribute to improving our understanding of iron biogeochemical cycle. I can recommend the paper for publication in Environmental Science: Atmospheres.

Specific comments
p.8: Heimburger et al. (2013) did not estimate higher acidity of the rainwater compared to aerosols. Please elucidate and rephrase it.

p.6, volcanic aerosol emission around HIMI: The under-representation of LFe sources around HIMI in a global model could be associated with volcanic emissions. The model estimates are available from Ito et al. (2020). Please discuss this quantitatively.

p.7, conclusions: Similar sentence in introduction is repeated for the reference #11. How do you associate the atmospheric input of toxic metals with the rapid ecosystem growth?

References
Ito, A., Perron, M.M.G., Proemse, B.C. et al. Evaluation of aerosol iron solubility over Australian coastal regions based on inverse modeling: implications of bushfires on bioaccessible iron concentrations in the Southern Hemisphere. Prog Earth Planet Sci 7, 42 (2020). https://doi.org/10.1186/s40645-020-00357-9

Reviewer 2

Comments:
In the manuscript entitled “Atmospheric inputs of volcanic Fe around Heard and McDonald Islands, Southern Indian Ocean”, authors reported first measurements of the total concentration and labile fraction of Fe in aerosols collected in the vicinity and downwind of HIMI. Atmospheric measurements and air-mass trajectory modeling provided insight into the origin and transport pathway of Fe-containing aerosols along the voyage track between Australia and HIMI and around the volcanic islands. Metal ratios in Heard Island seabed rocks served as a reference to assess the contribution of volcanic emissions from the islands to aerosol loading during the campaign. The study raises awareness on the importance to consider the impact of volcanic emissions on atmospheric composition and on the subsequent deposition of bioavailable nutrients (including Fe) to remote oceanic areas.

Overall, the manuscript is generally well written with clear logic, full and reasonable discussions, and the conclusion could be well supported by the data. I consider that it fulfills the necessary requirements to be published. Therefore, I recommend it for publication on Environmental Science: Atmospheres after the authors address a few minor comments.

1. The author mentioned in the Ship-board aerosol sampling that aerosol sampling was only conducted under head winds to prevent contamination from the ship’s exhaust. Is there any requirement for wind speed? If the wind speed is too low, can possible contamination from the ship exhaust be avoided?
2. In the section of Definition of soluble, labile and total Fe content in aerosol samples, the same filter was then soaked in 1.1 mol per liter ammonium acetate (pH=4.7) for one hour. In this operation, why did the author choose ammonium acetate and need to adjust the concentration and pH to 1.1 mol per liter and 4.7, respectively? Another suggestion is that the author can change mol per liter to mol/L.
3. For the description that 12h-long heat-assisted HF/HNO3 digestion provided near 100% recovery of all target elements on the remaining filter, did the author do the recovery experiment or have other literature support?
4. The author needs to check whether the formula of enrichment factor is correct. In addition, the abbreviation of enrichment factor should be marked when it is shown first time.
5. For aerosols A8 to A11 were associated with BC concentrations between 0.04 - 0.15 µg m-3, the author explained that the samples were collected less than 100 km away from HIMI, and there was early volcanic activity. Why was the sampling location of A9 closest to HIMI, but the BC concentration of A9 was the lowest among the four samples A8-A11?
6. Enhanced Fe solubility in A1 and A13 may also result from ligand-mediated Fe dissolution in the presence oxalate-laden pyrogenic particles as suggested by significant concentration of biomass burning tracer, levoglucosan, (and BC) in those samples. For this description in the manuscript, I did not find that the author provided data analysis of levoglucosan in the sample.
7. The author should add more analysis of Figure S3 in the text so that readers can understand the contribution of oceanic and terrestrial sources to different aerosol samples. For example, how did HYSPLIT transport model indicate that the sampling locations of aerosols A3 to A12 predominantly receive oceanic air-masses?
8. Fe-to-Al concentration ratio (TFe/TAl) was presented in the text and table, but Fig 1. was the logarithm of the total concentration ratio of Fe-to-Al, Log10(TFe/TAl). Why not be consistent? What is the difference about?

This text has been copied from the PDF response to reviewers and does not include any figures, images or special characters.

We wish to thank Reviewer 1 and Reviewer 2 for insightful comments on this manuscript and
valuable suggestions for specific points to be revised as detailed below.
Response to Reviewer #1
p.8: Heimburger et al. (2013) did not estimate higher acidity of the rainwater compared
to aerosols. Please elucidate and rephrase it.
In the revised manuscript page 4, the sentence reporting Heimburger (2013) study was
modified to accurately reflect the study’s findings.
p.6, volcanic aerosol emission around HIMI: The under-representation of LFe sources
around HIMI in a global model could be associated with volcanic emissions. The model
estimates are available from Ito et al. (2020). Please discuss this quantitatively.
Ito and co-workers (2020) previously highlighted that aerosol measurements around HIMI
(same measurements as presented in this study) were associated at 80% with aeolian
sources currently unaccounted for in model parametrizations. We revised the manuscript to
include this study in the introduction page 1 as well as in the conclusion page 7. While we
can suggest that such a missing source is, at least partly, represented by volcanic emissions
from Heard Island in our study, our aerosol measurements do not allow an accurate
quantification of source apportionment in the samples but rather hint a prevailing source.
The reference suggested by Reviewer #1 was added to the reference list.
p.7, conclusions: Similar sentence in introduction is repeated for the reference #11. How
do you associate the atmospheric input of toxic metals with the rapid ecosystem
growth?
We apologize that our sentence was not clear enough. The authors have indeed misunderstood the sentence p7. In this study we suggest that enhanced Fe deposition in the
vicinity and downwind from Heard and McDonalds may be responsible for the observed
rapid phytoplankton growth reported during the same cruise (reference #11). We raise
awareness on the potential for volcanic eruptions to become toxic for marine ecosystem (as
previously reported in the literature) in the event of large volcanic eruptions. While our
study demonstrated atmospheric enrichment in ‘volcanic metals’ like Ni, Mo and Cr
compared to the crustal references, no enhanced atmospheric concentration was
highlighted in our samples. While the volcanic eruption observed in our study was small,
both the toxic and fertilizing potentials of larger volcanic eruptions were presented in order
to encouraging additional future studies on this topic. One sentence in the introduction
page one was modified to prevent such confusion. The conclusion page 7 was also adjusted
to prevent mis-interpretation.
Response to Reviewer #2
1. The author mentioned in the Ship-board aerosol sampling that aerosol sampling was
only conducted under head winds to prevent contamination from the ship’s exhaust. Is
there any requirement for wind speed? If the wind speed is too low, can possible
contamination from the ship exhaust be avoided?
During HEOBI, we used an automated sector sampling switch to ensure that only head
winds were sampled and to prevent contamination from the ship’s exhaust. As shown
below, during the voyage, wind speeds mostly fell between 10 and 40 knots which is
expected at this latitude in the Southern Ocean. Information on wind speed was added to
page 2 in the ‘Ship-board aerosol sampling’ subsection.
Figure : 5-minute average true wind speed measurement on both port and starboard
side of the RV Investigator during the HEOBI voyage. Please note that the data displayed
provides wind speeds for all sectors, not solely the 290-70º wind direction sector for
which aerosol sampling was allowed. Data : CSIRO voyage data – IN2016_V01 underway
data
2. In the section of Definition of soluble, labile and total Fe content in aerosol samples,
the same filter was then soaked in 1.1 mol per liter ammonium acetate (pH=4.7) for one
hour. In this operation, why did the author choose ammonium acetate and need to
adjust the concentration and pH to 1.1 mol per liter and 4.7, respectively? Another
suggestion is that the author can change mol per liter to mol/L.
In the leaching protocol assessment paper from the same authors (Perron et al (2020)), we
described in detail the different steps of the leaching protocol. The ammonium acetate
leach was chosen following a method by Sarthou et al (2003) as a model for Fe release from
aerosol in rainwater. The ammonium acetate solution obtained by mixing acetic acid with
ammonia is already defined by a concentration of 1.1 mol per liter and the pH close to 4.7 is
only adjusted to 4.7 for consistency (and reproducibility) of the leaching technique. We have
changed the text to mol/L
References :
Sarthou, G., et al., 2003. Atmospheric iron deposition and sea-surface dissolved iron
concentrations in the East Atlantic. Deep-Sea Res., Part I 50, 1339–1352
Perron MMG, Strzelec M, Gault-Ringold M, Proemse BC, Boyd PW, Bowie AR (2020a)
Assessment of leaching protocols to determine the solubility of trace metals in aerosols.
Talanta 208:120377. https://doi.org/10.1016/j.talanta.2019.120377
3. For the description that 12h-long heat-assisted HF/HNO3 digestion provided near
100% recovery of all target elements on the remaining filter, did the author do the
recovery experiment or have other literature support?
Both. The digestion protocol was previously assessed by the authors in an earlier publication
(Morton et al., 2013; reference #19) referred in the manuscript. The entire leaching protocol
method was then assessed by the authors, including digestion recovery (Perron et al., 2020;
reference #20). In addition, one aliquot of Arizona Test Dust (ATD) was digested alongside
aerosol samples from the HEOBI campaign, resulting in similar recoveries than those
reported in the references. A new supplementary table S2 was added to the supplementary
documents, providing recovery values this digestion study, from the digestion protocol
assessment (same laboratory) as well as the average digestion recovery obtained in the
laboratory to date (additional digestion recovery data).
4. The author needs to check whether the formula of enrichment factor is correct. In
addition, the abbreviation of enrichment factor should be marked when it is shown first
time.
Formula was corrected in the manuscript ; we thank the reviewer for noticing our mistake.
In addition, the abbreviation “EF” for enrichment factor was added when first presented.
5. For aerosols A8 to A11 were associated with BC concentrations between 0.04 - 0.15 µg
m-3, the author explained that the samples were collected less than 100 km away from
HIMI, and there was early volcanic activity. Why was the sampling location of A9 closest
to HIMI, but the BC concentration of A9 was the lowest among the four samples A8-
A11?
The question raised by Reviewer #2 was addressed in the manuscript page 6. Sample A9 not
only showed low BC but also lower EFs compared to other aerosols collected around HIMI.
Based on HYSPLIT back-trajectories associated with this sample we suggest that A9 is
composed by two end-member air-masses of different origin, one air-mass dominated by
crustal (non-volcanic) source from the Kerguelen or from long-range atmospheric transport
and another air-mass dominated by volcanic aerosols from HIMI. Such mixture of local airmass and prevailing air-mass from the Kerguelen/long-range transport was proposed to
explain a diluted BC signal as measured alongside A9. A sentence was added to the relevant
explanation in the revised manuscript
6. Enhanced Fe solubility in A1 and A13 may also result from ligand-mediated Fe
dissolution in the presence oxalate-laden pyrogenic particles as suggested by significant
concentration of biomass burning tracer, levoglucosan, (and BC) in those samples. For
this description in the manuscript, I did not find that the author provided data analysis of
levoglucosan in the sample.
Levoglucosan analysis was undertaken and presented in a previous study by the authors
#46 E. Sanz Rodriguez, M. M. G. M. M. G. Perron, M. Strzelec, B. C. B. C. Proemse, A. R. A. R.
Bowie and B. Paull, J. Chromatogr. A, 2019, 1610, 460557.
The sentence was modified to clarify that levoglucosan data (now displayed) does belong to
a previously published article.
7. The author should add more analysis of Figure S3 in the text so that readers can
understand the contribution of oceanic and terrestrial sources to different aerosol
samples. For example, how did HYSPLIT transport model indicate that the sampling
locations of aerosols A3 to A12 predominantly receive oceanic air-masses?
Authors wish to emphasize the approximative nature of HYSPLIT. Indeed, when HYSPLIT
captures large features relevant to global or basin scale studies, it does not resolve local
atmospheric air-masses of interest for such study as this presented here. For this reason,
authors only use HYSPLIT to complement measurements made on aerosols. Prevailing airmasses found for each sample were reported in Table 1 of the manuscript. In addition, we
revised the supplementary figure S3 to display all HYSPLIT air-mass back-trajectories
associated with individual samples.
8. Fe-to-Al concentration ratio (TFe/TAl) was presented in the text and table, but Fig 1.
was the logarithm of the total concentration ratio of Fe-to-Al, Log10(TFe/TAl). Why not
be consistent? What is the difference about?
The concentration ratio of Fe to Al in aerosols is the commonly tool used in atmospheric
studies for tracing the prevailing aeolian source in a sample. For a matter of consistency and
as suggested by Reviewer #2, Figure 1 now represent a log-scale view of the Fe-to-Al ratios
rather than the log10 value to the same ratio. The log scale view allow for a clearer
visualization of the difference existing between the Fe-to-Al ratios of individual samples
investigated. This information was added to the figure caption.

23-Aug-2021

Dear Dr Perron:

Manuscript ID: EA-ART-07-2021-000054.R1
TITLE: Atmospheric inputs of volcanic iron around Heard and McDonald Islands, Southern Ocean.

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. After considering the changes you have made, I am pleased to accept your manuscript for publication in its current form. I have copied any final comments from the reviewer(s) below.

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Reviewer 2

The revised manuscript can now be accepted for publication.

Reviewer 1

The paper has been improved substantially.