From the journal Environmental Science: Atmospheres Peer review history

22-Jul-2023

Dear Mr Koenig:

Manuscript ID: EA-ART-05-2023-000063
TITLE: Observed in-plume gaseous elemental mercury depletion suggests significant mercury scavenging by volcanic aerosols

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Dr Lin Wang
Associate Editor, Environmental Science: Atmospheres

Environmental Science: Atmospheres is accompanied by companion journals Environmental Science: Nano, Environmental Science: Processes and Impacts, and Environmental Science: Water Research; publishing high-impact work across all aspects of environmental science and engineering. Find out more at: http://rsc.li/envsci

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Reviewer 1

This publication reports on observations of the chemistry of the plume of the volcano Piton de la Fournaise (Réunion) made in 2018, including a late April – early May eruptive event. Observations were made both close to the source, and from the Maïdo observatory approximately 40 km downwind. A wide variety of parameters were measured including concentrations of Gaseous Elemental Mercury (GEM).

Central to this study is the observation of two GEM depletion events in the plume at Maïdo in the morning of the 29th of April 2018. Analyses of other parameters suggests that the cause of these is not halogen chemistry which has previously been hypothesised to cause significant oxidation of in-plume mercury. Instead, the authors present an alternative hypothesis: that this depletion is due to uptake of mercury onto or into particles which are abundant within volcanic plumes. Such uptake has been previously observed in urban settings.

This hypothesis is evaluated using the available data, in both qualitative and quantitative approaches. Quantitative analysis includes the calculation of PM0.95sulfate proxy – an estimate of the quantity of particulate. This estimate is based on measurements of SO2 and direct measurements of sub-700nm aerosol, and the authors argue for it being a more useful parameter than the direct aerosol measures alone. The quantitative analysis yields estimates of the rate of mercury depletion within the plume

There is due consideration of prior related work and a short but well-written introduction places this research in context for the reader. Later analysis also duly considers prior work in this field.

The methodology used is well explained, both in terms of the physical measurements made on Réunion and the subsequent analysis.

I find that the article’s conclusions are well supported by the data and its analysis, and there is appropriate acknowledgment of the limitations of these and the uncertainties in the results that these cause (both qualitatively and quantitatively). The results are interesting, highly significant for the field, and within the scope of the journal. I recommend this for publication.

I find there is one notable omission from the analysis, otherwise my comments are all minor typographical and formatting issues:

Section 3.3 should make mention of the “effective source region” (Bobrowski et al 2007) wherein high temperatures in the first few seconds of the plume create radicals. Such high-temperature radicals appear to be necessary to explain near-source BrO columns and O3 depletions. While I expect that high-temperature radicals would be insufficient to cause the GEM depletions seen, the discussion in section 3.3 is incomplete without evaluating the potential contribution (or lack thereof) of these radicals to the GEM depletion.

==Minor comments==
• Line 125: the text “at standard conditions of 273.14 K and 1013.25 hPa (STP)”. It may be helpful for readers not familiar with GMOS standard procedures to state how a conversion to STP is made from the environmental conditions at Maïdo.
• In section 2.2.2. it would be helpful to refer to the labels on Figure 1 regarding the locations of measurements.
• Line 228: “are operated” --> “has been operated”.
• Line 249: remove “Recently”
• Section 2.3.3. The calculation of PM0.95sulfate proxy, does not directly consider factors which could influence the gas-phase SO2 to sulfate ratio (such as plume age). These simplifications could be acknowledged.
• Line 369. The relationship of “local time” to UTC should be mentioned on first instance.
• Figure 2a and 2b could benefit from being wider. Perhaps the photographs could be moved below or to a separate figure.
• Figure 3a: the use of a horizontal dash for the 29th April markers is confusing, as from the key it looks like this should be a second continuous line.
• Figure 3e: it should be mentioned in the caption that vertically upwards implies north for the wind directions.
• Line 406: Remove “with”.
• Figure 4: This data would be better presented as discreet markers rather than continuous lines. As presented, these lines imply an almost instantaneous change has been observed, which I don’t believe is supported.
• Line 476: remove “(!)” (though I agree this is a very surprising and interesting result!)
• Line 546: Given that Eq. 3 determines the gradient to be -k, and all the gradients on Fig. 5 are negative, I would assume these values of the rate constants would be positive.
• Line 564: There is no Figure S4, I believe this reference should be S3.
• Line 592 and 608. “ppm” should be in lower case.

==References==

Bobrowski, N., von Glasow, R., Aiuppa, A., Inguaggiato, S., Louban, I., Ibrahim, O. W., and Platt, U. (2007), Reactive halogen chemistry in volcanic plumes, J. Geophys. Res., 112, D06311, doi:10.1029/2006JD007206.

Reviewer 2

In this manuscript, the authors report on their observation of complete depletion events of gaseous elemental mercury (GEM) in dilute and moderately aged volcanic plumes from Piton de la Fournaise on Réunion Island. The authors have ruled out the dominant role of bromine chemistry in this process, as they have determined the concentration of bromine in both the gaseous and particulate states. Instead, the authors suggest that the interaction of gaseous-particles with sulphate-rich volcanic particles, which are present in large numbers in the dilute plume, is the main cause of GEM depletion. Additionally, based on their observations, the authors estimate the global removal of mercury from the atmosphere by volcanic aerosols. They conclude that atmospheric aerosols can act as sinks for atmospheric mercury.

The manuscript is well-written, and the experiments performed and methods used are clearly described. The findings are highly interesting and provide valuable insight for further research in the field of earth and atmospheric sciences. Thus, we recommend that this article be published in Environmental Science: Atmospheres following minor revisions.

1. Line 78-87: It’d be helpful to briefly review some of the existing literature on the mercury emissions from volcanoes, e.g. previous studies investigating the impact of volcanic plumes on atmospheric mercury, or studies on the gaseous elemental mercury depletion in other environmental contexts. I recommend updating the reference list with more recent references and replacing outdated references (references 8, 9, 10, 15-18) with more current ones.
2. Line 96, 109, 214: Please identify valid numbers of latitude and longitude.
3. Line 157, 161, 171: Please verify the units.
4. Line 168, 362: Please use the correct date format.
5. Line 247: The Monte Carlo simulation is an important tool for reaching the conclusions. I recommend that the authors provide a detailed description and explanation of the simulation in the Methods 2.3.1 section.
6. Line 375: It is difficult to determine whether the points and lines in Figure 3a correspond to the left or right axis, respectively. Additionally, the authors need to confirm that using “ppbv” in both the figure and the text is correct.
7. Line 516: The authors investigated the first-order relationship between GEM removal rate and particle mass. I wonder if the authors have more direct evidence, such as the relevant atmospheric particle characterization, to support the hypothesis of “Particle-induced GEM depletion”.
8. Line 593-595: What are the similarities and differences between the authors' results and published research concerning Hg mass fractions in volcanic fine particles and suspended fine particles?

The authors would like to thank the two anonymous reviewers for evaluating the manuscript and providing valuable comments and suggestions. Below, we give point-by-point answers to the specific observations of each reviewer.

RESPONSE TO REVIEWER 1:
Section 3.3 should make mention of the “effective source region” (Bobrowski et al 2007) wherein high temperatures in the first few seconds of the plume create radicals. Such high-temperature radicals appear to be necessary to explain near-source BrO columns and O3 depletions. While I expect that high-temperature radicals would be insufficient to cause the GEM depletions seen, the discussion in section 3.3 is incomplete without evaluating the potential contribution (or lack thereof) of these radicals to the GEM depletion.

Authors’ response:
We agree that this should be mentioned and discussed. We added a mention of the “effective source region”, discussed its potential contribution (we agree that it is unlikely to create enough radicals to cause GEM depletion in the dilute plume, we refer to Surl et al. (2021) for this), and referred to Bobrowski et al. (2007). Please see below:

We added the following: “It has been proposed that quick conversion of HBr into radical species may occur in the so-called “effective source region”, the region of the crater near to the vent where hot and oxygen-poor volcanic gases first mix and interact with cool and oxygen-rich background air8. However, it is unlikely that this process can generate sufficient radical bromine species to completely deplete atmospheric GEM within a few hours16. For this to happen, a “bromine explosion would most likely need to occur14, which is a photochemical process that requires UV radiation.”


==Minor comments==
• Line 125: the text “at standard conditions of 273.14 K and 1013.25 hPa (STP)”. It may be helpful for readers not familiar with GMOS standard procedures to state how a conversion to STP is made from the environmental conditions at Maïdo.

Authors’ response: We added a short sentence after this statement to clarify that we converted the sampled volume from ambient conditions to volume at STP using the ideal gas law.


• In section 2.2.2. it would be helpful to refer to the labels on Figure 1 regarding the locations of measurements.

Authors’ response: We added this information at all instances.


• Line 228: “are operated” --> “has been operated”.

Authors’ response: We made the suggested change.


• Line 249: remove “Recently”

Authors’ response: We think the referee refers to line 349, not 249. We made the suggested change there.


• Section 2.3.3. The calculation of PM0.95sulfate proxy, does not directly consider factors which could influence the gas-phase SO2 to sulfate ratio (such as plume age). These simplifications could be acknowledged.

Authors’ response: Agreed. We added a sentence in section 2.3.3 to acknowledge for the omission of plume age in the calculation of the sulfate proxy, and slightly restructured the sentence explaining the assigned relative uncertainty of 50%.


• Line 369. The relationship of “local time” to UTC should be mentioned on first instance.

Authors’ response: We added the suggested information.


• Figure 2a and 2b could benefit from being wider. Perhaps the photographs could be moved below or to a separate figure.

Authors’ response: Agreed. We made the figure wider by moving the photographs below, as suggested.


• Figure 3a: the use of a horizontal dash for the 29th April markers is confusing, as from the key it looks like this should be a second continuous line.

Authors’ response: There is a reason for using the horizontal dashes instead of just points as markers. This way, the exact information about start and end of sampling time is maintained for each “data point” (which corresponds to the average over a 15-minute sampling interval). To make this clearer we added in the caption a short sentence indicating that the width of each black segment has a physical meaning.

We also agree that the marker was a bit confusing when compared to the legend entry, which was just a line. To make this clearer, we modified the legend entry to show a line with vertical error bars, thus corresponding visually to the markers.


• Figure 3e: it should be mentioned in the caption that vertically upwards implies north for the wind directions.

Authors’ response: We added the suggested information in the caption.


• Line 406: Remove “with”.

Authors’ response: We made the suggested change and added a “thus” to maintain a logical sentence structure.


• Figure 4: This data would be better presented as discreet markers rather than continuous lines. As presented, these lines imply an almost instantaneous change has been observed, which I don’t believe is supported.

Authors’ response: As in the case of GEM observations in Figure 3a, the horizontal lines have a meaning: they show the start and end times of each sampling interval (each “data point” is actually not one value at a certain instance, but an average value over a certain sampling interval). We would like this information to be maintained. However, we agree that the same meaning does not apply to the “vertical connectors”, which have only been added to visually aid the reader. Seeing that they appear to be confusing or misleading, we removed these “vertical connectors”. We also made this change to Figure S1 in the supplement.


• Line 476: remove “(!)” (though I agree this is a very surprising and interesting result!)

Authors’ response: We got a bit carried away there! We removed the (!) as suggested.


• Line 546: Given that Eq. 3 determines the gradient to be -k, and all the gradients on Fig. 5 are negative, I would assume these values of the rate constants would be positive.

Authors’ response: We agree that this was incongruent. We removed the “-“ sign from the equation (so that now it correctly represents an exponential decay when the negative k is inserted), but added a small clarification after the equation that “k” is expected to be negative for an exponential decay (like we assume here).


• Line 564: There is no Figure S4, I believe this reference should be S3.

Authors’ response: Yes, this should have been Figure S3 at the time. However, as we added a new supplementary figure in the revised supplement, this is now really Figure S4.


• Line 592 and 608. “ppm” should be in lower case.

Authors’ response: We changed “PPM” to “ppm” at all instances in the manuscript.


RESPONSE TO REVIEWER 2:

1. Line 78-87: It’d be helpful to briefly review some of the existing literature on the mercury emissions from volcanoes, e.g. previous studies investigating the impact of volcanic plumes on atmospheric mercury, or studies on the gaseous elemental mercury depletion in other environmental contexts. I recommend updating the reference list with more recent references and replacing outdated references (references 8, 9, 10, 15-18) with more current ones.

Authors’ response: Regarding the addition of information: We think that the main information to be retained (volcanoes emit Hg to the atmosphere; emitted Hg is initially mostly Hg0 but the speciation evolves; the bromine explosion can oxidize mercury, which has been observed in polar regions but has also been hypothesized to occur in volcanic plumes) is already contained in the introduction. As more information and references are given later in the text, we would like to maintain the introduction as focused as possible, to better guide the reader. That said, we noted that there was no specific mention that GEM oxidation can cause GEM depletion in the arctic, so we added the sentence below (alongside relevant references).

We added: “This process is regularly observed at polar sunrise, where it can cause complete depletion of GEM.”

Regarding updating the reference list: We agree to such an update, but we do not like to remove references already contained in the introduction, because they helped guide the study and because we do not consider references after ~2005 to be outdated (especially as the available research body regarding volcanic Hg emissions is relatively small; see Edwards et al. (2021)). Instead, we made some additions referring to more recent studies:
1) Wang, S., McNamara, S. M., Moore, C. W., Obrist, D., Steffen, A., Shepson, P. B., Staebler, R. M., Raso, A. R. W., and Pratt, K. A.: Direct detection of atmospheric atomic bromine leading to mercury and ozone depletion, Proc Natl Acad Sci USA, 116, 14479–14484, https://doi.org/10.1073/pnas.1900613116, 2019.
2) Steffen, A., Douglas, T., Amyot, M., Ariya, P., Aspmo, K., Berg, T., Bottenheim, J., Brooks, S., Cobbett, F., Dastoor, A., Dommergue, A., Ebinghaus, R., Ferrari, C., Gardfeldt, K., Goodsite, M. E., Lean, D., Poulain, A. J., Scherz, C., Skov, H., Sommar, J., and Temme, C.: A synthesis of atmospheric mercury depletion event chemistry in the atmosphere and snow, Atmos. Chem. Phys., 8, 1445–1482, https://doi.org/10.5194/acp-8-1445-2008, 2008.
3) Halfacre, J. W., Knepp, T. N., Shepson, P. B., Thompson, C. R., Pratt, K. A., Li, B., Peterson, P. K., Walsh, S. J., Simpson, W. R., Matrai, P. A., Bottenheim, J. W., Netcheva, S., Perovich, D. K., and Richter, A.: Temporal and spatial characteristics of ozone depletion events from measurements in the Arctic, Atmos. Chem. Phys., 14, 4875–4894, https://doi.org/10.5194/acp-14-4875-2014, 2014.
4) Martin, R. S., Witt, M. L. I., Pyle, D. M., Mather, T. A., Watt, S. F. L., Bagnato, E., and Calabrese, S.: Rapid oxidation of mercury (Hg) at volcanic vents: Insights from high temperature thermodynamic models of Mt Etna’s emissions, Chemical Geology, 283, 279–286, https://doi.org/10.1016/j.chemgeo.2011.01.027, 2011.


2. Line 96, 109, 214: Please identify valid numbers of latitude and longitude.

Authors’ response: We would like to maintain the use of “decimal degrees” notation, because they allow readers to just “copy-paste” the coordinates into the appropriate map browser to pinpoint the location (while the “sexagesimal degrees”-notation, i.e. degrees-minutes-seconds, often requires previous conversion). That said, we acknowledge that there was some mixing between the different notations (with the identifiers N,E,S,W usually not used in “decimal degree”-notation). We corrected this at the three instances, and also added “latitude” and “longitude” where needed, to avoid any confusion.


3. Line 157, 161, 171: Please verify the units.

Authors’ response: In line 157, seconds is correct. At 161, the units appear to be OK and are in the right SI format. At 171, the units appear to be OK, but we reformatted L/min to L min-1 to be consistent with the other units.


4. Line 168, 362: Please use the correct date format.

Authors’ response: Thanks for pointing out the incongruencies in the date format. We made changes at several instances in the manuscript to maintain a consistent date format in the whole manuscript (In the form of: “on the 29th of April 2018”)


5. Line 247: The Monte Carlo simulation is an important tool for reaching the conclusions. I recommend that the authors provide a detailed description and explanation of the simulation in the Methods 2.3.1 section.

Authors’ response: In fact, two different Monte Carlo simulations were performed (for the linear regression coefficients and for the resulting mass fraction in particles). We think that going into much detail for each of these would be too long and distractive at this point, and we would like to keep this section as generalized as possible. However, as we agree that providing more details on the MC simulations could be useful for the readers, we added a new figure in the supplement (now supplementary figure S2) that schematizes the variables going into each MC simulation for these three cases.


6. Line 375: It is difficult to determine whether the points and lines in Figure 3a correspond to the left or right axis, respectively. Additionally, the authors need to confirm that using “ppbv” in both the figure and the text is correct.

Authors’ response: In fact, the points and lines correspond to both axes at the same time, as the right axis is just the left axis expressed in other units (at STP, the conversion from ng/m3 to mixing ratio is linear and uniform with 1 ng/m3 ~ 112 ppQv). We see that this can be confusing (especially when compared to the other panels, where different curves actually correspond to different axis), so we decided to remove the right axis. For the readers interested in the mixing ratio, we added the conversion factor in the caption.


7. Line 516: The authors investigated the first-order relationship between GEM removal rate and particle mass. I wonder if the authors have more direct evidence, such as the relevant atmospheric particle characterization, to support the hypothesis of “Particle-induced GEM depletion”.

Authors’ response: We do have information about particle composition, most importantly we know that these were mostly sulfate rich (mostly H2SO4) and very acidic particles. We also know that the particles were mostly in the submicron size range, i.e. having a quite large surface-to-volume ratio. However, we acknowledge that more information about the particles (most importantly, actual observations of particle-bound-mercury) would be necessary to unequivocally assign the GEM depletion to gas-particle-interactions. At this stage, this remains a hypothesis, and further research is necessary to confirm (or refute) it.


8. Line 593-595: What are the similarities and differences between the authors' results and published research concerning Hg mass fractions in volcanic fine particles and suspended fine particles?

Authors’ response: Concerning research about Hg mass fractions in fine particles (here defined as at least < 10 µm) of volcanic origin, there is little published work to compare to. We only found the Ermolin et al. (2018) and Ravindra Babu et al. (2022) references to be useful in this context.

Compared to Ermolin et al. (2018), we estimate similar Hg mass fractions here (within the – admittedly large- uncertainties). However, there are also differences: Ermolin et al. (2018) analyzed settled volcanic ash, not suspended particles like in our case. The investigated particle size in the settled volcanic ash of Ermolin et al. 2018 was around ~ 100 nm, while the suspended particles sampled here are < 1000 nm, with most of the mass found between ~700 nm and 1000 nm.

Our results also indicate Hg mass fractions similar to the Hg mass fraction that can be derived from data in Ravindra Babu et al. (2022). This mass fraction also corresponds to suspended particles, with a relatively similar size range ( < 2.5 µm) to ours (< 1 µm). However, it must be said that the Hg mass fraction derived from Ravindra Babu et al. (2022) is based on little data, so it should be taken with care.

In general, it can be said that the Hg mass fraction estimated here appears to be congruent with the little research that is currently available on the topic. There is a clear lack of research regarding Hg in suspended volcanic fine particles (here we define “fine particles” as particles with diameter at least below 10 µm; note that this is different to the definition often used in volcanological studies: particles <65 µm). We hope that our study impulses further research on this subject.

06-Aug-2023

Dear Mr Koenig:

Manuscript ID: EA-ART-05-2023-000063.R1
TITLE: Observed in-plume gaseous elemental mercury depletion suggests significant mercury scavenging by volcanic aerosols

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. 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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Associate Editor, Environmental Science: Atmospheres

Environmental Science: Atmospheres is accompanied by companion journals Environmental Science: Nano, Environmental Science: Processes and Impacts, and Environmental Science: Water Research; publishing high-impact work across all aspects of environmental science and engineering. Find out more at: http://rsc.li/envsci

Reviewer 1

I believe that the comments by myself and the other reviewer have been well-addressed and I recommend this paper for publication in its current form. I would like to congratulate the authors on an excellent piece of research which will be significant for this field.