From the journal Environmental Science: Atmospheres Peer review history

08-Nov-2021

Dear Dr Maters:

Manuscript ID: EA-ART-09-2021-000071
TITLE: Volcanic ash ice nucleation activity is variably reduced by aging in water and sulfuric acid: the effects of leaching, dissolution, and precipitation

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.

After careful evaluation of your manuscript and the reviewers’ reports, I will be pleased to accept your manuscript for publication after minor revisions. One of the reviewer requested that more discussion be added to describe the aging procedure, the aging chemical species, and how that the experimental setup is similar to what happens in the real atmosphere. The reviewer also requested that you add a few key references.

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Dr Tzung-May Fu
Associate Editor
Environmental Science: Atmospheres
Royal Society of Chemistry

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Reviewer 1

Volcanic ash is well-recognised to be capable of acting as ice-nucleating particles and can therefore exert a profound influence on diverse aspects such as the dynamics of the eruption plume and cloud, dispersion of ash in the atmosphere, and properties of meteorological clouds. However, the ice-nucleating activity (INA) of ash likely evolves during atmospheric transport due to interaction with water and natural and anthropogenic acids, although up to now the effects of aqueous processing on the INA of volcanic ash have not been explored widely (there are some studies already not mentioned in the present paper). In this paper experimentally they study the effect in ice nucleation of aging by exposing two mineralogically distinct volcanic ash samples to water or aqueous sulfuric acid for atmospherically relevant timescales prior to ice nucleation measurements. The paper is really well structured and written and they connect geochemical literature with ice nucleation work to understand their results.
Below I give some comments and questions:

- I understand that here they are presented the first experiments to study the effect of aging in volcanic ash, but could you comment in which extend the procedure is used for aging is really reproducing what happen in the atmosphere? Would not be better to use an aerosol flow tube or a chamber to age the ash?
- Which other atmospheric species should be used to study aging of ash? Would not be better to combine several ones at the same time to simulate in a more realistic way?
- What is the effect of particle size in aging?
- Please include in the introduction and discussion the following reference related to the effect of aging of volcanic ash in ice nucleation:
https://www.researchgate.net/publication/258622210_Aging_affects_the_ice-nucleating_properties_of_volcanic_ash_aerosol
https://www.proquest.com/openview/6595233d85c483531485d0a2ac6cf483/1?pq-origsite=gscholar&cbl=18750&diss=y
https://www.sciencedirect.com/science/article/pii/S0012821X20305318?casa_token=iI9sRpt1uuYAAAAA:suLnXPEXDqnObDnd6Ke6Ljj7iPBFhgjUFyZGjLya3292AbwXuYyaDi3bNkG_DufXdw481kwwxhMN
They are quite relevant for the paper but they have not even been mentioned.

Reviewer 2

Dear Editor

I have read with great interest the manuscript “Volcanic ash ice nucleation activity is variably reduced by aging in water and sulfuric acid: the effects of leaching, dissolution, and precipitation” by Fahy, Maters and coworkers.

In their work, the authors performed experiments of ice nucleation on two different volcanic ash, characterised by contrasting composition and ice nucleation activity.
Before nucleation would take place, the authors exposed each volcanic ash to either
water or aqueous sulfuric acid.
In the former case, water decreased the capability of ash to act as a nucleating agent due to a lowering surface crystallinity and cation availability.
Whereas in the latter, the effect of sulfuric acid strongly depended on which mineral phase was dominating on each ash sample, since their supersaturation contributed altering the ash’s nucleation activity.

Moreover, the authors presented a novel method for ice nucleation data analysis, studying ice nucleation spectra.

I have found the article nicely written and the results clearly presented.
I have explicitly appreciated the summary reported in Figure 4.

To conclude, I would like to strongly support publication of the manuscript in “Environmental Science: Atmospheres” as is.

Response to Reviewers’ comments on “Volcanic ash ice nucleation activity is variably reduced by aging in water and sulfuric acid: the effects of leaching, dissolution, and precipitation” by Fahy et al.

We thank the Editor and Reviewers for their evaluation of our submission. We have addressed the minor comments raised by Reviewer 1 and enclose a carefully revised manuscript. The comments and our responses are given below. Examples of relevant text (existing or new) are presented between quotations; line numbers correspond to those in the revised manuscript.

REVIEWER 1

1. I understand that here they are presented the first experiments to study the effect of aging in volcanic ash, but could you comment in which extend the procedure is used for aging is really reproducing what happen in the atmosphere? Would not be better to use an aerosol flow tube or a chamber to age the ash?

We thank the Reviewer for raising this point. In the Introduction we explain our choice of an aqueous procedure to study the effect of aging on volcanic ash ice nucleation activity:

L71-75: “As airborne ash emerges from the hot eruption plume core and begins cooling and dispersing with the entrainment of air, ash particles are likely to develop a liquid film and/or become immersed in liquid droplets.44,45 The liquid phase can be acidified by uptake of acids (e.g., SO2/H2SO4, HCl, HF, HNO3) released during the eruption and encountered during atmospheric transport.”46–49

L115-122: “To gain insight specifically into how volcanic ash interaction with an aqueous droplet at near-neutral or low pH affects its INA, here we conduct experiments suspending two mineralogically-contrasting ash samples in water (H2O(l)) or sulfuric acid solution (H2SO4(aq)) over different atmospherically relevant time scales, prior to ice nucleation measurements. To explore how changes in INA between ‘non-aged’ and ‘aged’ samples might relate to alteration of specific mineral phases in the ash, dissolved element concentrations are analyzed in the aging solution from each experiment and compared with INA data.”

We have inserted the following line in the Materials and Methods to further clarify why we chose this aging procedure:

L157-161: “Aqueous aging was chosen to allow time-resolved chemical analysis of dissolved material, providing insight into the geochemistry occurring at the ash-liquid interface and in the aging solution. Moreover, this approach enables longer aging timescales to be studied compared to typical chamber or flow tube reactor experiments.”

We have also inserted text in the Conclusions and Implications to acknowledge that future aging experiments involving gas-ash interactions will be useful to provide insights into other mechanisms of chemical alteration of ash during its atmospheric transport:

L584-585: “Also, the influence of gas-phase chemical processing on the INA of silicate aerosol via flow tube or chamber experiments is worthy of future study.”

2. Which other atmospheric species should be used to study aging of ash? Would not be better to combine several ones at the same time to simulate in a more realistic way?

We have inserted the following line in the Materials and Methods to clarify why we performed experiments of ash interaction with H2O(l) or H2SO4(aq) alone rather than a mixture of species:

L163-165: “While mixtures of species would more accurately mimic atmospheric conditions, these simpler experimental conditions facilitate isolation of key alteration mechanisms that may modify ash INA.”

We also acknowledge in the Conclusions and Implications that airborne ash is exposed to other species and that their effects may interact in modifying ice nucleation activity, and we have revised the text to highlight the need for future studies under different/more realistic conditions:

L571-576: “In addition to aqueous chemical aging, exposure to gaseous acids, oxidants, and other pollutants can alter the INA of silicate materials,31,43,121 both in the wider atmosphere and – in the case of volcanic ash – at high temperatures in the eruption plume. When these processes occur in parallel or in series, they may change the INA of the airborne particles in unexpected ways, leading to additional uncertainty about their capacity for ice nucleation during atmospheric transport.”

L580-584: “Laboratory studies will play a key role in unravelling the complexity of such competing effects. Further investigations are needed on the influence of various factors (e.g., other mineralogies, other aging procedures, particle size, solution temperature and pH, presence of other solutes) that could affect the impact of aqueous aging on the INA of silicate materials.”81,118,122,123

3. What is the effect of particle size in aging?

The effect of particle size in aging and ice nucleation by volcanic ash is not examined in our study, although changes in particle size during the aging experiments are indirectly accounted for in normalising the ice nucleation activity of the ash samples to their surface area:

L242-250: “The SSABET measurements of the ash samples (Table S1, S2) enable detection of changes in their surface area due to chemical aging. Little change in SSABET is observed for FUE and AST ash samples exposed to H2O, except for a slight increase for AST ash aged for 120 h, whereas SSABET is raised by a factor of five for both ash samples exposed to H2SO4, meaning there is more ash surface area per unit mass post-aging. Since ice nucleation is localized to the ash surface, this change could impact the trends observed in INA in Figure 2. To deconvolute surface area changes from other aging effects on INA, we also present surface area normalized ice nucleation active site density (ns) freezing spectra as a function of temperature in Figure S1.”

L445-448: “The increase in SSABET over time of FUE and AST ash aged in H2SO4 also supports extensive dissolution of these silicate materials, as a reduction in overall particle size and selective dissolution at etch pits and other surface textures69,101–103 could both contribute to increasing the SSABET of the ash samples.”

4. Please include in the introduction and discussion the following reference related to the effect of aging of volcanic ash in ice nucleation:
https://www.researchgate.net/publication/258622210_Aging_affects_the_ice-nucleating_properties_of_volcanic_ash_aerosol
https://www.proquest.com/openview/6595233d85c483531485d0a2ac6cf483/1?pq-origsite=gscholar&cbl=18750&diss=y
https://www.sciencedirect.com/science/article/pii/S0012821X20305318?casa_token=iI9sRpt1uuYAAAAA:suLnXPEXDqnObDnd6Ke6Ljj7iPBFhgjUFyZGjLya3292AbwXuYyaDi3bNkG_DufXdw481kwwxhMN
They are quite relevant for the paper but they have not even been mentioned.

We thank the Reviewer for highlighting these references. The study of Maters et al. (2020) is already mentioned in the Introduction and Conclusions and Implications:

L68-71: “For example, the INA of ash might be enhanced or depressed by interaction with volcanic gases (e.g., H2O, SO2, HCl) at high temperatures in the eruption plume, with the effect influenced by the susceptibility of individual minerals in the ash to thermochemical alteration.43”

L576-579: “For example, a potential enhancement of INA due to formation of ice-active anhydrite (CaSO4) on ash surfaces by reaction with gaseous SO2 in the hot eruption plume43 may be offset by a loss of INA during subsequent ash leaching and dissolution in aqueous H2SO4 and H2O in the cooling plume and wider atmosphere.”

We have inserted the following line in the Discussion:

L514-517: “Maters et al.43 showed that exposure to H2O(g) on its own or mixed with HCl(g) reduced the INA of Astroni ash, while exposure to H2O(g) mixed with SO2(g) could enhance INA, but these high temperature (up to 800 °C) gas-ash interaction experiments are not comparable to aqueous aging at ambient temperature.”

We have also inserted text in the Introduction and Discussion referring to the PhD thesis of Jahn (2020):

L77-78: “A recent study found the INA of Fuego volcanic ash to be unaltered or reduced by aqueous chemical aging.56”

L517-521: “Experiments performed by Jahn56 showed a reduction in Fuego ash INA after five days immersed in 0.01 M H2SO4(aq) and no change in INA upon immersion in H2O(l), but the slow drying procedure used may have altered the pH and saturation conditions within the aging solution, thereby influencing the relevant mechanisms of chemical processing.”

We have not cited the non-peer-reviewed EGU conference abstract of Bingemer et al. (2012) as this corresponds to a study that was later published in Atmospheric Chemistry and Physics and then retracted (see https://acp.copernicus.org/articles/12/857/2012/acp-12-857-2012-editorial-note.pdf).

17-Dec-2021

Dear Dr Maters:

Manuscript ID: EA-ART-09-2021-000071.R1
TITLE: Volcanic ash ice nucleation activity is variably reduced by aging in water and sulfuric acid: the effects of leaching, dissolution, and precipitation

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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With best wishes,

Dr Tzung-May Fu
Associate Editor
Environmental Science: Atmospheres
Royal Society of Chemistry

Reviewer 1

All previous comments have been nicely addressed, so the paper is ready for publication.

Reviewer 2

Dear Editor

after having read the revised version of the manuscript, only requiring minor changes, and the authors' reply, I can confidently support its publication in "Environmental Science: Atmospheres" in its present form.