In this manuscript, Liu and co-authors examine SO2 uptake by carbonate-containing mineral dust, and explore the potential role of carbonate radical-mediated processes in enhancing sulfate formation. The premise of the article is interesting and worthwhile; at the same time, the manuscript in its current form is very difficult to follow and the experimental conditions are sufficiently different from those in the ambient atmosphere that the reported conclusions require much more in the way of discussion and qualification of limitations than is currently provided. Given these concerns, I think that the manuscript requires significant (major) revisions prior to publication. I hope that the authors find my comments useful; I have aimed to be constructive throughout.
The manuscript is extremely difficult to follow, primarily because data and results text are largely in the supplemental information. As written, the supplemental reads like a point-by-point response to reviewers rather than a clear and concise summary of supporting information. As a result of this separation of information, important points in the text are quickly glossed over. For example, to me, Figure S3 seems to show that there is substantial production of sulfate at the surface of TiO2–CaO upon illumination. Isn’t it possible that the difference between CaCO3/CaO is related to other factors? Specific surface area differences? Hygroscopicity? Solubility? In my opinion, the manuscript requires significant reorganizational effort to address this issue, since I (as a relative expert in the field) am finding it difficult to be sure that I am convinced by the data and the results as presented.
2—General comments and concerns
Why would the authors expect that carbonate radical would promote sulfate oxidation more than hydroxyl radical produced by TiO2? Perhaps I am missing something here, but if the idea is that carbonate is scavenging OH to produce carbonate radical, then why would the overall S(IV) to S(VI) conversion be any different?
The experiments shown in the main experimental figure (Figure 1) were performed at almost 10 ppm SO2. The mismatch between these experimental conditions and those relevant for the actual atmosphere warrants discussion. What challenges do the authors anticipate when using these results to make predictions about behaviour under more realistic conditions?
3—Detailed comments and suggestions
L18—Is there direct evidence for the proposed mechanistic pathways?
L20—Is there direct evidence for production of gas-phase carbonate radical?
L29—These references seem somewhat out of date.
L31—What does “unique chemical activity” mean? Are nanometer-sized particles relevant for dust chemistry?
L33—Are these references for the aquatic environment? Or for the atmosphere? What does “over the water surface” mean?
L37—Higher selectivity than what? What selectivity does the carbonate radical have?
L40—Phenol is presumably only one organic compound that is degraded by carbonate radical—why was this one chosen to highlight here?
L41—Are porphyrins relevant in an atmospheric context? Is this rate constant relevant? Higher than what?
L42—“great oxidation capability that may trigger atmospherically relevant chemical reactions”—this is vague/unclear.
L44—These references seem out of date as well. Are there any tropospheric modelling-type papers (CAPRAM? GAMMA?) that provide carbonate radical rate constants?
L53—Where does this underproduction occur? Under what sorts of conditions? Also, what about other sulfate production pathways, e.g., the DMS chemistry being explored in Timothy Bertram’s group?
L55–60—“unconsidered heterogeneous mechanism is very likely to narrow the gap …” — to me, this seems speculative, if only one aerosol study is cited (Zheng 2015). Is it certain that heterogeneous pathways will close this gap?
L66—Carbonate salt is enriched over what? What does “authentic dust aerosol” mean here?
L70—How does carbonate alkalinity “favour sulfate formation”? What exactly does this mean?
L71—Where is this information provided in the literature (that CO2/carbonate produces carbonate radical)?
L73—What does “fast SO2 oxidation” mean here? Why is it likely to be a driving force?
L76—What does “extend their ability” mean?
General—As I noted at the beginning of my comments, is there a reason why all of the techniques, etc., are in the supplement? It would be very helpful to have information regarding the samples (which “authentic dust” samples and which “authentic simulants”? What does “authentic” mean here?) as well as the experimental set-up in the main text. A reference for the carbonate radical measurement strategy is needed, as well (aniline as probe molecule).
Field Observations—Where is the department? Was sulfate determined in all size fractions? More information is needed here. Citations for the “ionization balance approach” are needed. Which sulfate is expected to be non-water-soluble? Were the authors concerned about S(IV) to S(VI) conversion in the sample extracts? How was this addressed? Some of this information may be in the supplement, but as a reader, I do not want to have to shift back and forth between documents to find it.
L102—At this point, no information about the mineral dust proxies has been presented. How are their properties “consistent with earlier studies”?
L105—What is the experiment being discussed here? What kind of experimental conditions were employed? “Upon irradiation” of what? What were the SO2 concentrations here? What was the humidity?
L106—How are the authors distinguishing between S(VI) and S(IV) in these experiments? What quality assurance/quality control-type experiments were performed? How did the authors prevent S(IV) to S(VI) oxidation during extraction/prior to analysis? Was S(IV) quantified in these experiments? I would assume that it would have been present in all extracts … why are these data not shown in Figure 1?
L109—What does “it remains unclear for the role of carbonate salt …” mean?
L110—“There is a prevailing view” — reference? What does this statement mean?
L113—These data (CaO/CaCO3 comparison) need to be presented clearly in the main text, since they are part of the authors’ overall argument. “as they are likely to present similar physical and chemical properties” — what surface pH do they present? Are they comparable? Do they have similar hygroscopicities?
L118—I find this use of “theoretical” and “experimental” confusing.
L125—Again, these data need to be presented in the text. The need to flip back and forth between the main text and the supplementary information is very frustrating for the reader.
How were these uptake coefficients determined? Are they surface-area scaled? How does this pathway compare to other heterogeneous SO2 oxidation mechanisms? This statement (that this is “likely a potential driving force to trigger fast SO2 oxidation …” seems like an overreach to me in the absence of supporting calculations.
Figure 1—How many trials were performed here? Why was this DRIFTS experiment not performed for TiO2/CaO, to account for any changes in speciation after extraction? To me, the data presented in Figure 1b–c don’t necessarily show that CaCO3 is enhancing the photochemistry over that of TiO2 alone via carbonate radical … how does the surface area of these mixed films compare to that of the single-component films? What is the surface water content of each? Is it possible that the surface reaction environment is different? What is the S(VI)/S(IV) ratio under each condition? What does “yield” mean in the caption?
L151—What does “a strong interaction” mean? What does “weak interplay” mean? These terms are vague/imprecise. Why did grinding not lead to a decrease in size of the CaCO3 particles?
L155—Is it possible that the overall exposed surface area of the system was larger after grinding, or that aggregates were disrupted, and that this was the cause of the results in Figure S8 rather than anything relating to the specific interface/interactions between TiO2 and CaCO3?
L157—What is “fast production”, exactly? This term is used throughout the manuscript, but in the absence of kinetic data / multiple timepoints, I do not have a sense of what “fast” means here.
L164—What exactly is meant by “the rapid SO2 oxidation pathway”? This echoes my previous comment (L157).
L169—What mechanism do the authors propose for the enhancement observed upon addition of CaCO3 to SiO2/Al2O3? Overall, I find this paragraph confusing; there are a lot of results presented in this manuscript that are briefly discussed / glossed over, and to fully convince the reader that all possibilities have been considered, all results should be discussed in terms of how they fit / do not fit with the overall/big-picture interpretation presented by the authors.
L174—“thus likely involving the reaction mechanism proposed in this work”—I do not follow this statement.
L184—Recent work by Abou-Ghanem and co-workers (ES&T 2020) has shown that Ti in mineral dust differs substantially from commercial TiO2. In this context, I don’t think it is reasonable to state that 1% TiO2 is “atmospherically relevant” without qualification of some sort. I also don’t know what “authentic dust simulants” means here.
L186—What does “fast oxidation channel” mean? What does “beyond the conventional regime of alkaline neutralization of H2SO4” mean?
L190—Which “mineral dust” was used here? Sulfate “yield” on what surface? The CO2 is atmospherically relevant, but what about the SO2 concentrations? This (the concentration dependence) should be discussed in the main text, rather than shown only in the supplementary information, because the experiments shown in Figure 1 were conducted at 10 ppm SO2. Why was this the case, and how might the results obtained under these highly unrealistic conditions differ from those in the real atmosphere?
L193—I don’t follow these arguments regarding the effects of gas-phase CO2. How can these observations be directly related to observations with carbonate particles? I would think that the effect of CO2 might relate to a competitive adsorption effect rather than being directly comparable to the effect of pre-existing (solid) carbonate on SO2 uptake …
L203—What is the speciation of this TiO2? Elemental Ti content is not necessarily an accurate predictor of photoreactivity.
L205–210—I find this argument difficult to follow. Are these results scaled to the surface area presented by each of these samples? I do not see how they can be quantitatively compared in this manner otherwise.
L231—Where is the evidence that “two water layers” “absorb” (adsorb?) on dust particles? On line 225, I find the terms being used here imprecise … what do “capture SO2 in the gas phase first” and “then stabilize it as adsorbed S(IV)” mean?
L247—These aniline results should be presented in the main text. What does “a promoted degradation” mean?
L245—I find the paragraph starting on this line extremely difficult to follow. Specifically, I can’t decipher the argument relating to the scavengers. In addition, how do these conditions (dust suspension in water) compare to the conditions explored in the previous sections of the manuscript (solid particles)? Is it reasonable to use information gathered in aqueous suspensions to interpret results obtained at the surface of solid mixtures? The potential limitations/biases in this approach should be addressed in the text.
L305—I do not understand the logic underlying these proposed reactions (in particular, equations 6–8). Where is the evidence for these species?
L325—I do not at all follow how these concentrations (for carbonate/hydroxyl radicals) were chosen here. The selected references do not make sense to me—one title is “The carbonate radical is a site-selective oxidizing agent of guanine in double-stranded oligonucleotides.” How is this relevant to the argument regarding the relative concentrations of these oxidizing species?
L350–367—I think that this section would be much strengthened by addition of discussion regarding the way(s) in which performing experiments at such elevated SO2 concentrations may have altered the surface pH of the particles employed.
L369—What does a “non-negligible contribution to sulfate aerosol formation” mean? Compared to what?
L375—This information should be presented in the methods.
L378—I do not understand how the maximum OH and carbonate radical concentrations were determined here (in this paragraph). Also, are there any other possible reasons (other than carbonate radical release) why the aniline loss may have been larger in the presence of CO2?
L391—I don’t understand how carbonate radical ions “strengthen” the oxidative capacity of TiO2-containing dust particles.
L410—How were S(IV)/S(VI) ratios preserved after sampling/during extraction/analysis?
L422—What does “undesired processes” mean?
L446—I do not understand how the negative correlations discussed here and the CO2 suppression results are related to one another.
L469—I don’t understand the statement regarding “CO2-derived bicarbonate species”. Would the authors expect CO2 to occupy PM surface sites? What about CO2 dissolving in an aqueous layer? How do the authors know that the bicarbonate species are CO2-derived?
L493—To what extent do the authors think that gas-phase carbonate radicals contribute to overall atmospheric oxidative capacity? I am not entirely convinced of its production in these experiments (as the evidence is indirect).