Iron Speciation in Different Saharan Dust Advections and Effect of the Procedural Blank on the Results From X-ray Absorption Spectroscopy and Selective Leaching Experiments

Round 1

Reviewer 1 Report

There are two major aims in the paper. The first one is to determine iron speciation in different Fe advection episodes using XAS analyses. The second one is related to the quantitative evaluation of the effect of the blank filter on the XAS spectrum. As described below the problems are ill-posed and there are numerous inaccuracies and errors in the manuscript. Therefore, I recommend the rejection of the paper.

Major issues:

he application of the XAS analyses to determine the oxidation state of Fe (Fe2+ vs Fe3+) is reasonable. However, this methodology cannot help addressing the key question: understanding dust Fe bioavailability and its impact on ocean productivity and human health (two problems mentioned in the abstract). Moreover, this methodology cannot give a clue on a source area of the dust (contrary to the claim made on lines 224 and 234). There are numerous sources and atmospheric processes that can enrich long-range transported dust by Fe2+ or Fe3+. The sequential leaching extraction used in the paper is also not applicable to atmospheric dust bioavailability. The four-step leaching used in soil sciences and aquatic sediments is not appropriate for atmospheric dust (e.g., Buck et al., 2006; Morton et al., 2013).

I think the second aims of the paper should be done in the lab using a well-defined methodological rubric, not the samples with different dust loadings, different transport times in the atmosphere, etc. The evaluation of the effect of the procedural blank on iron concentrations and speciation is an important issue. Any time when there is an issue of the filter contribution to the measurement results, it is expected that the importance will decrease from most loaded to the least loaded filters. However, when the Fe oxidation state is of interest, using samples from different sources and different atmospheric aging times could lead to inaccurate results.

There are numerous flaws in the introduction section. It is incorrect to say that “Saharan dust is responsible for 95% of the global iron cycle” and certainly such statement is not given in the cited reference. Even if we just focus on the oceans, iron supply reaches mainly from rivers as suspended sediment. There is also Fe from ocean upwelling, glacial sources, etc. Please see Table 1 in Jickells et al., 2005). The statement is not even correct for the atmospheric burden (or ocean fluxes) of Fe (see Table 1 in Mahowald et al., 2009).

It is also not true that “aeolian dust deposition represents the primary mechanism of iron fertilization of high nutrient low chlorophyll (HNLC) areas in open oceans”. Such a statement cannot be found in the cited reference. About a decade ago there were some papers published in atmospheric sciences that have been disproved later.

The statement that “atmospheric processing leading to Fe reduction during aerosol’s transport is directly proportional to the distance traveled” is not only incorrect (and not mentioned in the cited reference) but also careless. Such a statement ignores the complex atmospheric processes (such as acid dissolution, photochemistry, reductive processes), mineral composition at the source region, size of the aerosols, etc. all of which can influence Fe reduction during atmospheric transport of dust particles.

I am aware of several inconsistencies in the published literature, but one should never equate iron solubility and bioavailability (as done on Line 25). Please see Meskhidze et al. (2019) for the current understanding of this issue.

 

References

Buck, C.S., Landing, W.M., Resing, J.A., Lebon, G.T., 2006. Aerosol iron and aluminum solubility in the northwest Pacific Ocean: Results from the 2002 IOC cruise. Geochemistry, Geophysics, Geosystems 7, Q04M07. https://doi.org/10.1029/2005GC000977

Jickells, T.D., An, Z.S., Andersen, K.K., Baker, A.R., Bergametti, G., Brooks, N., Cao, J.J., Boyd, P.W., Duce, R.A., Hunter, K.A., others, 2005. Global iron connections between desert dust, ocean biogeochemistry, and climate. Science 308, 67–71.

Mahowald, N.M., Engelstaedter, S., Luo, C., Sealy, A., Artaxo, P., Benitez-Nelson, C., Bonnet, S., Chen, Y., Chuang, P.Y., Cohen, D.D., Dulac, F., Herut, B., Johansen, A.M., Kubilay, N., Losno, R., Maenhaut, W., Paytan, A., Prospero, J.M., Shank, L.M., Siefert, R.L., 2009. Atmospheric Iron Deposition: Global Distribution, Variability, and Human Perturbations. Annual Review of Marine Science 1, 245–278. https://doi.org/10.1146/annurev.marine.010908.163727

Meskhidze, N., Völker, C., Al-Abadleh, H.A., Barbeau, K., Bressac, M., Buck, C., Bundy, R.M., Croot, P., Feng, Y., Ito, A., Johansen, A.M., Landing, W.M., Mao, J., Myriokefalitakis, S., Ohnemus, D., Pasquier, B., Ye, Y., 2019. Perspective on identifying and characterizing the processes controlling iron speciation and residence time at the atmosphere-ocean interface. Marine Chemistry 217, 103704. https://doi.org/10.1016/j.marchem.2019.103704

Morton, P.L., Landing, W.M., Hsu, S.-C., Milne, A., Aguilar-Islas, A.M., Baker, A.R., Bowie, A.R., Buck, C.S., Gao, Y., Gichuki, S., Hastings, M.G., Hatta, M., Johansen, A.M., Losno, R., Mead, C., Patey, M.D., Swarr, G., Vandermark, A., Zamora, L.M., 2013. Methods for the sampling and analysis of marine aerosols: results from the 2008 GEOTRACES aerosol intercalibration experiment: GEOTRACES 2008 Aerosol Intercalibration. Limnology and Oceanography: Methods 11, 62–78. https://doi.org/10.4319/lom.2013.11.62

Author Response

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Reviewer 2 Report

The manuscript “Iron speciation in different Saharan dust advections and effect of the procedural blank on the results from X-ray Absorption Spectroscopy and selective leaching experiments” by C. Petroselli and coauthors deals with a characterization of airborne dust particulate carried out by XAS spectroscopy and selective leaching experiments, aimed at assessing the Fe speciation in relation to Sahara dust advections. The manuscript is innovative and well-conceived and it merits consideration. Nevertheless, I found some remarks that merits consideration before publication.

 

Line 52 – there’s an unnecessary repetition of the term “level”

Table 1 and text – The two terms HVS and SWAM should be defined the first time they are cited. Presently, only HVS is defined in the 4.1 paragraph

Table 1 – why the number in the last columns are expressed in brackets? I’m not used to such convention

Figure 1 – concerning the spectra of the blank samples, the spectrum of SWAM sample is shown, whereas that of HVS no. There’s a reason? It could be added?

Line 122 – ferrihydrite is a poorly crystalline material, while goethite is an hydrated mineral but usually well crystalline

Line 124 – concerning pre-edge fitting, the number of peaks used in the fit and the criteria followed to this choice should be specified (here, or in the methods)

Line 136 -  how the model chosen to fit EXAFS compares with the fact that XANES seems to reveal features marking some similarities with “more ordinate Fe³⁺ compounds”?

Lines 153 and following – XANES and EXAFS data on the SWAM filter seem partial contradictory.

Line 209 – The authors should give a possible explanation of the changes occurred in the samples here described: how the change from SH to non-SH sources affects Fe valence state and speciation?

Line 288 – sequential extractions were carried out on the same filters used for XAS? Otherwise there were two sets of filters? In which way was accounted the sample heterogeneity in this second case? Please specify this in the 4 paragraph.

Line 320 – Presently, the 5 paragraph is useless and it should be removed.

 

I would also add a more general concern. From Table 1, the differences in sampling events are clearly marked. For the SH event a complete set of samplings were operated; for the mix-SH events only PM10 was sampled, for the non-SH events only PM2.5 was sampled. Although in the text some statements accounting on the different speciation expected linked to the size of the sampled dusts are given, I would appreciate a more comprehensive consideration on the comparison operated between samples that are mainly different in  nature. It is well known that mineral dusts mainly contribute to the coarse samples, and thus on can expect that also sample solubilities and Fe speciation could be affected.

Author Response

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Reviewer 3 Report

Dear All,

this is an interesting paper dealing with the iron speciation in Saharan dust advections on aerosol samples by XAS and selective leaching experiments, with investigations on possible filter impurities.

In my opinion, the manuscript is well written and can be accepted after minor revisions, indicated in the attached PDF directly. 

Also, please improve the resolution of Fig. S2, which is very low.

 

Regards

 

 

 

Comments for author File: Comments.pdf

Author Response

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