Royer et al. used bulk and single particle analysis to investigate African dust particles transported to Barbados. Although bulk analysis seems to imply significant aging of dust particles after long range transport, single particle analysis suggested that most dust particles were not aged after arrival at Barbados. This work provides novel insights into the mixing state of dust particles after long range transport, and has important implications for their climatic and biogeochemical impacts. The manuscript, which I found very interesting, can be accepted after some minor revision.

Comments:

Line 38: It can be great to add 1-2 sentences at the end of the abstract to discuss the broad implications of the results this work reported.

Line 63: Tang et al. (2016) reviewed the effects of chemical aging on CCN activities of mineral dust, and the authors may also cite this paper.

Line 86: I would suggest using long range transported instead of “LRT” in the manuscript, as “LRT” really reduces the readability.

Line 179: Please add a reference for this equation.

Line 311: Our work (Zhang et al., 2023) found that similar Al contents in fine and coarse particles for Asian dust, supporting what this work found for African dust.

Line 324-373: Could the authors estimate the thresholds (mass fraction in %) when nitrate and sulfate became undetectable? Such information will provide a rough idea about the upper limit of nitrate and sulfate contents when they were not detected.

Line 491-498: We found that during Asian dust events, aerosol Fe solubility at Qingdao (a coastal city of Northwest Pacific) was not elevated compared to that at desert regions (Chen et al., 2024). This suggested that Fe solubility was not significantly increased when Asian dust aerosol was transported to Qingdao, be consistent with what the authors suggested.

References:

Chen, Y. Z., Wang, Z. Y., Fang, Z. Y., Huang, C. P., Xu, H., Zhang, H. H., Zhang, T. Y., Wang, F., Luo, L., Shi, G. L., Wang, X. M., and Tang, M. J.: Dominant Contribution of Non-dust Primary Emissions and Secondary Processes to Dissolved Aerosol Iron, Environ. Sci. Technol., 58, 17355-17363, 2024.

Tang, M. J., Cziczo, D. J., and Grassian, V. H.: Interactions of Water with Mineral Dust Aerosol: Water Adsorption, Hygroscopicity, Cloud Condensation and Ice Nucleation, Chem. Rev., 116, 4205–4259, 2016.

Zhang, H. H., Li, R., Huang, C. P., Li, X. F., Dong, S. W., Wang, F., Li, T. T., Chen, Y. Z., Zhang, G. H., Ren, Y., Chen, Q. C., Huang, R. J., Chen, S. Y., Xue, T., Wang, X. M., and Tang, M. J.: Seasonal variation of aerosol iron solubility in coarse and fine particles at an inland city in northwestern China, Atmos. Chem. Phys., 23, 3543-3559, 2023.

This research focused on studying the degree of dust mixing with sea salt and particulate pollutants. This includes several interesting techniques for the data analysis. Although the results are based on a rather short time measurements period, they are of interest. The topic is within the scope of Atmospheric Chemistry and Physics. This study may be published after addressing aa set of questions and comments listed below.

Specific Comments:

C1. In the introduction the term ‘aging’ should be formally defined.

Line 60-61 authors said: The chemical change in the physiochemical properties of dust, herein referred to as “aging”, can alter the water uptake properties of dust…

This is ambiguous and not a formal definition, the use of ‘…chemical change in the physiochemical properties…’ sounds ambiguous.

Do the authors use the term aging as equivalent to internal mixing?

If so, the study concludes that dust particles (67% of them) are internally mixed with sea salt. Then, the title (…Lacks Indicators of Chemical Aging) is confusing.

C2. Line 31-32: Ion chromatography (IC) analysis of bulk nitrate, sulfate, and oxalate increase when African dust reaches Barbados, indicating dust aging.

How was this increase detected or measured? By comparing with other sites?

Please, explain in the text.

C3. Line 31-32.

English is not my mother's thong, but I think that the sentence should be rewritten.

What did increase? The analysis or the concentrations

Please, consider rewriting as:

Ion chromatography (IC) analysis indicates that the concentrations of bulk nitrate, sulfate, and oxalate increase when African dust reaches Barbados after transatlantic transport, indicating dust aging.

C4. Line 80-81.

Authors cite Li-Jones & Prospero,(1998) describing that the sulphate mixed with dust may be linked to sulphur emissions in Europe.

More recent studies have linked the nss-sulphate mixed with Saharan dust to industrial emission in North Africa (see details in  https://doi.org/10.5194/acp-11-6663-2011 )

This updated information should be included.

C5. Line 85-92. Authors cite several studies addressing dust aging.

Authors may be interested in studies on the changes of iron solubility during the westward trans-Atlantic transport of dust, e.g. https://doi.org/10.1016/j.atmosenv.2020.118092 , which includes the Barbados site.

C6. Lines 103-112. Authors only cite biomass burning as a North African potential pollutant mixed with dust; the role of the North African industrial emissions is being omitted, and should be cited (see C4).

C7. Line 134-135. Authors cite NOAA’s HYSPLIT model.

A reference is needed, e.g. https://doi.org/10.1175/BAMS-D-14-00110.1

C8. Section 3.3 Aerosol Mixing State Analysis, actually defines a sampling system rather than an analysis system. Shouldn’t his section be called something as

Section 3.3 Aerosol sampling for mixing state analysis

??

How are the samples collected with this system analyzed and what is actually being determined?

This should be included.

C9. Line 301. What particle type is smoke? What type of analytical technique was used to measure it ?,.. is it elemental carbon?

C10. Line 310 and in other sections of results, it would be helpful to remind the size ranges of supermicron, 1-5 mm ?, not all > 1 mm

C11. Lines 312-314: … large difference in the number fractions of submicron and supermicron dust particles, where dust only makes up 21% and 4% of the submicron and supermicron aerosol loading by particle number, respectively.

This result is actually something expectable, not surprising, for any type of aerosol, not only for dust. Aerosol particles are more abundant in number concentration in the submicron than in the supermicron fraction, even of the supermicron fraction has a much large dust mass. Several studies are showing that the number size distribution is shifted to small particle size, whereas mass size distribution is shited to large/coarse particle size. For this reason, in my opinion, it is not justified to say what the authors said in lines 314-316: ‘This discrepancy is not only due to the difference in individual particle mass between sub- and supermicron particles but also due to the oversimplification of dust particles in bulk analysis’.

What does the author mean when saying `oversimplification of dust particles in bulk analysis´.

In my modest opinion, lines 314-316 could be deleted, such oversimplification is not true.

C12. Fig. 2 and text lines 317-373. Authors use N to refer to nitrogen and S for sulphur, this is strictly OK, however, N is usually used to refer the number of particles or number concentration (N) and S the particle surface (S), so, it is actually confusing for the reader, in fact I got confused. I suggest to the author describe in the figure caption that N and S reefer to nitrogen and sulphur, respectively. Even in the text (317-341) it would also be useful to state clearly that N and S actually mean. This is just a suggestion that will make the text more friendly to read.

C13. Lines 343-373. Can the author segregate the presence of natural calcium sulphate, i.e. gypsum or anhydrite? . This should, at least, be cited in the text. The same of natural sea-salt sulphate, which is cited in the text.

C14. Lines 368-373. Sulphate mixed with dust includes a portion of ammonium sulphate and other sulphate salts, as calcium sulphate linked to natural gypsum and dust coating with sulphate linked to industrial emissions in central Sahara, e.g. see the oil refineries located in Algeria (Ourgla, Hassi Mesahoud), fig. 2, fig 10 described in https://doi.org/10.5194/acp-11-6663-2011

Industrial emissions of SO2 and NOx into North Africa plays an important role in the mixing of dust sulphate and nitrate.

C15. Section 4.4 TOF-SIMS Imaging. This section may be summarized.

C16. Line 446. I would not say that traditional methods oversimplify, I actually think that the methodology used by the authors and traditional bulk analysis are complementary. In fact, traditional bulk analysis allows a better quantification and provides a global picture of the aerosol composition load, that may be complemented by the analysis presented by authors in this study.

C17. Lines 496-498. Dust may be an important source of nutrients, even if not aged. For example, dust provides iron to the ocean. Iron contained in dust has a low solubility (0.5% aprox), it may increase to 5-7% after several days in the air. Even if iron – dust is not processed these inputs of insoluble iron are important since some marine species may process such dust particle contains insoluble iron, this is the case of trichodesmium, who is able to digest insoluble iron – dust by using the trichomes ( https://doi.org/10.1038/s41396-019-0505-x  ,  https://doi.org/10.1016/j.isci.2021.103587   ). In fact, the migration of some tropical tuna tracks the seasonal shift of dust inputs to the Atlantic, even of dust is not aged (see details in https://doi.org/10.1016/j.atmosenv.2023.120022 ). In summary, to say (496-498) ‘The lack of aging on mineral dust observed in this study suggests African dust may not be as important of a source of bioavailable nutrients for the North Atlantic as previously assumed’ is too vague and general; in my modest opinion such sentence should be avoided.