Review of “Microphysical and Compositional Differences Between Saharan and Middle Eastern Dust Revealed by UAS Observations” by Kezoudi et al.

General comments

The manuscript describes ground-based remote sensing and airborne in situ measurements of dust particles above Cyprus. The manuscript reaches sound conclusions. However, the overall presentation of the results needs improvement. In its current form, the paper reads more like a measurement report than a scientific article. To my disappointment, there is limited integration of the remote-sensing and in situ observations; based on the abstract, I expected more blending of these complementary approaches. They both keep their supportive but distinct lines. The manuscript deserves publication, but major revision is necessary. Please find below my comments and suggestions for improving the manuscript.

There are several issues with indentation/formatting and inconsistent terminology (e.g., aerosol, dust, particles) throughout the manuscript.

The ordering of references in the text appears somewhat inconsistent. In some cases, references seem to be sorted by relevance, while in others they appear alphabetical and/or chronological. I suggest that the authors choose one consistent approach and apply it throughout the manuscript.

Specific comments

Abstact, there are acronyms that are used for the first time and are not expanded (GPAC, POPS, UCASS).

Page 3, starting line 77, What OPCs were used (manufacturer and model)? The same applies to COBALDs. Similarly, for the remote-sensing instrumentation: lidars, ceilometers, and sun photometers come in different models with different features. Were the same instrument sets used at all ground stations (CAO-AMX, CAO-Nicosia, CARO-LIM NF)?

Page 4, line 83, CAMS is an acronym, please expand.

Page 4, line 85, HYSPLIT is an acronym, please expand and use appropriate reference (e.g. Stein et al., 2015).

Page 4, line 85, authors use North Sahara (expanded) but later line 87 they use acronym ME, later they use NA (North Africa) not North Sahara on line 94.

Page 5, Table 1, there is a mixture of terms altitude (i.e. above sea level) and height (i.e. above ground) it is very confusing especially in remote sensing field. In the table, the term “Africa” is used for HYSPLIT source, should it be “North Africa” or the HYSPLIT analysis source is nonspecific?

Page 5, line 96, the sentence starting with “Both UAS belong…” sounds akward, did you mean something like: “Both UAS are fixed-wing aircraft, primarily made of foam with plywood reinforcements.”?

Page 5, line 99, “Relative Humidity” should be lower case, unless you are introducing a variable (Relative Humidity (RH)).

Page 5, line 96, The paragraph as a whole feels superficial for a scientific paper. Even though a reference is provided (Kezoudi et al., 2021a), it would still be helpful to include key details such as aircraft weight and wingspan, sensor manufacturers, operating ranges, and (ideally) calibration procedures and uncertainties.

Page 6, Section 2.3 The authors omitted a description of the sampling strategy. Did they use data from both ascent and descent? Were the flights helical or linear? Was remote sensing used to plan the flight strategy, and if so, how?

Page 6, line 106, uncomplete reference (POPS; et al., 2016)

Page 6, line 110, the flow rate unit is wrong, probably should be cubic centimeters per second, about 0.18 l/min.

The sentence: “POPS is able to accurately measure …” sounds very optimistic, the size range 0.1 to 3.4 um too, please check Pilz et al., 2022 and Pohorsky et al., 2024 for details on actual calibration of POPS.

Page 6, line 116, double parentheses after reference.

Page 6, line 117, the sentence beginning “Computational Fluid Dynamics (CFD)…” in the context of Girdwood et al. (2022) is misleading. No CFD simulation was conducted for the underwing setup, but only for the Talon top-nose setup. It would be better to base the argument on the angle of attack (AoA): if the AoA in the underwing UCASS setup remains within the recommended range, then it should be acceptable.

Page 6, line 130, when combining the size distributions from both instruments (POPS and UCASS), it is unclear what equivalent diameters were used. POPS was calibrated with PSL spheres, whereas UCASS was calibrated with dust particles of different refractive index. Was the POPS size range recalculated to dust-equivalent diameters using the same refractive index as UCASS? If yes, how? Please clarify. Also, in Figure 1b the size distributions were merged; please describe the merging procedure. Were the overlapping bins and counts simply averaged, or did the authors use a more sophisticated method?

Page 7, line 161, the sentence “In principle, there is no upper cut-off…”, followed by “However, the upper limit is mainly limited…”, is confusing. Is there an upper limit or not? Please clarify.

Page 9, line 171, The authors describe using multiple-substrate GPaC systems for sampling multiple atmospheric layers. From the current text, it is unclear how the second layer is identified during flight in order to deploy the second substrate. Additionally, there is a general discussion of sampling duration; could the authors provide basic statistics on successful samples (16 out of 22)?

Page 11, Section 2.4, I suggest moving/merging this section into Section 2.1. This would address my comments regarding Section 2.1 and improve clarity. The same applies to Section 2.5.

Page 12, Figure 4, it is unclear how the authors arrived at these specific end points. On which estimates are the end points based?

Page 13, line 254, please be specific which Lidar you are talking about on the first occasion you mention it. This comment holds for any other instrument thought the whole text, please be specific.

Page 16, starting line 287, the following paragraphs mix the terms “height” and “altitude.” Furthermore, Figure 9 uses the unusual combination “height ASL.” Typically, “altitude” is referenced to sea level, and “height” is referenced to ground level. This is not necessarily incorrect, but it is confusing—especially when the terms are used inconsistently throughout the text.

Page 16, line 298, above this line authors use kilometers, here they changed to meters, please be consistent.

Page 17, line 300, there is no Table 2.1, probably typo.

Page 17, lines 309-311, the authors use the terms “coarse aerosol” and “coarse dust particles.” Does this reflect different coarse-mode composition for NA and ME? “Coarse dust particles” is a specific aerosol type, whereas “coarse aerosol” may include dust, sea salt, pollen, etc.

Page 19, Figure 11, the lognormal fits appear to be far off (up to 5 orders of magnitude for the ME data) on the right-hand side of the size distribution. Is there an explanation for this? Were the fits applied to all data points, or only to averaged distributions? Could the authors include uncertainty estimates (e.g., error bars) for the averaged data points?

If I understand correctly, the authors used averaged number/volume distributions (Figures 10, 11, and 13) for the OPC/GPaC flights, whereas Figure 9 shows clearly stratified layers (in mass concentration). On what basis do the authors assume that layers (e.g., at 0.5 km and 2 km ASL) are of the same type? This also seems at odds with Section 4, where the authors claim that the method “enables high-resolution vertical profiling.” Why is such resolution needed if the analysis averages over the whole dust layer?

Page 22, Figure 13, could the authors add standard deviations (as error bars) to the figure?

Page 22, line 377, the authors introduced the terms “Cyprus cases” and “Cyprus samples”, first I had impression they were trying to build an argument that collected NA and ME samples are very distinct from “Cyprus samples” and the NA and ME samples are not contaminated from local sources (Cyprus samples). I was wrong. Please, try to be consistent if you introduce certain terms through the manuscript and keep using the same terms.  

References

Pilz, C., Düsing, S., Wehner, B., Müller, T., Siebert, H., Voigtländer, J., and Lonardi, M.: CAMP: an instrumented platform for balloon-borne aerosol particle studies in the lower atmosphere, Atmos. Meas. Tech., 15, 6889–6905, https://doi.org/10.5194/amt-15-6889-2022, 2022.

Pohorsky, R., Baccarini, A., Tolu, J., Winkel, L. H. E., and Schmale, J.: Modular Multiplatform Compatible Air Measurement System (MoMuCAMS): a new modular platform for boundary layer aerosol and trace gas vertical measurements in extreme environments, Atmos. Meas. Tech., 17, 731–754, https://doi.org/10.5194/amt-17-731-2024, 2024.

Stein, A.F., Draxler, R.R., Rolph, G.D., Stunder, B.J.B., Cohen, M.D., & Ngan, F. (2015). NOAA's HYSPLIT atmospheric transport and dispersion modeling system. Bulletin of the American Meteorological Society, 96(12), 2059-2077