General comments:

The manuscript by Dumont et al. discusses a new dataset which consists of Saharan dust samples collected over the Pyrenees and European Alps by several research laboratories and citizens. The analysis of the samples consisted of mass, particle sizes with distance from the source along the transport path, and elemental composition. The manuscript is within the scope of the journal and can be published after minor revision by the authors. Before that, the authors should, however, provide the critical information on the data set on the following questions, to ensure the usability of these valuable and unique data:

1. whether your 152 samples were from 152 different locations or is there more than one sample from one location (e.g., from Fig. 4 it is hard to say how many locations you have);
2. which samples and how many were from citizens and which and how many were from research laboratories;
3. how these sample collections by citizens and professional differ from each other by sampling or any other property related (altitude, accessibility to sample, slope, etc any other);
4. how citizen/professional samples might affect the gained results (e.g., any bias in results due to any differences);
5. Also, it remains unclear for the reader, which samples (and how many) were used for which analysis, why, and how that affected the gained results. A summary table could help to give that information.
6. which samples were dry deposition and which were wet deposited? What effect that might have on the results?
7. In addition, for the future usage of these valuable data by others, identification of the sample locations (coordinates or numbering the samples in the map) should be used. Unfortunately, I could not find this information from the current version of this manuscript nor in the dataset related to the manuscript (ref. p. 27, Data availability, “Data presented in this study are available at https://doi.org/10.5281/zenodo.7464063 (Dumont et al., 2022)”. If that information exists there, it should be presented more clearly in the manuscript. For example, from an individual section, it appears that only 2 samples were used for optical properties, and elsewhere, e.g., it is said that 136 samples out of 152 samples (152 is mentioned in the Abstract) were used for some of the analysis. This all would need to be clarified for the reader to easily see what analysis was made form which samples and where the sample locations are. Figure 6 is a great illustration of the particle mass, but there also is an open question if these numbers where produced using 3 closeby locations (as stated in p.3, line 3).

Hence, a careful clarification of these valuable data is needed to ensure the best possible usability of these data by others.

Specific comments:

Page 2 line 44-45. It could help the reader, if you clarify here that your data set presents results for the first Saharan dust event of 4-8 Feb 2021.

Page 3, 2.1 and Figure 1. The sample collection described in 2.1 is not the same as shown in Fig. 1. In page 3, you say “We asked participants to collect a snow sample of 10 x 10 cm2 50 area (or any known area)” and in Figure 1 you show a cylinder. A small correction of adding “appr. 10 x 10 cm2” would already improve this. Are there other clarifications on the sampling and sample area that you could provide? For example, if you had different sampling instructions to public and to researchers, it should be mentioned, too. Furthermore, did you use the sampling area information to calculate the dust deposition as mass per area, e.g. as g/m²? It could be mentioned what was critical in citizen sampling for the quality of the samples for this purpose. Did you ensure the quality of the area estimate from citizens, and how did you do that? What was the container the citizens used and how did they get the dust into that container?

Page 3, line3. You say: ”average of the three closest samples for each site (see Sect. 2.3).” Please specify what was the distance of these three closest samples? How representative this makes the combined sample dust mass deposition estimate per one location? How many locations were combined for the purpose? The whole sample set or some samples, how many samples/location and if so, how did you select these samples/locations? Were some masses determined per sample, too?

Page 3, line 70. You say “poor counting statistics above a certain size (example shown in Fig. 2).”. It would be helpful if you could say instead “a certain size” the same giving a value, then refer to Fig 2 for more details.

Page 3, line 70-71. The same as above: could you give a value for what is lower part of the distribution, when you say: “We thus separated the distribution in a lower part, usually well measured (typically better than 10% uncertainty), and the tail of the distribution, which is highly…”

Page 4. Figure 1. It is a bit confusing for me that the French text font is so big, it is almost 1/3 of the image. Could you make the same figure with English text (or reduce the French text font size)? The figure could then be “Sampling protocol…” instead “Original sampling protocol”.

Page 5, line 89. You say: “For the analysis of optical properties, two specific samples were taken to ensure that a sufficient dust mass was available.”. It could be helpful to mention the locations of these samples here.

Page 6, line 120. You say “Some of the filters were analysed”. Which filters, how many, why these were selected, what were the sample locations?

p. 7 lines 130- 131. “in spring 2021”, what were the dates? So this was much later then? Is it possible that some other dust events (Asian dust?) could have transported dust elsewhere, too?

p. 9, section 3.1. Here it is finally revealed that 136 samples were used for the analysis. Could you confirm here if these were from 136 different locations?

p.9 Section 3.3.1. and Fig 6. Now in p. 9 it is revealed that 114 sample derived masses were obtained. But now is there a contradiction with what you say in p.3 line3 “average of the three closest samples for each site” So was the mass determined separately for 114 locations or not?

p.12 line 206 “A total of 95 samples”. Does this refer to 95 different locations?

p.12 line 220 “105 samples were analyzed for the elemental composition”. Are these the same as for sample size distribution analysis and some additional ones (where from, why)?

p.20 line 339. Please correct an error here when you say: “To our knowledge, this is the first time that such a large number of dust samples from a single dust event were collected and analysed with multiple analytical tools.”. Please correct to say, e.g., “To our knowledge, this is the first time that such a large number of dust samples from a single dust event in the Pyrenees and the European Alps were collected and analysed with multiple analytical tools. In another study for Februry 2021 (Meinander et al. 2022), Saharan dust citizen science samples from over 500 locations in Finland have been collected and analysed with multiple tools." Reference: Meinander, O., Alvarez Piedehierro, A., Kouznetsov, R., Rontu, L., Welti, A., Kaakinen, A., Heikkinen, E., and Laaksonen, A.: Saharan dust transported and deposited in Finland on 23 February 2021, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4818, https://doi.org/10.5194/egusphere-egu22-4818, 2022.

The manuscript by Dumont et al. presents interesting and valuable data. It builds on a unique initiative realized at short notice after a transport event of desert dust to the Pyrenees and the Alps in February 2021.  Starting with a very simple call to mountaineers a significant number of snow samples could be collected and evaluated. Still a number of points remain unclear and need to be improved. These points are listed below in the more detailed comments.  Additionally, some comments about the ‘lessons learned’ for a follow-up citizen science campaign would be interesting to know to provide input for future citizen science campaigns.

Specific comments

Page 2, Introduction: a short comment should be added why the measurements of radionuclides are performed

Page 3, lines 62-63: Which distances were covered when the three closest samples were considered? And where were these closest samples situated – this could be important as differences were reported for the different facing slopes. Perhaps it is more suitable to give this information in section 3 (Results) – but it should be given at some point.

Page 3, lines 65 to 67: How was the filtration performed, which type of filters was used and how big was the loaded area? Did you actually determine mass concentrations or mass loadings per unit area? How was the analytical uncertainty determined?

Page 3, lines 68 ff: The method described in Delmonte et al (2004) covers the size range of 0,7 – 20 µm. This is quite different to the size range given in Figure 2. Furthermore, I do not understand the approach to differentiate between the two parts of the size distribution (i.e. the well measured part and the tail of the distribution being more uncertain). Why is the cut diameter set at the lowest diameter where none of the three measurements detected any particle. Variations seem to get really large when at least one of the three individual measurements does not detect any particle. How did this cut diameter vary when all samples are considered. Is it possible to draw some general conclusions (either here or in section 3.3.2)? Some information about the cutoff diameter is available in Figure S3, but it is never mentioned in the text.

Page 4, line 80: give a range of mass loadings on the filters

Page 5, line 85: add a reference where the calibration function can be found

Page 5, line 90: As far as I understand the samples collected within the citizen science campaign were not used to determine the optical properties. This should be mentioned more clearly.

Page 6, line 102: It is somewhat surprising to find a reference to Fig S5, as the references to Figures S1-S4 are only given later, in the Results section.

Page 6, line 123: It seems that just a limited number of the filters of the citizen science campaign (just one?) could be evaluated for radionuclide analyses. This should be mentioned more clearly.

Page 9, lines 171-172: 152 samples and 85 participants – I guess that some participants brought a lot of samples and others just one. Could you give some details here? This could help to get a feeling about the variability of the results.

Page 9, lines 195-196: This remains unclear. I guess this is the comparison done for the ‘three closest sites’ (see page 3, line 62). The actual number of the deposited dust mass per unit area (in-situ measurements) should be given and compared to the modelled data. In Fig. 5 (as well as Fig. S1 and S2) the cumulative mass depositions are reported as ‘computed as MOCAGE outputs and corrected according to the observations’. Does this correction already reflect the underestimation reported here?

Page 12, line 206: Why is the number of samples analysed with the Coulter counter smaller than the number of samples filtrated?

Page 12, line 211: At least a short discussion about this comparison should be added.

Page 12, lines 212 to 216: This section needs clarification. What is the 'mean dust diameter' mentioned here? Furthermore, I cannot relate the numbers given in the text to Fig 8A. Fig 8 needs some clarification as well. What means ‘Size-Surf. Distrib.’?

Page 12, line 213: the reference to section 3.3.2 is strange – this is section 3.3.2

Page 19, line 298: Is it possible to give reasons for this decrease in the mass fraction of iron

Page 36, Figure S3 legend: obviously the surface distribution is given on the right and the volume distribution on the left