Renard et al. present interesting information about the composition of free amino acids (AA) in cloud water from a French mountain site.  The LC-MS analytical work is sophisticated and sensitive.  Care is taken to avoid important artifacts associated with matrix effects commonly found in LC-MS analysis of complex environmental samples.  The results are novel and the authors do a good job exploring several hypotheses related to factors influencing the abundance and relative composition of the observed AA.  I do have several suggestions to improve the manuscript prior to publication.

1. Please provide more detail about experimental methods
   1. Add a short description of the cloud water collectors
   2. Provide more detail about the instruments and materials used, such as the manufacturer of the nylon filter and the model of the Shimadzu TOC analyzer
   3. Line 118: be more specific about storage temperature and storage times prior to analysis for particular analytes, especially the AAs.
   4. Please add a reference for the Gerber Scientific PVM-100.
2. The authors compare AA compositions of cloud water samples collected over 6 years. Are they confident that differences in AA abundance and relative composition are not affected by differing storage times/conditions?
3. Line 223 and abstract: Standard addition prevents introduction of analytical biases resulting from matrix effects. It does not actually prevent the matrix effects.
4. The authors need to better distinguish the two sections of the manuscripts discussing STD. On p. 7, it would help the reader if they clearly indicated that they are speaking about the precision of the measurements of AA while they later discuss the variability of concentrations across different cloud samples. In both places the authors tend to rely on the jargon of discussing STD.  More nuance in the descriptions would help.
5. The authors are correct that not many publications have reported concentrations of speciated AA in cloud, fog, or rain. They did, however, overlook an early, seminal paper by Mopper and Zika (1987) Nature 325, 246-249. They should review this early paper, add it to their comparison tables, and include its findings in their discussions of comparisons to their current work.
6. Please replace “hydropathy” with hygroscopicity throughout. Replace “multiphasic” with multiphase.
7. The authors switch between referring to the Cape Verde Islands and the Cabo Verde Islands. Please switch all to Cape Verde Islands.
8. Please change terminology so that you refer to trajectories within (not below) the boundary layer.
9. Liquid chromatography is coupled to mass spectrometry, not "hyphened to" it.
10. The manuscript contains numerous errors in grammar and syntax and several awkwardly phrased sentences. There are also unusual choices to capitalize certain words (e.g., Sea, Free, Continental, all of which should not be capitalized), some misspellings (e.g., De Hann), many cases of singular-plural disagreements between nouns and verbs, and numerous poor choices of prepositions. With a few exceptions, the authors’ meaning is clear, but the text would greatly benefit from English language editing.

Renard et al. present a very interesting study about free amino acids in cloud water at puy de Dome. Such data are rare and strongly needed. The data are well-presented and discussed. The general style is good and the English very well readable.

However, several things should be considered before publication:

Line 105 and following: The samples are spread over a wide temporal range. First of all, please add the year, whenever you are referring to a sample in the Tables and in the text (not just day and month), as several years are regarded. What about possible artefacts, especially due to the long sample storage time? Could you add more details how the samples were treated (e.g. filtered before freezing)? Did this sample treatment or storage affect e.g. biological measurements?  Did you check this?

Line 114: Please add information on the collection efficiency of the cloud water sampler.

Line 183/184: You report the linearity (R^2), However, this value alone is not sufficient to describe the correlation, as e.g. a good linearity can be obtained by a good fit of just one high calibration point. Please consider to add values that describe the significance (such as the p-value). You show only one calibration curve of the standard addition (Fig. S3) of a sample containing high concentrations. I am wondering how it looks like for lower concentrated samples.

Line 200: Why do “marine” clouds have a low ion concentration? I´d expect some levels of sea salt.

Line 255: How is the ABLH determined?

Please add information about the volumes of cloud water used for the analysis. Standard addition is a method with many advantages, as the authors pointed out, however it consumed a high amount of samples as several aliquots are needed. Could you please comment on that. And did you compare the standard addition to an external or internal calibration to check for possible matrix effects?

Line 299 and following: I am wondering what information can be obtained from this paragraph and Fig. 4. A statistical method is applied but I don´t see a clear new result. It just “confirms” Fig. 3 and this section is not clear to me. Please improve and show the results obtained here.

Line 328: I am in doubt that “analytical tools” can explain the explained discrepancies. Why should different analytical methods show non consistent results on the concentration and composition on the amino acids?

Tab. 2 line “Cabo Verde” HPLC-MS does not go together with OPA derivatisation. This seems to be an error, please correct. Same for Table S4.

Table 3: Is “R” or “R^2” shown here? To this: line 392: I am in doubt if a correlation of 0.4 is “significant”. If you use this expression, please state your definition of significance. In general I am not convinced from the shown correlations. Here again, p-values would help.

Line 391: “ … a potential influence of the photochemistry..” on what?

Line 394 and Fig. 5, similar comment as above (Fig.4): I am not familiar with this type of graphs and discussion but I don’t see the clear benefit from this chart and discussion. Could you more clearly show the results and its importance?

Figure 6 is more “result” that “discussion”.

Chapter 4.1: I have some difficulties with this section. It seems that some examples are picked to explain the AA composition in relation to biological processes. I wonder how representative these explanations are. More references are needed. For example, the explanation of peptidoglycans as a source strongly needs references. What kind of microorganisms have peptidoglycan in their cell walls? What other types of glycanes exist? In the next chapter you report serine as constituents of diatom. Also here references are missing. I´d recommend to the authors to perform a more thorough discussion of the possible biogenic connection rather than choosing some examples that seem to fit to their results. What about long range transport of glycine? What about other serine sources (besides diatom)? I believe there are also more references on the composition of amino acids in seawater during a bloom period besides the one from Ittekkot, 1982. Biological processes are certainly one source of amino acids, but this discussion does not seem to improve the current knowledge at its current state.

Line 495: specify “one”.

Line 524/524: It is not clear if you are referring to the present study or to a result from literature.

Line 539: same comment as above. If you refer to a literature study please add a reference.

Line 553: please replace or specify “behavior”

Line 563: There seems to be something missing at the end of the sentence (AAs concentration/composition?)

Tables in the SI: I strongly recommend to insert the actual LOQ (rather that stating “below LOQ”). The authors apply a method without any enrichment/preconcentration steps, therefore it is important to see the sensitivity of the method right away. It took me a while to find the values in the manuscript.

Why are several samples classified as “marine” (Tab. S1, page 2) although they spent a lot of time over continental surfaces (Tab. S1, page8), for example: the “13 Jun” sample?