General comments:

Organic aerosol (OA) constitutes a large fraction of particulate matter, impacting human health and global climate. Molecular-level insights into OA are crucial for understanding its formation and evolution in the atmosphere. The authors present an interesting study on the potential fragmentation pathways of deprotonated norpinonic acid, combining lab experiments with quantum chemical computations to elucidate the main fragments and their formation pathways. As mass spectrometry, e.g. tandem MS-MS, plays an increasingly crucial role in OA studies, insights into the fragmentation patterns of deprotonated organic acids can greatly aid in interpreting these datasets.

However, potential readers would benefit a lot from further refinement in writing and discussions of the manuscript. There are a few typos as well as inconsistencies and arithmetic errors in discussions and figures. Nevertheless, the manuscript is well-organized scientifically, and I anticipate that the results will be of interest to many readers in the community. Therefore, I recommend publication following minor revisions outlined below.

Minor comments:

• Page 1, line 8. According to EGU standards, abbreviations need to be defined at the first instance of use. Here, it should be “Secondary Organic Aerosol (SOA)”. Please also check other abbreviations, e.g. in the abstract, “CIE” was used the same way as SOA, and “ER-CID” was used without a definition.
• Page 1, line 10. “O₃” with subscript.
• Page 1, line 29. “Primary organic aerosols” are emitted directly into the atmosphere.
• Page 2, line 40. “a few”?
• Page 2, line 56. Since “O₃” has already been defined, it can be used directly here. Please also check other abbreviations used in this manuscript.
• Page 2, line 58. “0.2 – 1.1 ng∙m^-3” is not significant compared to the total organic mass (e.g. <5 µg m^-3 in clean areas or ~15 µg m^-3 in the paper you referred). In either case, norpinonic acid contributes less than 0.1%. You may want to mention that mono-/di- carboxylic acids are identified in SOA in large amounts, and norpinonic acid (containing one carbonyl and one carboxylic acid functional group) is chosen as an example to investigate how organic acids fragment to rationalize why you focused on this specific compound.
• Page 3, line 79. Can you provide/estimate the purity of the synthesized cis-norpinonic acid used in this study? In addition, do you think that there would be any potential by-products or contamination during the synthesis process that might affect the analysis in this study?
• Page 4, Section 2.3. Can you explain in detail how you deal with conformers, e.g., the criteria for potential duplicates based on electronic energy or electric dipole, so that others may reproduce the calculations if they want?
• Page 4, subtitle 3.1. Fragmentation pathway of “Norpinonic acid anion”.
• Page 5, line 127. Is Supplementary Information (SI) Section 2 related to this discussion? Please specify which section and/or figure you refer to in the manuscript, as it's hard for readers to find relevant parts within the 59-page supplementary material. Try to avoid simply stating "Please consult the SI". Check other parts in the manuscript (e.g., lines 161, 248, 256…) that refer to supplementary material without specifying which part(s).
• Figure S3, “m/z 57” should be “m/z 41”.
• Figure 3. Can you add which neutral part has been lost along with the arrows? For example, the arrow from m/z 169 to m/z 125 represents the loss of CO₂. This clarification may help non-chemist readers follow the process more easily.
• Page 7, lines 141 and 142. Are “237” and “245” correct? Or should they be swapped? In line 179, you used “245” for the pathway that forms m125, while in line 211, you also used “245” for the pathway that forms m99. It is very confusing. Also, in line 153, the onset energies “171” seem to contradict the values in Figure 3 (there was no “171” in Figure 3). Please double-check all these values in the figures, tables, and corresponding discussions.
• Page 7, line 152. Please change “followed by” to “along with”.
• Page 7, line 160. Why do you think “the ωB97XD hybrid density functional method reproduces the experimental observation the most accurately” than other methods?
• Page 8, line 173. Is “58” correct?
• Page 10, line 212. Should “P_99B” be “IC_99”?
• Page 13, line 257. “fragmentat”?

“Mechanistic insight into the kinetic fragmentation of Norpinonic Acid in the gas phase: An experimental and DFT study”, Kurzydym et al.

This manuscript describes the fragmentation of norpinonic acid in a mass spectrometer and provides the proton affinity of these fragments using a series of neutral reagents. Although the technique is not original the paper is important as it provides the chemical signature of norpinonic acid. This paper should be published after a minor revision. I have some comments below.

1) Norpinonic acid is a semi-volatile product which resides in both gas and particle phase. These fragmentation patterns refer to gas phase. How much important is its gas phase with respect to the particle phase? Do you have an estimation of how much volatile it is? (any measurements or estimations from the literature). This research becomes more valuable if you show that the gas phase is important (i.e., norpinonic acid resides in the SVOC regime). This discussion should be added in the Introduction.

2) Are there any previous literature describing the fragmentation pattern of other important a-pinene SOA products? (e.g., pinic acid, pinonic acid, norpinic acid, etc.?). If yes, how much similar, or different is their fragmentation? How, much easy or difficult would be to distinguish norpinonic acid from other a-pinene SOA products in an ambient sample?

 3) Technical corrections:

Through the whole manuscript please use small letters for norpinonic acid (i.e., norpinonic instead of Norpinonic).

Page 1, Line 10: Please replace “O3” with “O₃”.

Page 1, Line 31: “from a biomass burning” delete “a”.

Page 2, Line 34: Please replace “result with” with “result in”.

Page 2, Line 37: Please replace “among which, compounds with carboxylic functional groups are observed in large amount.” with “among which, a large number of compounds containing carboxylic functional groups”.

Page 2, Line 42: Pathak et al., 2007 is a chamber study, without chemical analysis, so, it doesn’t fit here.

Page 2, Line 44: Please add more recent studies.

Page 2, Line 54: Please replace “during the field or ambient samples analytical process” with “during ambient samples analytical process”.

Page 3, Line 69: “to initiate of each” please delete of.

Page 3, Lines 74-76: Expanding…. atmosphere” Please rephrase the sentence.

Page 3, Line 80: Please replace “method by Moglioni et al. (Moglioni et al., 2000)” with method by Moglioni et al. (2000)”.

Page 3, Line 96: Please replace “In presented studies” with “in the present study”.

Page 4, Line 119 and Page 5, Figure Caption 1: Please replace “3.54 ×10-4” with “3.54 ×10^-4” (in several places in the manuscript “10-4” should be replaced by “10^-4”.

Page 7, Line 148: Please replace “the m/z 69 anion” with “m/z 69 anion”.

Page 7, Line 149: Please replace “Second pathway,” with “The second pathway,”.

Page 7, Line 161: Please replace “comparison the other” with “comparison with the other”.

Page 8, Line 171: “Rearrange” with “be rearranged”.

Page 8, Line 174: Please replace “by Yasmeen, F. et al,” with “by Yasmeen et al. (2010)”.

Page 8, Line 176: “understanding of the” please delete “of”.

Page 8, Lines 176-178: While….is formed” The syntax of this sentence doesn’t seem correct, please rephrase it.

Page 8, Line 179: Please replace “fact the excellent” with “fact an excellent”.

There are more small grammar/syntax errors, in the whole manuscript which is not possible to mention all of them. Since I’m not a native English speaker maybe the journal team is more suitable for these corrections.

“Mechanistic insight into the kinetic fragmentation of Norpinonic Acid in the gas phase: An experimental and DFT study”, by Kurzydym et al.

This manuscript provides a new insight of the formation of some fragmented products which may proceeds via other chemical routes that do not require the presence of radicals. Kurzydym et al confirmed these products from Norpinonic acid by using experimental and DFT studies.

In general, I think this topic is of high importance. However, I do have some concerns that need to be addressed before it can be accepted for publication.

General comments:

1) Please improve the structure of the abstract. Abstracts should have fewer than 250 words. Also, authors focus more on the approach and results/observations, but it is not clear to me about the status of research, research gap, the importance and implications of the results.

2) In this study, you studied the Norpinonic acid anionic fragmentation pathway in the gas phase. But in the Introduction section, you mentioned more analytical techniques that used for OA products (Lines 44-47). This might be confusing. I suggest that you should focus more on the gaseous measurements.

3) Lines 52-53: “to the proper identification of OA components, mentioned MS/MS measurements should be made very accurately”, I suggest that you could add more descriptions/discussions about the MS/MS technique here.

4) Line 156: why do you apply three different DFT methods in this study?

5) Lines 160-161: For the comparison, please state the specific difference or improvement between these three methods here.

6) Section 3.1: Is this the first research that emphasize fragmentation pathways of norpinonic acid? Are there any other comparable studies, please add more discussion in this Section.

7) Lines 129, 161, 248, 286, 292: It is hard for readers to connect the main text with the SI, here you do not provide the relevant parts in the SI.

8) Line 29: should be: “Primary organic aerosols are emitted to the atmosphere…”

Technical corrections:

Line 8: “athmospheric”.

Line 11: please provide the full title of m/z.

Line 13: Is “CIE” right?

Line 21: please provide the full title of ER-CID when it occurs at the first time

Line 119, Line 128: superscript: “10-4”.

Line 257: “fragmentat”

A study of Mechanistic insight into the kinetic fragmentation of Norpinonic Acid in the gas phase: An experimental and DFT study was did by Kurzydym et al. This work uses the tandem mass spectrometry techniques to elucidate the gas-phase fragmentation pathway of norpinonic acid. In addition, it provides a new insight into the chemical processes occurring in the atmosphere, specifically the formation and fragmentation of norpinonic acid. It is helpful to improve the understanding of the mass spectrometer technology and organic molecules. I think this work could be accepted by ACP, before the below comments can be addressed. Here are my main comments.

General comments: This manuscript takes many contents to discuss results from computed potential energy and reaction energy. However, there is no any information to explain this in the introduction and method.

Special comments:

Line 50-55. It needs to give more information about MS/MS techniques.

Line 129, 292. When you mention the supplementary information. Please explain where the information comes from. It is hard to find out where it is.

Line 156-157. It should use some references to explain the different DFT methods and give more information about those methods.

Figure 7. This figure gives a little information. However, this figure is too big. It is better to make it more sample and useful.

General comments:

This article “Mechanistic insight into the kinetic fragmentation of Norpinonic Acid in the gas phase: An experimental and DFT study” written by Kurzydym et al. studied the norpinonic acid (important α-pinene oxidation product) anionic fragmentation pathway in the gas phase, and the exact ion fragment structures were identified using the secondary-order mass spectrum (MS/MS) recorded during energy resolved collision-induced dissociation mass spectrometry experiments (ER-CID). The research results are helpful for us to understand better about the possible chemical structures from the fragmentation of norpinonic acid However, I feel that the significance of the research is not well explained. How should we link the experimental results to the current atmospheric chemistry? Will the fragmentation of norpinonic acid in the gas phase have much effect on the natural atmospheric aerosol formation?  Overall, the research contents are suitable for ACP readers, but some revisions should be made to the main text before this paper can be published.

Other comments:

1. It is not proper to use the abbreviation “DFT” in the title directly. Also, I haven’t seen any explanation about “DFT” in the main text.
2. Line 80: More details should be described in the main text, i.e., what is the main mechanism used for the synthesis of cis-norpinonic acid, what is the purity of the synthesized product? Why the trans-norpinonic acid is not used here?
3. Lines 105-106: The description of the calculation is quite simple, could you add some more details about the Gaussian 09 suite of programs and the Cartesian coordinates of the initial geometries here?
4. Line 223-225: the variations in the averaged experimental fragmentation energies are quite large (with large averaged standard deviation), how will these uncertainties affect the results and discussions in Sect. 3.2?
5. Lines 227 and 255: please keep the table format the same.
6. In the conclusion section, please summarise more on the experimental results and their significance rather than the experimental method and procedures.