Sachin Mishra et al. report VOC measurements in the megacity Delhi, where they applied a high resolution PTR-ToF-MS for the first time, which allowed them to observe species that had so far not been reported from there. Since VOC observations from India and South Asia in general are scarce, this work is a valuable and important addition to the literature. However, I have some concerns that should be addressed before publishing the paper in ACP.

General comments:

The paper is well structured and readable. However, in some places, e.g. in the abstract or at the end of the introduction, the paper reads a bit like an advert for the company selling the PTR-ToF-10k. I would ask the authors to cut down on the advertising. E.g., it is not necessary to include the brand name (“10k”) in the abstract. This is only necessary in the method description.

I have a general concern about the identification of compounds that the authors claim to be “unambiguous”. For example, they claim to have observed 42 pure hydrocarbons. But nowhere do they mention that the PTR method is subject to fragmentation and that these hydrocarbon masses could easily be, in part, fragmentation products of higher masses (Coggon et al., 2024; Pagonis et al., 2019). This includes isoprene, which can have substantial interference from higher aldehydes as well as substituted cyclohexanes (Coggon et al., 2024). Coggon et al. also described that the acetaldehyde mass is subject to interference from ethanol fragments, and benzene to fragments of other aromatics. I think the authors need to characterize their instrument’s fragmentation at least for the most important compounds that they show diel plots for. The paper by Coggon et al. gives some pointers on how to correct for them, as well. Alternatively, if GC measurements were made at the same time, the authors could use those to identify compounds unambiguously.

Specific comments:

1. Title: I would recommend to change the title to “Observations of…” instead of “discovery of”, since these compounds were not newly discovered but just observed in that location for the first time.
2. L 14 should be “COVID” capitalized
3. L 22 why was this surprising? These compounds have been observed in other urban areas, so I do not find this surprising.
4. L 33 should somewhere mention that Delhi is in India
5. L 72-73: FAME and NCR are unexplained abbreviations
6. Fig 1: a) the legend is illegible and b) the legend is too small
7. Method section: Some more information of the inlet would be helpful. What was its length and diameter, was it heated for the whole length, what was the inlet residence time and flow rate?
8. L 117: the verbal description of color symbols is not necessary, these should be legible in the legend. Why are hospitals shown? Are they VOC sources?
9. L 179 this sentence is unnecessary and sounds again like an advert.
10. L 190 exchange “checked” with “compared”
11. L 199: What about fragments?
12. L 208: Reproducible within how many %?
13. L 215: what is the overall uncertainty of the measured VOC concentrations? I assume it differs between compounds with a gas standard and compounds without one?
14. fig 2: This is not a histogram, it is a bar chart?
15. L 249: Not just isomers, also fragments
16. fig 3: I am not sure what information I am supposed to read from it, it is very small.
17. L 331: mention that these are diel averages.
18. L 339 what does “CNG” stand for?
19. L 341: Why do traffic emissions “seem” to be a major contributor to acetaldehyde emissions? No reasoning is given.
20. L 346 and Fig. 4: The diel cycle of “isoprene” makes me strongly doubt that this is purely isoprene. Especially in the post-monsoon it is higher at night than during the day. This strongly indicates that there may be other compounds, e.g. cooking aldehydes, fragmenting on this mass (Coggon et al., 2024). As mentioned above, the authors should characterize their instrument’s fragmentation especially onto C5H8H+. In FIg. 4: What does "VC" stand for?
21. L 377: Missing a “been” after “has”
22. L 388 the description of the instrument is a repetition that has already been given in the method section, also it again reads almost like advertising.
23. L 390: Why is this surprising? As the authors describe themselves, these compounds have been observed in many places before and can be expected in a megacity. I do not think the claim of the “surprise” is necessary for the paper.
24. L 411 a reference is missing for the deodorant and pesticide sources
25. L 414-415 references missing
26. L 419: Do the authors have any hypothesis of why dimethyl disulfide was not observed although it should be a major product?
27. fig 6: The plots make pretty clear that there are still two separate correlations in the monsoon time. I would suggest fitting these separately, and use the more similar slope between both as an argument for the discussion.
28. Table 1: Are the reported values really emission ratios or enhancement ratios? The emission ratio would depend on the distance from the source and the photochemical processing that has happened in between emission and observation. How to calculate emission ratios from ambient observations is described in (Gouw et al., 2017). Real observed emission ratios from flux observations in urban areas are shown in the SI of (Karl et al., 2018)
29. L 530 ff: The discussion of enhancement ratios needs to take into account the different lifetimes of the aromatics. Therefore, a different enhancement ratio can be the result of different photochemical processing/oxidant levels.
30. L 534: Does “ER” stand for enhancement ratio here?
31. L 552: again, the claim of “surprising” is not supported by previous literature and is also not necessary for the manuscript.
32. L 564: Why? Some more discussion would help.
33. Data availability: The data policy of ACP clearly states that data needs to be publicly available on a repository with a DOI. Availability upon request is no longer enough. (https://www.atmospheric-chemistry-and-physics.net/policies/data_policy.html)

References

Coggon, M. M., Stockwell, C. E., Claflin, M. S., Pfannerstill, E. Y., Xu, L., Gilman, J. B., Marcantonio, J., Cao, C., Bates, K., Gkatzelis, G. I., Lamplugh, A., Katz, E. F., Arata, C., Apel, E. C., Hornbrook, R. S., Piel, F., Majluf, F., Blake, D. R., Wisthaler, A., Canagaratna, M., Lerner, B. M., Goldstein, A. H., Mak, J. E., and Warneke, C.: Identifying and correcting interferences to PTR-ToF-MS measurements of isoprene and other urban volatile organic compounds, Atmos. Meas. Tech., 17, 801–825, https://doi.org/10.5194/amt-17-801-2024, 2024.

Gouw, J. A. de, Gilman, J. B., Kim, S.-W., Lerner, B. M., Isaacman-VanWertz, G., McDonald, B. C., Warneke, C., Kuster, W. C., Lefer, B. L., Griffith, S. M., Dusanter, S., Stevens, P. S., and Stutz, J.: Chemistry of Volatile Organic Compounds in the Los Angeles basin: Nighttime Removal of Alkenes and Determination of Emission Ratios, J. Geophys. Res., 122, 11,843-11,861, https://doi.org/10.1002/2017JD027459, 2017.

Karl, T., Striednig, M., Graus, M., Hammerle, A., and Wohlfahrt, G.: Urban flux measurements reveal a large pool of oxygenated volatile organic compound emissions, Proceedings of the National Academy of Sciences of the United States of America, 115, 1186–1191, https://doi.org/10.1073/pnas.1714715115, 2018.

Pagonis, D., Sekimoto, K., and Gouw, J. de: A Library of Proton-Transfer Reactions of H3O+ Ions Used for Trace Gas Detection, Journal of The American Society for Mass Spectrometry, 30, 1330–1335, https://doi.org/10.1007/s13361-019-02209-3, available at: https://doi.org/10.1007/s13361-019-02209-3, 2019.

Sachin Mishra et al.’s work detailed the measurement of volatile organic compounds (VOCs) in the urban environment of Delhi, India using the Proton Transfer Reaction Time of Flight Mass Spectrometer. The measurement period covered an extended period with “clean” and “polluted” monsoons that are impacted by several emission sources such as biomass burning events. The authors also argued the discovery of a few VOCs that were previously unaccounted in India. According to the authors, the major significance of the work was the speciation of new VOCs that might influence the urban atmospheric environments, particularly in the global south where fewer studies are reported compared to its counterpart. The application of new instrumentation to uncover underlying atmospheric chemical mechanisms is commendable.

General Comments:

The research structure provided by the authors in its current version appears to be a measurement/instrument paper and/or regional research manuscript, in which the authors heavily detailed the capacity of IONICON’s PTR-ToF-MS 10K. The paper reads as an atmospheric measurement technique study that highlights an instrumentation with improved resolution and sensitivity,  although not developed by the same authors. I also believe that the authors failed to maximize the enhanced instrument capabilities based on the evidence provided in the manuscript, thus old versions of the PTR-ToF-MS can still capture the variability of VOCs discussed by the authors. A considerable portion of the manuscript was allotted to the discussion of the PTR-ToF-MS but with limited contribution of new information and implications that will improve the understanding of the state and behavior of the atmosphere and climate . The authors are highly recommended to present the study as a scientific paper with new and proper atmospheric implication/s and dissociate it as methods and regional paper. More importantly, the authors should limit providing several grand claims with little to no supporting evidence.  Significant manuscript revision and restructuring based on the following comments/suggestions is advised before publishing to ACP.

1. The most promising section of the manuscript is the discovery of methanethiol, dichlorobenzene, C6-amides, and C9 organic acids, which the authors attributed to the new instrumentation that was previously deployed in India. This is the main differentiation compared to prior VOC studies performed in India, where seasonal (monsoon vs post-monsoon) comparisons were already presented(Jain et al., 2022; Wang et al., 2020). The paper would have been more impactful if the authors focused on this section, instead of reporting previously detected VOCs (e.g. methanol, isoprene, etc.) in Delhi.
2. At the current state of the manuscript, there are a lot of concerns that need to be addressed in the discussion of the four previously unaccounted VOCs. First, no statistical merits of these compounds were presented. What are the instrument blanks of these compounds? What are their corresponding limit of detection (L.O.D.) values? Are the values of such VOCs presented in the manuscript (i.e. ~50 ppt methanethiol in line 395) beyond the L.O.D? Instrumental background was measured according to the authors, but the results were never discussed in the main text and supplemental section.
3. Besides the molecular attribution provided by IDA that is based on the closeness of the experimental and theoretical m/z, what are the other credible evidence of the authors in “unambiguously” identifying the four compounds in section 2.4? Did the authors use standard compounds to account the for the fragmentation pattern of these proposed compounds? Indeed, the PTR-ToF-MS is a soft-ionization technique, however, it also suffers from collision-induced dissociation (CID) that reduces the relative abundance/concentration of the primary molecular ion [M+H]. The relative contribution of acylium ions [M-OH] is more prominent in longer carboxylic acids, as observed in C2-C6 short-chain fatty acids(Hartungen et al., 2004). Thus, the C9 compound that the authors might not correspond to the real analyte measured in India.
4. The authors also argued that the four compounds were only measured because of the extended volatility range mass spectrometer design and high sensitivity due to the ion booster and hexapole guide of the PTR-TOF-MS 10K system that enabled the detection of the compounds that are all intermediate volatility range organic compound (IVOC) (Line 387-388). What is the basis of the IVOC designation of these compounds? Did the authors calculate the saturation concentration of these compounds? Statements in Lines 387-388 also imply that the proper detection of these VOCs requires EVR, ion booster, and hexapole, whereas prior studies with older PTR-ToF-MS systems already detected IVOCs. For instance, Salvador et al., accounted IVOCs from the thermal desorption of organic aerosols (see Figure 2a of Salvador et al) using a PTR-ToF-MS 8000 without these new components(Salvador et al., 2022). Also, did the 10,000 m/Δm mass resolution, 1000 cps/ppbv sensitivity, and 1-10 pptv detection limit of the PTR-ToF-MS 10 K really help in the detection of these compounds? Do these compounds have known nominal mass interference and typical low concentration, thus requiring a highly mass-resolved and sensitive mass spectrometer? The authors should illustrate the mass spectra of these compounds, similar to Figure S1, to showcase the high resolving power of the instrument and to support their claims. The authors should heavily consider restating these statements.
5. The authors relied heavily on the distinction between ‘’clean” monsoon and “polluted” post-monsoon season to explain their observation, even for VOCs that shouldn’t respond evidently to such atmospheric event changes. For instance, the emission of biogenic compounds such as isoprene are known to be strongly dependent on temperature, UV, and vegetation activities. The authors should properly account for the sources of the VOCs. In line 361, the authors even argued that the increased biomass burning and traffic controlled the abundance of isoprene. Any supporting study and/or study to support such a claim?

Specific and technical comments:

1. The title is misleading, particularly the word “discovery”. Analysis or a similar term might be more appropriate
2. Line 18: the term “unambiguously” is inappropriate for this case, particularly with the lack of standard compounds and/or supporting measurements (e.g. GC-MS) that will confirm the identities of the proposed compounds. I understand the PTR-ToF-MS 10K has an enhanced mass resolution, but molecular formula alone cannot be the sole evidence for the “unambiguously” identification of VOCs.
3. Line 70: What are NCR and FAME? An important note: the authors argue that these VOC data sets are unique since they were collected post-COVID, in which new programs were introduced that will impact air quality. However, nowhere in the succeeding section were these accounted for or even mentioned, except for an overarching statement in the conclusion that has no strong foundation.
4. Line 122: Atmospheric ventilation is not a common meteorological parameter, and thus requires further explanation. What is the physical meaning/basis of atmospheric ventilation?
5. Line 137: Section 2.2 is too long with unnecessary information regarding the technique. This is not the first study that utilized this technique/instrument. Authors should consider moving them to supplement file or totally removing them.
6. Line 206: The authors indicated that the instrument was calibrated eight times during the extended measurement period, which should account for the drift/changes/responses in the instrumentation. However, the results of the calibration were not presented, even at least in the supplement file. This information is important, as it would provide confidence in the dataset that the variability in the VOC concentration can be accounted primarily for changes in atmospheric events(monsoon vs post-monsoon), instead of the instrument signal drift.
7. Lines 229-255: These statements should be moved to the experimental section. Please note that several sentences were already mentioned and should not be repeated. The authors should consider providing a sample ion that was identified through the proposed method of the authors to visually guide the readers.
8. Figure 3: The baseline concentrations of some VOCs were evidently different for both monsoon and post-monsoon seasons. An obvious example is the isoprene, particularly during mid-October when baseline concentration is increasing even at nighttime while a clear flat baseline is observed during clean monsoon. The same insights can be applied to acetonitrile and acetaldehyde. Any possible explanation? Does this impact the measured concentration between the two periods?
9. Line 339: What does CNG mean?
10. Figure 4: The nighttime enhancement of isoprene during post-monsoon is questionable. As a biogenic VOC with a short atmospheric lifetime, isoprene typically only has daytime enhancement. Is this related to the increasing baseline indicated in comment 8?
11. Figure 5: The C6 amide is missing a superscript. The same goes for line 447.
12. Lines 411-417: The possible sources of methanethiol were based on assumptions even though the authors have enough data to possibly explain their results. Is there a particular wind direction where methanethiol is more dominant? Does this coincide with the possible locations of the manufacturing facilities?
13. Line 422-426: How did the authors arrive at such results? Do the authors have a relevant data set showing the enhancement of organic aerosols due to the methanethiol in Delhi? Similarly, do the authors have evidence of the interaction of organic haze with sulfur compounds such as methanethiol? Consider removing such statements.
14. Lines 428-431: These statements are unclear. The increasing sales of methanethiol are not relevant to the few ppt concentrations observed in Delhi.
15. Line 475-477: What is the physical meaning of “inverse of daytime relative humidity” and how does it indicate that it partitions back and forth between the gas phase and aerosol phase? How did the authors arrive at such results? Please note that temperature and relative humidity have an indirect relationship in most cases. The authors might be following the trend of temperature instead. Also, the daytime enhancement profile of the C9 acid is a clear indication that it has a photochemical source and I am not sure why the authors are arguing that biomass burning and “evaporation” from the aerosol phase is the major source of the C9 acid.
16. Line 500-506 and Figure 6: The statistical merits (slope and r²) for both monsoon and post-monsoon do not have evident different values yet the authors came up with unsupported claims and implications (i.e. additional sources).
17. Line 538-539: Again, how did the authors arrive at such conclusion (i.e., the influence of non-traffic sources) based on the comparison of emission factors?
18. Line 543: Unprecedented is inappropriate here given the prior VOC comparison studies performed in Delhi.
19. Line 551: Why provide again the molecular formula and name? It was already mentioned in the results and discussion.
20. The authors keep on arguing that their compounds are IVOCs without presenting information regarding the volatility/saturation concentration of these compounds. I suggest that the authors present or cite these values or calculate them using the identified molecular formula. See Mohr et al., 2019 for reference (Mohr et al., 2019)
21. Line 567: Where’s the PM2.5 data to support such a claim?
22. Line 573-576: The authors should be careful in asserting such statements (i.e. not as state of the art). Other techniques such as GC-MS, Orbitrap, and CIMS which have improved mass resolution and sensitivity can provide better quantification of the VOCs compared to the instrument utilized by the authors. I commend the expanded list of VOCs measured by PTR-ToF-MS 10K, however, the compounds listed by the authors are already measured in previous studies using other techniques that lead to a better understanding of the complex atmospheric systems across the globe.

 REFERENCES

Hartungen, E. v., Wisthaler, A., Mikoviny, T., Jaksch, D., Boscaini, E., Dunphy, P. J., and Märk, T. D.: Proton-transfer-reaction mass spectrometry (PTR-MS) of carboxylic acids: Determination of Henry's law constants and axillary odour investigations, Int. J. Mass spectrom., 239, 243-248, http://dx.doi.org/10.1016/j.ijms.2004.09.009, 2004.

Jain, V., Tripathi, S. N., Tripathi, N., Sahu, L. K., Gaddamidi, S., Shukla, A. K., Bhattu, D., and Ganguly, D.: Seasonal variability and source apportionment of non-methane VOCs using PTR-TOF-MS measurements in Delhi, India, Atmos. Environ., 283, 119163, https://doi.org/10.1016/j.atmosenv.2022.119163, 2022.

Mohr, C., Thornton, J. A., Heitto, A., Lopez-Hilfiker, F. D., Lutz, A., Riipinen, I., Hong, J., Donahue, N. M., Hallquist, M., Petäjä, T., Kulmala, M., and Yli-Juuti, T.: Molecular identification of organic vapors driving atmospheric nanoparticle growth, Nature Communications, 10, 4442, 10.1038/s41467-019-12473-2, 2019.

Salvador, C. M., Chou, C. C. K., Ho, T. T., Ku, I. T., Tsai, C. Y., Tsao, T. M., Tsai, M. J., and Su, T. C.: Extensive urban air pollution footprint evidenced by submicron organic aerosols molecular composition, npj Climate and Atmospheric Science, 5, 96, 10.1038/s41612-022-00314-x, 2022.

Wang, L., Slowik, J. G., Tripathi, N., Bhattu, D., Rai, P., Kumar, V., Vats, P., Satish, R., Baltensperger, U., Ganguly, D., Rastogi, N., Sahu, L. K., Tripathi, S. N., and Prévôt, A. S. H.: Source characterization of volatile organic compounds measured by proton-transfer-reaction time-of-flight mass spectrometers in Delhi, India, Atmos. Chem. Phys., 20, 9753-9770, 10.5194/acp-20-9753-2020, 2020.