the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Feb 2024
Discovery of reactive chlorine, sulphur and nitrogen containing ambient volatile organic compounds in the megacity of Delhi during both clean and extremely polluted seasons
Abstract. Volatile organic compounds significantly impact the atmospheric chemistry of polluted megacities. Delhi is a dynamically changing megacity and yet our knowledge of its ambient VOC composition and chemistry is limited to few studies conducted mainly in winter before 2020 (all pre-covid). Here, using a new extended volatility range high mass resolution (10000–15000) Proton Transfer Reaction Time of Flight Mass Spectrometer10K, we measured and analyzed ambient VOC-mass spectra acquired continuously over a four-month period covering “clean” monsoon (July–September) and “polluted” post-monsoon seasons, for the year 2022. Out of 1126 peaks, 111 VOC species were identified unambiguously. Averaged total mass concentrations reached ~260 µgm-3 and were >4 times in polluted season relative to cleaner season, driven by enhanced emissions from biomass burning and reduced atmospheric ventilation (~2). Among 111, 56 were oxygenated, 10 contained nitrogen, 2 chlorine, 1 sulphur and 42 were pure hydrocarbons. VOC levels during polluted periods were significantly higher than most developed world megacities. Surprisingly, methanethiol, dichlorobenzenes, C6-amides and C9-organic acids/esters, which have previously never been reported in India, were detected in both the clean and polluted periods. The sources were industrial for methanethiol and dichlorobenzenes, purely photochemical for the C6-amides and multiphase oxidation and partitioning for C9-organic acids. Aromatic VOC/CO emission ratio analyses indicated additional biomass combustion/industrial sources in post-monsoon season, alongwith year-round traffic sources in both seasons. Overall, the unprecedented new information concerning ambient VOC speciation, abundance, variability and emission characteristics during contrasting seasons significantly advances current atmospheric composition understanding of highly polluted urban atmospheric environments like Delhi.
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RC1: 'Comment on egusphere-2024-500', Anonymous Referee #1, 20 Apr 2024
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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:
- 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.
- L 14 should be “COVID” capitalized
- L 22 why was this surprising? These compounds have been observed in other urban areas, so I do not find this surprising.
- L 33 should somewhere mention that Delhi is in India
- L 72-73: FAME and NCR are unexplained abbreviations
- Fig 1: a) the legend is illegible and b) the legend is too small
- 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?
- 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?
- L 179 this sentence is unnecessary and sounds again like an advert.
- L 190 exchange “checked” with “compared”
- L 199: What about fragments?
- L 208: Reproducible within how many %?
- 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?
- fig 2: This is not a histogram, it is a bar chart?
- L 249: Not just isomers, also fragments
- fig 3: I am not sure what information I am supposed to read from it, it is very small.
- L 331: mention that these are diel averages.
- L 339 what does “CNG” stand for?
- L 341: Why do traffic emissions “seem” to be a major contributor to acetaldehyde emissions? No reasoning is given.
- 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?
- L 377: Missing a “been” after “has”
- 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.
- 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.
- L 411 a reference is missing for the deodorant and pesticide sources
- L 414-415 references missing
- L 419: Do the authors have any hypothesis of why dimethyl disulfide was not observed although it should be a major product?
- 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.
- 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)
- 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.
- L 534: Does “ER” stand for enhancement ratio here?
- L 552: again, the claim of “surprising” is not supported by previous literature and is also not necessary for the manuscript.
- L 564: Why? Some more discussion would help.
- 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.
Citation: https://doi.org/10.5194/egusphere-2024-500-RC1
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