The manuscript titled “Biogenic and anthropogenic sources of isoprene and monoterpenes and their secondary organic aerosol in Delhi, India” investigates the concentration trends in precursor gas-phase emissions of isoprene and select prominent monoterpenes, and the resulting secondary organic aerosol in Delhi.

The paper is properly structured and well-written, and, there is a nice effort on the part of the authors to thoroughly explain the trends they observed in their ambient measurements. I think this work is suitable for publication in ACP after some minor issues are resolved.

• One of my major concerns is that even though the title says “secondary organic aerosol”, the SOA section of the paper almost entirely revolves around the NOS and OS species in SOA from isoprene and/or monoterpenes, which constitute a small fraction of the total SOA. Either the relevant discussion sections should be expanded to include some broader details of SOA from these precursors, or, the sub-section titles should be changed to more accurately represent the discussion contained therein.
• Based on the title, I was very curious about the anthropogenic sources of isoprene and monoterpenes in Delhi. However, I think that potential anthropogenic contributions are not discussed sufficiently in the paper. For example, correlations with CO are briefly discussed, and biomass burning and VCPs are hinted at as likely contributors. But this is all general information and as a reader, I am not able to gain significant insights into anthropogenic sources of terpenes in Delhi. I would appreciate seeing some correlations of compounds of interest with D5 or Benzyl alcohol. D5 can be measured via GC techniques so it would be useful to include at least some information on it (e.g. temporal trends even if ion abundance-based). Delhi is densely populated and characterized by a lot of street activity with temperatures reaching 30-40 C (pre-monsoon) so I would assume that there should be D5 in Delhi’s air.

Additional comments:

1. Section 2.3: Please add some sentences about the mass resolution of the spectrometer. Also, it would be good to add a brief discussion (3-4 sentences) about the quantification method before citing Bryant et al. 2021.  
2. Figure 3 (e,f): a-pinene concentrations do not show significant dilution during pre-monsoon daytime conditions when compared to post-monsoon. This is despite the daytime PBLH being substantially higher during pre-monsoon than post. Shouldn’t your discussion in lines 405-408 apply here? Is there an explanation for this?
3. Lines 497-499: Shouldn’t low ventilation and stagnant conditions lead to greater accumulation and higher concentrations of isoprene and sulfate? Or the magnitude of sources also drops?
4. Line 505: Please be consistent in using “sulfate” versus its formula in the text.
5. Line 577: Adding a brief discussion on C9H16O6S (or citations) would be of help here to an unfamiliar reader, especially since it contributes a large fraction to the OS(MT) mass. Is this species consistently observed in OS(MT) across different sites?
6. Lines 582-587: The authors should discuss what changed between pre- and post-monsoon around the site that led to seasonal variation in the significance of atmospheric reaction chemistry (e.g. MT+NO3 being more important in post-monsoon).
7. Lines 636-638: This sentence is confusing. qSOA is all isoprene and monoterpene derived species. So how come the fractions are not all 100%?    
8. Line 608: Are the higher post-monsoon concentrations of monoterpenes only due to lower PBLH or are the source profiles any different?
9. Line 643: There should be some discussion in the methods section on how were the iSOA tracers quantified using an ACSM.
10. Please proofread the manuscript for typos (e.g. line 434: “tarcers”; line 476: “update”; line 477: “into to”).

Bryant et al. present a nice analysis showing the seasonality of isoprene, monoterpene, and subsequent SOA products in Delhi, India. The authors highlight the importance of anthropogenic sources on the mixing ratios of isoprene and monoterpenes, and demonstrate the contribution of isoprene and monoterpene organosulfate and nitroxy-organosulfate products in SOA filter extracts. These observations are important for identifying key contributors to PM2.5 in highly polluted regions, such as Delhi.

The paper is very well written and presented. The results will be useful to the community, and I support publication. My comments are generally minor, but I do wonder if there is additional information that could be extracted from the monoterpene measurements that could directly point towards the impact of anthropogenic monoterpenes on the SOA markers observed in filter extracts. I’m curious if the authors have considered exploring the distribution of monoterpene OS_MT and NOS_MT shown in Table 1 to assess an anthropogenic fingerprint in these measurements.

Main comments

Line 35 – 36. The statement “this is one of the first observations in Asia, suggesting monoterpenes are dominated by anthropogenic sources” should be refined or removed, since work by Stewart et al and Nelson et al. already detail these observations for Delhi. This statement doesn’t seem necessary given that the main focus of the manuscript is on isoprene and monoterpene SOA markers.

Lines 229- 246 and Fig 1. These mixing ratios (and the differences across seasons) are impressive, but it’s difficult to see the details in the seasonal patterns in just a time series. It would be very helpful to see a third column where these gas-phase measurements are presented as diurnal patterns in order to see the seasonal and hourly differences. I would recommend this for Fig S1 as well to give the reader a better visual reference for how the meteorology impacts these mixing ratios. The authors provide a very nice discussion of the meteorological impacts in section 3.1,  but I believe overlaying the diurnal patterns seasons would be very helpful.

Lines 383 – 408 : The results and discussion about monoterpenes presented here reiterate many of the conclusions drawn by Stewart et al. 2021 and Nelson et al. 2021 (i.e., abundant anthropogenic source of monoterpenes). Are there other insights that can be drawn from the monoterpene data presented here? It would be helpful to see how the monoterpene distribution changes between by season or time of day (perhaps a pie chart of nighttime mixing ratios). Figure 2 suggests that alpha-pinene is the dominant monoterpene observed during the pre-monsoon season, while limonene seems to dominate during post-monsoon season. Does this point to a specific source in Delhi? Limonene is a key component of fragrances (Gkatzelis et al. 2021, Coggon et al. 2021, Peng et al. 2022) and cooking spices (Klein et al. 2016), and could be a component of biomass burning. Does this differ from the expected distribution of biogenic monoterpenes from the vegetation in Delhi?

Line 407: Please provide references to the OH sources noted here.

Figure 5: The legend for the pie chart is very small and difficult to read. Please make this larger to help those of us with poor eyesight!

Lines 515 – 538: The authors mention here and elsewhere the role of high NO in quenching NO3 radicals. Indeed, the NO is very high, but I don’t have a sense for how this stacks up against the other species with high reactivity towards NO3. I think this discussion could benefit from a pie chart showing the distribution of NO3 reactivity based on the VOC and inorganic gas measurements, but recognizing that NO3 reactivity may be missing from the measurements (e.g. Fig 4, Liebmann et al.). I believe this would help to supplement the discussion of Figures 5 and 6.

Table 1: I feel like there could be very valuable information in these distributions of isoprene and monoterpene products, and specifically for the OS_MT and NOS_MT speciation. Have the authors considered exploring the speciation of MT SOA markers and relating these back to the monoterpene mixing ratios observed by GC?

Minor Comments:

Line 402: The reference to Coggon et al. (2018) should be updated: https://www.pnas.org/doi/10.1073/pnas.2026653118.

References:

Klein, F. et al. Indoor terpene emissions from cooking with herbs and pepper and their secondary organic aerosol production potential. Sci Rep 6, 36623 (2016). https://doi.org/10.1038/srep36623

Coggon M. et al. Volatile chemical product emissions enhance ozone and modulate urban chemistry. PNAS 18, 13 (2021). https://doi.org/10.1073/pnas.2026653118.

Gkatzelis, G. et al. Identifying volatile chemical product tracer compounds in U.S. Cities. Environ Sci Technol, 55, 1, (2021). https://doi.org/10.1021/acs.est.0c05467.

Peng, Y. et al. Source apportionment of volatile organic compounds and evaluation of anthropogenic monoterpene emission estimates in Atlanta, Georgia. Atmos. Environ. 228 (2022). https://doi.org/10.1016/j.atmosenv.2022.119324.

Liebmann, J. M. et al. Direct measurements of NO₃ reactivity in and above the boundary layer of a mountaintop site: identification of reactive trace gases and comparison with OH reactivity, Atmos. Chem. Phys., 18, (2018). https://doi.org/10.5194/acp-18-12045-2018