General comments

The manuscript presents a positive matrix factorization analysis of a consequent PTR-ToF-MS dataset, to which PM_2.5 and PM₁₀ data were added. The general outline, scope and main conclusions are very clear. The results are interesting, and each of the 11 obtained factors is thoroughly described, backed up with external data and source profiles, and well explained.

However, I do feel that the methodology is not described enough. There should be more details on how the uncertainties were calculated, what were the uncertainties for each compound and their range. Also, more information is needed on the PMF approach of adding PM_2.5 and PM₁₀ data to the VOC dataset, what were the steps leading to the solution (how many runs, how was the base case chosen if these runs gave different solutions, …), were there any challenges with this approach? In the abstract you mention “our novel source apportionment method”, but it is not very clear in the paper how novel or different it is.

Also, you mention that the factors are stable in the bootstrap repetitions; however, the uncertainties of the model in Figure 3 seem quite important. Also, the contribution of factors (i.e. paddy, residential) for PM_2.5 and PM₁₀ changes a lot when the number of factors varies, suggesting they may not be very stable. Do you have other information to back up the factors’ stability (i.e., low timeseries correlations between the factors)? How do the scaled residuals change when increasing the number of factors or between different runs?

The comparison of the PMF output with emission inventories results needs more justification. If I understand correctly, PMF results are concentrations and seem to be directly compared to emissions, which are different quantities and on different scales. The conclusions drawn here seem too strong (i.e. lines 536-539). Also, please justify why the PMF results are more correct than model outputs? (i.e. when you state that sources are under-/over-estimated in the models)

Be more concise when you present the description of the factors, the fact that all the values and VOC m/z are written in the main text makes it tedious to read. Use only VOC names (or formula if unclear what the compound is, but the m/z are already all listed in Table S1). Also, delete all the concentration and % values in the main text if they are already on the figures, except if it is useful to emphasize the point (example in line 632: “a considerable portion of the PM₁₀ (18%) and PM_2.5 (28%)”). Same for log₁₀C₀, find a clearer way to present them. Another option would be to put the extensive description of factors in SI and a summary and interpretation in the main text.

Specific comments/questions

Line 87-88: I would suggest adding a map of the receptor site with the surroundings (i.e. roads, industries, agriculture…), and referencing it when needed.

Line 108: I think it would be worth summarizing the main differences between the 3 wind sectors (in terms of typology, specificity, and later on results).

Section 3.1 & Figure 3: I would suggest putting Figure 3 in supplementary and replacing it with only this study’s factors profiles in concentration (instead of normalized). In text 3.1, I would add the R correlation (of profile and/or diurnal cycle) of this study’s factors with the mentioned reference factors to justify the factors’ interpretation.

Sections 3.1 & 3.2: Since you have a dedicated subsection for the comparison of the sources with references, you don’t have to repeat them when describing each factor.

Figure 3: How were the displayed compounds chosen for this graph? And please use the compounds’ names so that it is clearer.

I would suggest adding Figure S3 in the main text as it is referenced a lot, and that way you don’t need to put the % in the main text.

Line 224: “The source identity of the PMF factors was confirmed by matching the normalized PMF factor profiles with normalized source fingerprints”. Could you add more detail about this, did you check the R correlations? Or was it just by visually comparing them?

Line 235-236: Did you measure the Munirka furniture market and Dhobighat at Akshar Dham samples? If not, could you add their reference?

Figure 5:

I would suggest enlarging (by the x axis) the timeseries plot, to make them easier to read. You should keep the same order of the factors as in description (& throughout the paper). What do the lines/shaded areas for the diurnal cycles represent (mean, median…)?

3.2.2. There is a mention that this factor may not be always fresh, which I found interesting, you could add a few words at the end of the paragraph about the fresh/aged nature of the factors based on all the information.

3.2.3. Some of these compounds (i.e. aromatics) can also be associated with cooking activities (e.g. Crippa et al (2013), doi.org/10.5194/acp-13-8411-2013).

3.2.4. You could add one sentence about the interpretation of VOCs (i.e. methanol and ethanol) for this factor.

3.2.5. & 3.2.6. Add a sentence (or change existing text) to highlight the differences between 2-wheeler & 4-wheeler factors.

Line 422: Interesting! Could this last sentence mean that part of PM_2.5 for this factor would be SOA?

Line 432-433: Do you have references for this last statement?

3.2.9. Interesting, the last sentence suggests a possible link of the OVOCs with SOA?

3.3. It’s a little tedious to read with all the emission values, please select when it is truly important to have them.

Lines 500-505: A map could be useful here as well.

Line 536-539: “our PMF results indicate that the actual emissions are slightly smaller than those” “our PMF estimates fall in between those of the EDGARv6.1 inventory and the REASv3.2.1 inventory” I don’t understand how you come to these conclusions, did you calculate emissions out of the PMF concentrations? If yes, please state. If not, I don’t think you can directly compare PMF results and emissions, only in terms of contributions to the total “measured” compounds for each method.

Line 551: “The EDGARv6.1 inventory significantly underestimates PM_2.5 & PM₁₀ from agricultural activities” Please backup this statement with a map for example to justify that agricultural emissions should be high.

Line 554-556: There were any more results available from FINNv2.5? “between 15th and August and 26th November 2021 alone” please clarify, was it 15/08-26/11? Then it’s the same length as the current dataset…

Table S1: You could add calculated uncertainties and detection limits here. Also, if the “Sr. No” numbers are not used, you can delete them from the table. Are the “Mean” and range values here the detection limits or the averaged concentrations throughout the campaign?

Table S2 & S3: Same comment about the “Sr. No”.

Figure S1: Are these figures referenced in the paper?

Technical corrections

Throughout the paper, add · in units (ex µg·m^-3)

Title: There shouldn’t be an abbreviation in the title, please use volatile organic compounds instead of VOC.

Line 16: There is a repetition of the word “using”, please change.

Line 23: Replace “(<2)” by “at least by a factor of 2”.

Line 36: Please reformulate “continues to add”.

Line 70: Delete the first “source” in “quantify the source contribution of the different sources”.

Line 80: Delete “:” in the title and check all the titles.

Line 113: “in blue” aren’t there other colours used on the graph too?

Line 116: Correct to “solar radiation as photosynthetically active radiation (PAR)”.

Line 119: Please add the dates of monsoon and post-monsoon seasons.

Line 151-152: The structure of the sentence seems wrong, please correct.

Line 180-181: There is a repetition of the word “model”, please change.

Line 190: “T” to delete at the beginning of the paragraph.

Line 180-181: There is a repetition of the word “using”, please change.

Line 194: Change to “The secondary organic aerosol production (SOAP)” in small case.

Line 196 & 197: Replace NOx with NO_X and check this throughout the paper.

Line 197-199: This sentence is a bit unclear.

Line 219: Replace “while” by starting a new sentence with “In addition,”.

Line 220: Replace “are” with “were” and check that it is the right tense throughout the paper.

Line 247: Delete “,” in “(Fig. 4 a & d) were petrol”.

Figure 4: “Photo”, “P2W” & “P4W” could be written in full name.

Line 252: Delete “,” between “both” & “paddy”.

Line 293: Put “-3” in superscript.

Line 286 & l288: Delete “,” in “A recent study in Punjab indicated that” and “increased by 0.027 and 0.047 µg·m^-3 respectively”.

Line 357: There is a repetition of the word “identified”, please change.

Line 62: I would suggest deleting the sentence “this is consistent with our results”, as “confirms” in line 358 already suggests this.

Line 368-369: Keep “µg·m^-3)” on the same line.

Line 383: Delete the first “source” in “The source fingerprint of this source”.

Line 397: Correct the start of the sentence to “This factor contributes on average more than 30 µg·m^-3”

Line 397-398: The second part of the sentence, “due to…”, to reformulate and you could reference the added map of surroundings.

Line 399: Add space in “NO (R=0.7)” and correct “CH₄”.

Line 402 & 404: Once you have written full MTBE and MT, abbreviation is fine. For monoterpenes, you can also write only full name.

Line 403: There are 2 “,” after “acetaldehyde (1.2 µg·m^-3)”.

Line 415-418: This part is a little difficult to read, cf general comment about writing all the values.

Line 438: Use “acetone + propanal” as before.

Line 452-460: This part is quite difficult to read and understand, cf general comment about writing all the values.

Line 531-532: Keep “y^-1” on the same line.

Line 558: Delete “to” in “Our PMF results reveal that to agricultural”.

Line 608: “two criteria air pollutants” do you mean “critical”?

Line 622: What is EDGARv6.1 better than in this sentence?

Line 635: Add “in Delhi”: “Despite including the most comprehensive set of organic species in Delhi to date”

Line 644: Add “,” after “that”

Line 651: Replace “till date” by “to date”

The paper “Biomass burning sources control ambient particulate matter but traffic and industrial sources control VOCs and secondary pollutant formation during extreme pollution events in Delhi” discusses the sources responsible for air pollution problems in Delhi. For this, they made stationary ambient gas-phase measurements at a prominent location in urban New Delhi and performed source apportionment analysis on the collected data. The chemical profiles of the factors were compared with previous measurements and tracers to identify sources. The work is quite timely since New Delhi is one of the most polluted cities in the world, and regulatory policies are currently being restricted by our limited understanding of the sources in the region.

Yet I have significant concerns, which I think should be resolved prior to proceeding with publication. Some of my biggest concerns are with the conclusions drawn and stated quite imposingly in the conclusion section. Hence, I’ll discuss those first before moving to the next major ones.

Line 606-607: fresh paddy burning is shown to be a negligible source of VOCs but the largest sources of PM2.5 and PM10. This is highly confusing to me. PM2.5 would be formed from the secondary oxidation of a lot of gas-phase organic molecules emitted from paddy burning. As such it should be emitting precursors of SOA. Or are the authors suggesting that paddy-burning directly emits particulate matter into the atmosphere but no VOCs? Is it possible that the PTR-TOF did not measure or fragment a lot of precursor species emitted from paddy burning?

Line 620 (also 566-568): “The transport sector’s PM emissions are dominated by the non-exhaust emissions of the CNG-fueled commercial vehicle fleet.” This sounds somewhat unlikely. Which non-exhaust emissions are the authors referring to be emitting from CNG vehicles? I can think of break/tyre-wear as a possible source but that contributes primarily to coarse PM, not so much to fine. Are there evaporative emissions of some kind? I imagine CNG itself would have negligible potential to form ambient PM given its small molecular size. On the other hand, transport sector in Delhi would have diesel trucks which are known to be large emitters of SOA precursors. Dust-resuspension has been attributed to non-exhaust emissions, but I am not sure if I agree with that classification. Dust is not a vehicular source.  Hence, I would like the authors to extensively elaborate what forms PM from non-exhaust emissions from CNG vehicles. This also reads somewhat contrary to lines 260-264 where petrol vehicles are shown to be major contributors to SOA. Furthermore, while a distinction has been made between 2-wheeler and 4-wheeler petrol vehicles, no significant discussion exists on the contribution of diesel vehicles. This needs to be explained in more detail.

Line 650-651: Authors state that “all” previous studies from the region have attributed PM to BB or fossil-fuel burning, and that we need to look beyond these sources. While I agree that a larger set of sources need to be identified, I think there is already some work done on this front. Kumar et al. 2022 ACP https://acp.copernicus.org/articles/22/7739/2022/acp-22-7739-2022.pdf

Figure 5: I notice that road construction and solvents factors show opposing temporal trends. Road construction peaks in the afternoon while solvents are higher during early morning or night hours. The authors state in lines 425-426 that the solvents contribute the most to the VOC burden at night. Given that both these sources are evaporative in nature, how could they show opposing temporal trends? Are there any specific sources of solvents in Delhi that are prominent during nighttime? One can also check the temporal trends in PCBTF, Texanol and p-dichlorobenzene, D4- and D5-siloxane that are known tracers of VCP sources. Some of these can be measured with PTR-ToF.

The authors should more clearly discuss how they calculated the total VOC mass in the paper. This is important because the fractions of other measured species are drawn from the total, and this can introduce significant bias in the conclusions regarding source contributions if the total VOC mass is not comprehensive enough. The chemical profiles shown in Figure 3 run up to C10H16 and there is some additional discussion in the paper about IVOCs. However, sources such as road construction emit minimally in the VOC space, and more in the IVOC and SVOC space. The authors should discuss how they prevented biases from creeping into their conclusions. Also, there should be at least some discussion in the paper about the inlet system used upstream of the PTR-TOF as this can prove crucial in the detection of many species (lines 132-133).

Furthermore:

Lines 182-184: The “pulling up” and “pulling down” should be briefly explained. It sounds vague in its current form.

Lines 187-188: It is quite amazing that the bootstrap found all 100% of the runs stable and well-mapped to the base solution. In principle, this may suggest that your dataset yields only one solution which is super robust. Is this what you are saying? I acknowledge citations, but in lines 180-187, I recommend briefly describing the rationale behind application of different constraints to help the reader assess.

Lines 229-234: The comparisons stated here are very on point, which is great. But it is not clear how contributions from heavy vehicles, e.g. road construction vehicles, were separated from other diesel-based sources, such as transport trucks. I recommend to put some correlation plots in the SI that compare the chemical profiles of the source factors obtained in this study with the sources from literature that are discussed here.

Lines 252-253: As a reader, I was surprised to see a comparison with NW-IGP and Mohali. It was quite sudden and not consistent throughout the paper. This should be rephrased in a way that gives a reader some context on which regions are being compared and why.

Lines 262-270: Add error values to the average percentages to account for the variability in these fractions during the study period.

Line 284: I am not sure whether a correlation R of 0.5 could be considered significant.

Line 288: 0.027 and 0.047 are quite small values. What is your error bound on these numbers?

Figure 5: The increase in NOx in petrol 2W panel during morning commute hours is not reflected in 2W or 4W factors. Does this make sense? Also why are the 2-wheeler petrol vehicle factor contributions high throughout the night and drop near the morning commute hours? I would imagine the 2W vehicles on the road to decrease substantially during the night.

Line 326: 3.2.2 Title: By waste disposal, do the authors mean waste burning? These can be very different things with different mechanisms of emissions if combustion is not involved in one versus the other.

Line 354: BB emissions are attributed to solid fuel-based cooking and a cow dung-fired traditional stove is discussed. These measurements were made at IMD Lodhi Road, which appears to be a highly urbanized area. How do the authors justify BB-based cooking activities near such location? Is regional transport important for fresh emissions? Furthermore, cooking’s contribution to PM10 is discussed, which is understandably low. However, what about PM2.5 that can be formed from the oxidation of gas-phase cooking emissions?

Minor points:

Line 86: “at” Lodhi Road.

Line 190: extra “T” at the start.

Line 264: “Direct”, do you mean “Primary” ?

Line 642: ‘’at this time of the year…” Which time of the year? This is written casually.

Figure 3: Remove the word “PMF” from all figure legends.

Figure 5: Add y-axis labels to the wind rose plots.