| 15 Apr 2026
Chemical Characterization and Source Apportionment of Carbonaceous Aerosols during Post-Monsoon Biomass Burning and Diwali at an Upwind Site of Delhi
Abstract. Sonipat, located ~40 km northwest of Delhi, lies along the principal transport corridor linking post-monsoon agricultural burning regions of Punjab–Haryana with Delhi and serves as an intermediate receptor for regional pollution. We conducted intensive high time resolution measurements of composition-based PM₂.₅ (non-refractory PM₂.₅ plus black carbon) from 25 October to 15 November 2023 using a ToF-ACSM and an Aethalometer to characterize carbonaceous aerosol sources during the biomass-burning period. Two severe haze episodes occurred, with PM₂.₅ exceeding 300 µg/m³. Organic aerosol dominated the submicron mass (~65 % of non-refractory PM₂.₅), with daily mean concentrations peaking near 140 µg/m³ during the first haze episode. Positive Matrix Factorization resolved five components including hydrocarbon-like, biomass-burning, and solid-fuel combustion organic aerosol, and two oxygenated fractions representing semi-volatile and low-volatility aged aerosol. Secondary organic aerosol accounted for ~57–60 % of organic aerosol mass, with low-volatility oxygenated organic aerosol reaching 42.8 µg/m³ during peak haze, indicating substantial regional aging and accumulation. Biomass-derived black carbon contributed ~78 % of total black carbon (mean 10.9 µg/m³), far exceeding fossil-fuel contributions (~3.1 µg/m³). Trajectory and wind analyses consistently identified northwestern agricultural regions as dominant sources with minor traffic influence, indicating that extreme carbonaceous aerosol over Delhi-NCR largely forms outside the urban core through regional transport biomass and solid-fuel combustion emissions combined with sustained secondary processing, highlighting the need for coordinated airshed-scale emission reductions across the Indo-Gangetic Plain.
The manuscript is a 54-page Egusphere preprint posted on 15 April 2026 and presents high-time-resolution ToF-ACSM and Aethalometer measurements from Sonipat during 25 October–15 November 2023, with PMF, Aethalometer-model BC apportionment, and trajectory/wind analyses. Its central claim is that severe post-monsoon haze reaching Delhi-NCR is already strongly carbonaceous and regionally processed before entering the urban core.
The study is timely, the site is strategically important, and the dataset appears valuable. The paper’s strongest contribution is its framing of Sonipat as an upwind receptor that can separate regional transport from Delhi-core emissions, and the reported dominance of OA and biomass-related BC is potentially important for air-quality policy in the Indo-Gangetic Plain. However, several key methodological choices and interpretive steps need to be tightened before publication, especially around PMF constraints, uncertainty quantification, BC source apportionment assumptions, and the degree to which some conclusions go beyond what the evidence directly supports.
Major comments
The methods say SoFi was used, with FPEAK exploration plus displacement and bootstrap analyses, and the results mention a five-factor solution with a constrained HOA profile using a randomized a-value between 0 and 0.5. But the manuscript text shown here does not give the reader enough quantitative evidence to judge whether the chosen solution is truly robust: for example, how many factor numbers were tested, how Q changed, how often bootstrap runs reproduced the selected factors, whether DISP indicated swaps, or how sensitive source contributions were to the HOA constraint. That is especially important because the paper’s interpretation depends heavily on separating HOA vs SFCOA and fresh vs aged oxygenated OA. I would ask the authors to add a compact but explicit robustness section in the main paper, not only in supplementary material.
The manuscript uses fixed absorption Ångström exponents of 1.0 for fossil fuel and 2.0 for biomass burning. That is common practice, but it is also a known source of uncertainty, especially during complex mixed-source periods such as post-monsoon haze and Diwali. The reported result that eBCbb accounts for about 78% of BC overall and that eBCff falls to 0.03 µg m⁻³ during H2 is striking. Because such values can be quite sensitive to the assumed exponents, the paper should include a sensitivity analysis showing how the biomass/fossil split changes across a plausible AAE range. Without that, the qualitative conclusion may still be reasonable, but the quantitative confidence appears overstated.
The paper argues that haze over Delhi-NCR “largely forms outside the urban core” and that city-level controls alone cannot effectively mitigate peak PM. The transport evidence is strong enough to support an important regional component, but the phrase “largely forms outside the urban core” is stronger than what a single upwind receptor plus PMF/CWT can fully establish. The results show that Sonipat receives a major regional combustion plume before mixing with Delhi emissions, which is already a valuable conclusion. I would suggest recasting policy claims to emphasize that regional controls are necessary in addition to urban controls, rather than appearing to diminish the relevance of urban sources altogether.
I did not find a real limitations discussion in the main text. This paper especially needs one. Relevant limitations include: ACSM measures non-refractory PM and misses refractory species; the source apportionment pertains to one site and one season; PMF factor labels are not unique; Aethalometer source apportionment depends on fixed AAEs; and CWT identifies likely source regions but not emissions inventories or source strengths directly. A frank limitations section would strengthen the paper, not weaken it.
Minor comments
The manuscript briefly shifts into first person in a way that reads unpolished for a research article: “I have tried to answer some of those questions.” This should be rewritten in standard scientific style.
The methods would also read better with a clearer separation between what is reproduced from the prior Rathore et al. study and what is newly done here. Right now, several instrumental details are deferred to Rathore et al. (2025), which is understandable, but the present paper should still be as self-contained as possible for readers and reviewers.