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.