Impact of Biomass Burning on Fine Aerosol Chemistry and Aging Processes in the Indo‐Gangetic Plain

Post‐monsoon air pollution over the Indo‐Gangetic Plain (IGP) is strongly influenced by crop‐residue burning, but how much transported, aging smoke adds particle mass and contributes to forecast underestimation remains poorly quantified. We report continuous high‐resolution time‐of‐flight aerosol mass spectrometer (HR‐ToF‐AMS) measurements in New Delhi from September–December 2024, spanning a pre‐event baseline, an intense biomass‐burning episode (12–19 November), and a winter post‐event period. During the event, non‐refractory PM 2.5 increased about four‐fold to 203.6 μg m −3 , with organic aerosol (OA) contributing 75.3% of the non‐refractory mass. Observed PM 2.5 averaged 358.9 μg m −3 , compared with an operational forecast mean of 182.4 μg m −3 , giving a mean forecast underestimation of 176.5 μg m −3 . Positive matrix factorization resolved biomass‐burning OA (BBOA, 43.4%), oxygenated OA (OOA, 37.8%), and hydrocarbon‐like OA (18.8%). Diagnostic m/z 60/K + and m/z 60/OC ratios, combined with 72 hr concentration‐weighted trajectories, identified agricultural‐residue burning over the upwind Punjab–Haryana corridor as the dominant external source. Although period‐mean O:C decreased from 0.76 to 0.65 and carbon oxidation state from +0.10 to −0.34, low‐volatility OOA increased from 9.9 to 46.6 μg m −3 , indicating that fresh smoke diluted bulk oxidation metrics while chemically processed OA remained substantial. An organic‐mass‐to‐organic‐carbon (OM/OC) mass balance gives secondary organic enhancement of 33–40 μg m −3 and nitrate enhancement of 12 μg m −3 , together accounting for approximately 25%–30% of the mean forecast underestimation. Secondary formation during smoke aging therefore contributed appreciably to the transported pollution not captured by the operational forecast.