Molecular Evidence for the Atmospheric Transformation of Trash Burning Aerosol Into Organonitrate‐ and Organosulfate‐Rich Haze: A Case Study From the Eastern Indo‐Gangetic Plain

Abstract Open burning of solid waste is projected to contribute up to 29% of global anthropogenic PM 2.5 emissions, yet its role in secondary aerosol formation remains largely absent from current assessments. Using 21‐T Fourier transform‐ion cyclotron resonance mass spectrometry, we show that fresh trash burning organic aerosol (tbOA) in the Indo‐Gangetic plain, specifically West Bengal, India, during wintertime is dominated by more volatile, low‐oxygenation CHO species (66% of the formulae), including markers of plastic and biomass combustion. The downwind ambient aerosol in haze was dominated by less volatile, highly oxygenated organonitrates and organosulfates (>85% of the formulae), consistent with substantial atmospheric transformation. Single‐particle measurements show that freshly emitted tbOA is internally mixed with inorganic salts, implying enhanced hygroscopicity and aqueous‐phase processing as a mechanistic driver of this transformation. Our molecular‐level observations suggest a potential link between open trash burning and organonitrate‐ and organosulfate‐rich secondary aerosol in the regional winter boundary layer. Our results motivate targeted chamber experiments and regional modeling studies to better quantify the contributions and chemical evolution of trash burning emissions in air quality assessments.