Microscopic Insights into Firework- and Biomass-Burning-Derived Haze Particles during the Diwali Festival in Delhi, India

Delhi, one of the world's most densely populated megacities, experiences extreme haze during the Diwali Festival─a nationwide celebration marked by intense fireworks coinciding with postmonsoon biomass burning. Although bulk measurements routinely show sharp PM2.5 spikes during Diwali, direct microscopic evidence linking specific aerosol types to these concurrent sources remains limited. Here, we combined transmission electron microscopy (TEM) with bulk chemical analysis to identify particle types and track their physicochemical evolution throughout the Diwali period. Before Diwali, aerosols were dominated by potassium (K)-rich particles (25%), carbonaceous particles (primary organic aerosol (POA) and soot, 29%), and their internal mixtures (K-POA/soot, 37%), with frequent spherical POA (i.e., tar balls), indicating a strong biomass-burning influence. During Diwali, particle populations shifted abruptly to a pyrotechnic signature of fireworks, characterized by abundant Al₂O₃ monomers (30-300 nm) and their agglomerates, either as bare (36-40%) or uniformly coated by K₂SO₄ (Al₂O₃-K, 46-47%). After Diwali, ultrafine Al₂O₃ particles (<100 nm) persisted and underwent coagulation with aged biomass-burning particles, forming distinctive Al₂O₃-K-POA/soot internal mixtures. Therefore, Al₂O₃ nanoparticles can serve as a tracer of fireworks and were further internally mixed with carbonaceous particles derived from biomass burning during severe haze events of Diwali. These metal-containing particles warrant particular attention because of their potential toxicity and adverse respiratory health effects in the densely populated megacity.