Year-Long Source Apportionment of PM₂ . ₅ in Delhi: Insights from One of the World's Most Polluted Environments

Abstract Delhi experiences some of the world’s highest urban fine particulate matter (PM 2.5 ) concentrations, with most previous studies focusing on short-term source apportionment. Here, we present a long-term, multi-season characterization of PM 2.5 composition taking measurement from ACSM, Xact and Aethalometer (AXA). High time-resolved measurements of non-refractory PM 2.5 (NR-PM 2.5 ), elements, and black carbon (BC) were integrated to construct a composition-resolved metric (C-PM 2.5 = NR-PM 2.5 + elements + BC). Seasonal mean C-PM 2.5 concentrations were 210.2, 103.9, 60.6, 158.9, and 156.2 µg m⁻³ during early winter, spring, summer, autumn, and late winter, respectively. Organic aerosol (OA) dominated across all seasons (40–65%), with maxima in autumn (65.3%). Secondary inorganic aerosols (SO₄²⁻, NO₃⁻, NH₄⁺, Cl⁻) peaked in winter, while chloride also exhibited episodic summer events. Elemental contributions were elevated in summer (11%), with dust prevailing in summer. BC contributed 6–14% consistently. Positive matrix factorization (PMF) of OA was performed using rolling window approach using 21 days moving window in Sofi Pro, which resolved OA into hydrocarbon-like OA(HOA), fresh and aged biomass burning OA(BBOA-Fresh and BBOA-Aged), and oxygenated OA Less-oxidized oxygenated organic aerosol (LO-OOA) and more-oxidized oxygenated organic aerosol (MO-OOA). Elemental-PMF identified eight sources, including power plants, dust, biomass burning, traffic, secondary chloride, waste incineration, Pb-rich, and industrial emissions, with fireworks appearing episodically during Diwali in autumn. Source contributions exhibited strong seasonal variability, with dust dominating in summer and power plant (Sulphur rich) emissions in winter. This study provides the first integrated seasonal perspective on long-term PM 2.5 sources and transformation processes in Delhi, offering critical insights for air quality management.