Unveiling the chemical composition and sources in PM2.5 at an urban and sub-urban site in Indo-Gangetic Plain: Insights from Biomass Markers

Anthropogenic biomass burning (ABB) is a major yet least understood source of atmospheric pollution, with significant implications for air quality, visibility, atmospheric chemistry, the Earth’s radiation budget and biogeochemical cycling. This study examines the chemical composition of aerosols during biomass burning (BB) and non-biomass burning (NBB) periods to assess their impact on aerosol composition and their contribution in generating oxidative stress. Sampling of PM₂.₅ and PM₁₀ was carried out at two contrasting sites one of the highly polluted cities of Indo-Gangetic Plain (IGP), Agra: Dayalbagh (suburban) and Rambagh (urban), from December, 2022 to November, 2024. The samples were analyzed for dicarboxylic acids (DCAs), sugars, organic and elemental carbon (OC/EC), water-soluble inorganic ions (WSIIs), metals and polycyclic aromatic hydrocarbons (PAHs). Moderate Resolution Imaging Spectroradiometer (MODIS) and Fire Information for Resource Management System (FIRMS) data identified intense fire hotspots over northwestern India (Punjab–Haryana) during BB, coinciding with elevated particulate concentrations. The mean concentration of PM₂.₅ and PM₁₀ increased considerably during BB (PM₂.₅: 101.1 ± 74.4 µg m⁻³ at Dayalbagh, 120.0 ± 57.5 µg m⁻³ at Rambagh; PM₁₀: 161.2 ± 67.3 and 184.4 ± 61.7 µg m⁻³, respectively) compared to NBB. The Pearson correlation analysis showed that carbonaceous species and biomass tracers (DCAs, K⁺, levoglucosan) showed strong positive correlations (r > 0.8), confirming the influence of agricultural residue and biofuel combustion. Secondary ions (SO₄²⁻, NO₃⁻, NH₄⁺) displayed enhanced interrelationships (r = 0.75–0.77) during NBB, indicating increased secondary aerosol formation. The oxidative potential, assessed using the dithiothreitol (DTT) assay, exhibited markedly higher activity in fine particles (r² = 0.70 and 0.79) and during BB (DTTv: 18.2 ± 10.1 pmol min⁻¹ m⁻³ at Dayalbagh, 17.1 ± 15.0 pmol min⁻¹ m⁻³ at Rambagh) compared to NBB (7.0 ± 4.0 and 10.0 ± 2.4 pmol min⁻¹ m⁻³, respectively). The correlation between DTTv and biomass tracers (oxalic acid (C2), malonic acid (C3), adipic acid (C6), levoglucosan (Lev), arabitol (Arab); r = 0.50-0.83), OC/EC (r = 0.52-0.70), metals (Fe, Mn, K, Na, Ni, Mn, Cr; r = 0.52-0.64) during BB period suggest higher redox activity during this period. Positive Matrix Factorization (PMF) identified four dominant sources: biomass burning (31–35%), vehicular emissions (26–30%), industrial activities (18–22%), and crustal dust (10–13%). Thus, BB emissions significantly enhanced PM loadings and oxidative potential, posing elevated health risks. The findings highlight the synergistic role of biomass combustion and urban emissions in amplifying aerosol toxicity and degrading air quality over the IGP.