Comparative analysis of winter composite-PM2.5 in Central Indo Gangetic Plain cities: Combined organic and inorganic source apportionment and characterization, with a focus on the photochemical age effect on secondary organic aerosol formation

To gain insights into air quality dynamics, a high-end instrument such as High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) alongside an Aethalometer was used to measure the Composite PM_2.5 (C-PM_2.5) in Lucknow and Kanpur during winter. It encompasses non-refractive PM_2.5 (NR-PM_2.5) and Black Carbon (BC) mass concentrations. Significant variation was noted in average C-PM_2.5 concentrations at both sites, as 168.8 ± 61.3 μg m⁻³ in Lucknow and 90.7 ± 25.7 μg m⁻³ in Kanpur. Organics emerged as the predominant component, constituting ∼55%–65% of the C-PM_2.5 mass, followed by inorganics (24% and 30%) and BC (20% and 8%). The present study employs Positive Matrix Factorization (PMF) on combined organic and inorganic source apportionment, resolving eight and seven source factors at Lucknow and Kanpur, respectively. Both sites exhibit significant contributions from solid-fuel combustion organic aerosol (SFC-OA), with ∼11% in Lucknow and 8% in Kanpur. However, SFC-OA mass concentration in Lucknow, at 24.67 μg m^-³, is nearly double that in Kanpur (12.05 μg m^-³). This was likely due to domestic coal burning in the nearby households, unregulated open burning, and burning of garbage on roadsides. The study shows that both sites were affected by oxidized biomass burning OA (O-BBOA) emissions, with increased concentration during the night due to dark oxidation by NO₃ radicals. The diurnal variation of secondary organic aerosols (SOA), such as O-BBOA and semi-volatile oxygenated OA (SVOOA), shows increasing concentration during daytime hours. Therefore, the photochemical aging (t_a) role in SOA formation was analyzed, and it was revealed that the formation might primarily be driven by photochemical oxidation. Additionally, two inorganic-rich factors, sulfate and nitrate-related OA (SO₄-OA and NO₃-OA) at both sites and additional ammonium chloride-related OA (NH₄Cl-OA) resolved at Lucknow. Our study shows that NO₃-OA and SO₄-OA formation was dominated by aqueous phase processes due to high relative humidity and decline in concentration with increasing t_a (t_a > 30 h) during winter.