Seasonal Dynamics and Toxicity of PM₂.₅ in Urban Lahore: Insights from Physicochemical and Oxidative Potential Analysis

This study presents a comprehensive physicochemical and toxicological characterization of fine particulate matter (PM₂.₅) collected from August 2024 to April 2025 at the rooftop of the Syed Babar Ali School of Science and Engineering, Lahore University of Management Sciences, using a low-cost size-selective air sampler operating at 15 L/min. Particulate matter was analyzed for morphology and size distribution using scanning electron microscopy (SEM), elemental composition via X-ray fluorescence (XRF), and oxidative potential through an acellular ascorbic acid (AA) depletion assay applied to both water- and methanol-soluble extracts. SEM analysis revealed morphologically diverse particles, including soot agglomerates, carbonaceous chains, and mineral fragments, with aerodynamic diameter shifting from 2.5 µm at 15 L/min to 1.7 µm at 30 L/min. PM₂.₅ concentrations exhibited pronounced seasonal variation, peaking in late autumn and winter (up to ~473 µg/m³) and decreasing to ~58–80 µg/m³ in spring, far exceeding World Health Organization guidelines. Oxidative potential showed similar seasonality, with higher AA depletion rates in winter, particularly in methanol extracts, suggesting dominance of organic-soluble redox-active species such as quinones and PAHs. Weak correlations between PM₂.₅ mass and oxidative potential (R² ≤ 0.015) indicate that chemical composition, rather than particle mass, governs toxicity. Transition metals including Fe, Cu, and Zn were positively associated with PM₂.₅ mass, while Mn, Pb, and as displayed element-specific trends, highlighting source variability. These findings emphasize that health risk assessments and air quality regulations should incorporate compositional metrics alongside mass concentration, particularly in regions with severe seasonal pollution episodes.