Chemical composition characteristics and potential source areas of PM₂.₅ across different pollution levels in a typical industrial basin city of northern China

Background PM2.5 pollution persists in industrial basin cities of northern China, yet the evolution of its chemical composition across pollution levels remains unclear. Methods Based on one year (2025) of PM₂.₅ monitoring and chemical composition data for Linfen City in the Fenwei Plain of northern China, this study characterized water-soluble ions and carbonaceous components across five pollution levels and examined a representative heavy-pollution episode, with the aim of clarifying the evolution of PM₂.₅ composition and its exposure relevance in an industrial basin city. Results Secondary inorganic aerosols (SIA: SO₄ 2− , NO₃ − , and NH₄⁺) dominated the water-soluble ions, accounting for 85.19% of the total ionic concentration; the total SIA concentration increased from 5.32 μg m −3 on excellent days to 85.69 μg m −3 in the single valid heavily polluted sample, raising the SIA share of PM₂.₅ from 32.88% to 55.29%. The sulfur and nitrogen oxidation ratios increased from 0.214 to 0.757 and from 0.059 to 0.525, respectively, indicating intensified secondary transformation, although these increases partly reflected declining SO₂ and NO₂ concentrations during the Spring Festival holiday, when firework emissions (K⁺ up to 15.41 μg m −3 , 29 times the annual mean) and the industrial shutdown complicated source attribution. Ion-balance calculations showed an excess of ammonium equivalents over sulfate and nitrate equivalents in the reported summaries (CE/AE 1.12–1.43). The secondary organic carbon (SOC) fraction of organic carbon (OC) decreased from 52.53% during the clean period to 45.41% during the polluted period as primary organic carbon (POC) increased more rapidly. Backward trajectories from southern Linfen (Cluster 1, 38.94%) and pathways extending toward Shaanxi and Ningxia (Cluster 3, 27.43%) jointly accounted for 66.37% of trajectories. Conclusion The heavy-pollution composition sample corresponded to approximately 155 μg m −3 p.m.₂.₅, about ten times the WHO 24-h guideline. These results quantify the coupled roles of secondary formation, combustion emissions, regional transport, and holiday effects in basin-city haze and provide exposure-relevant evidence for coordinated air-quality management across the Fenwei Plain.