Compositional transition and coupled formation pathways of PM_2.5 pollution in a typical industrial city: An empirical...

Under ongoing emission-control measures, PM_2.5 pollution in industrial cities is increasingly shaped by changes in chemical composition and secondary aerosol formation. This study investigates the compositional transition of PM_2.5 and the coupled formation mechanisms of secondary aerosols in Handan, a typical industrial city in northern China, from 2018 to 2024. Based on long-term PM_2.5 samples capturing daytime-nighttime differences across heating periods (HP) and non-heating periods (NHP), multi-scale analyses were performed using positive matrix factorization (PMF), the thermodynamic equilibrium model ISORROPIA-II, and SEM/TEM-EDS. Results reveal a fundamental shift from a sulfate-dominated to a nitrate-driven regime for the PM_2.5 pollution. In 2024, sulfate concentrations plummeted from 26.37 μg m^-3 in 2018 to 0.70 μg m^-3, while the NO₃^-/SO₄^2- ratio rose from 1.5 to above 3.0. Industrial and mobile sources accounted for 36% and 21% of PM_2.5, respectively, emerging as the dominant contributors over the study period. Secondary organic carbon (SOC) showed pronounced aging, with SOC/OC ratios reaching 60-86% during heavy pollution episodes. The dominant SOC formation mechanisms varied seasonally: aqueous-phase dark reactions prevailed during heating periods, whereas photochemical oxidation prevailed during the non-heating periods. Thermodynamic simulations indicate that aerosol pH increased from weakly acidic (approximately 5.8) to near-neutral/weakly alkaline (approximately 7.5). The pH transition, together with suppression of the aerosol liquid water content (ALWC) growth, is identified as a key factor in air quality improvement. Multi-scale observational and microscopic evidence corroborate the synergistic nitrate-ALWC-SOC pathway consistent with haze evolution. Comparison with observations from other industrial cities suggests that the transition towards nitrate-dominated pollution is commonly accompanied by carbonaceous aerosol aging. The results implicate ammonia availability and seasonal temperature variability as important factors influencing SOC formation.