Compositional transition and coupled formation pathways of PM2.5 pollution in a typical industrial city: An empirical study based on Handan, China.

Abstract

Under ongoing emission-control measures, PM2.5 pollution in industrial cities is increasingly shaped by changes in chemical composition and secondary aerosol formation. This study investigates the compositional transition of PM2.5 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 PM2.5 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 PM2.5 pollution. In 2024, sulfate concentrations plummeted from 26.37 μg m-3 in 2018 to 0.70 μg m-3, while the NO3-/SO42- ratio rose from 1.5 to above 3.0. Industrial and mobile sources accounted for 36% and 21% of PM2.5, 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.