Incomplete Reduction of Particle-Bound PAH Toxicity under Lower PM_2.5 Conditions: Evidence from Kunming, China

Declining PM_2.5 mass concentrations do not necessarily ensure proportional mitigation of hazardous particle-bound components. We investigated 16 priority PM_2.5-bound PAHs in Kunming, China, and compared their concentrations, BaP_eq burden, particle-normalized toxicity-equivalent intensity, source-related patterns, and inhalation cancer risks between lower- and higher-PM_2.5 stages. From the higher- to lower-PM_2.5 stage, PM_2.5, ΣPAHs, and BaP_eq decreased by 32.7%, 27.4%, and 31.4%, respectively. Covariate-adjusted models confirmed significant reductions in ΣPAHs and BaP_eq of 28.8% and 33.5%, whereas BaP_eq/PM_2.5, which represents the BaP-equivalent burden of the measured particle-bound PAHs per unit mass of PM_2.5, showed no significant decrease (-7.6%, p = 0.274), and BaP_eq/ΣPAHs decreased only modestly (-6.5%, p = 0.026). The contribution of 4-6-ring PAHs remained above 81%, while BaP and dibenzo[a,h]anthracene together accounted for more than 82% of BaP_eq in both stages. PAH-PMF results, interpreted conservatively as source-related chemical patterns, indicated persistent combustion-related PAH signatures, while RF-SHAP analysis identified carbonaceous and selected metal-related components as the leading predictors of absolute BaPeq. Despite the lower particle loading, 40.3% of simulated adult ILCR values exceeded 10^-6. These results demonstrate that reductions in particle mass and absolute hazardous-component burden may coexist with persistent toxicity-equivalent intensity per unit particle mass. Joint evaluation of PM_2.5, BaP_eq, and BaP_eq/PM_2.5 can therefore provide a practical framework for toxicity-oriented management of hazardous particulate components.