Multi-media Distribution, Source Apportionment, and Risk Assessment of Polycyclic Aromatic Hydrocarbons in the Forest-Grassland Transition Zone of Inner Mongolia
This study investigated the occurrence, sources, and ecological risks of polycyclic aromatic hydrocarbons (PAHs) in soil, litter, and bark samples collected from the forest-grassland transition zone of Inner Mongolia. A total of 12 PAHs were detected in soil, with concentrations ranging from 34.9 to 465.3 ng·g⁻¹ (mean 168.8 ng·g⁻¹), predominantly 3–5 ring compounds. Litter contained 14 PAHs at concentrations between 106.4 and 2262.5 ng·g⁻¹ (mean 465.1 ng·g⁻¹), dominated by 3- and 4-ring PAHs. Both living and dead bark exhibited 14 PAHs, with concentration ranges of 119.2–240.0 ng·g⁻¹ and 123.3–241.6 ng·g⁻¹, respectively, mainly composed of 4-ring PAHs. Spearman correlation analysis revealed no significant correlations among PAH concentrations across the three media (P > 0.05). Source apportionment using diagnostic ratios and principal component analysis indicated that soil PAHs primarily originated from biomass, coal, and gasoline combustion; litter PAHs from petroleum volatilization and coal/natural gas combustion; and bark PAHs from petroleum volatilization and fossil fuel combustion, with high-molecular-weight PAHs dominating. Ecological risk assessment using the risk quotient (RQ) method showed that soil PAHs posed low overall ecological risk, though certain individual PAHs exhibited higher risk. The toxic equivalent (TEQ) method indicated that dead bark was the primary accumulation medium with high carcinogenic contribution, posing elevated ecological risk. Although litter and living bark had lower PAH concentrations, their long-term accumulation effects warrant attention. These findings provide crucial scientific evidence for understanding the environmental behavior and potential risks of PAHs in cold, high-latitude regions of northern China.