Multi-media Distribution, Source Apportionment, and Risk Assessment of Polycyclic Aromatic Hydrocarbons in the Forest-Grassland Transition Zone of Inner Mongolia

• • Soil PAH concentrations (mean 168.8 ng·g⁻¹, range 34.9–465.3 ng·g⁻¹) exceed those in several natural forest areas, indicating significant accumulation from combustion sources; this necessitates monitoring in remote ecosystems. • • Litter exhibited the highest PAH concentrations (mean 465.1 ng·g⁻¹, up to 2262.5 ng·g⁻¹), with 3- and 4-ring PAHs dominating, suggesting that litter acts as a major temporary sink and potential secondary source. • • Dead bark showed a concentration range of 123.3–241.6 ng·g⁻¹ and was identified as the primary accumulation medium with high carcinogenic contribution (TEQ), underscoring its role in long-term ecological risk. • • Spearman correlation analysis found no significant correlations (P > 0.05) among PAH concentrations in soil, litter, and bark, indicating independent accumulation processes and necessitating multi-media assessment for comprehensive risk evaluation.