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Polycyclic aromatic hydrocarbons (PAHs) in surface sediment from city catchment area were selected to research the environmental responses of PAHs contamination under an urban-suburb gradient. Total concentrations of 16 PAHs in the samples were found from 1.8 ng/g to 4 126.3 ng/g, with the average concentration of 403.2 ng/g. The total concentrations of 16 PAHs increased with urbanization levels, which were higher in the urban area than that in the suburban area. The composition of low molecular weight PAHs was higher in the suburban area than that in the urban area. These were probably because PAHs source differed during urbanization. The results of isomer ratios of PAHs and principal component analysis coupled with multiple stepwise linear regressions showed that PAHs in Ningbo were mainly from traffic emissions and coal combustion. The proportion of traffic emissions and coal combustion mixed sources was 78% in the urban area with high-level urbanization. In contrast, the proportion of diesel-power vehicular was 42%, and coal combustion was 33.9% in the suburban area with low-level urbanization. These showed a difference in PAHs sources with a gradient of urbanization. Ecological risk analysis by the sediment quality guidelines (SQGs) showed that the potential ecological risk of 16 PAHs in surface sediment from urban catchment area was low, and a significant difference was observed in the average of effect rang low (ERL) quotient between urban and suburban areas (P<0.05). The average of ERL quotient of H3 sampling site in the urban area exceeded 1, which might raise some ecological risk. While the average of ERL quotient of sampling sites in the suburban area did not exceed 1. The toxic benzo[a]pyrene equivalents (TEQBaP) of 16 PAHs (mean value) in urban area was 3 times higher than that in suburban area, and 7 carcinogenic PAHs were the main factors to the ecological risk. [ABSTRACT FROM AUTHOR]