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Five-stage size-segregated ambient aerosol particles were collected during January 2003 until October 2004 at one urban and one suburban site of Beijing to investigate the seasonal size distributions of aliphatic and polycyclic aromatic hydrocarbons. The concentrations of total n-alkanes ranging from C11 to C35 varied between 2.1 and 2446 ngm3, with higher values at the urban site than suburban site. A seasonal fluctuation was clearly evident, with higher concentrations occurring during the colder period. The carbon preference index (CPI) of the n-alkanes displayed the highest values in the spring and apparently decreased in winter. Furthermore, smaller size airborne particle-bound n-alkanes tend to have lower CPI values. In addition, the presence of pristane, phytane, n-alkylcyclohexanes, hopanes, and steranes, together with the major unresolved complex mixture (UCM) of branched and cyclic compounds, confirmed the contribution from fossil fuel use in ambient aerosol particles of Beijing. The concentrations of 18 unsubstituted PAHs (18-PAH) varied from 0.84883 ngm3, with the content at the urban site 1.16.6 times higher than those for the suburban site. The seasonal variation of 18-PAH concentrations was as follows winter > autumn > spring > summer. About 6788 of the PAHs were associated with particles having an aerodynamic diameter 2.0 m. PAHs concentration diagnostic ratios reflect that the major source of fine particles is from fossil fuel utilization. [ABSTRACT FROM AUTHOR]