In this study, we measured polycyclic aromatic hydrocarbons (PAHs) in aerosols in Xi'an, China from 2005 to 2007, by using a modified Soxhlet extraction followed by a clean-up procedure using automated column chromatography followed by HPLC/fluorescence detection. The sources of PAHs were apportioned by using the positive matrix factorization (PMF) method. The PM(10) concentration in winter (161.1+/-66.4microgm(-3), n=242) was 1.5 times higher than that in summer (110.9+/-34.7microgm(-3), n=248). SigmaPAH concentrations, which are the sum of the concentrations of all detected PAHs, in winter (344.2+/-149.7ngm(-3), n=45) was 2.5 times higher than that in summer (136.7+/-56.7ngm(-3), n=24) in this study. These strong seasonal variations in atmospheric PAH concentration are possibly due to coal combustion for residential heating. According to the source apportionment with PMF method in this study, the major sources of PAHs in Xi'an are categorized as (1) mobile sources such as vehicle exhaust that constantly contribute to PAH pollution, and (2) stationary sources such as coal combustion that have a large contribution to PAH pollution in winter.