Thirty six PM2.5 samples were collected at a roadside site of the heaviest traffic road in Qingdao. a coastal mega city in North China, from March 2004 to January 2005 to perform a detailed characterization of 16 priority polycyclic aromatic hydrocarbons (PAHs) proposed by the USEPA and n-alkanes. For PAHs, the significant increase in winter was observed with average PAH level of 32.3, 11.5, 48.9 and 263 ng m(-3) for spring, summer, autumn and winter, respectively. The average concentrations of n-alkane in PM2.5 at ground level were 232, 121, 309 and 369 ng m(-3) in spring, summer, autumn, and winter, respectively. The pyrogenic PAHs were mainly from the coal burning, and the liquid fossil fuel combustion was their second contribution even at the roadside of a busy street with heavy traffic in Qingdao. Petroleum residues were the dominant source for the n-alkanes. Principal component analysis results indicated that all the measured PAHs but naphthalene and those C-17-C-26 n-alkanes could be attributed to the fossil fuel burning while long-chain n-alkanes (C-27-C-35) were mainly derived from higher plant waxes. Good correlations between PAHs and n-alkanes in summer, autumn and winter suggested that they were mainly from local sources. The poor correlation of n-alkanes and PAHs in spring could be mainly due to the shift from the space heating season to non-heating season during the sampling period. This work indicated that the roadside environment in Qingdao was more deteriorated compared with non-roadside environment in other mega cities in China. (C) 2009 Elsevier B.V. All rights reserved.