PM 10 samples were collected and analyzed for polycyclic aromatic hydrocarbons (PAHs) at eight sampling sites in Great Xiamen Bay, China. Analyses of the seasonal and spatial variations of these compounds revealed the following results. Significantly high levels of PAHs were found in the winter compared to the summer, sometimes exceeding 100 ng m À3 , and the spatial variations were influenced most by the sampling site surroundings. Composition profiles of PAHs of an urban and a rural site were

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... shown to be very similar with a positive correlation coefficient larger than 0.9 at the 0.01 level of significance for the same season. Diagnostic ratios, together with principal component and multiple linear regression analysis, showed that more PAHs were from grass/ wood/coal combustion in winter than in other seasons. The ratios of benzo[a]pyrene to benzo[e]pyrene (BaP-BeP) in winter and fall were 0.6-1.7 times higher than those in spring and summer, suggesting the importance of local emissions of PAHs. The BaP-BeP ratios in Kinmen were generally lower than those in Xiamen, indicating that the aging degree of PAHs was higher in Kinmen than in Xiamen. The external input of PAHs from upwind urban and industrial areas was one of the key factors causing high levels of PAHs in PM 10 in Great Xiamen Bay in winter. Environmental impact Understanding the influence of local emission and monsoons on the seasonal variation of PAHs associated with PM10 in Great Xiamen Bay (mainly including Xiamen and Kinmen with completely different land use type, population and number of registered vehicles) is very important for regional air pollution management and control in the Western Taiwan Strait Economic Zone in China. Significantly higher levels of PAHs in PM10, as well as diagnostic ratios and HYSPLIT back trajectory, indicated that there was an external input of PAHs during the Northeastern Monsoon season. A very simple model was established to identify the influence of external input and thermal inversion on the high levels of PAHs in PM10 during the Northeastern Monsoon season.