Abstract:PM2.5 and PM10 were sampled in the ambient air of four national background sites (Changbaishan, Pangquangou, Shennongjia and Nanling) from north to south in China during four seasons in 2013. Filter samples were pretreated by ultrasonic extraction with acetonitrile and sixteen polycyclic aromatic hydrocarbons (PAHs) were analyzed by ultrahigh-pressure liquid chromatography (UHPLC). The concentration variation of PAHs in PM2.5 and PM10 of four background sites were 0.09~25.42ng/m3 and 0.13~30.16ng/m3, respectively. The PAHs concentration level of four background sites is similar to the other background site in China and foreign countries. The degree of spatial variation of PAHs was ranked as: Pangquangou>Changbaishan>Shennongjia>Nanling. The seasonal variation was that PAHs concentration in spring and winter of Pangquangou and Shennongjia, spring of Changbaishan, winter of Nanling were higher than others. There was a linear correlation between BaP and ∑PAHs of low concentration in both PM2.5 and PM10. PAHs were mainly composed of 4 and 5-ring PAHs (such as fluoranthene, pyrene, benzo (a) fluoranthene) in most seasons of background sites despite of the winter of Changbaishan and summer of Nanling in which the proportion of 3-ring PAHs was high. The toxicity assessment of PAHs was determined by benzo(a)pyrene equivalent for carcinogenicity (∑BaPTEF) and mutagenicity (∑BaPMEF). The result showed that the toxicity in spring and winter of Pangquangou (1.81~2.74ng/m3 and 2.92~4.36ng/m3) was much higher than that of other seasons of background sites. It is necessary to pay more attention to the relative area which Pangquangou site represents.