Based on the particulate matter and meteorological measurements at the Dongtan Atmospheric Composition Observing Station (shortened Dongtan Station) from 2008 to 2015, the arriving air mass is classified, the PM2.5 concentration level and its annual variation are examined, and the potential source of higher level of PM2.5 is also identified in this article. The long-term variation of PM2.5 concentration presents an in-significant trend from 2008 to 2015, but the percentage of fine particles (PM2.5) keeps increasing. The ratio of PM2.5/PM10 increases from 0.84 to 0.92, indicating more and more data of secondary aerosols are observed at Dongtan Station. The air mass of 8-year big sample data at Dongtan Station can be aggregated into 3 types of back trajectories, that is, land, ocean, and land/ocean mixing types, accounting for 32.0%, 38.8% and 29.3% respectively. Among them, the PM2.5 background mass concentration ranges stably within 11-15 μg·m-3 in ocean air mass, 〖JP2〗but 29-56 μg·m-3 in land air mass, showing largely seasonal variability. The potential source of relatively higher PM2.5 observed at Dongtan Station presents clear seasonal transition from areas north to Shanghai including North China and, Huanghuai regions as well as Jiangsu and Anhui provinces in autumn and winter to southern Yangtze River Delta region including northern Zhejiang Province and its seaboard extending to Fujian Province in summer by PSCF analysis. In general, higher PM2.5 loading at Dongtan Station is mostly contributed by air mass from Shanghai and its neighboring city clusters including Suzhou, Wuxi, Changzhou, Hangzhou, Jiaxing and Huzhou etc. It is noted that air mass recycled from Yellow Sea and Bohai Sea is also an important source area for the elevated PM2.5 observed at Dongtan Station in spring, autumn and winter.