PM_2.5 and its main chemical components have been investigated during six air pollution period across four seasons from Jun. 16, 2016 to Jun. 15, 2017 at a suburban site in Beijing, China. Daily PM_2.5 mass concentrations were measured with associated ions, OC, EC, 16 PAHs and seven heavy metals. Mass closure analyses were conducted using measured and reconstructed chemical components. Chemical ratios were used to identify possible pollution sources, while principal component analysis (PCA) coupled with multiple linear regression (MLR) method was used to estimate the source contribution to PM_2.5 mass. The annual mean PM_2.5 concentration (96.7 μg m⁻³) is about 2.8 times the national level-2 standard limit, with apparent seasonal variation of daily exceedances as winter > spring > autumn > summer. Generally, higher levels are observed in winter and autumn than in spring and summer for NO₃⁻, SO₄²⁻, NH₄⁺, Cl⁻, K⁺, OC, EC, PAHs and metals Zn, As, Cu, Pb, along with higher concentrations of NO_x, CO and SO₂. Secondary ions and carbon fractions are most abundant over all seasons, except levels of dust related species Ca²⁺, Mg²⁺, Ca, Fe and Mn are also high in spring. The reconstructed results show that PM_2.5 consists of mostly organic compounds (26–38%) and ammonium salts (NH₄NO₃ + (NH₄)₂SO₄) (43–53%) in winter and autumn, while summer is dominated mainly by ammonium salts (81%). In spring, very high PM_2.5 episodes are occasionally composed of large amount of soil/road dust and gypsum particles (23–55%) due to floating dust events with drier weather conditions and human activities. Fuel combustions and secondary sources are responsible for ∼70% of PM_2.5 mass while mobile source is mostly a regional origin.