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Daily PM_2.5 samples were collected during winter 2012 and summer 2013 at an urban site in Lanzhou and were analyzed for chemical compounds including water soluble inorganic ions (WSIN), trace elements, water soluble organic carbon (WSOC), carbonaceous species (OC/EC), polycyclic aromatic hydrocarbons (PAHs), and humic-like substances (HULIS). The seasonal-average reconstructed PM_2.5 mass was 120.5 μg m^− 3 in winter and 34.1 μg m^− 3 in summer. The top three groups of species in PM_2.5 were OC (35.4 ± 13.9 μg m^− 3), WSIN (34.89 ± 14.21 μg m^− 3), and EC (13.80 ± 5.41 μg m^− 3) in winter and WSIN (11.25 ± 3.25 μg m^− 3), OC (9.74 ± 3.30 μg m^− 3), and EC (4.44 ± 2.00 μg m^− 3) in summer. EC exceeded SO₄^2 − on most of the days. Several anthropogenic produced primary pollutants such as PAHs, Cl⁻, Pb, Cd and OCpri were 4–22 times higher in winter than summer. Carcinogenic …