Chemical characteristics and source apportionment of PM_2.5 in Lanzhou, China

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 120.5μgm^-3 in winter and 34.1μgm^-3 in summer. The top three groups of species in PM_2.5 were OC (35.4±13.9μgm^-3), WSIN (34.89±14.21μgm^-3), and EC (13.80±5.41μgm^-3) in winter and WSIN (11.25±3.25μgm^-3), OC (9.74±3.30μgm^-3), and EC (4.44±2.00μgm^-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 substances such as Arsenic, BaP, Pb, and Cd in PM_2.5 exceeded the WHO guideline limits by 274%, 153%, 23% and 7%, respectively. Positive Matric Factorization analysis identified seven source factors including steel industry, secondary aerosols, coal combustion, power plants, vehicle emissions, crustal dust, and smelting industry, which contributed 7.1%, 33.0%, 28.7%, 3.12%, 8.8%, 13.3%, and 6.0%, respectively, to PM_2.5 in winter, and 6.7%, 14.8%, 3.1%, 3.4%, 25.2%, 11.6% and 35.2% in summer. Smelting industry and steel industry were identified for the first time as sources of PM_2.5 in this city, and power plant was distinguished from industrial boiler and residential coal burning.