Daily PM2.5 samples were collected in 2014 at a suburban petrochemical industrial site and a downtown site in Lanzhou city, Northwest China. Major chemical components in PM2.5, including water-soluble ions, metal elements, and organic and elemental carbon (OC and EC) were determined. The chemical mass closure method and the ISORROPIA II thermodynamic equilibrium model were used to reconstruct PM2.5 mass and quantify the combinations of NH4+, SO42- and NO3- to PM2.5. Positive matrix factorization (PMF) model was employed to apportion potential sources of PM2.5. The annual average PM2.5 concentration was 93.7±49.6μgm-3 at the suburban petrochemical industrial site and 88.9±52.0μgm-3 at the urban site, with the highest seasonal average in winter and the lowest in summer at both sites. Mineral dust was identified as the highest contributor to PM2.5 in spring, while water-soluble inorganic ions and carbonaceous aerosols were the dominant chemical components in other seasons. The correlation relationships between OC and EC and between K+ and EC suggested that coal combustion and vehicle exhaust were the major sources of carbonaceous aerosols in Lanzhou. Six major sources were identified by the PMF model. Coal combustion, soil dust, traffic emissions, and secondary inorganic aerosols were the dominant contributors, together accounting for 82% of PM2.5 mass.
Keywords: Aerosol mass closure; Chemical composition; PM(2.5); Source apportionment; Thermodynamic equilibrium model.
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