Seasonal variations and source apportionment of PM_2.5 at urban area of Chengdu

Daily PM_2.5 samples were collected from 2009 to 2010 during a typical month of each season at an urban site of Chengdu. The mass concentration and chemical compositions (including carbonaceous species, water-soluble inorganic ions and elements) of PM_2.5 were determined. The results showed that the average PM_2.5 mass concentration was (165.1±85.1) μg·m^-3, which was 4.7 times the annual PM_2.5 National Ambient Air Quality Standard of China. The average concentrations of OC, EC and total secondary inorganic ions (SO₄^2-, NO₃^- and NH₄⁺) were (22.6±10.2) μg·m^-3, (9.0±5.4) μg·m^-3 and (62.8±44.3) μg·m^-3, accounting for 13.7%, 5.5% and 38.0% of PM_2.5 mass concentration, respectively. Higher levels of PM_2.5 mass concentration and their major chemical compositions were found in autumn and winter compared to those found in spring and summer. Positive Matrix Factorization (PMF) model was performed to identify the sources of PM_2.5 at urban Chengdu. Four sources significantly contributing to the observed PM_2.5 were: soil dust and fugitive dust, biomass burning, vehicle emissions and secondary nitrate/sulfate, with the contributions of 14.3%, 28.0%, 24.0% and 31.3% to PM_2.5 mass concentration, respectively. These four sources exhibited distinct seasonal patterns: the relative contribution of biomass burning to total PM_2.5 was relatively high for all seasons; the relative contribution of soil dust and fugitive dust increased significantly during spring; vehicle emissions contributed significantly during summer; and the highest relative contribution to PM_2.5 during autumn and winter was secondary nitrate/sulfate factor.