Chemical characterization and source apportionment of PM2.5 aerosols in a megacity of Southeast China

PM2.5 aerosol samples were collected during a haze–fog event in winter, as well as in spring, summer, and fall in 2013 within an urban area (Xianlin) and city center area (Gulou) of Nanjing, a megacity of SE China. The PM2.5 showed typical seasonality of waxing in winter and waning in summer or fall with annual average concentrations of 145 and 139μg/m3 in Xianlin and Gulou, respectively. Concentrations of SO42−, NO3−, NH4+, Cl−, and K+, EC, OC, secondary organic carbon, and most elements were elevated in winter. The sulfur oxidation ratio and concentrations of SO42− andCl− were significantly higher in Xianlin than Gulou (p<0.05), whereas the nitrogen oxidation ratio and NO3− concentrations were significantly higher in Gulou than Xianlin (p<0.05). A chemical mass closure construction was used to apportion PM2.5 fractions. Using the positive matrix factorization model, six source factors were identified as having contributed to PM2.5. These were secondary nitrate, road dust, sea salt and ship emissions, coal combustion, secondary sulfate, and the iron and steel industry, which contributed annual averages of 17.8±15.1, 10.6±9.53, 4.50±3.28, 12.4±9.82, 46.3±14.4, and 8.42±5.15%, respectively, to the PM2.5 mass in Xianlin, and 34.5±16.2, 7.82±7.21, 7.27±5.61, 10.5±9.35, 33.0±16.6, and 7.00±6.1%, respectively, in Gulou. Distinct seasonal patterns of the source factors in the two areas associated with the main chemical components were identified, which could be explained by various sources and meteorological conditions.