[Analysis of Seasonal Variations in Chemical Characteristics and Sources of PM 2.5 During Summer and Winter in Ji'nan City].

To investigate seasonal variations in the chemical compositions of aerosols in Ji'nan City, PM _2.5 samples were collected during summer and winter in 2015. The sampling period lasted one month during each season. PM _2.5 samples were analyzed for the composition, concentration, and sources of water-soluble inorganic ions, organic carbon (OC), elemental carbon (EC), and water-soluble organic carbon (WSOC). Results showed that mass concentrations of PM _2.5 in winter were about twice those in summer, and concentration levels varied between fine and excellent. The concentrations of total water-soluble inorganic ions were also higher in winter than in summer, with SO ₄ ^2- , NO ₃ ^- , and NH ₄ ⁺ being the dominant species and well correlated with each other. NH ₄ ⁺ in PM _2.5 mostly existed in the form of (NH ₄ ) ₂ SO ₄ and NH ₄ NO ₃ in both summer and winter. There was strong secondary oxidation of SO ₂ and NO ₂ . The sulfate oxidizing rate (SOR) was higher in summer than in winter, while the nitrate oxidizing rate (NOR) showed the opposite trend. The ratio of anions to cations in both summer and winter were less than one, suggesting that PM _2.5 were slightly alkaline. The ISORROPIA-Ⅱ mode showed that acidity in winter was stronger than in summer. Concentations of OC and EC were both higher in winter than in summer. The ratios of OC to EC and WSOC to OC and estimated concentrations of secondary organic carbon (SOC) showed that secondary pollution was more serious in winter than in summer. Principal component analysis(PCA)indicated that the major sources contributing to inorganic ions were secondary oxidation and biomass burning in summer, and coal combustion and secondary pollutants formed by chemical oxidation of precursors emitted from coal combustion in winter.