[Analysis of Pollution Characteristics and Sources of PM_2.5 Chemical Components in Chengdu in Winter]

Day-night PM_2.5 samples were continuously collected in Chengdu from January 1 to 20, 2017, and the concentrations of major chemical components (water-soluble ions and carbonaceous components) were measured in the laboratory. During the observation period, the average mass concentration of PM_2.5 was (127.1±59.9) μg·m^-3. The mass concentration of water-soluble ions was (56.5±25.7) μg·m^-3 and SO₄^2-, NO₃^-, and NH₄⁺ were the most dominant ions with a concentration of (13.6±5.5), (21.4±12.0), and (13.3±5.7) μg·m^-3, respectively, accounting for 85.6% of the water-soluble ions. The average mass concentrations of organic carbon (OC) and elemental carbon (EC) were 34.0 and 6.1 μg·m^-3, respectively, accounting for 26.8% and 4.8% of the PM_2.5 mass concentration, respectively. The comparison of the average day-night concentration shows that the daytime and nighttime mass concentrations of PM_2.5 are (120.4±56.4) and (133.8±64.0) μg·m^-3, respectively, and that the nighttime pollution is more serious. The SO₄^2-, NO₃^-, and NH₄⁺ concentrations are higher during the day than at night, which is related to daytime light, which promotes the formation of secondary ions. The Cl^-, K⁺, OC, and EC concentrations increase significantly, which may be affected by increased emissions from coal and material combustion. Based on the literature review and comparison of the winter chemical composition of PM_2.5 in Chengdu in recent years, the SO₄^2- concentration significantly decreases from 50.6 μg·m^-3 in 2010 to 13.6 μg·m^-3 in 2017. The NO₃^- concentration changes little; it is maintained at~20 μg·m^-3. The analysis of the acid-alkali ion balance shows that PM_2.5 in Chengdu is alkaline due to the relative overgrowth of NH₄⁺, which is different from previous partially acidic results. The average value of NO₃^-/SO₄^2- is 1.57. Mobile sources have a greater impact on the PM_2.5 pollution in Chengdu in winter. The correlation coefficients of OC and EC between daytime and nighttime are 0.82 and 0.90, respectively (P<0.01), which indicates that the OC and EC sources are consistent. The SOC estimation shows that the SOC concentrations during the day and night are 8.5 μg·m^-3 and 11.9 μg·m^-3, respectively, accounting for 28.1% and 30.8% of the OC, respectively. The K⁺/EC average value is 0.31 and the correlation coefficient between K⁺ and OC is 0.87 (P<0.01), indicating that biomass combustion has a certain influence on the carbonaceous aerosol in Chengdu in winter. The principal component analysis shows that the winter PM_2.5 in Chengdu mainly originates from combustion sources (coal burning, biomass burning, etc.), secondary inorganic sources, and soil and dust sources. The contribution rates are 32.8%, 34.5%, and 21.5%, respectively.