成都冬季PM_(2.5)化学组分污染特征及来源解析

Analysis of Pollution Characteristics and Sources of PM_(2.5) Chemical Components in Chengdu in Winter

2017年1月1~20日在成都地区分昼夜对PM_(2.5)进行连续膜样品采集,并在实验室测定了其主要化学组分(水溶性离子和碳质组分)的质量浓度.观测期间,PM_(2.5)的平均质量浓度为(127.1±59.9)μg·m^(-3);总水溶性离子的质量浓度为(56.5±25.7)μg·m^(-3),其中SO2-4、NO-3和NH+4是最主要的离子,质量浓度分别为(13.6±5.5)、(21.4±12.0)和(13.3±5.7)μg·m^(-3),一共占到了水溶性离子的85.6%;有机碳(OC)和元素碳(EC)的平均质量浓度分别为34.0μg·m^(-3)和6.1μg·m^(-3),分别占PM_(2.5)质量浓度的26.8%和4.8%.昼夜污染对比显示,PM_(2.5)白天和夜晚质量浓度分别为(120.4±56.4)μg·m^(-3)和(133.8±64.0)μg·m^(-3),夜间污染更为严重.SO2-4、NO-3和NH+4白天浓度高于夜间,这与白天光照促进了二次离子的形成有关;而Cl-、K+、OC和EC浓度夜间明显升高,可能是受夜间煤和生物质燃烧排放增加的影响.通过对近年来成都冬季PM_(2.5)化学组分的研究进行文献总结和比较后发现,SO2-4浓度显著降低,从2010年的50.6μg·m^(-3)降低到2017年的13.6μg·m^(-3);而NO-3浓度变化不大,维持在20μg·m^(-3)左右.PM_(2.5)中离子酸碱平衡分析表明,成都冬季PM_(2.5)由于NH+4的相对过剩而呈现出碱性,与以往呈偏酸性结果存在差异.对成都冬季NO-3/SO2-4的比值进行计算,NO-3/SO2-4平均值为1.57,表明移动源对PM_(2.5)污染影响更大.OC与EC的相关性表明,白天和夜间OC与EC的相关系数分别为0.82和0.90(P<0.01),OC与EC来源具有一致性.SOC估算结果显示,白天和夜间SOC浓度分别为8.5μg·m^(-3)和11.9μg·m^(-3),占到OC的28.1%和31.8%.K+/EC平均值为0.31,并且K+与OC之间相关系数为0.87(P<0.01),说明生物质燃烧对成都冬季碳质气溶胶有一定影响.主成分分析表明,成都冬季PM_(2.5)主要来源于燃烧源(燃煤、生物质燃烧等)、二次无机污染源以及土壤和扬尘源,其贡献率分别为32.8%、34.5%和21.5%.

Day-night PM2.5 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 PM2.5 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 PM2.5 mass concentration, respectively. The comparison of the average day-night concentration shows that the daytime and nighttime mass concentrations of PM2.5 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 PM2.5 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 PM2.5 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 PM2.5 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 PM2.5 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.