南京北郊PM2.5中有机组分的吸光性质及来源

对南京北郊2018年9月~2019年9月PM_2.5中有机组分的吸光性质进行了研究,并利用PM_2.5化学组成及主成分分析法分析该地区吸光性有机碳（棕碳,brown carbon,BrC）的主要来源.结果表明,水溶性有机碳（water-soluble organic carbon,WSOC）和甲醇可提取有机碳（methanol extractable organic carbon,MEOC）在365 nm处光吸收系数（Abs_365,w和Abs_365,m）的平均值分别为（3.22±2.18）Mm^-1和（7.69±4.93）Mm^-1.Abs_365,w和Abs_365,m分别与WSOC（r=0.72,P<0.01）和MEOC（r=0.62,P=0.04）的质量浓度显著相关,均表现为冬高夏低,夜高昼低的时间变化特征.这可归结于冬季和夜间的气象特征（例如边界层高度降低和大气稳定度升高）、冬季一次源排放的增加以及夏季和白天更强的"光漂白作用".Abs_365,m/Abs_365,w的年均值（2.60±0.92）远高于MEOC/WSOC（质量浓度比值,1.37±0.30）,表明MEOC中非水溶性组分的吸光作用更强,在BrC的吸光作用中占主导地位.WSOC、MEOC、Abs_365,m和K⁺均未表现出强相关性（r<0.60）,因此生物质燃烧不是该地区BrC的主要一次来源.WSOC和MEOC质量吸收效率（MAE_365,w和MAE_365,m）及其比值（MAE_365,m/MAE_365,w）的季节变化和Abs₃₆₅相同.MEOC中非水溶性组分的MAE₃₆₅［（4.10±5.15）m²·g^-1］分别是MAE_365,w和MAE_365,m的6.0和2.9倍,支持BrC的吸光作用受非水溶性有机组分主导这一推断.和WSOC的埃氏吸收指数（Å_WSOC）相比,MEOC的埃氏吸收指数（Å_MEOC）随时间变化更显著,这可能与非水溶性吸光组分排放的季节变化有关.主成分分析结果显示,本研究PM_2.5中有机组分的吸光作用主要来源于二次形成过程和人为活动相关的一次排放,而不是生物质燃烧.

Light-absorbing Properties and Sources of PM2.5 Organic Components at a Suburban Site in Northern Nanjing

The light absorption of organic components in PM_2.5 was investigated at a suburban site in northern Nanjing from September 2018 to September 2019, and PM_2.5 compositional data and principal component analysis (PCA) were used to identify the sources of light-absorbing organic carbon (brown carbon, BrC). The results showed that the average light absorption coefficients of water-soluble organic carbon (WSOC) and methanol extractable organic carbon (MEOC) were (3.22±2.18) Mm^-1 (Abs_365,w) and (7.69±4.93) Mm^-1(Abs_365,m), respectively. Significant correlations were observed between Abs_365,w and mass concentrations of WSOC (r=0.72, P<0.01) and between Abs_365,m and mass concentrations of MEOC (r=0.62, P=0.04). Both Abs_365,w and Abs_365,m exhibited seasonal variations, with higher values during winter than during summer,and higher diel variations at night than during the day. This can be attributed to meteorological characteristics during the winter and nighttime, i.e., decreased boundary layer height and increased atmospheric stability, enhanced primary emissions in winter,and stronger photobleaching effects during the summer and during the day. The annual average Abs_365,m/Abs_365,w ratio (2.60±0.92) was much larger than the average mass ratio of MEOC/WSOC (1.37±0.30), indicating that the water-insoluble fraction of MEOC had a stronger light absorption effect and dominated BrC absorption. No strong correlation (r<0.60) was observed between WSOC, MEOC, Abs_365,m, and mass concentrations of K⁺, indication that biomass burning was not the main source of BrC in the study location. The mass absorption efficiency of WSOC (MAE_365,w) and MEOC (MAE_365,m) and their ratios (MAE_365,m/MAE_365,w) showed similar seasonal variations to Abs₃₆₅. The average MAE₃₆₅ value of the water-insoluble fraction of MEOC (4.10±5.15) m²·g^-1 was 6.0 and 2.9 times higher than that of MAE_365,w and MAE_365,m, respectively, suggested that BrC absorption was primarily attributable to water-insoluble components. In comparison to the absorption Ångström exponent of WSOC (Å_WSOC), Å_MEOC displayed marked temporal variability, which might be related to the seasonal variation in the emission of water-insoluble chromophores. According to the PCA results, the light absorption of PM_2.5 organic was mainly attributed to secondary formation and anthropogenic primary emissions rather than biomass burning.