[Light-absorbing Properties and Sources of PM_2.5 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.