A yearlong study of water-soluble organic carbon in Beijing II: Light absorption properties

- Light absorption by WSOC was found to be higher in Beijing than other regions.
- Mass absorption efficiency of WSOC showed apparent temporal and spatial variations.
- Biomass burning contribution could enhance the light absorption efficiency of WSOC.
- Precursors of WSOC could have significant influence on its MAE value.

Light absorption properties of water-soluble organic carbon (WSOC) in Beijing were investigated by 24 h-averaged fine particulate matter (PM2.5) samples collected from October 2010 to November 2011. The light absorption spectra of WSOC exhibited strong wavelength dependence such that the absorption Ångstrom exponent was approximately 7.5. The light absorption at 365 nm (Abs365), which is typically used as a proxy of water-soluble brown carbon, was found to correlate strongly with WSOC (R2 > 0.75, p < 0.01). Moreover, the correlation between Abs365 and levoglucosan (especially in fall and winter) indicated that biomass burning could contribute significantly to water-soluble brown carbon. Source apportionment with Positive Matrix Factorization (PMF) model showed that biomass burning and mixed sources contributed 58.0% and 20.8% to total Abs365, compared with 21.2% from WSOC associated with sulfate and oxalate. The mass absorption efficiency (MAE) of WSOC in Beijing showed distinct temporal variations (averaging 1.26 m2/g and 0.51 m2/g during winter and summer, respectively), and was approximately 2-3 times the values of that observed in the southeastern United States, but was substantially lower than the summertime results from Los Angeles. Influence factors responsible for the temporal and spatial variations of MAE were investigated. MAE were calculated for each PMF factor. It was found that the MAE for WSOC from biomass burning (1.19 m2/g) and mixed primary sources (2.89 m2/g) was much higher than that of WSOC associated with sulfate or oxalate (0.32-0.33 m2/g) in Beijing. In addition, it was concluded that differences in the precursors of WSOC might also be responsible for the observed variation of MAE such that WSOC associated with anthropogenic precursors are more light-absorbing compared with WSOC biogenic sources.