Mass absorption efficiency of elemental carbon and water-soluble organic carbon in Beijing, China
1State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, China
2College of Environmental Sciences and Engineering, Peking University, Beijing, China
3Key Laboratory of Computational Geodynamics, College of Earth Science, Graduate University of Chinese Academy of Sciences, Beijing, China
4School of Earth and Atmospheric Sciences, Georgia Inst. of Technology, Atlanta, GA, USA
5School of Civil and Environmental Engineering, Georgia Inst. of Technology, Atlanta, GA, USA
Abstract. The mass absorption efficiency (MAE) of elemental carbon (EC) in Beijing was quantified using a thermal-optical carbon analyzer. The MAE measured at 632 nm was 8.45 ± 1.71 and 9.41 ± 1.92 m2 g−1 during winter and summer, respectively. The daily variation of MAE was found to coincide with the OC (organic carbon) abundance, especially the OC to EC ratio, perhaps due to the enhancement by coating with organic aerosol (especially secondary organic aerosol, SOA) or the artifacts resulting from the redistribution of liquid-like organic particles during the filter-based absorption measurements. Using a converting approach that accounts for the discrepancy caused by measurements methods of both light absorption and EC concentration, previously published MAE values were converted to the equivalent MAE, which is the estimated value if using the same measurement methods as used in this study. The equivalent MAE was found to be much lower in the regions heavily impacted by biomass burning (e.g., India), probably due to the influence of brown carbon. Optical properties of water-soluble organic carbon (WSOC) in Beijing were also presented. Light absorption by WSOC exhibited strong wavelength (λ) dependence such that absorption varied approximately as λ−7, which was characteristic of the brown carbon spectra. The MAE of WSOC (measured at 365 nm) was 1.83 and 0.70 m2 g−1 during winter and summer, respectively. WSOC in Beijing has been demonstrated to be strongly linked to SOA; and the seasonal pattern of its MAE was attributed to the difference in the precursors of SOA such that anthropogenic volatile organic compounds (AVOCs) should be more important as the precursors of SOA in winter. Moreover, the MAE of WSOC in Beijing was much higher than results from the southeastern United States which were obtained using the same method as used in this study, perhaps due to the influence of biomass burning.