Optical properties of elemental carbon and water-soluble organic carbon in Beijing, China

Abstract. The mass absorption cross-section (MAC) of elemental carbon (EC) in Beijing was quantified using a thermal-optical carbon analyzer and the influences of mixing state and sources of carbonaceous aerosol were investigated. The MAC measured at 632 nm was 29.0 and 32.0 m² g⁻¹ during winter and summer respectively. MAC correlated well with the organic carbon (OC) to EC ratio (R² = 0.91) which includes important information about the extent of secondary organic aerosol (SOA) production, indicating the enhancement of MAC by coating with SOA. The extrapolated MAC value was 10.5 m² g⁻¹ when the OC to EC ratio is zero, which was 5.6 m² g⁻¹ after correction by the enhancement factor (1.87) caused by the artifacts associated with the "filter-based" methods. The MAC also increased with sulphate (R² = 0.84) when the sulphate concentration was below 10 μg m⁻³, whereas MAC and sulphate were only weekly related when the sulphate concentration was above 10 μg m⁻³, indicating the MAC of EC was also enhanced by coating with sulphate. Based on a converting approach that accounts for the discrepancy caused by measurements methods of both light absorption and EC concentration, previously published MAC values were converted to the "equivalent MAC", which is the estimated value if using the same measurement methods as used in this study. The "equivalent MAC" 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 mass absorption efficiency (σ_abs) of WSOC (measured at 365 nm) was 1.83 and 0.70 m² g⁻¹ during winter and summer respectively. The seasonal pattern of σ_abs was attributed to the difference in the precursors of SOA, because WSOC in Beijing has been demonstrated to be strongly linked to SOA. Moreover, the σ_abs 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.