Mixing State of Black Carbon Aerosol in a Heavily Polluted Urban Area of China: Implications for Light Absorption Enhancement

Black carbon (BC) is important for climate forcing, and its effects on the Earth's radiative balance remain a major uncertainty in climate models. In this study, we investigated the mixing state of refractory black carbon (rBC) and aerosol optical properties in a polluted atmosphere at Xi’an, western China. The average rBC mass concentration was 9.9 μg m⁻³ during polluted periods, 7.6 times higher than that in clean periods. About 48.6% of the rBC was internally-mixed or coated with non-refractory materials during polluted periods; this was 27% higher than in clean periods. Correlation analysis between the number fraction of thickly-coated rBC particles (<i>f_BC</i>) and the major particulate species indicate that organics may be the primary contributor to rBC coatings during polluted periods. The average mass absorption cross section of rBC (MAC_BC) particles at λ = 870 nm was 7.6 ± 0.02 m² g⁻¹ for the entire campaign. The MAC_BC showed a positive correlation with <i>f_BC</i>, and the enhancement of MAC_BC due to internal mixing was 1.8 times. These observations suggest that an enhancement of BC absorption by a factor of ∼2 could be appropriate for climate models associated with high PM_2.5 levels.