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 Xian, western China. The average rBC mass concentration was 9.9 g 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 nonrefractory materials during polluted periods; this was 27% higher than in clean periods. Correlation analysis between the number fraction of thickly-coated rBC particles (fB_C) 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 (MACB_C) particles at = 870 nm was 7.6 ± 0.02 m² g^-1 for the entire campaign. The MACB_C showed a positive correlation with fB_C, and the enhancement of MACB_C 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.