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 mu 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 (f(BC)) 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 lambda = 870 nm was 7.6 +/- 0.02 m(2) g(-1) for the entire campaign. The MAC(BC) showed a positive correlation with f(BC), 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 similar to 2 could be appropriate for climate models associated with high PM2.5 levels.