<div><p>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 <b>μ</b>g m<sup>−3</sup> 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 (<i>f</i><sub>BC</sub>) 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<sub>BC</sub>) particles at <b>λ =</b> 870 nm was 7.6 <b>±</b> 0.02 m<sup>2</sup> g<sup>−1</sup> for the entire campaign. The MAC<sub>BC</sub> showed a positive correlation with <i>f</i><sub>BC</sub>, and the enhancement of MAC<sub>BC</sub> 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<sub>2.5</sub> levels.</p><p>Copyright 2014 American Association for Aerosol Research</p></div
