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

• Published in: Aerosol science and technology 2014-07, Vol.48 (7), p.689-697
• Main Authors: Wang, Qiyuan, Huang, R.-J., Cao, Junji, Han, Yongming, Wang, Gehui, Li, Guohui, Wang, Yichen, Dai, Wenting, Zhang, Renjian, Zhou, Yaqing
• Format: Article
• Language: English
• Subjects: Absorption cross sections; Aerosols; Air pollution; Atmospheric aerosols; Capture cross sections; Carbon; Carbon black; Climate models; Correlation; Correlation analysis; Optical properties; Urban areas
• ISSN: 0278-6826; 1521-7388
• DOI: 10.1080/02786826.2014.917758

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 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 λ = 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 ∼2 could be appropriate for climate models associated with high PM 2.5 levels. Copyright 2014 American Association for Aerosol Research