Size-resolved mixing state and optical properties of black carbon at an urban site in Beijing

Hang Liu; Xiaole Pan; Yunfei Wu; Dongsheng Ji; Yu Tian; Xueshun Chen; Zifa Wang

doi:10.1016/j.scitotenv.2020.141523

Size-resolved mixing state and optical properties of black carbon at an urban site in Beijing

Sci Total Environ. 2020 Dec 20:749:141523. doi: 10.1016/j.scitotenv.2020.141523. Epub 2020 Aug 5.

Authors

Hang Liu¹, Xiaole Pan², Yunfei Wu³, Dongsheng Ji⁴, Yu Tian¹, Xueshun Chen⁴, Zifa Wang⁵

Affiliations

¹ State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China; College of Earth Sciences, University of Chinese Academy of Sciences, Beijing 100049, China.
² State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China. Electronic address: panxiaole@mail.iap.ac.cn.
³ Key Laboratory of Regional Climate-Environment for Temperate East Asia (RCE-TEA), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China.
⁴ State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China.
⁵ State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China; College of Earth Sciences, University of Chinese Academy of Sciences, Beijing 100049, China; Center for Excellence in Regional Atmospheric Environment, Chinese Academy of Science, Xiamen 361021, China.

PMID: 32827831
DOI: 10.1016/j.scitotenv.2020.141523

Abstract

The size-resolved (200-700 nm) mixing state and optical properties of black carbon (BC) in Beijing in the spring of 2019 were investigated using a tandem system consisting of an aerodynamic aerosol classifier, a nephelometer, and a single particle soot photometer. The results showed that the coating thickness distribution exhibited a clear bimodal pattern for BC-containing particles with a fixed aerodynamic diameter (D_ae). Based on the coating thickness, BC-containing particles can be classified as having external and internal mixing states. The number fraction of internal BC-containing particles increases with increasing D_ae and reaches 95% when D_ae = 700 nm. Both the BC core diameter and coating thickness simultaneously increased with an increasing D_ae of BC-containing particles. The dynamic shape factor (χ) of BC-containing particles decreased from 1.43 to 1.0 as D_ae increased from 200 nm to 400 nm and varied around 1.0 when D_ae = 500-700 nm. This demonstrated that thickly coated BC-containing particles were more likely to have regular shapes. An observation-constrained simulation on the basis of Mie theory showed that the coating plays an important role in light absorption. The amplification of BC absorption by the coating increased from 1.21 to 1.75 with increasing D_ae due to the thicker coating of BC-containing particles with a larger D_ae. The single-scattering albedo was dependent on size, increasing from 0.83 to 0.98 with increasing D_ae. The size-dependent characteristics of BC-containing particles were similar under different pollution conditions, but BC-containing particles tended to be larger with a thicker coating and have a larger absorption enhancement under polluted conditions (PM_2.5 > 75 μg/m³) than under clean conditions (PM_2.5 < 35 μg/m³). This study highlights the strong dependence of the microphysical and optical properties of BC on size.

Keywords: Black carbon; Mixing state; Optical properties; Size distribution.