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

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, and reaches 95% when D-ae = 700 nm. Both the BC core diameter and coating thickness simultaneously increased with an increasing D, of BCcontaining particles. The dynamic shape factor (x) 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, 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 coaling increased from 1.21 to 1.75 with increasing Da, clue to the thicker coating of BC-containing particles with a larger D, The single-scattering albedo was dependent on size, increasing from 0.83 to 0.98 with increasing Dae. 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 (PM2.5 > 75 mu g/m(3)) than under clean conditions (PM2.5 < 35 mu g/m(3)). This study highlights the strong dependence of the microphysical and optical properties of BC on size. (C) 2020 Elsevier B.V. All rights reserved.