Recently, Liu Dantong (School of Earth Sciences, Zhejiang University) and his team, working with Beijing Key Laboratory of Cloud, Precipitation and Atmospheric Water Resources, China Meteorological Administration (CMA), published "Environmental Science & Technology Letters, a top journal of Environmental Science and Ecology, Identifying the Fraction of Core−Shell Black Carbon Particles in a Complex Mixture to Constrain the Enhancement by Coatings, The optical absorption effect of refractory black carbon (rBC) with complex cladding core-shell structure is further elucidated.

The optical absorption effect of BC has a drastic impact on climate change. It has a complex chain fractal structure. BC and other substances (such as sulfate) continue to mix and form core-shell structure during the aging process of the atmosphere. The BC particles in the mixed state may enhance the absorption of light due to the prism effect (ammonium sulphate-like coating focuses more light on rBC nuclei).

The dynamic shape factor (χ) of all black carbon particles with single-particle accuracy in the actual atmosphere was obtained online by simultaneous measurement of the electromigration diameter and isovolumetric diameter of black carbon particles. χ is the ratio of the resistance of a particle in an electric field to that of an equivalent spherical particle. The more irregular the shape of the particle itself is, the greater the resistance and the greater the χ value (χ = 1 indicates that the particle is a sphere).

At the same time, the particles were heated to 350°C to remove the inclusions on the black carbon particles, so as to study the effect of the inclusions on their properties. It is found that there are two distribution states of black carbon shape factor (FIG. 1), and χ=1.75 can be used as the critical value to effectively distinguish the shape of black carbon particles: when χ is lower than 1.75, black carbon particles can be considered as spheroid, and vice versa.

Figure 1: The actual atmosphere and the morphological characteristics of black carbon particles after the removal of black carbon inclusions

Only when the inclusion is completely coated with black carbon particles, the prism effect of black carbon particles exists, which corresponds to the absorption enhancement of black carbon particles. At the same time, black carbon particles also show spherical morphology. Therefore, χ=1.75 can also be used to distinguish whether there is an absorption enhancement effect of black carbon particles. However, the mass concentration ratio (Fns) of non-spherical black carbon particles can be used to describe the proportion of black carbon particles without enhanced absorption. Thus, after introducing the partial core-shell model, the proportion of black carbon in accordance with the traditional core-shell model at each particle size can be effectively extracted (FIG. 2).

Figure 2: Mass concentration ratio of non-spherical black carbon particles

The original English topic：Identifying the fraction of core-shell black carbon particles in complex mixture to constrain the absorption enhancement by coatings

This study used the SP2 (Single Particle Soot Photometer) of DMT Company to obtain the physical characteristics of particles coated with black carbon particles, including rBC size (BC core diameter) and cladding thickness (shell thickness) of individual particles.