AS&T Article Highlight: Constraining the Complex Refractive Index of Black Carbon Particles…

Constraining the complex refractive index of black carbon particles using the complex forward-scattering amplitude

Article by: Dong Gao, Yale University

Authors of the Manuscript Featured: Nobuhiro Moteki, Sho Ohata, Atsushi Yoshida, Kouji Adachi

http://doi.org/10.1080/02786826.2023.2202243

Black carbon (BC), despite accounting for a small fraction of aerosol particles, plays a significant role in climate change due to its ability to absorb and store heat from sunlight. A study by researchers at the University of Tokyo has shed light on the critical optical property of BC – its refractive index. Previous measurements of BC’s optical properties were fraught with difficulties, owing to the complexity of the particles’ shapes and impurity of the samples. Moteki et. al tackled this challenge by capturing BC particles in water and isolating them with sulphates or other water-soluble chemicals, allowing for a detailed and more accurate analysis of light scattering by the particles. Their findings indicated that the refractive index of BC at visible wavelengths is higher than widely used values in climate models. This suggests that current models may underestimate solar radiation absorption by a significant 16%, underscoring the need for refining these models for accurate climate predictions. The method used to ascertain the refractive index can also be applied to other particles, offering the potential for identifying unknown particles in various environments and assessing the optical properties of various powered materials.

Figure 1. Transmission electron microscope images of ambient aerosols collected by an aerosol-impactor sampler installed on the research vessel Shinsei Maru. Red arrows indicate individual black carbon aggregates, most of which were mixed with sulfate (green arrows) and/or organic materials (light blue arrows).