Newswise — Our modern society emits numerous and diverse contaminants into the environment. Combustion generates aerosol mass, which contains black carbon. Despite constituting only a small portion of aerosol particles, black carbon is particularly concerning because it can absorb heat and hinder the ability of surfaces like snow to reflect heat. Therefore, understanding how black carbon interacts with sunlight is crucial. Scientists have measured the refractive index of black carbon with the greatest precision to date, which could influence climate models.
Numerous factors contribute to climate change. While some, such as carbon dioxide emissions from fossil fuel combustion, sulfur dioxide from cement production, and methane emissions from animal agriculture, are well-known, the significance of black carbon aerosol particles from combustion is often overlooked. These particles, essentially soot, are highly efficient at absorbing heat from sunlight and retaining it, thereby increasing atmospheric warmth. Additionally, black carbon covering lighter surfaces, including snow, reduces their ability to reflect heat back into space, as darker colors are less reflective.
Assistant Professor Nobuhiro Moteki from the Department of Earth and Planetary Science at the University of Tokyo stated that comprehending the interplay between black carbon and sunlight is fundamentally crucial in climate studies. He further added that the refractive index of black carbon is the most significant property in this context, which determines how it deflects and disperses incoming light waves. However, the existing measurements of black carbon's refractive index were imprecise. To rectify this, Professor Moteki and his team conducted comprehensive experiments to enhance the measurements. Their improved findings suggest that the current climate models might underestimate the absorption of solar radiation by black carbon by a considerable 16%.
Previous assessments of the optical characteristics of black carbon were often complicated by various factors, such as impure samples and challenges in measuring light interactions with particles of varying complex shapes. However, Professor Moteki and his team overcame these obstacles by encapsulating black carbon particles in water and then isolating them using water-soluble chemicals such as sulfates. This isolation enabled the team to illuminate the particles more effectively and analyze their scattering behavior, which provided the necessary data to determine the refractive index value with greater accuracy.
According to Professor Moteki, "We evaluated the amplitude or intensity and phase or step of light scattered from black carbon samples that were isolated in water. This approach enabled us to determine the complex refractive index of black carbon. The refractive index is considered complex since it comprises two components, one of which is imaginary and relates to absorption. Although it may seem intangible, its impact is substantial. In the realm of optical science and beyond, complex numbers with imaginary components are prevalent."
By revealing that existing climate models are underestimating the impact of black carbon on atmospheric warming, the team anticipates that their findings will benefit other climate researchers and policymakers. Furthermore, the team's technique for determining the intricate refractive index of particles has wider applicability beyond black carbon. This enables the optical recognition of unfamiliar particles in ice cores, oceans, or the atmosphere, as well as the assessment of optical characteristics of powdered substances, extending beyond climate change concerns.
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Journal article: Nobuhiro Moteki, Sho Ohata, Atsushi Yoshida & Kouji Adachi. “Constraining the complex refractive index of black carbon particles using the complex forward-scattering amplitude”, Aerosol Science and Technology. DOI: 10.1080/02786826.2023.2202243
Funding:
Funds were provided by the Environment Research and Technology Development Fund (JPMEERF20202003) of the Environmental Restoration and Conservation Agency, the Japan Society for the Promotion of Science (JSPS) KAKENHI program (JP19H04236, JP19KK0289, Accepted Manuscript JP19H04259, JP19H05699, 22H03722, and 22H01294), and the Arctic Challenge for Sustainability ArCS II project (JPMXD1420318865) of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan.
Useful links:
Graduate School of Science - https://www.s.u-tokyo.ac.jp/en/
Department of Earth and Planetary Science - https://www.eps.s.u-tokyo.ac.jp/en/
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