Our industrial society releases many diverse pollutants into the world. Combustion in particular produces a hazy mass including black carbon. Although this accounts for only a few percent of aerosol particles, black carbon is particularly problematic because of its ability to absorb heat and impede the thermal reflection capabilities of surfaces such as snow. Therefore, it is necessary to know how black carbon interacts with sunlight. The researchers have estimated the refractive index of black carbon to the most accurate point yet that may influence climate models.
There are many factors that lead to climate change; Some are all too familiar, such as carbon dioxide emissions from burning fossil fuels, sulfur dioxide from the cement industry or methane emissions from animal farming. Carbon black mist particles, also from combustion, are less covered in the news but are particularly important. Black carbon, which is basically soot, is very good at absorbing and storing heat from sunlight, which heats up the atmosphere. At the same time, because darker colors are less effective at reflecting light and thus heat, as carbon black coats lighter surfaces including snow, it reduces the ability of those surfaces to reflect heat back into space.
“Understanding the interaction between black carbon and sunlight is of fundamental importance in climate research,” said Assistant Professor Nobuhiro Moteki of the University of Tokyo’s Department of Earth and Planetary Sciences. “The most important property of black carbon in this regard is its refractive index, which is how it redirects and scatters incoming light rays. However, current measurements of the refractive index of black carbon have been imprecise. My team and I have conducted detailed experiments to improve this. Using our improved measurements, we now estimate that Current climate models may reduce solar radiation absorption due to black carbon by as much as 16%.”
Previous measurements of the optical properties of black carbon have often been confounded by factors such as a lack of pure samples, or difficulties in measuring the interactions of light with molecules of various complex shapes. Motiki and his team improved on this situation by capturing black carbon particles in water, then isolating them with sulfate or other water-soluble chemicals. By isolating the particles, the team was better able to highlight them and analyze their way of scattering, which gave the researchers the data to calculate the refractive index value.
“We measured the amplitude, or strength, and phase, or pitch, of the scattered light from the carbon black samples isolated in water,” Motiki said. “This allowed us to calculate what is known as the complex refractive index of black carbon. Complex because instead of being a single number, it is a value with two parts, one of which is ‘imaginary’ (meaning absorption), although the effect is very real. Such complex numbers Imaginative components are very common in the visual sciences and beyond.”
Because new optical measurements of black carbon indicate that current climate models underestimate its contribution to atmospheric warming, the team hopes that other climate researchers and policymakers can build on their findings. The method the team developed to ascertain the particle’s complex refractive index can be applied to materials other than carbon black. This allows visual identification of unknown particles in the atmosphere, ocean or ice cores, and assessment of the optical properties of powder materials, not just those of the ongoing problem of climate change.