| Abstract | The optical properties of soot are crucial in estimating its climate impact through direct radiative forcing. Soot light absorption is typically quantified by the mass absorption cross-section (MACλ) or the absorption function E(m λ), which are wavelength dependent. Light absorbed by soot can be predicted from its MACλ using mass-concentration measurements, or from its E(m λ) using material density and an optical model accounting for soot-aggregate morphology. Recent work has shown that the soot MACλ shows a size dependency, due to a size-dependent degree of graphitization. We therefore hypothesized here that a similar size dependency may be observed for E(m λ), which we quantify here. To test this hypothesis, we present a novel approach to obtain size-resolved MACλ and E(m λ) of soot from a gas turbine engine by combining pulsed laser-induced incandescence signals with total mass-concentration measurements. E(m λ ) was found to vary with soot-particle size, with values ranging between 0.23 to 0.31 for the smallest (≈ 0.13 fg) and largest (≈ 3 fg) particles measured. To our knowledge, these measurements are the first to demonstrate that E(m λ) not only varies between soot samples, but also within a population of soot particles, which impacts the interpretation of optical diagnostics and prediction of the radiative properties of soot. |
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