Size-resolved measurements of the light absorption properties of soot aerosols from a gas turbine engine

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 (MAC_λ) or the absorption function E(m_λ), which are wavelength dependent. Light absorbed by soot can be predicted from its MAC_λ 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 MAC_λ 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 MAC_λ 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.