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. Aircraft engines are a unique source of carbonaceousaerosols in the upper troposphere. There, these particles can moreefficiently interact with solar radiation than at ground. Due to the lack ofmeasurement data, the radiative forcing from aircraft exhaust aerosolremains uncertain. To better estimate the global radiative effects ofaircraft exhaust aerosol, its optical properties need to be comprehensivelycharacterized. In this work we present the link between the chemicalcomposition and the optical properties of the particulate matter (PM)measured at the engine exit plane of a CFM56-7B turbofan. The measurementscovered a wide range of power settings (thrust), ranging from ground idle totake-off, using four different fuel blends of conventional Jet A-1 andhydro-processed ester and fatty acids (HEFA) biofuel. At the two measurementwavelengths (532 and 870 nm) and for all tested fuels, the absorption andscattering coefficients increased with thrust, as did the PM mass. Theanalysis of elemental carbon (EC) and organic carbon (OC) revealed asignificant mass fraction of OC (up to 90 %) at low thrust levels, whileEC mass dominated at medium and high thrust. The use of HEFA blends induceda significant decrease in the PM mass and the optical coefficients at allthrust levels. The HEFA effect was highest at low thrust levels, where theEC mass was reduced by up to 50 %–60 %. The variability in the chemicalcomposition of the particles was the main reason for the strong thrustdependency of the single scattering albedo (SSA), which followed the sametrend as the fraction of OC to total carbon (TC). Mass absorptioncoefficients (MACs) were determined from the correlations between aerosollight absorption and EC mass concentration. The obtained MAC values(MAC 532 = 7.5 ± 0.3  m 2  g −1 and MAC 870 = 5.2 ± 0.9  m 2  g −1 ) are in excellent agreement with previousliterature values of absorption cross section for freshly generated soot.While the MAC values were found to be independent of the thrust level andfuel type, the mass scattering coefficients (MSCs) significantly varied withthrust. For cruise conditions we obtained MSC 532 = 4.5 ± 0.4  m 2  g −1 and MSC 870 = 0.54 ± 0.04  m 2  g −1 ,which fall within the higher end of MSCs measured for fresh biomass smoke.However, the latter comparison is limited by the strong dependency of MSC onthe particles' size, morphology and chemical composition. The use of the HEFAfuel blends significantly decreased PM emissions, but no changes wereobserved in terms of EC∕OC composition and radiative properties.

Chemical composition and radiative properties of nascent particulate matter emitted by an aircraft turbofan burning conventional and alternative fuels