Abstract. Aircraft engines are a unique source of carbonaceous
aerosols in the upper troposphere. There, these particles can more
efficiently interact with solar radiation than at ground. Due to the lack of
measurement data, the radiative forcing from aircraft exhaust aerosol
remains uncertain. To better estimate the global radiative effects of
aircraft exhaust aerosol, its optical properties need to be comprehensively
characterized. In this work we present the link between the chemical
composition and the optical properties of the particulate matter (PM)
measured at the engine exit plane of a CFM56-7B turbofan. The measurements
covered a wide range of power settings (thrust), ranging from ground idle to
take-off, using four different fuel blends of conventional Jet A-1 and
hydro-processed ester and fatty acids (HEFA) biofuel. At the two measurement
wavelengths (532 and 870 nm) and for all tested fuels, the absorption and
scattering coefficients increased with thrust, as did the PM mass. The
analysis of elemental carbon (EC) and organic carbon (OC) revealed a
significant mass fraction of OC (up to 90 %) at low thrust levels, while
EC mass dominated at medium and high thrust. The use of HEFA blends induced
a significant decrease in the PM mass and the optical coefficients at all
thrust levels. The HEFA effect was highest at low thrust levels, where the
EC mass was reduced by up to 50 %–60 %. The variability in the chemical
composition of the particles was the main reason for the strong thrust
dependency of the single scattering albedo (SSA), which followed the same
trend as the fraction of OC to total carbon (TC). Mass absorption
coefficients (MACs) were determined from the correlations between aerosol
light absorption and EC mass concentration. The obtained MAC values
(MAC532=7.5±0.3 m2 g−1 and MAC870=5.2±0.9 m2 g−1) are in excellent agreement with previous
literature values of absorption cross section for freshly generated soot.
While the MAC values were found to be independent of the thrust level and
fuel type, the mass scattering coefficients (MSCs) significantly varied with
thrust. For cruise conditions we obtained MSC532=4.5±0.4 m2 g−1 and MSC870=0.54±0.04 m2 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 on
the particles' size, morphology and chemical composition. The use of the HEFA
fuel blends significantly decreased PM emissions, but no changes were
observed in terms of EC∕OC composition and radiative properties.
Optical Properties, Chemical Composition and the Toxicological Potential of Urban Particulate Matter