Abstract
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.
Highlights
The rapid rise of the aviation industry in the last decades and the continuous growth projected for the 20 years (Leahy, 2016) have motivated the study of aircraft engine emissions and their related effects on the environment and human health
All the results presented in this work were corrected to the engine exit plane, taking into account the dilution in the particulate matter (PM) line and the particle losses in the sampling system
It is important to note, that most gaseous and particle species measured at the engine exit plane will rapidly evolve in the atmosphere, and their radiative effects can largely vary from those of the direct emissions presented in this work, in which the collected data correspond to a time after emission of approximately 0.1 to 0.6 s (Brem et al, 2015)
Summary
The rapid rise of the aviation industry in the last decades and the continuous growth projected for the 20 years (Leahy, 2016) have motivated the study of aircraft engine emissions and their related effects on the environment and human health. Aviation BC emissions are very small relative to other anthropogenic sources like road transport, industry or biomass burning (Balkanski et al, 2010; Hendricks et al, 2004; Karagulian et al, 2017), their radiative effects can be enhanced when emitted at high altitude and over high surface albedo such as snow and ice surfaces or clouds. Several model studies have shown that the direct radiative forcing (DRF) of BC strongly increases with altitude A detailed understanding of the optical properties of the carbonaceous particles emitted from aircraft exhaust is essential to estimate the related climate effects
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