Flugzeuggetragene Messungen von Eis- und Rußpartikeln in Kondensstreifen bei Verwendung konventioneller und synthetischer Treibstoffe

Kurzfassung

With a share of more than 2% contrail cirrus currently yields the largest single component contribution from aviation to total anthropogenic climate forcing. According to global simulations the radiative forcing due to contrail cirrus correlates considerably with the number of associated ice particles. Hence, reducing ice particle numbers in contrails represents a fast acting measure to decouple - at least to some extent - the climate forcing of global aviation from its growth. In this regard, the use of alternative synthetic fuels with substantially lower aromatic content seems promising, since, compared to conventional Jet A-1, these fuels cause much lower numbers of emitted soot particles, which are often assumed - but not yet experimentally verified - to control ice particle nucleation in contrails. The present work analyzes if switching from conventional to synthetic fuels leads to systematically reduced ice particle numbers present in contrails. To this end, in situ measurements were performed employing instruments installed on the DLR research aircraft Falcon to sample contrails originating from an Airbus A320 burning both conventional and semi-synthetic fuels. Conducted within the DLR project Emissions and Climate Impact of Alternative Fuels (ECLIF), measurements in contrails from semi-synthetic fuels are the first of their kind. In order to obtain a high quality data set of contrail and environmental parameters, both the Cloud and Aerosol Spectrometer (CAS) and the impact of ice residues on measurements of interstitial soot particles were characterized in-depth. Before focussing on fuel effects, microphysical and dynamical processes governing contrail formation and evolution are studied in detail. By juxtaposing vertical profiles of measured ice and soot particle numbers during the vortex descent, strong evidence is provided that soot particles indeed control ice particle formation for soot emissions in the range of 2-5 × 10^15 particles per kg of fuel burned. At temperatures close to 215 K around 90 % of the emitted soot particles are found to contribute to ice particle formation. Furthermore the results provide direct observational evidence of sublimation substantially reducing ice particle abundances in descending aircraft wakes due to adiabatic heating. Based on these analyses it is shown that, compared to conventional Jet A-1, ice particle numbers in contrails originating from semi-synthetic fuels are systematically reduced. Concerning a widespread type of aircraft (A320, IAE V2527-A5 engines) and conditions typical for cruise flights in the northern extratropics, marked reductions of the number of emitted soot particles of 52 (±3) %, initially formed ice particles of 50 (±4) % and of wake-averaged ice numbers towards the end of the vortex phase of 36 (±5) % are found when comparing semi-synthetic to conventional fuel. The results of this study will help to assess the overall potential of synthetic fuels as a measure to mitigate the climate impact of aviation.