In-flight measurements of contrail ice nucleation on low particle emissions from modern engines and sustainable aviation fuels – the ECOCON experiment

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We propose to measure young contrails from modern lean-burn engines and sustainable aviation fuels (SAF), to study the effects of strongly reduced particle number emissions on contrail ice nucleation, contrail properties and their climate impact. Contrails have the largest share in the climate impact of aviation. In contrails, ice crystals form by heterogenous nucleation on emitted particles. Due to their relatively large particle size, non-volatile soot particles serve as the main condensation nuclei (CN) for ice crystals in contrails. At current soot emission levels above 10^14 particles per kg fuel, other emitted or ambient aerosols only play a minor role in the contrail ice nucleation process. But modern engine technologies and SAF promise much lower soot emission levels. Ground emission tests revealed, that staged lean-burn engines can have by two to three orders of magnitude lower non-volatile soot particle emissions and higher total particle emissions. In-flight data are required in order to measure the engine particle emissions at cruise altitudes and to investigate microphysical contrail properties and their effect on the atmospheric radiation budget, both for conventional kerosene and for SAF. We will answer the question on the increasing importance of CN types other than soot particles in future contrail ice nucleation, including ion-induced nucleation of ultra-fine aqueous particles or ambient aerosols. We will further investigate at which emission levels and temperatures ice nucleation on volatile particles or ambient aerosol will kick in. To this end, we will perform and evaluate in-flight measurements of contrails during an experiment led by NASA. The experiment will focus on contrails from a modern lean-burn combustion system. The airborne measurement laboratory DC-8 by NASA will serve as the measuring aircraft. Contrail ice particle concentrations will be measured with the Fast Forward Spectrometer Probe (FFSSP) and the Cloud Aerosol and Precipitation Spectrometer (CAPS). By using the lean-burn combustion technology and possibly combining it with the burn of 100% SAF, we expect to achieve soot emissions below 10^14 particles per kg fuel. This gives us the possibility to analyse, whether the activation of volatile aerosol contributes to ice nucleation in contrails in the low soot regime. We also plan to evaluate the contrail model CoCiP with the in-situ data and we will derive the radiative forcing and energy forcing of contrails from reduced emissions. The results will shape the way for future climate friendly aviation.