Properties of contrails from aircraft with modern engines and alternative fuels

Kurzfassung

Contrails forming from aircraft engine emissions can evolve into long-living contrail cirrus when ambient conditions are ice-supersaturated. Their net effective radiative forcing has been shown to directly correlate with the number of contrail ice particles under similar atmospheric conditions. As one of the largest single contributors to radiative forcing from aviation, contrail ice crystal reduction is a major lever in mitigating aviation's climate impact. In the course of this work, the first ever in-situ measurements of contrails and emissions from the combustion of 100 % sustainable aviation fuel (SAF) were conducted within the framework of the ECLIF3 project. In a series of research flights, the DLR-operated Dassault Falcon served as the measurement platform in chasing the emission source aircraft Airbus A350-941 equipped with Rolls-Royce Trent XWB-84 engines and burning 100 % HEFA-SPK (Hydro-processed Esters and Fatty Acids - Synthetic Paraffinic Kerosene) fuel. The resulting contrails were probed by in-situ instrumentation for the detection of contrail ice particles, (non-)volatile particulate matter, trace gases, and water vapor onboard the DLR Falcon. Hereby, a 56 % reduction of ice particle numbers per mass of burned fuel compared to the combustion of reference Jet A-1 fuel was measured at cruise conditions. Simultaneously, a lower reduction of 35 % for soot particle numbers was detected, thereby suggesting less ice activation by the low aromatics and low sulfur HEFA-SPK fuel. Beyond CO2 footprint reductions of 100 % non-fossil origin SAF, global climate model simulations based on these experimental results estimate an additional decrease in contrail radiative forcing. Modern aircraft engines employing lean-burn combustion technology are expected to reduce soot emissions into the low-soot regime of 10^11 - 10^14 particles per kg of burned fuel. The implications for the formation of contrails by the use of SAF and modern lean-burn aircraft engines are investigated in the German-French NEOFUELS-VOLCAN (VOL avec Carburants Alternatifs Nouveaux) project. In the course of two research campaign phases, an Airbus A319neo and an Airbus A321neo equipped with lean-burn CFM LEAP-1A engines served as emission source aircraft which were chased by the DLR Falcon measurement platform. Despite large soot particle emission reductions for the lean-burn combustion mode compared to the forced rich-burn mode in non-contrail forming conditions, a high number of contrail ice particles is measured in both combustion modes. At the same time, a correlation of ice particle numbers is found to total aerosol particle (nonvolatile + volatile) emissions. SAF use in lean-burn combustors showed a decrease in contrail ice particle numbers compared to conventional jet fuel combustion in the lean-burn and forced rich-burn combustion modes. The results of this work will contribute to the assessment of benefits and caveats of the use of sustainable aviation fuels and modern combustion technologies regarding contrail formation and properties as well as the associated contrail climate forcing. This will help industry and policymakers to make informed decisions on the development of future technologies and the regulation of non-CO2 effects currently burdened by large uncertainties.