Soot particles emitted from aircraft engines serve as condensation nuclei for small droplets and ice crystals, driving the formation of contrails. The ice crystals of the contrails can remain for several hours in cold and humid conditions at altitudes of between 8 to 12 kilometers, and form high clouds known as contrail cirrus.

The global aviation sector contributes approximately 5% of the current anthropogenic radiative forcing, owing to direct emissions of fossil-fuel CO₂ (28 mW m⁻²) and the formation and evolution of contrails and contrail-induced cirrus clouds (50 mW m⁻²). Of these effects, the largest uncertainties are associated with aviation-induced cloudiness, both directly from contrail-induced cirrus clouds and indirectly from the contribution of black carbon, organic and sulfate aerosols that may act as cloud condensation nuclei and ice nuclei.

—Moore et al.

Contrail cirrus clouds have a similar impact on the climate than all aviation carbon dioxide emissions that have accumulated over more than 100 years in the atmosphere. The soot emissions largely determine the number of ice crystals in contrails. The possibility of reducing the engine exhaust soot emissions by more than half using biofuels paves the way for reducing the climatic impact of contrails.

—co-author Hans Schlager from the DLR Institute of Atmospheric Physics

In May 2014, the DLR research aircraft Falcon took to the air, equipped with advanced instrumentation and operated by a crew of specialists from the DLR Institute of Atmospheric Physics and DLR Flight Operations in the United States. They followed a NASA DC-8 research aircraft at close distances ranging from 30 to 150 meters to measure exhaust composition from each of the inboard engines. The DC-8’s CFM56 engines were alternately powered by normal Jet A1 fuel and a 1:1 blend of Jet A1 fuel with the biofuel of hydroprocessed esters and fatty acids (HEFA). The HEFA was produced from Camelina plant oil.

Previous measurements had only provided information on soot formation using biofuels on the ground, whereas different environmental conditions prevail in flight.

This is the first time we have quantified the amount of soot particles emitted by jet engines while burning biofuel-blended fuel in flight.

—Rich Moore, NASA scientist and lead author

The tests were conducted as part of the Alternative Fuel Effects on Contrails and Cruise Emissions Study (ACCESS) at NASA Armstrong Flight Research Center in Palmdale, California. DLR was invited to participate in the research project by NASA. The DLR Falcon 20E, together with NASA’s Falcon HU-25C measuring aircraft and the T-33 of the Canadian NRC, measured the emissions of the preceding DC-8.

More than a dozen instruments mounted on the trailing aircraft characterised the exhaust particles and gases emitted by the engines of the preceding DC-8.

DLR and NASA have been cooperating in the field of atmospheric research for 18 years. In aeronautics research, both partners are particularly involved in joint research projects in the areas of air traffic management, as well as low-noise and low-emission flying. The joint research flights in May 2014 were a highlight of this longstanding cooperation.

In the future, close cooperation is also planned in the investigation of biofuel emissions. In early 2018, NASA will participate with the DC-8 in a series of research flights in Germany conducted as part of DLR’s own project, ECLIF (Emission and Climate Impact of alternative Fuel). Here, the researchers want to more closely investigate how the composition of different alternative fuels influences the emissions and the climate-relevant properties of contrails.

Resources

• Richard H. Moore, Kenneth L. Thornhill, Bernadett Weinzierl, Daniel Sauer, Eugenio D’Ascoli, Jin Kim, Michael Lichtenstern, Monika Scheibe, Brian Beaton, Andreas J. Beyersdorf, John Barrick, Dan Bulzan, Chelsea A. Corr, Ewan Crosbie, Tina Jurkat, Robert Martin, Dean Riddick, Michael Shook, Gregory Slover, Christiane Voigt, Robert White, Edward Winstead, Richard Yasky, Luke D. Ziemba, Anthony Brown et al. (2017) “Biofuel blending reduces particle emissions from aircraft engines at cruise conditions” Nature 543, 411–415 doi: 10.1038/nature21420