Climate-savvy travelers are likely aware of the global impact of flying. The aviation industry is working hard to decarbonize, but carbon dioxide emissions are only half the story. Contrails—clouds of condensed water that trail behind a jet—may be just as important as carbon dioxide when it comes to global warming. This makes eliminating contrails an attractive target for climate action.
New research led by scientists at the German Aerospace Center suggests that it won’t be easy: Removing soot emissions doesn’t prevent contrail formation (Nature 2026, DOI: 10.1038/s41586-026-10286-0).
For decades, scientists have known that soot particles play a pivotal role in forming contrails. When the atmospheric conditions are right, the hot water vapor in engine exhaust will condense onto the tiny solid particles and freeze into wispy cirrus clouds that can spread across the sky and persist for hours.
“The soot particles are the nuclei for contrail ice crystals,” explains Christiane Voigt, scientific leader of the project. In theory, reducing the number of nuclei should reduce the number of ice crystals and limit contrail formation.
Researchers have found a couple of ways to reduce soot, Voigt says. One is by burning sustainable aviation fuels, which have lower aromatic and sulfur content. Another is by designing new, lean-burn jet engines that use fuel more efficiently and change how the heat in the engine is distributed. These modifications produce less soot.
Ground-based experiments show that these new engines reduce soot emissions significantly. But “nobody had measured either the particle emissions from those lean-burn engines, nor the contrails that form in flight,” she says. So, Voigt and her team set off to chase a jet.
Collecting contrail data in flight
In spring 2023, the scientists packed a research aircraft with their instruments and took flight, following a commercial-sized plane equipped with engines that could switch between lean- and rich-burn modes. They flew at about 10 km in altitude at various sites off the coast of France.
Every research flight is an adventure, Voigt says. These were no exception. At times, the pilots brought the research aircraft only 50 m away from the plane they chased, “almost poking the engines of the preceding aircraft,” she says. And even when they trailed the jet by several kilometers, the flight was bumpy as the research aircraft rode the wake of the plane ahead.
The data the scientists collected surprised them. The soot emissions from the airplane they chased were 1,000 times lower when the engines were in lean-burn mode. But the plane still made contrails.
“You could look even out of the window of the aircraft, and you would see the contrail,” Voigt says. Something besides soot was providing nuclei for clouds to form.
Parsing through the data back on the ground, the scientists found the culprit: liquid sulfate aerosols and tiny droplets of engine oil.
“Liquid particles can trigger contrail formation as long as soot is reduced,” Voigt says.
Sustainable aviation beyond soot reduction
To make aviation more sustainable, the industry will have to look beyond just reducing soot emissions, Voigt says. Tweaking the engine design to limit the amount of engine oil that escapes and developing sulfur-free fuels could help make that a reality. But scientists will need to run more experiments to understand exactly how such changes will impact air travel and climate, she adds.
Despite the evidence that contrails still form behind lean-burn engines, the collected data also tell a success story, says Richard Moore, a research scientist at NASA’s Langley Research Center who was not involved in the new work. Lowering soot emissions by three orders of magnitude is a “huge win” for air quality, he says.
“This is the cleanest engine technology that’s currently on the market,” Moore adds. Just because it did not reduce contrails as expected, “we shouldn’t detract from the technological successes they show here.”
2026 American Chemical Society
