Mysterious “onion-like” pellets found in aircraft exhaust

Scientists have identified previously unknown nanoparticles, which have a unique “onion-like” structure in aircraft exhaust, that have the potential to change our understanding of aviation environment and health effects.

New research published in ACS ES&T air on April 8 shows that these tiny particles dominate the aircraft exhaust plume and that the interaction with previously studied soot particles may differ from the interactions between the atmosphere and human respiratory systems.

Beyond Smoke: Four Different Particle Types Determined

The researchers examined exhaust particles from commercial turbofan engines at Zurich Airport, Switzerland using a high-resolution transmission electron microscope. Although soot particles from jet engines have been studied for decades, the study found three other types of particle that have never been found in aircraft exhaust.

“Four aircraft engine exhaust particles with different internal structures were observed,” the team noted. These include well-known turbine-like (soat) particles, as well as newly identified onion-like particles, amorphous particles and trace amounts of amorphous particles.

The most surprising thing is the presence of “onion-like” particles of concentric spherical carbon layers similar to structures of onions. These particles are abundant 15 meters downstream of the engine, where they and other non-fixed particles account for more than 99% of the sample.

Small scale, big problem

The newly discovered particles are only 10-20 nanometers in diameter – the size of cedar pollen and invisible to the naked eye. Their tiny size allows them to penetrate deep into the human lungs and potentially enter the bloodstream.

What makes these particles particularly attractive is their unique internal structure, which is different from the soot ash produced by typical combustion. The study shows that these particles may be formed through different mechanisms, which may involve nucleation and condensation of engine lubricants rather than direct combustion processes.

Key discoveries about newly discovered particles

• Onion-like particles have an orderly concentric carbon layer similar to artificially synthesized carbon nanomaterials
• Most particles are single (non-condensing) spheres, unlike typical soot ash, forming grape-like clusters
• Soot particles are mainly engine outlets, but less than 1% of the particles are 15 meters.
• There is evidence that these particles are formed from burning of lubricating oil rather than fuel

Health and environmental impacts

The finding raises important questions about the health effects of aviation emissions. With millions of people living near airports around the world, air traffic continues to increase, so understanding these particles has taken a new urgency.

Do these apparently structured particles interact with human lung tissue differently than ordinary soot? Do they have different atmospheric lifespan or climatic effects than previously studied aerospace particles? The research team stressed that the unique structure “may affect the physicochemical properties of particles, including fluctuations, surface reactivity, and solubility, and may affect the interaction of particles with the human respiratory tract.”

The scientific mystery of practical applications

Interestingly, similar onion-like carbon structures were intentionally used in laboratories for advanced material applications. They are usually produced by applying strong energy to soot particles. Discovering these naturally occurring structures in aircraft exhausts not only brings scientific problems but also potential application opportunities.

“The formation mechanism of onion-like particles in aircraft engines is scientifically interesting and has potential implications for materials and other fields,” the researchers noted.

The study builds on previous work that identified lubricating oil as a major component of aircraft exhaust nanoparticles, published in 2019 by lead author Akihiro Fushimi and colleagues. The international research team includes scientists from the National Institute of Environment, the Metropolitan University of Tokyo, and Applied Sciences from Zurichland, the Swiss University of Switzerland.

As air travel continues to expand globally, it is becoming increasingly important to understand the full range of aircraft emissions. These newly identified particles add another dimension to ongoing research on the environmental footprint and health impacts of aviation, while potentially opening the door to novel materials science applications inspired by their unique structure.