Mysterious ‘Onion-Like’ Particles Discovered in Aircraft Exhaust

Scientists have identified previously unknown nanoparticles with unique “onion-like” structures in aircraft exhaust, potentially changing our understanding of aviation’s environmental and health impacts.

The new research, published April 8 in ACS ES&T Air, revealed that these tiny particles dominate aircraft exhaust plumes and may interact differently with both the atmosphere and human respiratory systems than previously studied soot particles.

Beyond the Soot: Four Distinct Particle Types Identified

Using high-resolution transmission electron microscopy, researchers examined exhaust particles from commercial turbofan jet engines at Switzerland’s Zurich Airport. While soot particles from jet engines have been studied for decades, this investigation uncovered three additional particle types never before identified in aircraft exhaust.

“Four types of aircraft engine exhaust particles with different internal structures were observed,” the research team noted. These included the well-known turbostratic (soot) particles, plus newly identified onion-like particles, amorphous particles, and trace amorphous particles.

Most surprising was the presence of “onion-like” particles with concentric, spherical carbon layers resembling an onion’s structure. These particles were abundant 15 meters downstream from the engine, where they and other non-soot particles made up more than 99% of the sample.

Tiny Size, Big Questions

The newly discovered particles measured just 10-20 nanometers in diameter—roughly 1/1000th the size of cedar pollen and invisible to the naked eye. Their minuscule size allows them to penetrate deeply into human lungs and potentially enter the bloodstream.

What makes these particles particularly intriguing is their unique internal structure, which differs from typical combustion-generated soot. The research suggests these particles likely form through different mechanisms, possibly involving nucleation and condensation from engine lubricating oil rather than direct combustion processes.

Key Findings About the Newly Discovered Particles

• Onion-like particles feature well-ordered, concentric carbon layers similar to artificially synthesized carbon nanomaterials
• Most particles were single (non-agglomerated) spheres, unlike typical soot which forms grape-like clusters
• Soot particles dominated at the engine exit but comprised less than 1% of particles 15 meters downstream
• Evidence suggests these particles form from lubricating oil rather than fuel combustion

Health and Environmental Implications

The discovery raises important questions about the health impacts of aviation emissions. With millions of people living near airports worldwide and increasing air traffic, understanding these particles has taken on new urgency.

Could these distinctly structured particles interact differently with human lung tissue than ordinary soot? Might they have different atmospheric lifetimes or climate effects than previously studied aviation particles? The research team emphasized that the unique structures “may affect the physicochemical properties of the particles, including volatility, surface reactivity, and solubility, and potentially affect the interaction of the particles with the human respiratory tract.”

A Scientific Mystery with Practical Applications

Interestingly, similar onion-like carbon structures are deliberately manufactured in laboratories for advanced materials applications. They’re typically created by applying intense energy to soot particles. Finding these structures naturally occurring in aircraft exhaust presents both a scientific puzzle and potential application opportunity.

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

This study builds on previous work identifying lubricating oil as a major component of aircraft exhaust nanoparticles, published by lead author Akihiro Fushimi and colleagues in 2019. The international research team included scientists from Japan’s National Institute for Environmental Studies, Tokyo Metropolitan University, and Switzerland’s Zurich University of Applied Sciences.

As air travel continues expanding globally, understanding the full spectrum of aircraft emissions becomes increasingly important. These newly identified particles add another dimension to ongoing research into aviation’s environmental footprint and health impacts, while potentially opening doors to novel materials science applications inspired by their unique structures.