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$(a) Turbostratic (soot) particles (diameter 67 nm). (b) Onion-like particles (diameter 17 nm). (c) Amorphous particles (diameter 26 nm). (d) Trace amorphous particles (diameter 17 nm). The four particle types were divided into single and agglomerated particles, and their fractions are shown. Credit NIES/ZHAW/TMU$

(a) Turbostratic (soot) particles (diameter 67 nm). (b) Onion-like particles (diameter 17 nm). (c) Amorphous particles (diameter 26 nm). (d) Trace amorphous particles (diameter 17 nm). The four particle types were divided into single and agglomerated particles, and their fractions are shown. Credit NIES/ZHAW/TMU

Abstract:
1. Background and purpose of the study

Aircraft emit nanoparticles (<50 nm Note 1 in diameter) into the atmosphere, from the ground to the upper troposphere. Studies in Europe, the U.S., and Japan have reported high concentrations of particles in and around airports Ref 1, and there is worldwide concern regarding the effects on human health. The atmospheric heating effects by contrails generated from aircraft exhaust particles are also known, and research is being conducted to assess their potential impacts on climate. Particle number emissions from turbofan jet engines, which are commonly used in civil aviation, are generally dominated by volatile particles (sulfate or organics) rather than non-volatile particles (mostly soot). However, the emission and formation mechanisms of the volatile particles are not well understood. Our research team investigated the physicochemical properties of aircraft exhaust nanoparticles (volatile and non-volatile) to obtain information on the emission and formation mechanisms of volatile particles.

Onion-like nanoparticles found in aircraft exhaust

Tsukuba, Japan | Posted on May 14th, 2025

2. Methods

We measured the morphology and internal (microphysical) structure of exhaust particles at the engine exit and 15 m downstream of commercial turbofan jet engines at a test facility in Zurich Airport, Switzerland. The morphology and internal structure of the particles were observed by high-resolution transmission electron microscopy (HRTEM) using bulk particulate samples collected on thin films.

3. Results and discussion

Four types of aircraft engine exhaust particles with different internal structures were observed (Fig. 1). Type (a) represented turbostratic particles with scattering-layered graphene-like structures Note 2 and were considered typical for soot (non-volatile particles). Type (b) consisted of onion-like particles with partial graphite-like structures, which are well-ordered graphene-like spherical multilayers. Type (c) particles were amorphous (non-crystalline), and type (d) were trace amorphous particles (images are thin and non-crystalline). Before this study, onion-like particles were not identified in the combustion exhaust or atmosphere. Although graphitic soot with the turbostratic structure emitted from aircraft has been studied for many years, three other types of particles were identified for the first time in this study.

The number fraction of turbostratic (soot) particles was high at the engine exit and lower than 1% at 15 m downstream (Fig. 1). Fifteen meters downstream, the remaining fraction was dominated by onion-like, amorphous, and trace amorphous particles. These three types of particles were mostly single (non-agglomerated, Note 3) spherical particles with diameters of 10–20 nm. Further analysis suggested that these three types of particles are volatile particles formed via nucleation and condensation downstream of the engine and mainly consist of organic compounds originating from the lubrication oil.

4. Outlook

These unique internal 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. Our research team found that aircraft emit onion-like, amorphous, and trace amorphous particles in addition to soot particles. However, there are many questions regarding the physicochemical characteristics, origin, and formation mechanism of onion-like particles, and whether they are similar in nature to soot or volatile organic particles, such as oil mist. The onion-like particles may have different dynamics in the atmosphere and in the body compared with other particles; therefore, further research is needed to understand their climate and health implications.

In the field of nanomaterials, onion-like particles are synthesized by applying high energy to soot, among other methods. The formation mechanism of onion-like particles from aircraft engines is scientifically interesting and has potential implications in materials and other fields.

5. Annotations

Note 1. nm: Nanometer. One nanometer is one-millionth of a millimeter (mm). Particle diameter (size) of 50 nm is about 1/1000th the size of cedar pollen (about 30 μm in diameter). One micrometer is 1/1000th of a millimeter.

Note 2. Graphene: A thin sheet of material with a crystalline structure of carbon atoms linked together in a hexagonal shape.

Note 3. Agglomeration: Adhesion and coalescence of particles to form large aggregates. Soot particles produced by incomplete combustion often form large agglomerates that are shaped like grape bunches.

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Contacts:
Media Contact

Daisuke Hayashi
National Institute for Environmental Studies

Expert Contact

Akihiro Fushimi
National Institute for Environmental Studies

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