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A research team from the National Institute for Environmental Studies (Japan), Zurich University of Applied Sciences (Switzerland), Tokyo Metropolitan University (Japan), and other institutions conducted emission tests on aircraft engines and examined the shapes and internal structures of the exhaust particles with high-resolution transmission electron microscopy.

The research team identified novel onion-like (multiple concentric spherical crystal layered) particles and amorphous (non-crystalline) particles, in addition to the well-known soot in the aircraft engine exhausts. The exhaust particles were typically very small (approximately 10–20 nm in diameter) and single spherical particles.

Although the detailed physicochemical properties of the onion-like particles are currently unknown, their behaviors in the atmosphere and in the body may be different from other particles. Therefore, further research is required to assess their potential impacts on climate and health.

The research paper was in ACS ES&T Air.

Aircraft emit nanoparticles (<50 nm 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, and there is worldwide concern regarding the effects on human health.

The atmospheric heating effects caused by contrails generated from aircraft exhaust particles are also known, and research is being conducted to assess their potential impacts on the climate.

Particle 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.

The 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.

The researchers 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.

Four types of aircraft engine exhaust particles with different internal structures were observed. Type (a) represented turbostratic particles with scattering-layered graphene-like structures 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 fraction of turbostratic (soot) particles was high at the engine exit and lower than 1% at 15 m downstream. 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, 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.

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.

The 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.