| May 14, 2025 |
Aircraft engines emit nanoparticles, including newly identified onion-like structures. Research explores their formation, health effects, and climate impacts.
(Nanowerk News) Aircraft emit nanoparticles smaller than 50 nanometers—one nanometer being a millionth of a millimeter—into the atmosphere, from ground level to the upper troposphere. Studies conducted in Europe, the United States, and Japan report high concentrations of these particles near airports, raising global concern about potential health risks. Additionally, contrails formed by aircraft exhaust particles contribute to atmospheric heating, prompting research into their climate impacts.
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Civil aviation primarily utilizes turbofan jet engines, whose particle emissions are predominantly volatile particles composed mainly of sulfates or organic compounds rather than non-volatile soot particles. The specific processes behind the emission and formation of volatile particles, however, remain unclear.
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Researchers investigated the physicochemical properties of nanoparticles emitted by aircraft engines, both volatile and non-volatile, to better understand these mechanisms. At a test facility at Zurich Airport in Switzerland, the team measured the morphology and internal microphysical structure of exhaust particles directly at the engine exit and again at a point 15 meters downstream. High-resolution transmission electron microscopy (HRTEM) enabled detailed observation of the particles collected on thin films.
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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. (Image: NIES/ZHAW/TMU)
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The researchers identified four distinct types of exhaust particles based on their internal structures. The first type exhibited turbostratic structures—layered graphene-like forms typically characteristic of soot. The second type, never previously identified in combustion exhaust, had onion-like structures with well-ordered spherical multilayers similar to graphite. The remaining two types were amorphous (non-crystalline), with the fourth being trace amorphous particles characterized by their particularly thin and non-crystalline appearance. While graphitic soot had previously been studied extensively, the discovery of the onion-like and amorphous particles was novel.
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Initially, turbostratic soot particles were abundant at the engine exit but represented less than 1% of the particles observed 15 meters downstream. The downstream particle population primarily consisted of the onion-like and amorphous particles, which were predominantly single, spherical particles measuring between 10 to 20 nanometers in diameter. These were identified as volatile particles formed through nucleation and condensation processes downstream from the engine, primarily from organic compounds originating from lubrication oil.
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The unique internal structures of these particles likely influence their physicochemical properties, such as volatility, surface reactivity, and solubility, potentially altering their interactions with human respiratory systems. While the identification of onion-like and amorphous particles significantly expands knowledge about aircraft emissions, critical questions remain about their precise physicochemical characteristics, origins, and formation mechanisms.
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Specifically, understanding whether onion-like particles behave similarly to soot or other volatile organic particles such as oil mist is essential. Due to their distinct structural characteristics, onion-like particles may interact differently within the atmosphere and human body compared to other particle types, necessitating further research into their health and environmental impacts.
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Moreover, onion-like particles are of particular scientific interest because similar particles are intentionally synthesized in nanomaterials research through methods involving the application of high energy to soot. Clarifying the mechanism behind their formation in aircraft engine emissions could yield important insights applicable to materials science and broader industrial applications.
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The findings are published in (ACS ES&T Air, “Unique Microphysical Structures of Ultrafine Particles Emitted from Turbofan Jet Engines”).
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