Aged Aircraft Exhaust Particles Grow More Toxic as They Drift in the Atmosphere

Fine particulate matter has long been recognized as one of the most dangerous forms of air pollution, but a new study from the University of Basel suggests that the danger it poses is not fixed at the moment of emission. Tiny particles with diameters in the nanometer range, produced by fuel combustion in engines of all kinds, can remain suspended in the atmosphere for hours or even several days before they are inhaled. During that time, they do not simply drift passively. They undergo chemical transformations driven by sunlight and reactive components of the air, and those transformations can fundamentally change how harmful the particles are to human health. A research team led by Professor Markus Kalberer set out to quantify exactly how this happens, focusing on two very different combustion sources: a small aircraft jet engine and a marine diesel engine. Their findings, published in the journal Environmental Science: Atmospheres, reveal a striking asymmetry between the two emission types that could reshape how regulators think about aviation’s contribution to air pollution.

The health stakes of this question are considerable. Fine particles can penetrate deep into the respiratory tract, where they trigger oxidative stress, a biochemical process in which reactive molecules damage cells and tissues. Oxidative stress promotes inflammation, and over the long term it is linked to serious conditions including respiratory and cardiovascular diseases. Scientists summarize this harmful capacity in a single measurable property called oxidative potential, which describes how effectively a particle can drive damaging oxidation reactions once it reaches living tissue. Because people rarely inhale exhaust particles in their fresh state, the oxidative potential of aged particles is arguably the more relevant quantity for public health. Yet, as the Basel team notes, little has been known about how this property evolves over the hours and days that particles spend in the atmosphere before reaching human lungs.

To close that knowledge gap, the researchers used a photochemical reactor to simulate atmospheric aging over periods ranging from one to twelve days. Inside the reactor, exhaust particles from the aircraft turbine and the ship engine were exposed to controlled conditions mimicking sunlight and atmospheric chemistry, allowing the team to observe in the laboratory what would normally happen in the open sky. The central challenge was analytical: oxidative potential is difficult to measure, particularly in real time and on freshly generated aerosols. Conventional approaches often require collecting particles on filters and analyzing them later, which risks altering the very chemistry under investigation. The Basel group therefore needed an instrument capable of tracking the oxidative properties of exhaust emissions continuously as the particles aged.

That instrument was developed specifically for this purpose at the University of Basel. Known as the Online Oxidative Potential Ascorbic Acid Instrument, or OOPAAI, it measures how much a given aerosol sample can trigger harmful oxidative reactions, using ascorbic acid, a biological antioxidant, as the reactive target. By running the instrument online, the researchers could compare the oxidative potential of fresh exhaust against that of the same emissions after simulated atmospheric aging, without the delays and artifacts of offline sampling. Battist Utinger, one of the two first authors of the study from the Department of Environmental Sciences and the Swiss Nanoscience Institute, is shown in laboratory imagery operating the device, which the university describes as capable of quantifying the particles’ capacity to provoke oxidative stress.

The results divided cleanly along source lines. For emissions from the marine diesel engine, the health-relevant oxidative properties remained largely stable throughout the simulated aging period. The ship exhaust particles, in other words, were about as harmful after days in the simulated atmosphere as they were when freshly emitted. The aircraft emissions told a very different story. As the jet exhaust particles aged, their oxidative potential increased significantly, meaning the particles became progressively more capable of inducing the oxidative stress that underlies inflammation and chronic disease. The team attributes this increase to changes in the chemical composition of the particles as they reacted with components of the air under sunlight over an extended period, a process known in atmospheric chemistry as secondary aerosol formation and aging.

This asymmetry has immediate implications for how aviation pollution should be assessed. If freshly emitted aircraft exhaust understates the true health burden, then measurements taken at airport fences or in aircraft plumes shortly after emission may systematically underestimate the toxicity that downwind populations actually experience. Utinger emphasized this point in the university’s release: the study shows it is important to look beyond freshly emitted exhaust particles, because atmospheric processes can significantly alter the oxidative properties of particles and thus their potential to harm human health. In practical terms, a community located tens or hundreds of kilometers from a flight corridor could be exposed to particles more toxic than those measured near the source, complicating both exposure assessment and epidemiological studies that rely on proximity to airports as a proxy for aviation-related pollution.

Kalberer drew an even sharper regulatory conclusion. Current legal frameworks, he stated, such as the Swiss Ordinance on Air Pollution Control, do not adequately account for the most toxic sources of fine particulate matter and their properties. Air quality standards typically regulate particle mass or number concentrations without distinguishing between particles of different chemical character or different oxidative potential. The Basel findings suggest that two particles of identical mass can carry very different health risks depending on their origin and atmospheric history, and that a regulatory regime blind to this distinction may misallocate attention away from the sources that matter most. As aviation traffic grows and shipping continues to burn heavy fuels, the composition of the global particulate burden is shifting in ways that mass-based metrics cannot capture.

The broader public health context underscores why such refinements matter. The World Health Organization estimates that more than 7 million premature deaths occur annually due to exposure to high concentrations of air pollution particles, a toll the organization notes is more than twice the number of global deaths from AIDS, malaria and tuberculosis combined. Against that backdrop, understanding which emission sources produce the most oxidatively potent particles, and how that potency evolves in the atmosphere, is not an academic nicety but a question with direct consequences for millions of lives. The Basel study provides a methodological template for answering it: pair realistic atmospheric aging simulations with online, source-resolved measurements of oxidative potential, and compare across engine types rather than treating all combustion particles as equivalent.

The study, titled Online characterisation of oxidative potential and reactive oxygen species of fresh and aged aircraft turbine and ship engine particulate emissions, was published on 22 September 2026 in Environmental Science: Atmospheres. Its central message is deceptively simple: the toxicity of an exhaust particle is not a constant but a moving target, shaped by days of photochemical processing in the atmosphere. For aircraft emissions in particular, that target moves in the wrong direction, with oxidative potential climbing as particles age. For regulators, airport operators, and public health authorities, the implication is that the full health cost of aviation may only become visible downwind and down time, in the aged, chemically transformed particles that people actually breathe.

Subject of Research: Atmospheric aging and oxidative potential of fine particulate emissions from aircraft turbine and marine diesel engines

Article Title: Fine particles from aircraft exhaust become more harmful over time

Article References: Fine particles from aircraft exhaust become more harmful over time. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: fine particulate matter, aircraft emissions, oxidative potential, atmospheric aging, air pollution, oxidative stress, ship engine exhaust, photochemical reactor, public health, Environmental Science Atmospheres, University of Basel, aerosol chemistry