PSI researchers Benjamin Brem (left) and Sarah Tinorua in one of their laboratories at the PSI Center for Energy and Environmental Sciences.
PSI researchers conducted air quality measurements in the area surrounding Zurich Airport in the winter of 2022 and the summer of 2024. In these two measurement series, each lasting several weeks, they used special equipment to capture particulate matter in the air approximately one kilometre downwind from runway 28, the airport’s busiest. They then analysed the particulate matter in near-real time, that is, immediately after capture, using spectrometers to determine particle size and chemical composition. This approach has the advantage over previous studies of this type – which collect data throughout the day and only measure it later – of revealing the influence of changing wind directions or current aircraft approach patterns. Thus the sources can be mapped more precisely.
The measurements confirm findings already obtained from several other European airports, such as Amsterdam and Frankfurt: During takeoff and landing, aircraft especially emit ultrafine particles (UFP) with sizes below 100 nanometres from their engines. These are a mixture of solid soot and volatile sulphate particles produced during the combustion of fuel in the turbines and in the subsequent cooling of the exhaust gas. The researchers detected up to 300,000 UFPs per cubic centimetre of air. This is more than ten times the normal background level at the airport at night when no aircraft are operating.
In cities, typical UFP measurements range from about 5,000 to 40,000 particles per cubic centimetre of air, significantly lower than the values measured at the airport. However, urban air contains a larger proportion of the coarser particulate matter categories below 2.5 and below 10 micrometres. “So in terms of particle mass, the particulate matter pollution at the airport is lower than in the city, where it originates mainly from road traffic, heating systems, and industry,” says study author Sarah Tinorua from the PSI Center for Energy and Environmental Sciences. “But the number of particles at the airport is significantly higher.” This is relevant to human health, since smaller particles, when inhaled, can penetrate more deeply into the fine branches of the lungs. This is therefore a meaningful distinction for airport workers and the nearby residents.
Every plane brings a surge of particulate matter
The researchers had more or less expected to see the high concentration of ultrafine particles at the airport. What is new about their study is primarily that they also detected, for the first time, lubricating oil adhering to the ultrafine particles. “A previous study at Frankfurt Airport had already found lubricating oil in the particulate matter. However, in that study, the particles were collected and daily average values were determined,” reports study leader Benjamin Brem, also from the PSI Center for Energy and Environmental Sciences. “We, on the other hand, recorded measurements of the lubricating oil in real time, that is, immediately after a plane had flown by.” Every time a plane flew over the measuring station at an altitude of around 80 to 100 metres, the researchers recorded a sharp increase in the UFP concentration. “This is because the turbine exhaust gases are forced downwards behind the wings by the downdraft – essentially, directly into the funnel of our capture device,” explains Brem. Only at altitudes above 300 metres are such peaks no longer detectable, as the particles disperse rapidly in the air, depending on the wind, and no further exchange takes place with air at ground level.
Wind also proved to be an important influencing factor: The peaks were more pronounced during landings than during takeoffs. They only occurred strongly during takeoffs when the wind blew the UFP cloud from the runway to the measuring station. Furthermore, the counts of fine particles were higher when measured in strong winds, since in light winds particles clump together more easily over time, forming larger clusters.
The fact that aircraft emit primarily ultrafine particles is due to the relatively complete combustion of fuel in the hot gas turbines compared to other kinds of engines. “However, aircraft engines are optimised for high-altitude flight,” says atmospheric chemist Brem. This means that combustion on the ground during takeoff and landing is less complete and consequently produces more particulate matter. Evidently, some engines are also releasing lubricating oil residue – a semivolatile organic compound – into the exhaust. The measured values can be clearly attributed to airport influence and elevated particle concentrations. “The potential health effects of such lubricants being inhaled deep into the lungs have not yet been fully investigated,” says Tinorua. As a precautionary measure, in any case, it is imperative to eliminate such emissions as much as possible.
Previous measurements taken directly at the engines showed that lubricating oil emissions do not occur in all aircraft types. Their occurrence could therefore depend on the engine design or its maintenance status.
