A team of researchers from Goethe University Frankfurt analysed the chemical composition of ultrafine particles and came across a group of organic compounds which, according to their chemical fingerprints, originated from aircraft lubrication oils.
For several years, the Hessian Agency for Nature Conservation, Environment and Geology (HLNUG) has been measuring the number and size of ultrafine particles at various air monitoring stations in the vicinity of Frankfurt International Airport.
These particles form during combustion processes, for example when wood or biomass is burned, as well as in power and industrial plants. Alongside road traffic, large airports are a major source of these ultrafine particles, which are less than 100 millionths of a millimetre (100 nanometres) in size.
Because the particles are so small, they can penetrate deep into the lower respiratory tract, overcome the air-blood barrier, and, depending on their composition, cause inflammatory reactions in the tissue. Furthermore, ultrafine particles are suspected of being capable of triggering cardiovascular diseases.
How do lubrication oils cause air pollution?
Now, the team has corroborated this finding by means of further chemical measurements of the ultrafine particles. They originated to a significant degree from synthetic jet oils and were particularly prevalent in the smallest particle classes, i.e. particles ten to 18 nanometres in size.
Such lubrication oils can enter the exhaust plume of an aircraft’s engine through vents, where nanometre-sized oil droplets and gaseous oil vapours are not fully retained.
In laboratory experiments, the researchers also succeeded in reproducing the formation of these particles from lubrication oils.
To this end, a common engine lubrication oil was first evaporated at around 300°C in a hot gas stream, which simulated the exhaust plume of an aircraft engine, and subsequently cooled down. The number-size distribution of the freshly formed particles was then measured.
Can these particles be reduced?
Alexander Vogel, Professor for Atmospheric Environmental Analytics at the Institute for Atmospheric and Environmental Sciences of Goethe University Frankfurt, explained: “When the oil vapour cools down, the gaseous synthetic esters are supersaturated and form the nuclei for new particles that can then grow fast to around ten nanometres in size.
“As our experiments indicate, these particles constitute a large fraction of the ultrafine particles produced by aircraft engines. The previous assumption that ultrafine particles originate primarily from sulphur and aromatic compounds in kerosene is evidently incomplete.”
Vogel concluded: “According to our findings, lowering lubrication oil emissions from jet engines holds significant potential for reducing these particles.”
The experiments show that the formation of ultrafine particles in jet engines is not confined to the combustion of kerosene alone, and potential mitigation measures should consider this. This means that using low-sulphur kerosene or switching to sustainable aviation fuel cannot eliminate all the pollution caused by ultrafine particles.
A comprehensive scientific study by the Federal State of Hesse, which starts in 2023, will examine pollution from ultrafine particles and their impact on health. In this context, the current study’s results can help identify airport-specific particles and derive possible mitigation measures.