Nucleation of jet engine oil vapours is a large source of aviation-related ultrafine particles

Various studies identified airports as a major source of ultrafine particles (UFPs – aerodynamic diameter <100 nm), whereby little is known about their chemical composition and formation processes ^{[1] [2] [3] [4] [5]}. In a previous work, we determined the organic chemical composition of aviation-related UFPs and identified jet engine oils as a major contributor ^[6].

Here, we show the nucleation and particle formation potential of jet oil vapors, supported by a quantitative analysis of the full spectrum of jet engine oil components in particles with diameters <56 nm. We used a common synthetic lubrication oil to analyse the jet oils gas-to-particle partitioning behavior using laboratory based thermodenuder-experiments and quantified the oil contribution to ambient UFPs originating from Frankfurt International Airport.

We sampled UFPs on aluminium-filters at an air quality monitoring station 4 km north of the Frankfurt Airport using a 13-stage cascade impactor system (Nano-MOUDI). Quantitative characterization of UFPs in the size ranges 10–18 nm, 18–32 nm and 32–56 nm was performed by standard addition combined with liquid chromatography (UHPLC), followed by heated electrospray ionization (HESI) and mass analysis using a high-resolution Orbitrap mass spectrometer (HRMS). In parallel to filter sampling, the particle size distribution was monitored to determine the size-resolved total particle mass.

Thermodenuder-experiments enable the monitoring of the gas-to-particle partitioning behavior of jet engine oils at different temperatures. We observed a fivefold increase in total particle number at 300 °C, with a significant increase in the number of particles with a mean diameter of ~12 nm compared to the same experiments performed at 20 °C.

Particle diameters of UFPs from other directions (e.g. winds originating from the city centre) are larger compared to the UFPs downwind of large airports ^[4].They are rather in the same size region as the newly formed oil particles in our laboratory experiment. Quantification of the jet oil compounds in ambient samples was achieved by standard-addition of purchased original standards to the native sample extracts. Besides two ester base materials, additives were also quantified, including two amines serving as stabilizers and an organophosphate used as wear inhibitor/metal deactivator. The two homologous ester series were quantified using one ester compound and cross-calibration. We characterized the Nano-MOUDI to determine loss factors and corrected the ambient jet oil contribution to the total particle mass for each UFP size stage accordingly.

Results indicate that aircraft emissions have a strong influence on the total mass of the 10-18 nm particles. The aircraft fraction decreases with larger particles (e.g. 18-56 nm), implying that jet oils form new particles in the cooling exhaust of aircraft engines.

[1] Habre, R., et al. (2018) Environ. Int., 118, 48–59.

[2] Ditas, F., Rose, D. & Jacobi, S. (2022) Hessian Agency for Nature Conservation, Environment and Geology.

[3] Fushimi, A., et al. (2019) Atmos. Chem. Phys., 19, 6389–6399.

[4] Stacey, B., (2019) Atmos. Environ., 198, 463–477.

[5] Rivas, I., et al. (2020) Environ. Int., 135, 105345.

[6] Ungeheuer, F., et al. (2021) Atmos. Chem. Phys., 21, 3763–3775.