Ice nucleating properties of aircraft turbine engine soot particles with respect to cirrus clouds formations

Aircraft operate mainly in the upper troposphere/lower stratosphere — altitudes where the aerosol loading is rather low — emitting gases (mainly H₂O and CO₂) and soot particles (a result of the incomplete combustion of aviation fuel). At these altitudes, clouds composed of micrometric ice crystals (known as cirrus clouds) originate from the freezing of small liquid droplets and/or from the deposition of water vapor onto solid particles (called ice nucleating particles, INPs). Aircraft soot particles are thought to be efficient INPs for cirrus-cloud formation, therefore potentially disturbing the cirrus cloud coverage, resulting in a modified cloud radiative budget, hence affecting climate. To date, the ice-nucleating abilities (INAs) of aircraft soot have not been quantified partly because of the challenge to sample such particles behind a turbine engine.

In this work, we present a series of experiments conducted at the aircraft engine test cell of SR Technics at Zurich airport, aiming at quantifying the INAs of aircraft turbine soot particles. Exhaust from commercial turbofan engines was sampled using a traversable probe within 1.5 m downstream of the exhaust nozzle over a range over power levels from medium to high thrust. The exhaust sample was drawn through trace-heated lines and a series of driers into a stirred stainless steel tank, allowing the coagulation of the particles, similar to that thought to occur in the restricted volume between aircraft wingtip vortices. The stainless steel tank also acts as a reservoir for the rest of the ice nucleation experiment. The coagulated particles were then size-selected according to their electrical mobility diameters in all experiments and injected into a cloud chamber where they experienced cirrus-relevant temperature (T < -40 °C) and relative humidity (RH_ice > 100%) conditions, allowing them to form ice crystals. Together with the inline particle size and mass distribution measurements, the fraction of soot particles forming ice crystals at different RH_ice levels has been measured. A catalytic stripper operating at 350°C removing volatile material and sulfur was used upstream of the cloud chamber, helping to infer the effect of the mixing state on the soot INAs. In parallel, soot samples were collected for additional offline measurements. Microscopy and gas adsorption techniques were used to characterize the morphology of the soot particles (e.g., primary particle size, pore size distribution) as well as their surface properties (e.g., water affinity, organic/inorganic content) which are known to be critical parameters for the freezing mechanism of soot particles in the cirrus regime. Preliminary results show that samples after the conditioning with the catalytic stripper are more active INPs than the unstripped, suggesting that mixing state and organic/sulfur content could be important for determining the role of aircraft soot as INPs in the upper troposphere.