Characterisation of aircraft APU aerosol emissions relevant to contrail formation

Contrail cirrus, aviation-induced condensation trails and their associated cloudiness form when ice crystals nucleate on exhaust and ambient aerosols. The size, morphology, and chemical composition of these aerosols influence contrail formation and the resulting atmospheric perturbations, and are controlled by multiple factors, including fuel type and combustion conditions. To investigate these physicochemical effects, emissions from an aircraft Auxiliary Power Unit (APU) were characterised under full-load and ready-to-load operating conditions. Particle size distributions (PSDs) were measured for conventional Jet A-1 and aviation fuel surrogates composed of n-paraffins and iso-paraffins, to assess the influence of fuel composition on emissions. Under full-load conditions, n-paraffin surrogates produced the smallest particles, with a modal diameter of 22 nm, compared with 28 nm and 32 nm for iso-paraffin surrogates and Jet A-1, respectively.

The fractal structure of the emitted particles was examined using a tandem Aerodynamic Aerosol Classifier (AAC) - Scanning Mobility Particle Sizer (SMPS) system and Transmission Electron Microscopy (TEM). In parallel, the ice-nucleating behaviour of APU emissions was investigated using the Portable Ice Nucleation Experiment (PINE) chamber.

The IATA Net Zero Roadmap projects that 62% of aviation-sector CO₂ reductions by 2050 will rely on replacing 80-90 % of conventional aviation fuel with sustainable alternatives (IATA, 2023). As higher sustainable aviation fuel (SAF) blend ratios are increasingly adopted, understanding how fuel-dependent emission properties and ice-nucleating behaviour influence contrail formation is essential for assessing the full climate impact of these mitigation strategies.

Reference: IATA: Energy and New Fuels Infrastructure Net Zero Roadmap, International Air Transport Association, https://www.iata.org/en/programs/sustainability/roadmaps/, 2023.