Contrail cirrus account for the major share of aviation’s climate impact. Yet, the links between jet fuel composition, contrail microphysics and climate impact remain unresolved. Here we present unique observations from two DLR-NASA aircraft campaigns that measured exhaust and contrail characteristics of an Airbus A320 burning either standard jet fuels or low aromatic sustainable aviation fuel blends. Our results show that soot particles can regulate the number of contrail cirrus ice crystals for current emission levels. We provide experimental evidence that burning low aromatic sustainable aviation fuel can result in a 50 to 70% reduction in soot and ice number concentrations and an increase in ice crystal size. Reduced contrail ice numbers cause less energy deposition in the atmosphere and less warming. Meaningful reductions in aviation’s climate impact could therefore be obtained from the widespread adoptation of low aromatic fuels, and from regulations to lower the maximum aromatic fuel content.

Powering aircraft by sustainable aviation fuels (SAFs) is a pathway to reduce the climate impact of aviation by lowering aviation lifecycle CO₂ emissions and by reducing ice crystal numbers and radiative forcing from contrails. While the effect of SAF blends on contrails has been measured previously, here we present novel measurements on particle emission and contrails from 100 % SAF combustion. During the ECLIF3 (Emission and CLimate Impact of alternative Fuels) campaign, a collaboration between the Deutsches Zentrum für Luft- und Raumfahrt (DLR), Airbus, Rolls-Royce, and Neste, the DLR Falcon 20 research aircraft performed in situ measurements following an Airbus A350-941 source aircraft powered by Rolls-Royce Trent XWB-84 engines in 1 to 2 min old contrails at cruise altitudes. Apparent ice emission indices of 100 % HEFA-SPK (hydro-processed esters and fatty acids–synthetic paraffinic kerosene) were measured and compared to Jet A-1 fuel contrails at similar engine and ambient ice-supersaturated conditions within a single flight. A 56 % reduction in ice particle numbers per mass of burned fuel was measured for 100 % HEFA-SPK compared to Jet A-1 under engine cruise conditions. The measured 35 % reduction in soot particle numbers suggests reduced ice activation by the low-sulfur HEFA fuel. Contrail properties are consistently modeled with a contrail plume model. Global climate model simulations for the 2018 fleet conservatively estimate a 26 % decrease in contrail radiative forcing and stronger decreases for larger particle reductions. Our results indicate that higher hydrogen content fuels as well as clean engines with low particle emissions may lead to reduced climate forcing from contrails.

Although traffic exhaust emissions in Europe have been drastically reduced, airborne particle emissions caused by brakes and tires are still increasing with the number of vehicles. The measurement of non-exhaust emissions is an emerging technological challenge. We present a custom measurement setup to investigate the brake- and tire-wear emissions of an in-use battery electric vehicle. A separate brake housing and HEPA ventilation enabled airborne brake wear emissions to be measured under realistic conditions without external influences. The emission tests on a chassis dynamometer included particle number concentrations and particle size distribution for diameters of 4 nm to 10 μm. Emission indices were determined for three driving cycles: WLTC Class 3b, WLTC Brake Part 10, and a real driving cycle. Further investigations focused on emission control through regenerative braking and brake coating. Driving with regenerative braking reduced emissions by up to 89.9%, which related to the concentration of particles in the ultrafine/fine size range. Hard-metal brake coating led to a further significant reduction in emissions of up to 78.9%. The results point the way to future RDE measurement of non-exhaust emissions and show the potential of regenerative braking and brake coating to reduce airborne brake wear emissions.