Fuel sulfur content can modulate contrail ice crystal numbers

In recent efforts to reduce the radiative forcing of aviation, fuel design has gained increased attention. Sustainable Aviation Fuels (SAF) are seeing wider adoption, and their positive impact on CO2 and non-volatile soot particle emissions is well-established. However, the effects of the reduction in fuel sulfur content on volatile particle emissions and contrails are unknown. This study presents new observations from in-flight measurements of emissions and contrails of an Airbus A350-900 burning fuels with different sulfur contents. We find a reduction in volatile particles and contrail ice crystals for low-sulfur fuels. For higher fuel sulfur contents, our findings demonstrate an additional contrail ice particle source through activation of sulfate aerosols. Our data-driven results need to be consolidated by in-flight observations with different fuels and engines. Eventually, climate impact estimates as well as regulations should account for the modulating effect of the fuel sulfur content on contrail ice particle numbers.