The use of more sustainable aviation fuels, or SAFs, is critical to decarbonising aviation, especially in the nearer term. With multiple production pathways already producing fuel to be ‘dropped in’ to existing Jet A-1 kerosene supplies, what’s the scientific latest on the impact of SAFs on emissions — and non-emission warming effects — from aviation?
Professor Volker Grewe, senior scientist at the DLR, the German Aerospace Center, tells us that the science continues to advance at speed. At the highest level of the current scientific consensus, Grewe says, referring to a recent paper by Lee et al in the journalAtmospheric Environment, “is that all aviation from 1940 to 2018 led to an impact on climate that is roughly driven 1/3 by CO2 and 2/3 by non-CO2 effects — i.e., contrails and NOx emissions.”
(This article, intended for digital publication, does not use the scientific subscript notation for abbreviations such as NOx for oxides of nitrogen or CO2 for carbon dioxide in order to aim for cross-platform digital formatting compatibility.)
Grewe emphasises that it is important to “note that the assessment of non-CO2 effects are still associated with large uncertainties,” but that “non-CO2 effects should be addressed in addition to a reduction of CO2 in order to significantly reduce the climate impact of aviation.”
Nonetheless, it is clear that reducing non-direct emissions like contrails and inhibiting cloud formation is an important part of reducing aviation’s warming contributions, and the science behind cleaner burning fuels — a contributor to these reductions in emissions and emissions equivalents alongside their lower carbon production — is developing alongside these new technologies.
In terms of “sustainable alternative fuels with similar properties as Jet-A1 — for example, drop-in,” Grewe tells us, “those are still different from type to type. Depending on the production of SAF there is still some net CO2 emitted and that amount varies between the individual SAF types. The combustion is similar to that of burning normal kerosene. Therefore the amount of emitted nitrogen oxides (NOx) is also similar to kerosene.”
The use of more sustainable fuels not only reduces the CO2 impact via its lower carbon industrial process — defossilisation of atmospheric CO2 in plant feedstocks, waste recycling plus methane reduction from solid waste pathways, and so on — but the chemical makeup of sustainable versus fossil kerosene is also different.
The amount of emitted soot particles are reduced leading also to fewer but larger ice particles in a forming contrail,” Grewe notes, “Note that [whether] a contrail forms or not doesn’t significantly differ between using SAF or normal kerosene, but the properties of the contrail are changed.”
Research on this topic, including new models enabled by digital technologies, atmospheric sensors and improved processing, continues apace. Indeed, recent research from MIT using NASA’s GOES-16 satellite — which captured images of the reduced contrail levels during COVID-19 travel restrictions — has shown promising advances on contrail mapping.
“Science is progressing,” Grewe enthuses. “We are also working on numerical models that are connected to weather models and predict and estimate what we call climate sensitive regions. This is actually a wide ranging research.”
As one example, Grewe cites research into dynamic, realtime mapping of “regions in which contrails form that have a large warming potential or [where] the emitted NOx leads to a large production of the greenhouse gas ozone.”
“Those maps are similar to weather maps and can be used for the planning of aircraft trajectories to avoid climate sensitive regions, and by that reduce the climate impact of non-CO2 effects during the flight,” Grewe explains. “This is work in progress and the quality of those forecasts has to be assessed carefully. But it is a quite promising approach.”
Author: John Walton
Published 16th March 2023
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