Non-CO2 climate impacts of aviation: Contrails
Eliminating the aviation sector’s contribution to global climate change will require two major changes on the technology side. The first is a wholesale shift from conventional fossil fuel-derived aviation fuel to a more climate-compatible set of energy carriers, including hydrogen and hydrogen-derived synthetic drop-in fuels made via processes that have very low net greenhouse gas emissions, biofuels made from waste material, and electricity from low- and zero-carbon power generators. The second strategy — which is the focus of this Briefing Paper — is a reduction in the formation of high-altitude, line-shaped clouds known as contrails. As discussed in this paper, contrails can be reduced by removing chemical compounds called aromatics from aviation fuel and by routing aircraft around regions of the atmosphere that are especially susceptible to contrail formation. To achieve midcentury decarbonization targets, the aviation sector needs to pursue both strategies — fuel shifting and contrail abatement — concurrently.
Greenhouse gas (GHG) emissions from the aviation sector have increased rapidly in recent decades, making up 920 Mt of global carbon dioxide (CO2) emissions from fossil fuels in 2019, compared to only 706 Mt in 2013. However, aviation has other non-CO2 effects on climate change resulting from NOx, water vapor, and aerosol emissions. According to recent studies, these non-CO2 effects can have a much larger impact on climate than CO2 emissions and could represent two-thirds of the total climate change impact from aviation.
What are contrails?
Contrails (or condensation trails) are the visible line-shaped clouds that form behind an aircraft when flying at high altitudes. At typical flight levels (~35000 feet / 10000 meters, upper troposphere), aircraft sometimes encounter so-called ice-supersaturated regions (ISSR). In these regions, the relative humidity with respect to ice exceeds saturation (i.e., very cold and moist atmospheric conditions) and cirrus clouds made of ice crystals can start forming. On the other hand, when jet fuel is combusted incompletely in the engine, it results in the emission of a number of components like CO2, H2O, SOx, NOx, CO, other hydrocarbons, and non-volatile particulate matter (nvPM) like soot. When water vapor and soot are emitted from the aircraft exhaust in ISSR, the soot particles act as condensation nuclei for the water vapor, freezing and forming ice crystals. When the humidity of the surroundings is high, these ice particles will continue to grow and eventually form contrails that can persist in the air (see Figure 1). Depending on the humidity conditions of the atmosphere, some contrails can be short-lived (up to 10 minutes) evaporating soon after being formed, while others can extend for kilometers and last many hours in the sky (long-lived contrails). When long-lived contrails maintain a linear shape, they are called persistent contrails and can last up to 10 hours. When they evolve into irregular shapes, they are called contrail cirrus and are not easily distinguishable from natural occurring cirrus clouds, persisting for longer periods of time. Contrails can still form outside ISSR in drier or warmer air, but they are only short-lived, covering small areas and thus, having a negligible impact on climate. Contrail cirrus have a similar impact on our climate as natural cirrus clouds that form at high altitudes (15000-30000 ft). Cirrus clouds reflect a small part of incoming solar radiation back to space and trap a large share of the infrared radiation from Earth, producing a net warming effect, as opposed to the net cooling effect from low clouds (see a simplified schematics in Figure 2).
What is the radiative forcing from contrails?
Radiative forcing (RF) is a measure of the difference between incoming and outgoing radiation of the Earth and is the term commonly used by scientists to quantify global warming. When a climate forcing factor (e.g., CO2) results in incoming energy being larger than outgoing energy, the planet will warm up (positive RF); and when the outgoing energy is larger, it will cool down (negative RF). Climate models have shown that the RF associated with contrail cirrus is about nine times larger than that from persistent line-shaped contrails alone. A recent study that evaluated the RF from the different CO2 and non-CO2 effects of global aviation, estimated that the RF from contrail cirrus is the largest component among all, and larger than the RF from CO2 accumulated in all aviation’s history, representing more than half of the total radiative forcing in this sector (see Figure 3). However, it is also the term with the largest uncertainty as it depends on many factors.
Aromatic content in jet fuel
The formation of soot appears to be largely associated with the aromatic content in jet fuel (especially naphthalene), which depends on the composition of the crude oil or crude blends used to produce the jet fuel. Since there is a large variation in crude oil compositions depending upon what a refinery buys, there can likewise be a large variation in the aromatic content in different batches of jet fuel supplied across the globe. The exact composition of the jet fuel used in a particular aircraft is normally unknown, since different batches of fuel are mixed in an airport’s storage facilities. International jet fuel standards today set a maximum limit of 3% and 25% in volume of naphthalene and aromatic content respectively (with an average actual content of 2%-3% and 15%-20% in jet fuel sold in Europe). A minimum of 8% aromatic content has also been historically established for safety considerations. Aromatics increase the density of the jet-fuel and encourages the seals to swell, providing more protection from leakage and avoiding damage to the fuel system. However, there is no consensus in the industry regarding the need for this minimum limit and a number of modern aircraft and engines today already use sealings that do not require aromatics in the fuel.
Researchers have estimated that burning a low-aromatic aviation fuel can reduce soot formation by 50-70%. In turn, an 80% reduction of soot particles in the exhaust could reduce contrail formation by 50%, although there is a high uncertainty in the magnitude of the effect that requires better measurements and modeling. When the soot concentration is low, fewer and larger ice crystals are formed, increasing the sedimentation rate and, thus, reducing the contrail’s lifetime and its overall RF. Therefore, lowering the aromatics and naphthalene content in jet fuel would reduce contrail formation.
How can aromatics be reduced?
Aromatics in fuels can be reduced through a process called hydrotreatment. This process is already widely used for reduction and removal of sulfur and aromatics in diesel. In hydrotreatment, hydrogen is used to saturate the aromatic hydrocarbons in the fuel in the presence of a catalyst. This process also reduces the fuel’s sulfur content, improving air quality as a positive side effect (by reducing the formation of SOx and thereby reducing acid rain). Untreated diesel has a significantly higher content of aromatics and napthalene than kerosene and hence further reduction of aromatics in jet fuel (e.g., down to 8% aromatics and near 0% naphthalene) should theoretically not pose a significant technical barrier. However, it is important to note that further reduction of aromatics/napthalene during the hydrotreatment process has the following consequences for refinery operators:
- More hydrogen is needed (consumed) to remove more aromatics/napthalene (~7.4 times more hydrogen to reduce aromatics and naphthalene compared to conventional hydrotreating for jet fuel).
- Due to more severe operating conditions required to reduce aromatics/naphthalene.
- More catalyst is needed to maintain the same jet fuel production rate and catalyst run time.
- Without increasing the catalyst quantity, more frequent changeout of fresh catalyst would be required.
- There could be metallurgical/operating constraints in the hydrotreating reactor due to higher heat release from more aromatics saturation.
Hydrogen supply can be one of the challenges for aromatics reduction, as it is a costly element of refining, and most refineries are usually short on supply. Current hydrogen production also emits significant amounts of GHG that will need to be reduced by way of low-carbon hydrogen production methods (e.g., electrolyzers powered by low-carbon electricity or applying carbon capture and storage to natural gas reforming processes and strict upstream methane emissions control to minimize leakage). As mentioned above, an increase in the amount of catalyst used for kerosene hydrotreating would also be needed in order to maintain the same production rate of jet fuel in the refinery. This could be done by purchasing more reactors (or revamping idle ones), or by replacing the catalyst more often, which would require shutting down the unit more frequently, increasing the production cost.
In economic terms, the catalyst replacement needed to achieve lower aromatic content in jet fuel should not have major impacts on refineries. The typical cycle length of jet fuel hydrotreaters today is between four to six years; after this time, the catalysts need to be swapped out. Reducing the aromatic content further would reduce this cycle length to two to three years, similar to diesel units. Furthermore, the quantities of catalysts applied in kerosene units are significantly lower than for diesel units. Nevertheless, time and financial investment would be needed in refineries to reduce the aromatic content in jet fuel, and currently there are no legal or financial incentives to decrease the amount of these components further. Regardless of the environmental benefits and the slightly higher energy content of a fuel low in aromatics, airlines may not find significant incentives to purchase a more expensive fuel. Limiting and monitoring aromatic content would require establishing new regulations and policies. However, if not done right, there is a risk that fuel producers decide not to sell in the regulated markets, producing fuel shortages, or that airlines decide to refuel in airports outside of the regulated regions.
How can sustainable aviation fuels help to reduce contrails?
Sustainable Aviation Fuels (SAF) are a loosely defined set of sustainably produced, non-fossil-derived fuels that are compatible with existing jet engines and fuel storage technologies because their chemical and physical properties are almost identical to those of conventional jet fuel. SAF, which can be made from biomass or from power to liquid (PtL) systems, are typically low in aromatics, producing less soot when combusted. This means that the use of SAF would have a similar effect to lowering the aromatic content in conventional jet fuel, although SAF can also increase the contrails occurrence by 1-8% due to higher water vapor emissions. The net effect of this higher contrail occurrence but lower soot emissions (~ -52%) result, however, in an annual decrease of net radiative forcing of around -44%.
As discussed in CATF’s 2022 report, biofuels alone will not be able to cover the whole aviation fuel demand due to limited biomass feedstock and other competing uses (cars and trucks currently consume the majority of fuels derived from biomass, but there is growing demand within the maritime, heavy industry, and power sectors). Meanwhile, PtL are still in their infancy and will take time to scale up. The SAF that is available for blending with conventional jet fuel can help to reduce aromatic content. For instance, a 50% HEFA blending with conventional jet fuel was found to decrease soot by 50–70%. SAF also has the added benefit of reducing CO2 emissions. The full life cycle emissions of SAF must be evaluated to make sure that the benefits from contrails reduction are not outweighed by increased emissions in SAF production and delivery (e.g., higher energy consumption for hydrogen production and fuel synthesis, long transportation routes, etc.).
Beyond SAF, hydrogen may play an important role decarbonizing aviation in the long term. In the next 10-15 years, we could see hydrogen powered commuter, regional, and short-range aircrafts commercially available. The first two would use fuel cells, while short-range aircraft could be powered by a hybrid system of fuel cells and hydrogen turbines, with the fuel cell used in cruise and the turbines during take-off to deliver the required higher thrust. As hydrogen becomes a feasible option in aviation, it is reasonable to wonder about contrail formation from hydrogen-powered aircraft. The main product from hydrogen fuel cells is water vapor while for hydrogen turbines are water vapor and nitrogen oxides. Water vapor emissions are 4.3 times higher in hydrogen aircraft than in conventional jet fuel; a recent study found that the global area covered by contrails would be expected to increase by 70% for liquid hydrogen combustion and fuel cell powered planes. However, as soot is not produced, the ice crystals created would be larger, and there would be fewer of them. As is the case with low aromatics jet fuel and SAF, the sedimentation rate is faster with large ice crystals, resulting in short-lived contrails. The net effect is a reduction of ~25% and ~20% in contrail cirrus RF for hydrogen turbines and fuel cells respectively, compared to conventional kerosene aircrafts. However, more analyses and flight measurements are needed to obtain a better characterization of hydrogen contrails. As part of the Airbus ZEROe roadmap, in 2022 the company launched the “Blue Condor” test program in collaboration with the German Aerospace Center (DLR) to compare the contrails formed from hydrogen and kerosene turbines hoping to shed some light on the impacts. In March 2023, Airbus and the DLR also launched the new VOLCAN flight test campaign to gather information on the non-CO2 emissions and the creation of contrails from an A321neo powered by 100% SAF.
Atmospheric factors that affect contrails formation
However, the formation of contrails depends not only on the amount of soot emitted by the aircraft but also on the atmospheric conditions. The number of initial ice crystals formed is proportional to the number of soot particles emitted, but atmospheric conditions determine the formation and persistence of the contrail. It has been estimated that only ∼12% of all flights over the North Atlantic region are responsible for 80% of the annual contrail energy forcing. Aside from the amount of soot in the exhaust, strong warming/cooling contrails are associated with factors like seasonal changes in radiation and meteorology, time of the day and background clouds. In the north Atlantic region, strong warming contrails appear to be more commonly formed at night, during winter and above low-level clouds. For instance, during the day, contrails reflect back to space part of the energy from the sun adding a cooling component, an effect that does not happen during the night. In winter, ISSR are larger in this region, which increases the amount of time that a transatlantic flight can create contrails. ISSR are typically a few hundred meters thick and can cover tens to hundreds of kilometers wide.
Route optimization to avoid these regions where large and long-lasting contrails can be formed, can be achieved through effective flight planning. Atmospheric data (temperature and humidity) and good forecasting models can be used to identify and predict the location of ISSR to deviate the route accordingly. In 2021, EUROCONTROL’s Maastricht Upper Area Control Centre (MUAC), in partnership with the DLR conducted the world’s first operational trial to investigate the feasibility of contrail prevention by Air Traffic Controllers. The trials ran from January to December 2021 between 3pm-5am UTC (when contrails formation probability is higher) covering the MUAC airspace (Belgium, Luxembourg, the Netherlands and north-west Germany), used by 16% of all flights passing through Europe. The objectives of the trial were evaluating if contrail formation can be predicted with reasonable accuracy and testing the procedures to avoid persistent contrails in real time by modifying the flight level. During the trial, flights were tactically requested to deviate their flight level (~2000 feet / 600 meters up or down) based on ISSR forecasts. Despite the low air traffic in 2021 due to the COVID pandemic, results from the trial showed that contrail prevention is operationally feasible, although better ISSR prediction is needed. It is also important to consider that route detours have a penalty in terms of fuel consumption, translated in higher cost for the airline. Higher fuel consumption also increases CO2 emissions, and therefore, the tradeoff between contrail avoidance versus higher CO2 emissions needs to be carefully analyzed in each case. Another solution suggested by some researchers is to use the scarce SAF available in the small proportion of flights that cause the strongest warming contrails. This smart allocation (e.g., applying a 50% blending rate to 2% of the flights associated with the highest warming contrails) would multiply the climate benefits by 9-15 times.
What measures can be taken to reduce contrails?
Aviation contrails warm the planet two times more than CO2 emissions on average. Despite the high uncertainty in the radiative forcing estimations, it is well established that contrails produce a net positive warming effect in the atmosphere, similar to that from cirrus clouds. The two factors responsible for contrail formation are the amount of soot emitted from the engine exhaust and the atmospheric conditions during the flight. Optimised flight planning can help reduce contrails formation by avoiding ISSR, where the most persistent contrails are created. It’s been demonstrated that ISSR can be predicted, and contrail formation avoided, although higher accuracy in the modelling is needed. To mitigate soot formation, refiners can reduce the amount of aromatics (especially naphthalene) in jet fuel or airlines can use SAF which has low levels of aromatics.
Legislators around the world should establish measures to implement adequate monitoring rules and limitations to the aromatic content in jet fuel as a near-term measure while also pushing forward on measures that promote or require uptake of climate compatible forms of SAF. This could be done by amending the aviation fuel standards (e.g., ASTM) or by creating new ones. Non-CO2 effects are starting to be looked at in some regions, mainly to improve our understanding of the impacts. As per the recently revised EU ETS Directive, EU aircraft operators have to monitor and report non-CO2 effects from each aircraft they operate as of 2025. Based on the data collected, the European Commission may put forth a legislative proposal to include non-CO2 emissions within the scope of the EU ETS. In the United Kingdom, one of the six measures introduced in the jet-zero strategy focuses on working closely with academia and industry to better understand the science and potential mitigations of non-CO2 impacts. Similarly, the U.S. Federal Aviation Administration (FAA) is leading a research program aimed at improving the scientific understanding of non-CO2 aircraft emissions and their impacts. While much of this research will involve satellite measurements, FAA “will also support aircraft flight measurements of contrails and industry partners to evaluate and validate the performance of [a predictive] tool such that the tool can be used more widely.” The uptake of contrails-reducing SAF in the United States could be accomplished by the 2022 Inflation Reduction Act’s Sustainable Aviation Fuel Tax Credit, which provides producers of qualifying SAF with tax credits worth between USD $1.25 and $1.75 per gallon, depending on the carbon intensity of the fuel.
Additional work needs to be done in both Europe and United States on the potential development of policies and other measures that encourage or require aircraft to avoid ISSR where feasible, and/or to prioritize the use of SAF on the flight paths that are responsible for the most contrail formation. The efficacy of both these strategies hinges on the ability of flight planners to predict the location of ISSR, however—so more research is needed from government agencies like DLR in Germany and NASA in the United States on ways to improve the tools and techniques behind ISSR prediction.
CATF is helping to tackle these challenges through rigorous analysis, public policy design, public education, and advocacy. As part of this effort, CATF is currently working on a study to understand the requirements in existing refineries to reduce contrails precursors (i.e.: aromatics) in jet fuel at the levels needed with current technologies.
