Industry Steps Up Efforts To Understand Non-CO2 Effects Better

Beginning on New Year’s Day, European airlines will face a new requirement: As part of the European Emissions Trading System legislation, carriers will have to conduct monitoring, reporting and verification of aviation non-CO₂ effects.

The European regulatory policy is just one indication that the effects of contrails and aviation’s other non-CO₂ effects are increasingly in the spotlight. As awareness grows about the climate impact of contrails, a doubling down is expected in the new year of research into potential mitigation methods and into the fundamental atmospheric physics of the phenomenon.

• Better modeling and forecasting can help carriers avoid contrail formation
• Environmentalists want faster progress on contrails

The pace of contrail research is accelerating as aviation recognizes that improving sustainability depends as much on addressing climate-influencing non-CO₂ emissions as it does on reducing CO₂ emissions by expanding the use of low-carbon fuel and improving aircraft efficiency.

But environmentalists argue that although progress is being made, it is not happening rapidly enough. While the COP29 climate conference was underway in November in Baku, a group of aviation and climate scientists wrote an open letter urging faster action on contrails, calling it an opportunity to make a big difference at a relatively low cost to industry.

The problematic effects of contrails have been known for decades, but the issue has simply been swept under the rug, says environmental nongovernmental organization Transport & Environment (T&E), which published the letter.

“Crucially, if decisive action is taken, these effects may be partly mitigated in a faster and more cost-effective way than other climate issues, thanks to slightly rerouting a small number of targeted flights,” the letter said. “This would entail minimum impact on the aviation industry and passengers and a negligible risk of doing more climate harm than good.”

Contrails are “aviation’s climate opportunity of the decade,” T&E Aviation Technical Manager Carlos Lopez de la Osa said. “There are very few climate solutions that can be implemented so quickly at so little cost.”

Recent research efforts aimed at better understanding contrails support T&E’s position. Contrails form when hot engine exhaust mixes with the surrounding colder air to form ice crystals. In ice-supersaturated regions of the atmosphere, these crystals grow, and the contrail can persist and expand into an aircraft-induced cirrus cloud.

During the day, this thin layer of cirrus reflects sunlight and cools the Earth; at night, however, the cloud traps the heat and warms the planet. This imbalance, or “radiative forcing,” stemming from contrails is considered to have an overall warming effect on climate.

A major focus for investigators is perfecting techniques to measure and assess contrails produced by the current generation of aircraft and engines, thereby setting a baseline against which improvements can be measured and mitigation achieved.

In the U.S., NASA and GE Aerospace began evaluating aircraft-wake scanning technology in November during flight tests under the Contrail Optical Depth Experiment (CODEX). Conducted from Norfolk, Virginia, the tests involved NASA Langley Research Center’s Gulfstream III aircraft and GE’s Boeing 747-400 flying testbed.

Operating at altitudes over 30,000 ft. in restricted airspace off the East Coast, the Gulfstream trailed at various distances behind and above the 747, scanning the GE testbed’s wake with light detection and ranging (lidar) sensors to generate 3D imaging of the contrails. For CODEX, the 747 was configured with four standard GE CF6-80CB1F engines powered by regular Jet A fuel.

Besides improving knowledge of contrail formation and behavior over time, CODEX’s data is also expected to help GE pave the way for future flight-testing of lower-emission combustors and other advanced propulsion technologies planned with CFM International partner Safran under the Revolutionary Innovation for Sustainable Engines (RISE) program.

“We know from recent modeling assessments and consensus reports that contrails have a significant climate impact, and we think from current model estimates the magnitude of that impact from contrail cirrus clouds today is of similar magnitude to that from the accumulated emissions of aviation CO₂ over the past century,” says Richard Moore, physical research scientist at NASA Langley Research Center.

“So while it’s highly uncertain, we know that it’s a potentially important effect how the contrail interacts and evolves in the atmosphere and interacts with the radiation and the sunlight coming down and the radiation coming up from the Earth,” Moore explains. “This directly depends on the optical properties of that cloud. With the lidar, we’re bringing our own light source. Instead of having to rely on the Sun and specific solar angles, we can shoot a laser down at a green wavelength, 532 nanometers, and we can measure the amount of light that is backscattered up by the ice crystals in the contrail.”

The G-III was configured with Langley’s High-Altitude Lidar Observatory (HALO), an instrument developed to characterize distributions of greenhouse gases, clouds and small particles in the atmosphere. HALO provides nadir-viewing profiles of water vapor, methane columns and profiles of aerosol and cloud-optical properties. Until NASA conducted a test with Boeing under the ecoDemonstrator program in late 2023, HALO had never been applied to contrail analysis.

“We get a profile of that light being scattered back to the aircraft, and then we are able to relate that to the amount of light being not only scattered back but also absorbed and extinguished,” Moore says. “If we take that profile of light extinction through the contrail, we can add it up, and that’s something we call the optical thickness of the cloud, or the contrail. That’s an important parameter for understanding the radiative impacts.”

Together with information from the high-spectral-resolution lidar channels, the sensor suite included a laser tuned to specific absorption bands for water vapor. “So in addition to dragging the laser beam across the aircraft’s track to give us the profile of the contrails, we’re continuously measuring along the track the curtain of the water vapor field below the aircraft,” Moore says. To complete the dataset, the NASA aircraft also dropped radiosondes to measure temperatures.

The atmospheric data from the contrail surveys will help validate models that predict contrail formation conditions and forecast the optical thickness properties that feed into radiative-forcing calculations. The work is targeted not only at comprehending contrail formation but also, in the longer run, providing hard data to an industry, which is wary of encouraging contrail-mitigation policies if it turns out that the warming cirrus would have formed naturally anyway.

“There may be 70% uncertainty in the effective radiative forcing of contrails, but that should not prevent us [from] starting to reduce them,” Christiane Voigt, head of the cloud physics department at German aerospace center DLR, said at the ICAO Symposium on Non-CO₂ Aviation Emissions in Montreal in September. “At the lowest point on the error bar, they are still important.”

While many industry insiders believe immediate action is necessary to tackle contrails, some argue that a deeper comprehension of the phenomenon should take first priority.

A recent research study by scientists at Imperial College London highlighted the complexity of understanding how reducing contrails fits in with broader efforts to limit aviation’s climate impact.

The study, based on machine-learning analysis of satellite data on contrails over the North Atlantic, showed that modern commercial aircraft create longer-lived contrails at high altitudes than older aircraft do. Although modern aircraft emit less carbon than older aircraft, they may be contributing more to climate change through contrails, the researchers said in August.

“This study throws a spanner in the works for the aviation industry,” the study’s lead author, Edward Gryspeerdt, a Royal Society University Research Fellow at Imperial College London’s Grantham Institute-Climate Change and the Environment, said at the time. “Newer aircraft are flying higher and higher in the atmosphere to increase fuel efficiency and reduce carbon emissions. The unintended consequence of this is that these aircraft flying over the North Atlantic are now creating more longer-lived contrails, trapping additional heat in the atmosphere and increasing the climate impact of aviation.

“This doesn’t mean that more efficient aircraft are a bad thing,” he added. “Far from it, as they have lower carbon emissions per passenger mile. However, our finding reflects the challenges the aviation industry faces when reducing its climate impact.”

The study found that one simple step—reducing the amount of soot emitted from aircraft engines—could shorten the lives of contrails.

“Our study provides the first evidence that emitting fewer soot particles results in contrails that fall out of the sky faster compared to contrails formed on more numerous soot particles from older, dirtier engines,” said study co-author Marc Stettler, a professor of transport and the environment in Imperial College London’s department of civil and environmental engineering.

Mitigating strategies for contrail formation are focused primarily on avoiding ice-supersaturated conditions in the atmosphere and reducing the particles in the engine exhaust on which ice crystals can nucleate. Avoiding areas prone to contrail formation would require rerouting only a small fraction of flights, Voigt said.

Aircraft can avoid contrail-forming regions by flying higher or lower, but that will require improvements in modeling and forecasting to enable airline operations centers to predict atmospheric hot spots where contrails could form and to reroute flights with minimum impact on fuel consumption and emissions.

It also will also require collaboration with air traffic control, which must approve the changes. Operational trials are underway to determine the practicality and effectiveness of altitude adjustments. “Operationally, it is very challenging, and weather and contrail models must be improved and tested,” Voigt said.

A T&E study released in November found that slightly altering the routing of just a few flights could halve contrail warming by 2040 at a cost of less than €4 ($4.20) per flight. The report said that 80% of contrail warming is generated by only 3% of flights; geography, flight latitude, time of day and seasonality all play a role in their climate warming effects. The study noted that the extra fuel expended to avoid contrails would be less than 0.5% across the whole fleet over a year.

Overall, the T&E study showed that the climate benefits from contrail avoidance would be 15-40 times greater than the CO₂ penalty. Changing the flightpaths of only the worst culprits would have a disproportionately big impact.

The second key to contrail formation, soot in exhaust plumes, is already being tackled through engine technology improvements and fuel changes. Cleaner-burning engines and sustainable aviation fuel (SAF) have been shown to generate less soot and contrails. But the results include some surprises.

The latest generation of commercial turbofans with lean-burn combustion, such as GE’s GEnx 1 and CFM’s Leap 1, generate significantly less soot, also known as nonvolatile particulate matter (nvPM). Reducing soot should reduce ice crystals and therefore contrail formation, but recent flight tests show there is a limit.

As nvPM emissions fall, some models predict that ice crystal formation will diminish until reaching a lower limit set by naturally occurring aerosols in the atmosphere. Other models predict that ice formation will reduce at first but then increase as volatile particles in the plume take over from soot as nucleation sites.

The latter effect was confirmed by flight tests of a Leap 1A-powered Airbus A320neo in France under the Volcan project and a Leap 1B-powered 737-10 in the U.S. under the NASA--Boeing ecoDemonstrator program. The Volcan flights indicated that volatile particles become nucleation sites as soot reduces. The ecoDemonstrator flights showed 100% SAF reduces ice crystal formation, but contrails still form due to volatile particles.

In contrast to nvPM emissions, volatile particulate matter (vPM) consists of condensable gases in the exhaust plume that can form new particles or coat existing soot particles, making them more able to form ice crystals. They depend on both the combustion process and fuel composition, Richard Miake-Lye, principal scientist and vice president at Aerodyne Research, told the ICAO symposium.

Examples of vPM include unburned and partially combusted fuel, sulfuric acid from the fuel and lubrication oil vented from the engine. Oil is currently not considered an emission and is vented differently among engine manufacturers. There are certification standards for nvPM emissions, which are measured on the ground at the exhaust exit, but not for vPM, which evolves after emission.

Fuel composition is also an important factor. SAF has lower sulfur and higher hydrogen content than fossil jet fuel, reducing soot and ice formation. When fewer ice crystals are in the plume, those crystals grow larger and sediment out more quickly, reducing the lifetime and radiative forcing of the contrail, Stettler told the symposium.

All these tests, including those undertaken for the EU’s Emission and Climate Impact of Alternative Fuel (ECLIF) experiments, indicated the need for future campaigns to map a wide range of fuel compositions, engine types and combustor technologies. In particular, the surprise discovery of contrail formation even from lean-burn engines using SAF has led to calls for studies of other species of vPM, including engine oil. Such programs as CODEX have added to the database for evaluating contrails from older engines and have broadened the knowledge about their properties over time.

Plans to gather more data to improve climate and contrail models include launching satellite atmospheric infrared sounders and deploying water vapor sensors on in-service aircraft. “We have limited data in cruise with different aircraft, engines and fuels,” Moore says. “There are going to be some surprises, just as oil has risen to the surface with lean-burn engines.”