Research Provides Complex Answers To The Effects Of Aircraft Contrails

Efforts to understand and mitigate aviation’s non-CO₂ emission effects have focused on contrails, one of the most visible signs of aviation’s impact on the Earth’s atmosphere.

Contrails form through the mixing of warm, moist engine exhaust 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 spread out to produce aircraft-induced cirrus cloud.

During the day, the thin layer of cirrus reflects sunlight and cools the Earth. But at night, the cloud traps heat trying to escape into space and warms the planet. Overall, this imbalance, or “radiative forcing,” caused by contrails is considered to have a warming effect on climate.

It is difficult to compare the climate effect of CO₂, which stays in the atmosphere for thousands of years, with that of contrails, which last a few hours. But the consensus is that the effect of aviation’s non-CO₂ emissions is on the same order of magnitude as its CO₂ emissions.

But where aviation’s CO₂ emissions are hard to abate compared with other sectors, there is promise that contrails could be virtually eliminated by changing the altitudes of a small percent of flights to avoid ice-supersaturated regions, albeit at the cost of a small increase in CO₂ emissions from rerouting aircraft.

There is significant uncertainty in the understanding of contrail formation and the climate effect of contrail cirrus, however, and industry is wary of encouraging policies and regulations that would require mitigation of contrails 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 starting to reduce them. At the lowest point on the error bar, they are still important,” Christiane Voigt, head of the cloud physics department at German aerospace center DLR, told the ICAO Symposium on Non-CO₂ Aviation Emissions in Montreal in September.

OPERATIONAL MITIGATION

Contrail formation requires two key things: ice-supersaturated conditions in the atmosphere and particles in the engine exhaust on which the ice crystals can nucleate. Avoiding one and reducing the other offers the potential to mitigate the problem, but with caveats.

Ice-supersaturated regions are thin layers of atmosphere many miles wide but only a couple of hundred feet deep. They are only found in the troposphere. Further north, where the tropopause is lower and aircraft cruise in the stratosphere, contrails do not form because the air is too dry.

Ice supersaturation can occur in frontal and high-pressure regions where moist air is lifted to cruise altitudes. The regions depend on actual weather conditions and have a strong regional dependence, but they are small and avoiding them would require rerouting of only a small fraction of flights, Voight said.

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

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

TECHNICAL MITIGATION

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 are not quite as expected.

The latest generation of commercial turbofans with lean-burn combustion—GE’s GEnx and CFM’s LEAP—generate significantly less soot, also known as non-volatile 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 reduce, some models predict ice crystal formation should reduce until it reaches a lower limit set by naturally occurring aerosols in the atmosphere. Others predict 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 family aircraft in France under the Volcan project and a LEAP-1B-powered Boeing 737 MAX-10 in the US under Boeing’s ecoDemonstrator program. The Volcan flights indicated volatile particles take over as nucleation sites as soot reduces. The ecoDemonstrator flights showed that using 100% SAF reduces ice crystal formation but contrails still form because of volatile particles.

Volatile particulate matter (vPM) is 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 symposium.

Examples of vPM include unburned and partially combusted fuel, sulfuric acid from sulfur in the fuel, and lubrication oil vented from the engine. Oil is currently not considered an emission and how it is vented differs between engine manufactures. 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 an important factor. SAFs have lower sulfur and higher hydrogen content than fossil jet fuel, reducing soot and ice formation. When there are fewer ice crystals in the plume they grow larger and sediment out more quickly, reducing the lifetime and radiative forcing of the contrail, Marc Stettler, a reader in transport and the environment at the Imperial College London, told the symposium.

NEXT STEPS

To develop technical solutions to the contrail issue, the UK’s government-backed Aerospace Technology Institute has launched a £20 million ($26 million) non-CO₂ program.

“If we wait too long, we risk not having technology ready for the next generation of aircraft,” Adam Morton, ATI head of technology for sustainability and strategy, told the symposium. “Do we wait for the science or start developing the technology? Doing it in parallel is the right decision.”

The near-term need is for more data to improve climate and contrail models. Plans 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. There are going to be some surprises, just as oil has risen to the surface with lean-burn engines,” NASA’s principal investigator for ecoDemonstrator, Rich Moore, told the symposium.