DLR Study Reveals New Drivers of Contrail Formation Beyond Soot

Aviation’s climate impact is heavily influenced by contrail cirrus clouds, which form when hot engine exhaust meets cold, humid air at cruising altitudes. For years, the prevailing scientific consensus held that soot particles were the primary drivers of ice crystal formation in these contrails. However, a new study published in the scientific journal Nature challenges this long-held understanding, revealing that reducing soot does not automatically equate to fewer contrail ice crystals.

According to an official press release from the German Aerospace Center (DLR), recent measurement flights demonstrate that volatile organic compounds and lubricating oil vapors play a crucial role in contrail formation, particularly when aircraft utilize extremely low-sulfur fuels and modern lean-burn engines. The findings stem from the NEOFUELS/VOLCAN project, a collaborative research initiative involving DLR, Airbus, CFM International, and academic partners.

The research highlights a critical gap in current climate models, which may underestimate the environmental impact of contrails by failing to account for ice formation on liquid volatile particles. As the aviation industry pushes toward climate-compatible flight, we expect these insights to shape future engine designs, fuel compositions, and oil venting architectures.

Chasing Emissions at Cruising Altitude

To investigate the emissions of modern lean-burn engines, researchers conducted a series of complex flight tests in the spring of 2023. The NEOFUELS/VOLCAN campaign marked the first time emissions and resulting contrails from a lean-burn engine were measured in flight.

High-Speed Chase Maneuvers

The DLR utilized its Falcon 20E research aircraft to trail an Airbus A321neo equipped with CFM LEAP-1A engines. Over the course of 15 flights, the Falcon 20E performed high-speed chase maneuvers at an altitude of 10 kilometers above the Mediterranean and the Atlantic. The research aircraft sampled the exhaust plume at distances ranging from 40 to 250 meters and intercepted fully developed contrails several kilometers downstream.

By modifying engine control settings, CFM International enabled the researchers to compare emissions under both lean-burn and rich-burn operations. The engines were also tested using fuels with varying levels of sulfur and aromatics, providing a comprehensive dataset on how different variables affect contrail properties.

Beyond Soot: The Role of Volatile Particles

The flight measurements yielded unexpected results regarding the relationship between soot and contrails. While lean-burn operations successfully reduced soot emissions by three orders of magnitude compared to rich-burn conditions, the number of contrail ice crystals remained high.

A Shift in Scientific Understanding

The data indicated that the concentration of ice crystals far exceeded the number of measured soot particles. Instead, researchers observed a massive formation of liquid volatile particles in the cooling exhaust plume.

“The defining moment came when the initial data revealed no soot, but plenty of contrail ice crystals,” said Christiane Voigt, scientific lead of the project at DLR and Johannes Gutenberg University Mainz (JGU), in the DLR press release. “It immediately became clear that advancing our understanding of contrail formation will be essential for shaping the technological future of aviation.”

The study found that when using ultra-low-sulfur fuels, volatile organic compounds and lubrication oil vapors become increasingly significant in the formation of new particles. While lower sulfur content in fuels did reduce the number of contrail ice crystals, the presence of these other volatile elements means that soot reduction alone is insufficient to mitigate contrail-related climate impacts.

Updating Climate Models and Mitigation Strategies

The findings from the NEOFUELS/VOLCAN project extend the classical theory of contrail formation. Because most current climate models do not incorporate ice formation on liquid particles, they likely underestimate the true climate impact of aviation contrails.

Engineering Levers for Climate-Compatible Flight

To address these newly identified drivers of contrail formation, future mitigation strategies will need to look beyond current emission standards, which primarily regulate gases and non-volatile particles. The DLR notes that while current fuel sulfur content is capped at 0.3 percent by mass, with typical levels around 0.046 percent, further reductions may be necessary.

Additionally, optimizing lubrication oil venting systems could provide engine developers with a new engineering lever to minimize volatile particles and, consequently, the climate impact of contrails.

AirPro News analysis

At AirPro News, we note that the aviation industry has heavily invested in lean-burn engine technology as a primary means to reduce soot and nitrogen oxide emissions. However, this Nature study underscores the complexity of atmospheric chemistry and the unintended consequences of optimizing for a single emission metric. If volatile organic compounds and lubricating oils are significant contributors to contrail cirrus clouds, engine manufacturers may need to redesign oil venting architectures, a component previously overlooked in climate mitigation discussions. Furthermore, we believe this could accelerate regulatory pressure to mandate ultra-low-sulfur sustainable aviation fuels (SAF) globally, as traditional jet fuel may no longer align with the industry’s net-zero climate targets once these updated contrail models are adopted by policymakers.

Frequently Asked Questions

What are contrails and why do they matter?

Contrails are line-shaped ice clouds that form behind aircraft at cruising altitudes when hot engine exhaust mixes with cold, humid air. They are a major contributor to aviation’s overall climate impact because they can trap heat in the Earth’s atmosphere.

Did lean-burn engines reduce contrail formation?

While lean-burn engines reduced soot emissions by three orders of magnitude during the tests, the number of contrail ice crystals remained high. This indicates that other factors, such as volatile organic compounds and oil vapors, drive contrail formation when soot levels are low.

How were the measurements taken?

The German Aerospace Center (DLR) used a Falcon 20E research aircraft to fly closely behind an Airbus A321neo. The Falcon sampled the exhaust plume and contrails at distances between 40 and 250 meters during 15 flights at an altitude of 10 kilometers.

Sources

• German Aerospace Center (DLR)