Cleaner burning aviation fuels can reduce contrail cloudiness

Aviation's contribution to climate change is substantial, with contrail cirrus clouds—ice clouds formed from aircraft exhaust—being the primary contributor to its non-CO2 climate impact. For every kilogram of fuel burned, aircraft engines release significant amounts of water vapor and soot particles. At high altitudes, the water vapor condenses onto these particles and freezes, forming contrails. The persistence and extent of these contrails are influenced by atmospheric conditions, particularly temperature and humidity. Persistent contrails can spread to form extensive cirrus clouds, impacting the Earth's radiation budget. This impact is predominantly warming, contributing significantly to aviation's overall climate forcing. The regional impact of contrail cirrus can be considerably higher than the global average, particularly in busy air corridors. Air traffic volume has been increasing steadily, leading to a substantial rise in contrail cirrus-induced radiative forcing, with projections indicating a substantial increase in the future. Therefore, mitigating the climate impact of contrails is crucial. Several mitigation strategies are being explored, including optimizing flight routes and altitudes to avoid contrail formation. However, these strategies might come with drawbacks, such as increased fuel consumption and reduced airspace efficiency. A more promising approach involves reducing the radiative impact of the contrails that do form. Model simulations suggest that reducing the number of ice crystals within contrails can significantly decrease their radiative forcing. One potential mechanism to achieve this is to modify the composition of jet fuel to reduce soot emissions, which serve as nucleation sites for ice crystal formation. Previous studies have shown that biofuel blends can reduce soot emissions, but a direct experimental link between soot reduction and contrail ice crystal reduction hasn't been definitively established. Moreover, concerns have been raised that increased water vapor emissions from biofuels, due to their higher hydrogen content, might offset the benefits of reduced soot. This study directly addresses these gaps in knowledge and examines the relationship between fuel composition, soot emissions, and contrail properties to explore practical methods of mitigating the climate impact of aviation.

The literature extensively documents the climate impact of aviation, emphasizing the significant contribution of contrail cirrus. Studies using satellite data and climate models have quantified the radiative forcing of contrails, highlighting their warming effect. Research has explored various approaches to mitigate this impact, including operational strategies like altering flight paths and altitudes to minimize contrail formation. The influence of fuel composition on soot particle emissions has also been investigated, with some studies showing a reduction in soot using alternative fuels like biofuels. However, there's been debate about the net effect of these biofuels, considering the potential trade-off between reduced soot and increased water vapor emissions. Previous work has established a theoretical link between soot particle emissions and contrail ice crystal formation, suggesting that decreasing soot could effectively reduce contrail radiative forcing. However, direct experimental evidence supporting this link with alternative jet fuels has been lacking. This research builds upon these existing studies by directly measuring the impact of different fuel blends on soot and ice crystal formation under realistic flight conditions, aiming to provide conclusive evidence on the effectiveness of fuel composition as a mitigation strategy.

This research involved two flight campaigns, ECLIF1 (2015) and ECLIF2/ND-MAX (2018), conducted in collaboration between DLR and NASA. The campaigns used a state-of-the-art Airbus A320 (ATRA) equipped with a comprehensive suite of sensors, which measured real-time aircraft performance data and emissions. The aircraft flew oval patterns at a constant altitude and speed while sampling aircraft (DLR Falcon 20 for ECLIF1 and NASA DC-8 for ECLIF2/ND-MAX) probed the exhaust plume and contrails at varying distances. Five different fuels were tested: two standard Jet A1 fuels and three blends of Jet A1 with either synthetic (Fischer-Tropsch) or sustainable bio-based (HEFA) kerosene. The blends were designed to vary the aromatic content (and specifically naphthalene content) while controlling for sulfur content, allowing researchers to assess their effect on soot emissions and contrail formation. Measurements included ice crystal number concentrations and size distributions (using Cloud and Aerosol Spectrometer Probe (CAS) and Fast Forward Scattering Spectrometer Probe (FFSSP)), soot particle number concentrations (using modified condensation particle counters), and carbon dioxide (CO2) concentrations (using cavity ring down spectroscopy (CRDS)). Ambient conditions like temperature, pressure, and relative humidity with respect to ice (RHI) were also recorded. Data were analyzed in terms of emission indices (EI), which express the amount of a species (e.g., soot particles, ice crystals) emitted per kilogram of fuel burned, accounting for plume dilution. The study focused on contrails formed under conditions favorable for persistence, ensuring reliable measurements of contrail microphysical properties. Statistical analyses were performed to quantify the relationships between fuel composition, soot emissions, and contrail properties. The emission indices for soot particles were directly compared to the so-called apparent ice emission indices (AEIice), which are calculated similarly by scaling to CO2 emissions but represent the number of ice crystals formed per kg fuel consumed. This approach provided a direct comparison of the impact of fuel composition on both soot emissions and contrail ice crystal formation. Log-normal distributions were fitted to ice crystal size distribution data to characterize contrail microphysical properties. Finally, the study compared the properties of contrails formed from different fuels to assess the impact of fuel composition on contrail extinction and overall warming.

The study revealed a strong correlation between fuel composition, soot emissions, and contrail ice crystal formation. The reference Jet A1 fuel had high soot emission indices, with most soot particles being activated into ice crystals in the contrail. In contrast, the alternative fuel blends (SSF1, SAF1, and SAF2) showed significantly lower soot emission indices (45-53% lower than the reference fuel for the HEFA-based blends, and ~50% lower for the FT-based blend). Correspondingly, these alternative fuels displayed substantial reductions in apparent ice crystal emission indices (45-74% lower). The SAF2 fuel, specifically designed with low naphthalene content and high hydrogen content, exhibited the lowest soot and ice emissions among all tested fuels, demonstrating that bi-cyclic naphthalenes are more effective soot precursors than mono-cyclic aromatics or aliphatic hydrocarbons. Importantly, the study found that while the higher hydrogen content in the alternative fuels resulted in a small increase in water vapor emissions (around 4% for SSF1), this increase had a minor effect on ice crystal size. Instead, the reduction in soot particles led to fewer ice crystals, but those ice crystals grew larger due to the same amount of water vapor being distributed over a smaller number of ice crystals. The larger ice crystals in contrails formed from alternative fuels lead to a 30% reduction in contrail extinction compared to contrails formed from Jet A1. The study shows that reduced ice crystal numbers translate to reduced contrail extinction, less energy deposition into the atmosphere, and consequently, less warming. The relationship between soot and ice particle emission indices were found to be linear. This study therefore provides the first experimental evidence confirming the impact of fuel composition on contrail ice crystal numbers, a key parameter in determining contrail climate impact.

The findings directly address the research question regarding the relationship between aviation fuel composition, soot emissions, and contrail climate impact. The substantial reductions in soot and ice crystal numbers observed with the sustainable aviation fuel blends offer compelling evidence for the effectiveness of fuel modification as a mitigation strategy. These results strongly support previous modeling studies suggesting that reducing contrail ice crystal concentrations is a key factor in lowering contrail radiative forcing. The observed increase in ice crystal size with lower ice crystal number concentrations also aligns with theoretical predictions, indicating faster sedimentation and sublimation of larger ice crystals, leading to shorter contrail lifetimes. The strong correlation between fuel aromatic content (particularly naphthalene) and soot emissions highlights the potential for targeted fuel design to further reduce the climate impact of aviation. The relatively minor effect of the increased water vapor emissions from the hydrogen-rich fuels on ice crystal size, indicates that the reduction in soot emissions is the primary driver of the observed climate benefits. The study highlights the importance of considering both soot and ice crystal properties when assessing the effectiveness of fuel mitigation strategies. The results suggest that using low aromatic and high-hydrogen sustainable aviation fuels in contrail outbreak situations could considerably reduce their regional warming impact.

This study provides definitive experimental evidence demonstrating that using low-aromatic sustainable aviation fuels significantly reduces soot and ice crystal concentrations in contrails, thereby mitigating their warming effect. The higher energy content of these fuels further enhances their climate benefit by improving fuel efficiency. The reduced naphthalene content in the fuels proved particularly impactful in lessening soot and ice production. The findings strongly support the adoption of low-aromatic sustainable aviation fuels and stricter regulations on maximum aromatic fuel content as effective strategies to reduce the climate impact of aviation. Future research could focus on further refining fuel design to minimize soot precursor families, and on exploring the interactions between contrails and existing clouds to better understand the complex climate impacts of aviation emissions in diverse atmospheric conditions.

The study's focus on specific fuel blends and atmospheric conditions might limit the generalizability of the results. While the experiments covered a range of conditions conducive to persistent contrail formation, the findings might not be directly applicable to all flight scenarios. Also, the study mainly focuses on the near-field contrail characteristics, while the long-term evolution of contrails into cirrus clouds is complex and might be influenced by various factors not considered here. Furthermore, the detailed microphysical processes involved in ice crystal formation are complex and simplified by focusing on soot's role and considering only particles greater than 0.5-1.0 µm. The impact of volatile particles remains less clear, particularly in soot-poor conditions where their contribution might be more significant. Finally, the study primarily focuses on the radiative impact, and other potential impacts such as the influence of contrails on precipitation or atmospheric chemistry, are not fully addressed.