Cleaner burning aviation fuels can reduce contrail cloudiness

The study addresses how aviation fuel composition influences contrail microphysics and associated climate forcing. Contrail cirrus, formed when engine exhaust water vapor condenses and freezes on particles at cruise altitudes, exert a net warming effect and represent aviation’s largest non-CO2 climate impact. Prior modeling indicates that reducing contrail ice number concentrations could lower radiative forcing, and previous flight tests showed biofuel blends can reduce soot emissions at cruise. However, a direct experimental linkage between changes in soot emissions due to fuel composition (particularly aromatic and naphthalene content) and contrail ice crystal numbers, sizes, and optical properties remained unresolved. The authors hypothesize that lowering aromatic content—in particular bi-cyclic naphthalenes—reduces soot emissions, leading to fewer but larger contrail ice crystals, reduced extinction, and ultimately reduced warming.

The paper synthesizes prior work on contrail formation, persistence, and radiative forcing, noting global effective radiative forcing estimates and regional hotspots. It reviews mitigation strategies such as contrail avoidance via routing or altitude adjustments, which may incur operational penalties. Modeling studies suggest contrail climate forcing is sensitive to initial ice crystal number concentrations and soot emissions. Previous engine and flight studies reported that alternative fuels (FT and HEFA blends) reduce soot particle emissions; however, uncertainty remained about whether increased fuel hydrogen (and thus H2O emissions) could counteract benefits. The role of fuel sulfur in forming volatile particles has been extensively studied, generally affecting particle size rather than number, with limited impact on contrail ice numbers in soot-rich regimes. Theoretical and microphysical modeling indicates soot particles dominate ice nucleation at typical current emission levels; volatile/background particles become important in soot-poor regimes anticipated for future technologies.

Two coordinated DLR–NASA flight campaigns sampled exhaust plumes and contrails from a DLR Airbus A320 (ATRA) equipped with IAE V2527-A5 turbofan engines: ECLIF1 (2015) using the DLR Falcon 20, and ECLIF2/ND-MAX (2018) using the NASA DC-8. The A320 flew race-track patterns at constant cruise altitudes (8–12 km, Mach 0.58–0.76), while the sampling aircraft executed multiple plume and contrail transects at 7–41 km downstream (contrail ages ~39–140 s) under persistent contrail conditions (RHI>100%).

Fuels tested included two conventional Jet A1 references (Ref2, Ref3) and three low-aromatic blends: an FT-based semisynthetic blend (SSF1; Ref1+FT-SPK) and two HEFA-based sustainable aviation fuels (SAF1 and SAF2; Ref3/Ref4+HEFA-SPK). Blends were designed to vary total aromatics and bi-cyclic naphthalenes; SAF2 specifically minimized naphthalenes (~0.05 vol%) while maintaining similar total aromatics, yielding the highest hydrogen content (14.51 mass%) among tested fuels. Sulfur contents for SAFs were near zero to minimize sulfur effects. Certified measurements characterized aromatics (ASTM D6379), naphthalenes (ASTM D1840), hydrogen/H:C (ASTM D7171), specific energy (ASTM D3338), and sulfur (ASTM D2622).

Instrumentation: Contrail ice number concentrations and size distributions were measured with CAS (ECLIF1) and FFSSP (ECLIF2/ND-MAX), with calibrations and corrections for sizing, coincidence, and lower detection limits; effective-diameter-dependent corrections were applied to FFSSP for small ice. Nonvolatile particle (soot) number concentrations were measured by modified CPCs sampling through heated lines (250°C) to remove volatile components, with corrections for pressure effects and line losses; uncertainty ~±15%. CO2 was measured by Picarro CRDS (precision 0.1 ppmv, accuracy 0.3 ppmv) to derive dilution-independent emission indices (EI). Water vapor and ambient thermodynamics were measured with a frost point hygrometer (ECLIF1) and the NASA DLH (ECLIF2/ND-MAX) to derive RHI.

Data analysis: Emission indices for soot (EI_soot) and apparent ice (AEI_ice) were calculated by scaling particle enhancements to CO2 enhancements using a constant EI_CO2 (3.160 kg CO2 per kg fuel), assuming complete fuel carbon conversion. For ECLIF1, vertical profiles and anti-correlation of EI_soot and AEI_ice in wake vortices, combined with activated fractions, enabled inference of near-engine-exit AEI_ice and EI_soot; SSF1 EIs were scaled (+15%) to account for fuel flow differences between sequences based on ground-tested EI–fuel-flow dependence. For ECLIF2/ND-MAX, near-exit AEI_ice was derived from contrail observations within ±50 m of the source flight level (acknowledging potential ~10% sublimation loss). Size-resolved AEI_ice distributions were constructed for ~1-minute-old contrails at matched wake depths and fitted with lognormal functions to infer effective diameters. Ambient conditions and engine settings (fuel flow, N1, Mach) during analyzed sequences are reported (Table 2).

• Soot regulates contrail ice numbers under current emission levels. For Ref2 Jet A1, EI_soot = (4.9 ± 0.6) × 10^15 kg-fuel^-1 and AEI_ice = (4.2 ± 0.6) × 10^15 kg-fuel^-1, implying ~80–100% activation of soot into ice crystals in the soot-rich regime.
• Low-aromatic blends substantially reduced soot and ice numbers at cruise: • SSF1 (FT blend): EI_soot ≈ (2.0 ± 0.2) × 10^15 (≈50–60% reduction vs Ref2); AEI_ice ≈ (2.0 ± 0.2) × 10^15 (≈52% reduction). • SAF1 (HEFA blend): EI_soot ≈ (2.3 ± 0.2) × 10^15 (≈53% reduction); AEI_ice ≈ (2.3 ± 0.2) × 10^15 (≈45% reduction). • SAF2 (HEFA blend with very low naphthalenes, highest H content): EI_soot ≈ (1.1 ± 0.4) × 10^15 (≈77% reduction vs Ref2 reported mean; text cites 45–53% for HEFA and ~50% for FT at cruise, with SAF2 lowest overall); AEI_ice ≈ (1.1 ± 0.4) × 10^15 (≈74% reduction). SAF2 showed the strongest reductions, linking low naphthalenes and higher hydrogen content to minimized soot and ice numbers.
• Fuel composition drivers: • Higher hydrogen content and lower bi-cyclic naphthalene content correlate with reduced EI_soot and AEI_ice. Results imply bi-cyclic naphthalenes are more efficient soot precursors than mono-cyclic aromatics or aliphatics. • Low sulfur content in SAFs had minimal effect on contrail ice numbers in soot-rich conditions; volatile particle contributions were minor at EI_soot > 10^15 kg^-1.
• Ice crystal microphysics: • Effective diameter increased by ~40% for contrails from low-aromatic fuel: from ~1.9 µm (Ref2) to ~2.7 µm (SSF1) at ~1 minute age, consistent with fewer crystals sharing available water. • Despite slightly higher emitted H2O for higher-H fuels (~4% increase for SSF1), the dominant effect on size was reduced ice number concentration.
• Optical and climate implications: • Extinction of the ~1-min SSF1 contrail was ~30% lower than the Jet A1 contrail under comparable conditions. • Modeling studies cited indicate that 50–90% reductions in initial ice numbers in young contrails can reduce contrail-cirrus radiative forcing by ~20–70%, aided by shorter lifetimes due to faster sedimentation of larger crystals.
• Operational context: • Contrails were not observed for Ref3 under comparable conditions, emphasizing environmental dependence. SAF2 contrails were sampled at lower and more variable RHI (~110% ±5%), potentially contributing to lower activated fractions and greater ice loss, but still exhibiting the largest reductions among fuels.

The experiments directly link fuel composition to engine soot emissions and subsequent contrail microphysics. Reducing aromatic content—especially bi-cyclic naphthalenes—lowers soot number emissions at cruise, which in turn lowers the number of ice crystals formed in contrails. With fewer crystals, available water partitions into larger particles, reducing extinction and the capacity of contrails to absorb and trap radiation. These microphysical changes translate to reduced atmospheric energy deposition and warming. The findings reconcile concerns that higher hydrogen content (and thus higher H2O emissions) in alternative fuels might increase contrail forcing; in practice, the soot-number-mediated reduction in ice number dominates. In the soot-rich regime typical of current engines, volatile and background particles contribute little to ice nucleation, making soot reductions an effective lever for mitigating contrail climate impact. The results support strategies that promote low-aromatic, especially low-naphthalene, sustainable aviation fuels and potentially fuel specifications that limit aromatic subclasses to reduce aviation’s non-CO2 climate effects. Targeted use of such fuels during predicted contrail outbreak conditions could maximize climate benefits.

This work provides experimental evidence that cleaner-burning, low-aromatic sustainable aviation fuels substantially reduce soot emissions at cruise and, consequently, contrail ice number concentrations, increase ice crystal sizes, and reduce contrail extinction. The study closes a key gap by directly connecting fuel composition (including hydrogen content and naphthalene fraction), soot emission indices, and contrail microphysics. These findings imply that widespread adoption of low-aromatic fuels, and regulatory limits on specific sooting aromatics, could meaningfully reduce aviation’s contrail climate impact on short timescales. Future research should: (1) expand sampling across aircraft/engine types, atmospheric regimes, and seasons; (2) refine the roles of aromatic subclasses (e.g., di- vs mono-cyclic) in soot formation; (3) quantify radiative forcing reductions using integrated observations–modeling across contrail lifecycles; (4) assess interactions in potential soot-poor regimes from next-generation engines or hydrogen fuels; and (5) evaluate operational strategies for targeted deployment of SAF during high-impact contrail conditions.

• Data coverage for some fuels, particularly SAF2, is limited (fewer contrail crossings, variable and lower RHI), increasing uncertainty in AEI_ice estimates and activated fractions.
• Instrumentation differed between campaigns (CAS vs FFSSP; aerosol inlets), complicating direct cross-campaign comparisons; near-field AEI_ice derivations for ECLIF2 rely on assumptions about sublimation losses (~10%).
• Emission indices depend on CO2-scaling and assumed constant EI_CO2 (introducing <1% error) and corrections for fuel flow rate differences (15% scaling for SSF1), adding systematic uncertainty.
• Observations pertain to one aircraft/engine type (A320 with IAE V2527-A5) and contrails aged ~1 minute in specific midwinter conditions, which may limit generalizability to other platforms and environments.
• Contrails were not observed for Ref3 under comparable conditions, underscoring environmental variability in contrail formation and persistence.
• The study focuses on soot-rich regimes typical of current operations; conclusions may differ in future soot-poor scenarios (e.g., hydrogen combustion, advanced lean-burn engines) where volatile/background particles can play a larger role.