Substantial aircraft contrail formation at low soot emission levels

Substantial aircraft contrail formation at low soot emission levels

Article Date: 01 April 2026
Article URL: https://www.nature.com/articles/s41586-026-10286-0
Article Image: https://www.nature.com/articles/s41586-026-10286-0

Summary

This in-flight study reports the first measurements of emissions and contrails from a modern lean-burn engine (CFM LEAP-1A on an A321neo). The key finding is that, despite soot (non-volatile particle) emissions dropping by about three orders of magnitude in lean-burn cruise, persistent contrails with large numbers of ice crystals still form. Measured contrail ice emission indices (EI_ice) were roughly 1.6 × 10^15 ice crystals per kg fuel for Jet A-1 under lean-burn conditions — about 1,000 times higher than the measured non-volatile soot emission index (EI_nv).

The study shows that volatile particles (d > 5 nm) formed from fuel sulfur oxidation, lubrication oil vapours and low-volatility organics can nucleate and grow in the exhaust plume and act as ice-nucleating sites when soot is rare. Low-sulfur, low-aromatic HEFA blends reduced initial ice numbers by roughly a factor of three compared with higher-sulfur Jet A-1, and ultralow-sulfur fuels (few ppmm) are predicted to reduce EI_ice by an order of magnitude. The authors updated microphysical models (ACM and MoMiE) to include sulfuric, organic and oil-vapour pathways and found good agreement with observations when these volatile sources are included.

The implications are broad: fuel composition (especially fuel-sulfur content and aromatics), oil venting and volatile-particle formation must be considered alongside soot reductions when estimating contrail climate impacts and when setting fuel and engine regulations and design choices.