Mitigating aviation's environmental impact through alternative fuels and cleaner engines.

Lean-burn engines are designed to reduce emissions, and sustainable alternative fuels provide additional potential for lowering particle emissions. However, the magnitude of the benefits from these mitigation levers remains poorly constrained, limiting robust predictions of aviation's climate impact. We present in-flight measurements of particle emissions from modern lean-burn LEAP-1A engines operated on Airbus A319neo and A321neo aircraft, burning conventional and sustainable aviation fuels. Emissions of nonvolatile particles larger than 14 nanometers from lean-burn engines were reduced by three orders of magnitude compared to rich-burn operation. Under rich-burn conditions, soot particle number emissions show an exponential dependence on fuel hydrogen content, with engine-specific variations. Volatile particle emissions scale with fuel sulfur content, and substantial differences were observed even between two engines of the same type. Optimizing fuel composition and combustor technology may reduce particle emissions, improve airport air quality, and mitigate contrail radiative forcing.