Substantial aircraft contrail formation at low soot emission levels

Contrail cirrus are a major contributor to the climate forcing from aviation. Yet, the number of contrail ice crystals forming behind aircraft with modern lean-burn engines is unknown. Theory spans a five orders of magnitude range in ice crystal numbers – rendering related climate effects unpredictable. Here, we present first in-flight observations of contrails formed behind an aircraft with lean-burn engine technology. We find a massive reduction in soot particle number emissions, three orders of magnitude lower than soot emissions from conventional rich-quench-lean engines. In contrast, volatile particle number emissions - as well as contrail ice crystal numbers - exceed 10^15 particles per kilogram of burned fuel. We provide first experimental evidence and theoretical explanations for contrail ice activation on volatile aerosol in the absence of soot. Our results demonstrate the impact of lean-burn engine configurations, fuel composition, and ambient conditions on contrail ice crystal numbers. The integration of our data in models will enable to reliably predict the fleet-wide contrail climate effect. Our findings point to the need to minimize volatile particle emissions and affect industrial decisions on engine and fleet design for competitive and clean future aviation.