In-flight particle emission measurements behind a hydrogen-fueled turbojet engine during the Blue Condor campaign

Hydrogen-powered aircraft are considered a key technology for reducing the climate impact of aviation, particularly for short- and medium-range applications. However, the assessment of their environmental effects critically depends on a robust characterization of emissions under realistic operating conditions. To date, in-flight emission data for hydrogen combustion engines at cruise altitude are essentially nonexistent, leading to large uncertainties in climate impact assessments and model representations.

Here, we present results from the Blue Condor campaign, a collaboration of DLR, Airbus, The Perlan Project, and AV Experts, which aimed to characterize in-flight emissions and contrails from a hydrogen-fueled turbojet engine and to directly compare them with emissions from a kerosene-fueled engine. Both engines were mounted on Arcus J gliders and operated under the same atmospheric and operational conditions.
An autonomous and modular emission measurement system was successfully developed and integrated onto the high-altitude research aircraft Grob EGRETT. The system included instrumentation for non-volatile and total particle number (tPM) concentrations and particle size distributions, optimized for formation-flight sampling in aircraft exhaust plumes.

The measurement system was deployed during formation flights behind the hydrogen-powered turbojet engine, enabling the first in-flight measurements of a hydrogen combustion engine at cruise altitude (FL 180-320). Particle emissions were sampled at distances of several tens to hundreds of meters downstream of the engine exhaust, ensuring representative plume conditions. For reference, analogous measurements were conducted behind a kerosene-fueled engine at similar flight levels and ambient conditions.

The tPM emissions were found to be three orders of magnitude lower for the hydrogen engine. However, the tPM emission indices of the hydrogen engine range from 10¹¹ to 10¹⁵ particles of kg fuel burned, depending on engine conditions.

The hydrogen engine particle emissions exhibit a distinct

particle size distribution, in contrast to the particle size distributions typically observed in kerosene combustion, both in terms of modal structure and emission indices.

the observed particle emissions most likely originate from non-combustion-related sources, such as lubrication oil. This interpretation is supported by complementary laboratory measurements.

These results provide the first observational constraints on in-flight particle emissions from hydrogen-fueled aircraft engines which are precursors for contrail formation and represent an important step toward assessing their atmospheric and climate impacts.