Aviation forces the climate via multiple pathways: The emission of carbon dioxide (CO<sub>2</sub>) from conventional propulsion technologies has a warming effect, but also non-CO<sub>2</sub> effects like contrail cirrus play a crucial role. For new hydrogen (H<sub>2</sub>) based propulsion concepts there are no direct CO<sub>2</sub> and soot emissions, but higher water vapor emissions, which can affect the formation, micro-physical properties and the climate impact of contrails. In-flight measurements of relevant contrail and emission parameters of fuel cell propulsion systems are non-existent. In order to study contrail formation in the wake of fuel cell powered aircraft, a fuel cell exhaust emulator (FCEE) was designed by Airbus to produce the low temperature and high humidity exhaust conditions expected from these systems. Due to their unique exhaust composition and thermodynamic emission characteristic, processes such as plume mixing with ambient air are crucial to understand contrail formation. Further, to compare engine and fuel-dependent contrail properties, in situ methods derive a dilution corrected apparent ice emission index (AEI) using conservative tracers, like CO<sub>2</sub> or nitrogen oxides (NO<sub>X</sub>). As the FCEE emits no such inert trace gas, a newly developed artificial dilution tracer system was used. The design and characterization of this system, as well as the significance and application for exhaust dilution and emission index assessment of fuel cell-powered aircraft in flight, are described.</p> <p>As part of the Nephele flight test campaign, in December 2023 in Minnesota, USA, the prototype FCEE and artificial dilution tracer release system were installed on board a Convair C131. The sampling system was installed on a Piper Cheyenne 400LS chaser aircraft, together with instrumentation for emission and contrail measurement, both aircraft operated by AV Experts LLC. A proof-of-concept of the artificial tracer system was demonstrated in contrail and emission flights. Contrails formed on aerosol particles emitted by the FCEE near the homogeneous freezing threshold (at ambient temperatures around 230 K) were sampled at 400 hPa in distances between 50 and 1500 m behind the aircraft. An increase of AEI from 1.4·10<sup>13</sup> to 4·10<sup>15</sup> kg<sup>-1</sup><sub>H<sub>2</sub></sub> with increasing temperature difference of 3 to 5 K below the homogeneous freezing temperature was observed. As contrails from fuel cell systems already form at higher ambient temperatures compared to conventional propulsion systems, these are the first contrail measurements in this temperature range. Complementary model simulations suggest that droplet freezing is responsible for the evolution of ice crystal numbers in this temperature range.