Simon Unterstrasser

Targeted flight rerouting for contrail avoidance relies on computational models to predict which avoidance actions would provide the most warming reduction. In the absence of empirical data for model validation, we propose using intercomparison to evaluate whether the lifetime total extinction of contrails (a proxy for climate impact) – as predicted by a high-fidelity contrail model – can be robustly predicted by lower-fidelity models when simulating contrails individually without feedbacks with a coupled weather model. We compare three contrail-cirrus models in the diffusion regime: CoCiP (a 0D plume model), APCEMM (a 2D simplified model), and EULAG-LCM (a large eddy simulation model applied on a 2D grid) under prescribed meteorological conditions. We identify that differential sedimentation and turbulence might be necessary to simulate a contrail in a physically consistent way. Since monodisperse 0D models are unable to represent differential sedimentation, the CoCiP and EULAG-LCM results have a low Pearson correlation coefficient of 0.50, compared to 0.93 between APCEMM and EULAG-LCM. The 0D and monodisperse nature of CoCiP affects its response by varying amounts in different regions of the parameter space. The presence of an explicit turbulence solver in EULAG-LCM enhances the ice crystal loss and increases the total ice mass, leading to a contrail with a higher settling velocity than in APCEMM, which parametrises turbulence. APCEMM contrails hence last longer and are larger. Hence, APCEMM overpredicts the lifetime total extinction by ~30 % relative to EULAG-LCM, demonstrating that a strong correlation between models does not imply agreement in the predicted plume-scale properties.

Contrail cirrus clouds are a major contributor to the climate impact of aviation. Large-scale models, such as general circulation models (GCMs) with an integrated contrail module, are used to estimate the radiative forcing of these clouds. However, small-scale processes cannot be explicitly resolved in these models and must therefore be parameterized. In this study, we develop a novel parameterization for the number of contrail ice crystals formed on ambient aerosols entrained into the exhaust plume behind aircraft burning hydrogen. The continuous entrainment of ambient aerosols, combined with higher supersaturation in hydrogen exhaust plumes, results in longer-lasting ice crystal formation than in conventional kerosene combustion, where ice crystals predominantly form on emitted soot. We construct the parameterization from a comprehensive database of time-resolved contrail formation simulations conducted with the Lagrangian Cloud Module in a box model approach. Shallow neural networks are used to reproduce the simulation outcome, complemented by analytical scaling relations to extend the applicability of the parameterization. The parameterization incorporates dependencies on ambient conditions, ambient aerosol properties and aircraft-related parameters. We compare the new parameterization with an existing one originally developed for conventional kerosene combustion that treats ice crystal formation as a single nucleation pulse. The comparison reveals that the assumption of a nucleation pulse is not reasonable for hydrogen combustion scenarios. We find it essential to base the parameterization on time-resolved simulations, as this realistically captures ice crystal formation on continuously entrained ambient aerosols.

Aviation forces the climate via multiple pathways: The emission of carbon dioxide (CO₂) from conventional propulsion technologies has a warming effect, but also non-CO₂ effects like contrail cirrus play a crucial role. For new hydrogen (H₂) based propulsion concepts there are no direct CO₂ 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 CO₂ or nitrogen oxides (NO_X). 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. 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 1.4·10¹³ to 4·10¹⁵ kg^-1H₂ 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.

Aviation forces the climate via multiple pathways: The emission of carbon dioxide (CO₂) from conventional propulsion technologies has a warming effect, but also non-CO₂ effects like contrail cirrus play a crucial role. For new hydrogen (H₂) based propulsion concepts there are no direct CO₂ 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 CO₂ or nitrogen oxides (NO_X). 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. 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 1.4·10¹³ to 4·10¹⁵ kg^-1H₂ 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.

We analyze ice crystal formation behind future hydrogen fuel cell-propelled aircraft via simulations using the particle-based Lagrangian Cloud Module (LCM) in a box model approach. Unlike conventional kerosene combustion, the exhaust of these aircraft could be significantly more humid and contain no soot particles, resulting in larger supersaturation values during plume cooling. These conditions can promote water droplet formation via Homogeneous Droplet Nucleation (HDN), which may substantially increase the number of contrail ice crystals. We implement the HDN into the LCM box model and validate the implementation against benchmark cases from the International Wet Steam Modeling Project. For the first time, the impact of HDN on ice crystal number in nascent contrails is quantified for exhaust parameters representative of fuel cell-propelled aircraft. We demonstrate that a low exhaust temperature together with high exhaust humidity leads to a large increase in ice crystal numbers due to additional droplet formation via HDN and their subsequent freezing. This would amplify the climate-forcing potential of an individual contrail. Conversely, reducing water vapour emissions suppresses HDN and effectively reduces the ice crystal number. The increase in ice crystal numbers is higher in cold ambient conditions Tamb ⪅ 218 K, which are, however, not expected to occur often on low flight levels of propeller aircraft. A sensitivity analysis reveals that faster plume dilution can lead to an increase of ice crystal number by orders of magnitude through an increased HDN rate. We propose that the slope G of the mixing line, which describes the thermodynamical conditions of the diluting plume, should be kept below around G < 10 Pa K−1 by technical means to ensure that HDN does not occur, regardless of the ambient conditions.

Aviation forces the climate via multiple pathways: The emission of carbon dioxide (CO2) from conventional propulsion technologies has a warming effect, but also non-CO2 effects like contrail cirrus play a crucial role. For new hydrogen (H2) based propulsion concepts there are no direct CO2 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 CO2 or nitrogen oxides (NOX). 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. 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 1.4·1013 to 4·1015 kg-1H2 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.

Contrail cirrus clouds are a major contributor to the climate impact of aviation. Large-scale models, such as general circulation models (GCMs) with an integrated contrail module, are used to estimate the radiative forcing of these clouds. However, small-scale processes cannot be explicitly resolved in these models and must therefore be parameterized. In this study, we develop a novel parameterization for the number of contrail ice crystals formed on ambient aerosols entrained into the exhaust plume behind aircraft burning hydrogen. The continuous entrainment of ambient aerosols, combined with higher supersaturation in hydrogen exhaust plumes, results in longer-lasting ice crystal formation than in conventional kerosene combustion, where ice crystals predominantly form on emitted soot. We construct the parameterization from a comprehensive database of time-resolved contrail formation simulations conducted with the Lagrangian Cloud Module in a box model approach. Shallow neural networks are used to reproduce the simulation outcome, complemented by analytical scaling relations to extend the applicability of the parameterization. The parameterization incorporates dependencies on ambient conditions, ambient aerosol properties and aircraft-related parameters. We compare the new parameterization with an existing one originally developed for conventional kerosene combustion that treats ice crystal formation as a single nucleation pulse. The comparison reveals that the assumption of a nucleation pulse is not reasonable for hydrogen combustion scenarios. We find it essential to base the parameterization on time-resolved simulations, as this realistically captures ice crystal formation on continuously entrained ambient aerosols.