03 Jul 2023
 | 03 Jul 2023

Contrail formation on ambient aerosol particles for aircraft with hydrogen combustion: A box model trajectory study

Andreas Bier, Simon Unterstrasser, Josef Zink, Dennis Hillenbrand, Tina Jurkat-Witschas, and Annemarie Lottermoser

Abstract. Future air traffic using (green) hydrogen (H2) promises zero carbon emissions but the effects of contrails from this new technology has hardly been investigated. We study contrail formation behind aircraft with H2 combustion by means of the particle-based Lagrangian Cloud Module (LCM) box model. Assuming the absence of soot and ultrafine volatile particle formation, contrail ice crystals form solely on atmospheric background particles mixed into the plume. While a recent study extended the original LCM with regard to the contrail formation on soot particles, we further advance the LCM to cover the contrail formation on ambient particles. For each simulation, we perform an ensemble of box model runs using the dilution along 1000 different plume trajectories that are based on 3D Large Eddy Simulations using the FLUDILES solver.

The formation threshold temperature of H2 contrails is by around 10 K higher than for conventional contrails (which form behind aircraft with kerosene combustion) due to a factor of 2.6 higher energy-specific water vapor emission. Therefore, contrail formation becomes primarily limited by the homogeneous freezing temperature of the water droplets formed on the ambient particles such that contrails can form at temperatures down to around 234 K.

The number of formed ice crystals varies strongly with ambient temperature even far away from the contrail formation threshold. The latter is because the water-supersaturation in the plume lasts longer for colder conditions and, hence, more of the entrained aerosol particles can form droplets and ice crystals. The contrail ice crystal number clearly increases for a higher ambient aerosol number concentration. The increase becomes weaker for higher number concentrations (>≈ 200 cm−3) and lower ambient temperatures (< 230 K). The ice crystal number decreases significantly for ambient particles with mean dry radii <≈ 10 nm due to the Kelvin effect.

Besides simulations with one aerosol particle ensemble, we analyze contrail formation scenarios with two co-existing aerosol particle ensembles that differ either in their mean dry size or hygroscopicity parameter. We compare them to scenarios with a single ensemble that is the average of the two ensembles. We find that the total ice crystal number can differ significantly between the two cases, in particular if nucleation mode particles are involved.

Due to the absence of soot particle emissions, the ice crystal number in H2 contrails is typically reduced by more than 80–90 % compared to conventional contrails. The contrail optical thickness is significantly reduced and H2 contrails either become later visible than kerosene contrails or are not visible at all for low ambient particle number concentrations. On the other hand, H2 contrails can form at lower flight altitudes where conventional contrails would not form.

Andreas Bier et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-1321', Anonymous Referee #1, 21 Jul 2023
  • RC2: 'Comment on egusphere-2023-1321', Anonymous Referee #1, 21 Jul 2023
  • RC3: 'Comment on egusphere-2023-1321', Anonymous Referee #2, 20 Aug 2023

Andreas Bier et al.

Andreas Bier et al.


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Short summary
Hydrogen (H2) is a promising technology to reduce the climate impact by contrails. We study contrail formation behind aircraft with H2 combustion. Due to absence of soot emissions, contrail ice crystals form only on ambient particles mixed in the plume. The ice crystal number, which strongly varies with temperature and aerosol number density, is decreased by more than 80–90 % compared to kerosene contrails. However, H2 contrails can form at lower altitudes due to higher water vapor emissions.