Contrail formation for aircraft with hydrogen combustion – Part 1: A systematic microphysical investigation

Overview

This research addresses the microphysical processes governing ice crystal formation in contrails produced by hydrogen-combustion aircraft. While parameterizations exist for conventional kerosene-based aviation to estimate ice crystal numbers during early contrail development, no such framework has been established for hydrogen combustion. The study is motivated by the potential role of hydrogen as an aviation fuel in climate mitigation strategies, particularly given that non-CO2 effects such as contrails account for over half of aviation's total effective radiative forcing. Unlike kerosene engines, hydrogen combustion produces no soot emissions, fundamentally altering the ice nucleation process. Ice crystals must instead form on ambient aerosol particles entrained into the exhaust plume. The research systematically investigates these microphysical processes to establish a foundation for parameterizing ice crystal number in hydrogen-combustion contrails, which is essential for assessing the climate impact of this potential future aviation technology in general circulation models.

Methods and approach

The investigation employs a particle-based Lagrangian Cloud Module implemented in a box model approach to simulate contrail formation. The modeling framework conducted more than 20,000 simulations spanning a comprehensive range of atmospheric conditions and aerosol properties. The simulations account for the absence of soot particles in hydrogen combustion exhaust, requiring ice crystal formation to occur exclusively on entrained ambient aerosols. The study examines how aerosol characteristics including size distribution, hygroscopicity, and number concentration influence ice nucleation. Both single-population and multi-population aerosol scenarios are investigated to determine whether ice crystal formation from complex aerosol mixtures can be reconstructed from simpler single-population cases. The parameter space includes variations in ambient relative humidity, temperature, and aerosol properties to identify governing regimes and sensitivities in the ice formation process. The approach also evaluates conditions under which homogeneous droplet nucleation can be excluded as a relevant ice formation pathway.

Results

The simulations reveal that the total number of entrained aerosol particles primarily governs the nonlinear depletion of water vapor during contrail formation. Coarse-mode particles exert negligible influence on ice crystal formation due to their low abundance in the atmosphere. Ice crystal formation from multiple aerosol populations can be reconstructed from single-population simulations by employing population-specific properties such as particle size and hygroscopicity along with total number concentration. The research identifies atmospheric conditions where homogeneous droplet nucleation can be safely neglected as a potential ice formation pathway, simplifying the parameterization requirements. Most significantly, the study identifies a regime where ice crystal formation becomes nearly independent of ambient relative humidity, aerosol size, and hygroscopicity. This regime simplification has important implications for developing robust parameterizations that do not require detailed knowledge of all atmospheric and aerosol variables across the full parameter space.

Implications

The findings provide a systematic physical basis for developing a data-driven parameterization of ice crystal number in contrails from hydrogen-combustion aircraft, to be detailed in an accompanying publication. This parameterization capability is essential for incorporating hydrogen aviation into general circulation models that estimate the climate impact of contrail cirrus. The identification of regimes where ice crystal formation exhibits reduced sensitivity to multiple environmental variables enables more tractable parameterization schemes for large-scale climate modeling. Understanding these microphysical processes is critical for properly assessing hydrogen as an aviation fuel from the perspective of non-CO2 climate effects. While hydrogen combustion eliminates direct carbon dioxide emissions, the contrail-related radiative forcing remains a significant climate consideration. The research contributes to the broader evaluation of hydrogen aviation technology by providing the necessary tools to quantify one of its primary non-CO2 climate impacts, enabling more complete assessments of its overall climate benefit relative to conventional kerosene-based aviation.