Atmospheric Impacts of Hydrogen as an Aviation Fuel

Abstract

Hydrogen is being investigated as a promising zero-carbon aviation fuel, offering the potential to eliminate direct CO₂ emissions while being produced with low lifecycle greenhouse gas emissions. Despite these benefits, there are additional indirect climate and air quality costs associated with direct hydrogen emissions which are often overlooked. We quantify the perturbation in the atmospheric composition associated with the introduction of hydrogen-fueled aircraft, broadening the current understanding of the non-CO₂ effects of these fleets. We use the GEOS-Chem High Performance (GCHP) global chemistry-transport model to conduct a spatially discretized, multi-year impact assessment of the atmospheric impacts of hydrogen-fueled aviation. We implement a flux surface boundary condition for hydrogen to provide an improved representation of the soil sink, relative to the default fixed boundary condition. This results in a net surface exchange of-16.7 Tg H₂ per year. Two hydrogen scenarios are evaluated using the updated GCHP implementation, which are representative of a high and low mitigation scenario for direct hydrogen emission rates. For the two scenarios, respectively, we observe increases in the mean atmospheric methane mixing ratio of 3.34 ppbv and 10.7 ppbv, corresponding to an increased methane lifetime of between 0.24% and 0.77%, respectively. The increased methane lifetime as well as in-situ oxidation of stratospheric hydrogen results in an increased stratospheric water vapor burden of 0.42 Tg and 2.3 Tg (or 0.052% and 0.28%) for the high and low mitigation scenarios, respectively. Additionally, we show the perturbation to tropospheric ozone levels to be between-0.047% and +0.30%, where the decreased ozone results from the removal of NOₓ emissions associated with fuel cells and low hydrogen emission rates. This analysis provides the foundation for understanding the implications of potential future hydrogen-based aviation fleets on climate and air quality.