Atmospheric Impacts of Hydrogen as a Sustainable Aviation Fuel

Hydrogen is being investigated as a promising zero-carbon sustainable aviation fuel (SAF), offering the potential to eliminate direct CO₂ emissions with low lifecycle greenhouse gas emissions. Additionally, the combustion of hydrogen can remove all emissions of SO_x, nvPM, CO, and unburned hydrocarbons. There are, however, climate and air quality costs to the use of hydrogen. Combustion of hydrogen is still expected to result in emissions of NO_x, and the degree to which contrails will be increased or mitigated is unknown. In addition, there has been little research into the long-term consequences of direct emission of hydrogen.

Hydrogen emissions originate from leakage, venting, and purging that occur both within the supply chain and on the aircraft. Though not a direct greenhouse gas, hydrogen reacts in the atmosphere via mechanisms that increase the effects of other potent greenhouse gases and air pollutants. Specifically, increased emissions of hydrogen are projected to increase the lifetime of methane, the tropospheric burden of ozone, and the stratospheric burden of water vapor – all greenhouse gases. No study to date has assessed the additional climate impacts associated with direct emission when considering hydrogen as an aviation fuel. Furthermore, with the dominant loss mechanism of emitted hydrogen being based in soil, no study has evaluated the degree to which these indirect mechanisms might differ for hydrogen which is emitted at altitude. These considerations have the potential to change the perceived sustainability of hydrogen as an aviation fuel.

In this study, we quantify the climate and air quality impacts (excluding contrails) of a hypothetical future hydrogen aircraft fleet compared to equivalent kerosene and hydrocarbon-based SAF fleets. We use the GEOS-Chem global chemistry transport model, modified to represent hydrogen’s surface soil sink, to conduct a spatially discretized, multi-year impact assessment. Multiple scenarios are evaluated to address uncertainty in factors such as hydrogen leakage rates and NO_x emissions from hydrogen aircraft. This assessment will provide the foundation for understanding the magnitude of the environmental benefits and costs of hydrogen-fueled aviation.