Imprint of Anthropogenic Sources and Soil Removal on the Surface Concentration of H₂ in the Contiguous US

Hydrogen (H₂) is experiencing renewed interest throughout the world as a low carbon fuel alternative or complement to fossil fuels. Significant uncertainties remain regarding the environmental impact of increasing H₂ usage, in part due to gaps in our understanding of the H₂ atmospheric budget, including the H₂ release from industrial activities and the H₂ soil removal, the most important sink of H₂. This study focuses on H₂ dry air mole fractions measured by the NOAA Global Monitoring Laboratory in discrete ambient air samples collected every few days at sites located in the contiguous United States between 2010 and 2022. We take advantage of the long-term observations from this network to study the regional distribution of H₂ sources using the potential source contribution function (PSCF). We find that H₂ PSCF is consistent with a large anthropogenic source of atmospheric H₂ from the incomplete combustion of fossil fuel. However, episodic air masses with high H₂ dry air mole fractions (>700 ppb) recorded at some sampling locations in the Western and Southeastern US are not associated with elevated CO, suggesting significant noncombustion anthropogenic or geological sources of H₂. Air masses depleted in H₂ are recorded in rural or remote continental sites. Our analysis of observations from a tall tower in NE Colorado suggests that the degree of H₂ depletion not only reflects differences in soil exposure but also the regional inhibition of the soil sink under arid conditions.