Imprint of Anthropogenic Sources and Soil Removal on the Surface Concentration of H ₂ in the Contiguous US

Hydrogen (H 2 ) 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 H 2 usage, in part due to gaps in our understanding of the H 2 atmospheric budget, including the H 2 release from industrial activities and the H 2 soil removal, the most important sink of H 2 . This study focuses on H 2 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 H 2 sources using the potential source contribution function (PSCF). We find that H 2 PSCF is consistent with a large anthropogenic source of atmospheric H 2 from the incomplete combustion of fossil fuel. However, episodic air masses with high H 2 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 H 2 . Air masses depleted in H 2 are recorded in rural or remote continental sites. Our analysis of observations from a tall tower in NE Colorado suggests that the degree of H 2 depletion not only reflects differences in soil exposure but also the regional inhibition of the soil sink under arid conditions.