Reanalysis of NOAA H2 observations: implications for the H2 budget

Hydrogen (H-2) is a promising low-carbon alternative to fossil fuels for many applications. However, significant gaps in our understanding of the atmospheric H-2 budget limit our ability to predict the impacts of greater H-2 usage. Here we use NOAA H-2 dry air mole fraction observations from air samples collected from ground-based and ship platforms during 2010-2019 to evaluate the representation of H-2 in the NOAA GFDL-AM4.1 atmospheric chemistry-climate model. We find that the base model configuration captures the observed interhemispheric gradient well but underestimates the surface concentration of H-2 by about 10 ppb. Additionally, the model fails to reproduce the 1-2 ppb yr(-1) mean increase in surface H-2 observed at background stations. We show that the cause is most likely an underestimation of current anthropogenic emissions, including potential leakages from H-2-producing facilities. We also show that changes in soil moisture, soil temperature, and snow cover have most likely caused an increase in the magnitude of the soil sink, the most important removal mechanism for atmospheric H-2, especially in the Northern Hemisphere. However, there remains uncertainty due to fundamental gaps in our understanding of H-2 soil removal, such as the minimum moisture required for H-2 soil uptake, for which we performed extensive sensitivity analyses. Finally, we show that the observed meridional gradient of the H-2 mixing ratio and its seasonality can provide important constraints to test and refine parameterizations of the H-2 soil sink.