Constraining potential geological sources of atmospheric hydrogen using a box-model approach

With the proposed increased use of hydrogen energy, a more accurate representation of the atmospheric hydrogen budget is crucial to evaluating potential climate impacts. To this end, we extend the study by Sand et al. (2023), where five different chemical models were used to calculate the global warming potential of hydrogen. A box-model (SimpleH2 model) has been developed using those model results to calculate how atmospheric hydrogen concentrations and hydrogen isotopic compositions change for different sources and sinks. The sources included in the model are anthropogenic sources, biomass burning, nitrogen fixation (over land and ocean), photochemical production in the atmosphere, and geological sources. The two sinks are soil uptake and oxidation by OH. In this study, we will simulate the box model with different combinations of sources and sinks (both in terms of concentrations and isotopic values) to evaluate the feasibility of those inputs, focusing on the contributions of different geological sources and soil sinks. For example, adding a geological source of 20 Tg/year with an isotopic composition of -600 per mil and increasing the soil uptake by 20 Tg/year, will modify the isotopic composition of atmospheric H2 (to ~10 per mil) far from the observed range.  These provide us with useful constraints that can be tested in future measurement campaigns.