Multi-model and box modeling evaluation of the tropospheric hydrogen budget

Hydrogen (H₂) is expected to play an important role in the transition to low-carbon energy systems. Tropospheric H₂ is either emitted directly or produced in situ in the atmosphere through chemical reactions, while the two sinks are soil uptake and reaction with the hydroxyl radical (OH). Large uncertainties persist in the global H₂ budget, particularly due to limited direct observations of atmospheric H₂ production, soil uptake, and the global OH abundance.

In this study, we investigate the global H₂ budget using a suite of three-dimensional atmospheric chemistry models to evaluate the key species involved in atmospheric hydrogen production (such as formaldehyde) and loss through OH-related chemistry (such as nitrogen dioxide and carbon monoxide). We then use a box model incorporating isotopic compositions with sources and sink estimates to test different plausible H₂ budget scenarios. Combining model evaluations and box model constraints, we suggest atmospheric H₂ production of 37-60 Tg yr-1, and atmospheric losses of 15-30 Tg yr-1. Finally, we evaluate the uncertainty in these estimates using the box model framework.