Comment on egusphere-2025-4898

Abstract. Understanding the global hydrogen (H₂) budget is critical as H2 is expected to play an important role in future energy systems. Tropospheric H2 sources include direct emissions and atmospheric production via chemical reactions, while sinks are soil uptake and removal by hydroxyl radical (OH). Large uncertainties remain in quantifying the atmospheric production and loss of H2 largely due to the lack of global-scale knowledge of the abundance of OH. We use a suite of global three-dimensional Atmospheric Chemistry Models to evaluate key reactive species involved in atmospheric production and loss – formaldehyde (HCHO), nitrogen dioxide (NO2), and carbon monoxide (CO) – with satellite retrievals. A box model is then used to simulate the evolution of global mean tropospheric H2 from pre-industrial to present day; to test different relative contributions in atmospheric production from methane and Volatile Organic Compounds; and to assess atmospheric loss with different OH concentrations. Isotopic compositions are further used to constrain these sources and sink terms and assess the possibility of geological sources. Models generally match satellite retrievals for HCHO, though model diversity exists for NO2 and CO. From model evaluations and box model constraints, we estimate atmospheric H2 production of 37–60 Tg/year, and atmospheric losses of 15–30 Tg/year, suggesting that some top-down literature estimates may overestimate production. Box model results suggest an upper bound 9 Tg/year for geological sources, considerably lower than the 23 Tg/yr proposed previously. We recommend more isotopic observations and targeted measurement campaigns to further refine the budget.