the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Oct 2025
A multi-model approach to constrain the atmospheric hydrogen budget
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.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: open (until 10 Dec 2025)
- RC1: 'Comment on egusphere-2025-4898', Maarten Krol, 20 Nov 2025 reply
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- 1
This paper deals with the global hydrogen budget. An important subject acknowledging potential leakages in large scale deployment of hydrogen in the energy transition. First, the paper presents an evaluation of a multi-model ensemble that simulated the global hydrogen budget. Second, a box model is tuned on the global models, and the global budget is constrained by the observed hydrogen isotopic composition. The authors claim to have tightened the atmospheric hydrogen production, potential geological sources, and the soil sink.
Although the subject and the employed methods are interesting, I find the paper rather messy (e.g. typos, overall structure) in its current state. Moreover, the overall claim that the geological sources are constrained to < 9 Tg H2/yr is not very well substantiated. Below and in the attached annotated pdf file I provide more detailed comments.
Global model evaluation
The different parts in the manuscript are not very well connected. The paper starts with an evaluation of the global models, showing comparisons to CHCO, NO2 and CO satellite data. The paper remains rather vague here. I read that the model is forced to H2 (and CH4) surface observations, and that the soil sink is tuned to reproduce reasonable hydrogen concentrations. This seems a double constraint, and it remains unclear how that impacted the H2 budget. Moreover, no detailed H2 budget terms are presented here (used in the box model?), and the model evaluation is limited, showing figures with many panels that are not very informative. Moreover, in comparing to NO2 (and CHCO) satellite products, Averaging Kernels (AK are important, as well as co-sampling. The authors acknowledge this (and use 3 hr output still without AK). According to me, this evaluation does not add much current knowledge (and the results presented in Sand et al. (2024)). If anything, the model results could be used to estimate the uncertainty in the OH sink and other uncertainties in the H2 budget.
Box model
The box modelling is an interesting way to constrain the H2 budget. But the way the box model is presented and used needs improvement. First, it seems that the starting condition in the box model is not consistent, specifically for the “lifetime 3” case (which should be lifetime 2, but there are many of these mistakes). Second, it is unclear how the H2 budgets differ in the models (needs to be part of the global model evaluation). More detailed information is needed why the isotopic composition differs in the ACM-based box models. Some reasons are given, but this is actually a good way to give the ACM results a decent place in the manuscript. Third, large geological sources are considered unlikely, but I think this is an overstatement of the ability of the box model. Figure 9 is rather misleading, because this show the geological result of the -1000 ‰ signature (at least if I trust Figure A5). Furthermore, Figure 10 suggest that the impact of geological emissions on the isotopic composition is opposite to the effect of OH oxidation. This would imply that a stronger OH sink could compensate for the geological emissions. However, the OH sink is not included in the re-tuning of the model. The soil sink is included, but the soil sink does not enrich the atmosphere. I would suggest allowing the OH-sink to vary at least over the ACM results (you write: There is a larger diversity in OH indicating more uncertainty and a bigger spread for atmospheric losses).
Other issues
The box model should account for the enrichment of H2 due to inflow of stratospheric air. The authors mention that this can account for a 29-37‰ offset in δD. This is not particularly large but introduces a bias. Overall, I earlier pointed to the work of Pieterse et al., to which the paper refers to now. However, I would expect some further discussion, e.g. about the used isotopic values (Figure A4 looks quite different).