Constraining the Hydrogen Soil Sink and Photochemical Source: Insights from Atmospheric H<sub>2</sub> Inversions (2003&ndash;2023)

Stroo, Firmin T.; Hooghiem, Joram J. D.; Krol, Maarten C.; Peters, Wouter

doi:10.5194/egusphere-2026-3077

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https://doi.org/10.5194/egusphere-2026-3077

Preprints

15 Jun 2026

| 15 Jun 2026

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Constraining the Hydrogen Soil Sink and Photochemical Source: Insights from Atmospheric H₂ Inversions (2003–2023)

Firmin T. Stroo, Joram J. D. Hooghiem, Maarten C. Krol, and Wouter Peters

Abstract. Hydrogen (H₂) is expected to become an increasingly important energy carrier during the energy transition, likely leading to higher atmospheric H₂levels due to losses during production, transport, storage and usage of hydrogen. Multiple studies have shown this could impact atmospheric composition through interactions with the hydroxyl radical. However, the magnitude of this impact remains uncertain due to large uncertainties in the global H₂budget, particularly in the soil sink and photochemical source. To address this, we present a spatiotemporally resolved H₂budget derived using atmospheric inversions with the TM5 chemical transport model. With this approach, we infer a global mean soil sink of 52.8 [47.8–56.7] Tg yr⁻¹ and a photochemical source of 34.6 [29.2–38.2] Tg yr⁻¹ over 2003–2023. Relative to Ouyang et al. (2025), we estimate a soil sink that is 45 % and 35 % weaker in the Middle East and Oceania, and 45 % and 70 % stronger in South America and Russia, respectively. Our results further suggest that variability in the observed H₂growth rate between 2003–2023 was primarily driven by changes in the photochemical source from CH4 oxidation, together with a declining global soil sink at a mean rate of 0.23 Tg yr⁻² . Finally, we infer a sensitivity of the soil sink to the El Niño–Southern Oscillation, strongest over diffusion-limited soils in tropical South America, with increased uptake during drier El Niño conditions and reduced uptake during wetter La Niña conditions.

Received: 28 May 2026 – Discussion started: 15 Jun 2026

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Firmin T. Stroo, Joram J. D. Hooghiem, Maarten C. Krol, and Wouter Peters

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CC1: 'Comment on egusphere-2026-3077', Alexander Archibald, 16 Jun 2026 reply

Stroo et al. present an important study on the use of inverse modelling to constrain two of the major uncertainties in the hydrogen budget: the magnitudes of the photochemical source and the soil sink.

This is a nice study and adds to our currently expanding understanding of the atmospheric chemistry of hydrogen.

I had a couple of small point I thought worth discussing, regarding the section on the photochemical source of hydrogen (4.2).

1. Photolysis rate versus photolysis rate constant (or frequency).

Please check throughout section 4.2 as to the use of photolysis rate (J*[HCHO]) versus photolysis rate constant (or frequency; J). On reading this section I was confused if the rate of HCHO photolysis was being talked about and or the rate constant for photolysis was being discussed. My guess was that most of this section is discussing the rate constant for photolysis of HCHO to form H2 (some times called J-molecular).

2. Evaluation against ATom data.

I found the evaluation of HCHO mixing ratios from the model and ATom to be useful and help clarify the point that the reduction in photochemical source (in the posterior) could be coming from an overestimate in J-molecular. This can and could be investigated. I would encourage the authors to consider comparing the modelled J values with the ATom determined J values or potentially equally useful, combining the ATom HCHO and J values to create an observed photochemical H2 source to compare against the model. Both I think would help clarify whether the issue with the model is coming from incorrect treatment of HCHO photolysis or something else.

References:

Hall et al. (2018), "Cloud impacts on photochemistry: building a climatology of photolysis rates from the Atmospheric Tomography mission," Atmos. Chem. Phys. 18:16809–16828, doi:10.5194/acp-18-16809-2018

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Citation: https://doi.org/10.5194/egusphere-2026-3077-CC1
RC1:
'Comment on egusphere-2026-3077', Alexander Tardito Chaudhri, 02 Jul 2026 reply
This useful work is an great contribution that helps to deepen our understanding of atmospheric hydrogen by constraining its soil uptake and chemical source.
In this you have used a model inversion to better constrain the atmospheric hydrogen photochemical source and soil uptake. The model uses 100 ensemble members with Kalman filtering to follow observed atmospheric hydrogen over 20 years to 2023.
You base prior assumptions of soil uptake from implementations of two commonly used moisture-temperature-diffusivity limited soil uptake schemes. Different implementations of these schemes have appeared in various studies so I am glad to see that you directly compare them here.
I particularly liked how you were able to identify particular years and regions where your observation-based method disagrees with the theoretical prior.

Specific comments:
These first comments concern experimental configurations that could be commented on and would strengthen these work, and possible substantial conclusions that may have been overlooked in this article:
Paulot et al. (2024) and Tardito Chaudhri and Stevenson (2025) both identify constraints in hydrogen uptake from its observed seasonality. Presumably your setup is ideal to explore this. I see in Figs E5 g and h that the posterior uptake schemes require distinct seasonality compared to the assumptions based on physical parameterisations. What do your results suggest about the seasonality of soil uptake?

You find faster soil sinks in South America and Russia than the synthesis (Ouyang et al. 2025) – how sensitive are these to the choice of scaled ocean H2 emission from nitrogen fixing?

The time evolution of posterior fluxes from non-isoprene photochemical source (Fig 7d) and the soil sink (Fig 7e) are confusing. These require a ~10% increase in the non-isoprene source in 2016-2024 versus 2008-2016, and a 0.23 Tg/yr decrease in soil uptake over the analysis period. The priors had <5% increase in non-isoprene source and an increase in soil uptake.
Please can you give an explanation for how credible the posterior non-isoprene source variability is (fig.7d)? From line 209, does your choice of the low state vector uncertainty in the methane source imply that this should be well constrained already?

The prior and intuition (specific comment for Line 317 included) would suggest that the soil uptake should have increased over the period. Given that value chain emissions estimates of up to 20% are provided by both Trapani (2025) and Esquivel-Elizondo et al. (2023, Front. Energy Res), and the range of bottom-up and top-down studies (Table 2) that have higher H2 fossil fuel source – would it be feasible to explore configurations with a varied industrial source to estimate how likely changes here are to explain the trend? The approach of Paulot et al. 2024 ACP also includes industrial H2 in a revision CEDS v20210421. A substantial result might explore whether the recent trend is better described by an unaccounted H_2 emission, or a downturn in the soil uptake rate.

Specific comments line-by-line:
Line 12, 296, 360: El Nino ~ wet western but dry equatorial-eastern (looking at Cai et al. 2020)?
Line 22 and 35: It would be helpful to point to the existing constraints for these summarised in Table 2.
Patterson et al. 2025 also used new observations to find constraints on how sources and deposition might have evolved in the last century.
Line 24: The distribution of ocean and soil emissions might be poorly constrained. Does the calculation with seawater CO concentration match with REVISED Paulot et al. 2024 ACP supplementary figure 3 where SH ocean H_2 emission is reduced versus GFED4s?
Line 40: Northern Hemisphere subtropics + midlatitudes expected to have comparable or higher uptake (Paulot et al. 2024 ACP and Tardito Chaudhri and Stevenson 2025 ACP).
Line 96: Here you take the pattern of ocean emissions from CO, then introduce a scaling. My understanding is that we don’t necessarily have good constraints on ocean H2 emissions, and inventories for these add a source mostly south of the 25N land bisector (line 189). Increasing this source requires increased uptake south of this divide – previous comment, the revision in Paulot et al. reduces higher-latitude SH ocean emissions to meet observations.
Considering this, how robust is your South America posterior uptake to this choice of scaling?
Line 171 - 184: A linearisation is made to the uptake scheme, a posterior scheme is then found and the average deviation from linear uptake is then presented. Is it worth checking there aren’t particular regions where this linearisation has performed better and worse? This would reassure the reader that your conclusions are sound at the particular locations with the biggest changes in updated fluxes.
Line 213-221: Another possible error comes from flask observations typically being made at similar times of day at each site, Forster et al. 2012 record ~10ppb variations in hydrogen from diurnal cycles, with notable drops during night-time inversions.
Line 247-248: Please can this sentence be made clearer? ‘Due to the larger freedom to adjust sources in sinks …’
Line 267: Would this suggest that the prior configuration would lead to too-high H_2 in the southern hemisphere versus Northern Hemisphere? Would it be clearer to also include the latitudinal distribution that the prior would have achieved and what the posterior achieves?
This might be beyond scope, but would a configuration with higher anthropogenic H_2 emission (more NH) see increased but similar pattern of H_2 uptake as the prior?
Line 317 and Fig 7e: This result is counter-intuitive. If we assume rates of microbial activity increase up to about 36C and that global patterns of soil moisture have not generally changed over 2003-2023 then intuition would suggest that it would be more likely that soil uptake would have increased rather than decreased?
Line 325: However, this could be a useful result. Is it easy to estimate HD depletion rates that would be expected for the constraints found in this work versus an emission change explanation?

References:
Patterson et al. 2025, https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2025JD043662
Paulot et al. 2024, ACP, https://acp.copernicus.org/articles/24/4217/2024/
Esquivel-Elizondo et al. 2023, Front. Energy Res, https://www.frontiersin.org/journals/energy-research/articles/10.3389/fenrg.2023.1207208/full
Cai et al 2020, https://www.nature.com/articles/s43017-020-0040-3
Patterson et al. 2025, https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2025JD043662
Forster et al. 2012, https://www.tandfonline.com/doi/full/10.3402/tellusb.v64i0.17771

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Citation: https://doi.org/10.5194/egusphere-2026-3077-RC1

Firmin T. Stroo, Joram J. D. Hooghiem, Maarten C. Krol, and Wouter Peters

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Short summary

Measurements and computer modelling were used to improve quantification of the global hydrogen budget from 2003–2023. We estimate a global soil sink of 52.8 Tg/yr and a photochemical source of 34.6 Tg/yr. Changes in the hydrogen growth rate were mainly driven by shifts in photochemical production and a weakening soil sink. Soil uptake responds to the El Niño–Southern Oscillation, especially in tropical South America, likely controlled by changes in soil moisture.

Constraining the Hydrogen Soil Sink and Photochemical Source: Insights from Atmospheric H2 Inversions (2003–2023)

Constraining the Hydrogen Soil Sink and Photochemical Source: Insights from Atmospheric H₂ Inversions (2003–2023)