the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Nov 2024
Soil Deposition of Atmospheric Hydrogen Constrained using Planetary Scale Observations
Abstract. Quantifying soil deposition fluxes remains the greatest source of uncertainty in the atmospheric H2 budget. A new method is presented to constrain H2 deposition schemes in global models using observations of the zonal mean H2 distribution and seasonality. A "best-fit" scheme that reproduces the observed zonal-mean seasonality of atmospheric H2 at the planetary scale is found by perturbing a prototype deposition scheme based on soil temperature and moisture dynamics. Comparing the best-fit and prototype schemes provides insight for how the prototype scheme may be improved to better reproduce observed seasonality.
The H2 signal driven by the prototype scheme is accurate compared to observations in the Northern Hemisphere extra-tropics but shows discrepancies in the Southern Hemisphere, with too high surface mixing ratios and too weak seasonality. A best-fit scheme indicates that the function capturing the soil microbial consumption of H2 requires a shift of approximately +3 months in the seasonality in the tropics, where the prototype scheme is sensitive to seasonal soil moisture driven by the shifting of the ITCZ. New constraints on the H2 surface flux at low-latitudes are key to accurately modelling the H2 cycle in the Southern Hemisphere.
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RC1: 'Comment on egusphere-2024-3247', Anonymous Referee #1, 20 Jan 2025
Soil deposition of atmospheric hydrogen constrained using planetary scale observations by Chaudhri and Stevenson
This manuscript uses the recently updated global NOAA H2 dataset to help constrain the soil sink by latitude and season. The observational data are compared with what would be predicted using a prototype soil deposition scheme that applies current functions of moisture, texture, and temperature. Emissions, atmospheric chemistry, and atmospheric transport of H2 are included. Discrepancies between predicted and observed atmospheric concentrations then identify shortcomings in our understanding of the budget, especially regarding the magnitude and seasonality of the soil sink. This research extends the Xiao et al (2007) study, both in time and in latitudinal resolution. The approach taken here seeks to take full advantage of the latitudinal distribution of the NOAA data sites, slicing the world into 64 latitudinal bands instead of 2 to 6, while meanwhile keeping computation costs low. The prototype deposition scheme was thoughtfully applied and utilize the most recent published works. There is a lot of information densely packed into this manuscript, providing a wealth of insights. This appears to be a novel approach to addressing the major uncertainties in the global atmospheric budget of H2.
I only have a few minor recommendations, largely to support the readability of the manuscript.
1) Figure 1 shows the sites with different deposition rates, anomalous means, and late peaks. Can you please identify these stations so the reader can cross-compare with the NOAA dataset? This information can be provided in Supplementary Materials. There may be some local factors that can explain the statistically anomalous sites. For example, there may be local pollution sources, strong local soil sinks, variable sampling times, or potential sampling issues. Sites identified by your work might be tagged for further scrutiny. It would also be helpful to describe what is meant by ‘low’ vs ‘high’ deposition, and how this was assessed for each site.
2) Line 43. The Meredith et al., 2017 paper, appears to be the appropriate reference here, in lieu of their 2014 paper.
3) Line 101: “The seasonal H2 signal does not depend significantly on zonal variations in local deposition”. Can you please clarify what is meant by this sentence? What does it mean for local deposition to vary across latitudes?
4) Line 153-154: In the model, the “long lifetime of H2 compared with timescales of horizontal mixing in the atmosphere indicating that H2 is reasonably well mixed across zonal bands”. Is this true for all latitudes, including the tropics which have a much larger circumference (and hence mixing time)? There can also be large variability within single latitudinal bands even without identified strong local deposition. (e.g., it appears there can be 30 ppb variation in the mean at specific latitudes, even those that have not been identified as anomalous).
5) “Line 224-226: “In particular, the seasonality due to soil temperature captures the strong seasonality of BF1 in the NH mid-latitudes, which has been identified in microbial laboratory studies (Smith-Downey, 2006).” This sentence may need to be reworded. The microbial laboratory studies reveal the temperature sensitivity of H2 deposition, but do not themselves identify the strong seasonality in NH mid-latitudes.
6) Line 311-313. “Without a seasonally varying soil uptake, zonal mean surface H2 would peak with a similar amplitude during the late summer to early autumn in both the NH and SH extra-tropical regions”. Is this based primarily on estimated emissions, oxidation and mixing? What are the most important factors strongly influencing the modeled seasonality when soils are not included?
7) [optional] Lines 235-237. It would help the reader if the specific locations discussed are visually identified in figures 8a and 8b, such as with a circle, arrow or by indicating “areas of darker blue shading”.
8) Lines 240-243 “In the NH, better agreement is achieved where this lag and the amplitude of the seasonality of the peaks are decreased with increasing latitude; the peak at 52N (D) agrees well with BF1 for a ~1 week lag and a 60% multiplier”. This statements suggests a steady decrease in the amplitude and lag time as you go towards the poles. Is this true, or does it refer to a specific high latitude band?
9) Lines 264-265: “enhanced deposition with a maximum of around 0.2 Tg yr-1/ lat focused in the NH mid-latitudes, and peaking in both hemispheres between December and February (Fig 9b)”. A Dec-Feb peak in deposition is not obvious in Figure 9b.
10) Lines 322-323. “In the SH the prototype scheme results in too-high surface mixing ratios in the annual mean, and differences in phase and a weak amplitude of the seasonality in the Southern tropics and subtropics”. Are your results consistent with a small but widespread additional sink in the Southern Hemisphere, such as a weak ocean sink? This may be too speculative for the manuscript, but it may be helpful to question the role of the oceans in the H2 budget.
Citation: https://doi.org/10.5194/egusphere-2024-3247-RC1 -
RC2: 'Comment on egusphere-2024-3247', Anonymous Referee #2, 28 Jan 2025
Tardito Chaudhri and Stevenson present a study to better constrain the soil sink of atmospheric H₂, a critical yet highly uncertain component of the global H₂ budget. The authors employ a relatively simplified 2D (latitude-height) model and leverage NOAA observations of atmospheric H₂ to test existing modeling schemes and identify numerical modifications that improve the model-observation agreement. Overall, I am very supportive of the goal of the work: utilizing observational data and the latitudinal/seasonal variation of H₂ to evaluate soil uptake schemes, extending the analysis of Paulot et al. (2024, ACP) with a simpler and more flexible model. (I suggest the authors better articulate this work's distinctions and advancements compared to Paulot).
However, there is a major scientific flaw in the current implementation of the soil sink. The authors model the soil sink based on bacterial activity as a function of moisture f(s) and temperature g(T). However, this approach confuses bacterial activity (the potential sink) with the actual soil sink, which is modulated by both biological activity and diffusion processes—the rate at which H₂ becomes available to bacteria. This fundamental distinction (and interplay) between diffusion (a physical process) and bacterial activity (a biotic process) has been known for decades (e.g., Yonemura et al., 2000, Tellus B; Smith-Downey et al., 2006, GRL; Yashiro et al., 2011, ACP; Ehhalt et al., 2011, 2013, Tellus B and more recently by Bertagni et al. 2021, GBC, which showed the critical role of diffusive limitations in humid regions). Although the authors reference some of these works, they have lost a non-negligible component of the processes involved.
By neglecting diffusion, the authors exclude a critical physical factor that substantially impacts soil sink representation. This omission introduces large biases in calculated soil uptake rates, particularly in humid temperate and tropical regions with more pronounced diffusion limitations. As a result, both the prototype scheme and the subsequent best-fit scheme are fundamentally flawed, rendering the conclusions unreliable. Addressing this issue requires incorporating diffusion into the soil sink parameterization and rerunning the entire modeling framework (prototype and best fit).
In addition to this central issue, the clarity and structure of the manuscript could be significantly improved. Just as examples: i) the results in terms of atmospheric concentrations are presented before introducing the underlying soil sink calculations, which disrupts the logical flow of the paper; ii) section 5 on the best-fit scheme includes mathematical equations that appear simultaneously overly simplistic and excessively detailed for inclusion in the main text (appendix?); iii) the abstract could be simplified to be less technical.
In conclusion, although the authors’ goal is valuable and the topic of significant scientific interest, the major flaw in the representation of the soil sink —the omission of diffusive processes— invalidates the study’s results. I recommend a comprehensive revision of the soil sink modeling framework to address this issue, along with a restructuring of the manuscript for clarity. At this stage, I cannot recommend publishing the paper in its current form. I am hence refraining from providing minor suggestions.
Citation: https://doi.org/10.5194/egusphere-2024-3247-RC2 -
CC1: 'Preliminary Reply on RC2', Alexander Tardito Chaudhri, 30 Jan 2025
Dear Reviewer,
This is a preliminary reply while I implement your valuable suggestion, and prepare revisions and detailed response.
Thank you for your insightful comment regarding diffusivity. I have attached an updated version of fig.4 where normalised soil diffusivity for different soils -- based on the Moldrup et al. (2013) version communicated in Bertagni et al. (2021) with soil parameters also communicated in the latter paper – is plotted. To maintain some simplicity (in the spirit of the paper – where we restrict ourselves based on the limits of available observations) this is plotted considering a constant diffusion in free air and neglecting the inclusion of diffusive barriers – which would particularly reduce high diffusivity at low soil moisture contents. The role of a diffusive barrier is carefully discussed in Bertagni et al. (2021).
Therefore, we consider soil diffusivity proportional to (porosity)^2 (1 – soil moisture content)^(2 + 3/b), where porosity and b are communicated for different soil textures is discussed in the aforementioned paper. Diffusivity is limited as soil moisture content increases. When we consider the three terms in the prototype scheme, uptake is prevented for very dry soils due to the low moisture limit on biological uptake. At higher moisture content restricted diffusivity and weaker biological uptake compound to reduce uptake – this slightly decreases the soil moisture content of peak uptake. Our original intention to neglect this term was based on the resulting uptake still having the form of: none below a low-moisture limit; then a sharp uptick in uptake with moisture; then a decline as moisture continues to increase. Of course, we scale our prototype deposition to achieve a global 57.2 Tg/yr avg.
When we include this in our zonally integrated prototype scheme, we see that – as you predicted – deposition is reduced in damp deep-tropical soils and this changes some of the distribution in the global uptake. Seasonality in the uptake is still dominated by f(soil moisture) in the tropics and h(soil temperature) in the extra tropics, this form of diffusivity contributes some changes in seasonality in the dampest soils (deep tropics, and some mid-latitudes). I will redo the integrations and produce a more detailed response.
Thank you again,
ATC
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CC1: 'Preliminary Reply on RC2', Alexander Tardito Chaudhri, 30 Jan 2025
- EC1: 'Comment of reviewer #3 on egusphere-2024-3247', Jens-Uwe Grooß, 27 Feb 2025
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