the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Jul 2025
Water vapour dynamics as a key determinant of atmospheric composition and transport mechanisms
Abstract. Concentrations of any dry-air component (gas i) are often defined with reference to dry air, excluding water vapour. Here it is shown that water vapour is air and indeed—with regard to air’s sources and sinks—it can be said that air is water vapor. Alone among atmospheric components, water vapor shifts from a trace constituent (practically negligible) to a bulk gas that meaningfully reduces gas i’s fraction and abundance within sultry, tropical air. Customary exclusion of humidity when expressing gas concentrations has practical justification, but biases assessments of gas i’s content within air. Overwhelmingly dominating surface exchanges, water vapor dynamics (WVD) influences gas i’s distributions, concentration gradients, and transport mechanisms, but this has been overlooked due to reliance on gas fractions within dry air. Important implications of this include physical decoupling of leaf gas exchanges under very hot conditions, with extreme humidity inside stomatal pores acting physically to boost transpiration and simultaneously inhibit photosynthesis by suppressing carbon dioxide, consequences that ecology’s stomatal conductance modelling framework has not elucidated. We accentuate the environmental significance of WVD and urge quantifying humidity whenever assessing fractional air composition.
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CC1: 'Comment on egusphere-2025-2695', Dan Yakir, 10 Aug 2025
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AC1: 'Reply on CC1', Andrew Kowalski, 19 Aug 2025
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Please see the attached PDF file that includes Dr. Yakir's comments in bold, italic font followed by our replies.
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AC1: 'Reply on CC1', Andrew Kowalski, 19 Aug 2025
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RC1: 'Comment on egusphere-2025-2695', Anonymous Referee #1, 15 Sep 2025
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Summary
The authors outline current shortcomings in this discussion paper from excluding the impacts of water vapour by the use of dry air equivalents. In certain cases, this may impede understanding of measurements or cause inappropriate model outcomes especially for ecosystem-atmosphere interactions. Using thought-experiments, they discuss this issue, its effects and call to improve such models as well as use mole fractions to include water vapour.
There indeed seem to be cases where the outlined issues occur, which could be further highlighted with specific examples. However, many of these cases are only found in very specific or extreme conditions. Furthermore, the current availability of observational data from such cases are still few, requiring theoretical considerations and some speculation as to the magnitude and frequency of their impacts. The inclusion of further supporting data and concrete examples using measurements would be helpful to illustrate the authors arguments.
Overall, the paper acts as an incentive to increase data collection of such cases, as well as creating awareness for the current model shortcomings, which the community need to consider when evaluating their own data. The paper should be accepted after some revision and linguistic editing.
Specific points
- Generally, a well-conceived idea to raise awareness of such issues. That is why basing arguments on concepts of “air is water vapour” detracts from the overall discussion and aim. Because water vapour can in some cases reach bulk concentrations, does not warrant it to assume parity with the main constituents in all cases. Naturally, the addition of a gas will change the relative distribution among the other constituents. The discussion could be dedicated less to this effect and more to its impacts.
- The authors omit any discussion considering humidity corrections in observational data to account for the impacts of water vapour in the aforementioned cases and references to relevant method papers or examples where such effects were observed could be added.
- Replacing dry air measures simply with molar fractions that include water vapour would likely cause more issues than it would solve, except in the few circumstances outlined in the manuscript in which cases molar fractions could be used. Also, this method would remove this useful measure for independent comparisons. If decoupling of CO2 and H2O at stomatal interfaces has been observed due to this effect, then there should be data available to model these processes with the improved approach suggested by the authors to illustrate the importance of such effects.
- In Fig. 4. the CO2/H2O decoupling graph is not very clear. Please improve the illustration/graph/labelling to highlight the argument and link it with the examples given.
Citation: https://doi.org/10.5194/egusphere-2025-2695-RC1
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Review of Kowalski et al., Water vapor dynamics…
This Viewpoint paper offers a provocative perspective of the role of water vapor in atmospheric and leaf-scale gas exchange. It argues that water vapor dynamics (WVD) can be a driver of gas transport phenomena, using first-principles reasoning and thought experiments. The paper challenges the conventional (and useful) practice that expresses gas concentrations relative to dry air, especially under very humid conditions. It raises valid conceptual challenges to current modeling frameworks in both atmospheric chemistry and plant ecophysiology.
However, the discussion of using mole fraction and partial pressure of dry air is not new, and the cases raised here apply mostly to extreme and rare cases, and remain speculative due to limited direct empirical validation. Some of the claims may be too strong like the relevance WVD in driving bulk airflow in atmospheric boundary layer dynamics. Or the suggestion of widespread invalidation of top-down flux inversion models (without demonstrating practical model biases attributable to neglecting WVD).
The authors use sound theoretical reasoning to develop the WVD idea but extending these principles into stomatal decoupling, and macro-scale transport, relies on indirect support, and many of the supporting studies (e.g., Kowalski 2017; Kowalski et al., 2021, 2025) are from the same authors.
Similarly, the idea that WVD, under high humidity and temperature, could lead to a physical decoupling of CO₂ uptake and H₂O loss in leaf gas exchange is intriguing. But validation with field or chamber measurements is badly missing.
In fact, the widely used model of Farquhar et al 1980 deals with some of the aspects of CO2 dilution by water vapor in the substomatal space, but is not cited or discussed. in fact, all leaf gas exchange measurements are also corrected for humidity dilution in calculating net assimilation.
In discussing the links to biochemical rates, specifically in photosynthesis, some reference should be made to the fact that dissolved CO2 is the end member via Henry’s law and other local factors at the site.
Rhetoric like “air is water vapor; water vapor is air” is somewhat distracting
The schematic showing decoupled CO₂ and H₂O fluxes is conceptually useful but would benefit from real data overlay (e.g., from gas exchange measurements during heatwaves) to illustrate feasibility.
Equations A5, and 2, could benefit from some relevant quantitative examples.
Overall, the manuscript presents a thought-provoking argument that challenges long-standing assumptions in atmospheric and plant sciences. It presents a sound physical and conceptual relevance of water vapor as an important player in gas transport and exchange. It would benefit from: Stronger empirical support through simulations or re-analysis of published data. Moderation of rhetoric in places where established practices are critiqued.
I think the manuscript should be accepted for publication after moderate revision, particularly with a better balance of conceptual divergence with empirical grounding.