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.