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.