To what extent do molecular collisions arising from water vapour efflux impede stomatal O3 influx?

Pre-requisite for reliable O3 risk assessment for plants is determination of stomatal O3 uptake. One unaddressed uncertainty in this context relates to transpiration-induced molecular collisions impeding stomatal O3 influx. This study quantifies, through physical modelling, the error made when estimating stomatal O3 flux without accounting for molecular collisions arising from transpiratory mass flow of gas out of the leaf. The analysis demonstrates that the error increases with increasing leaf-to-air water vapour mole fraction difference (Δw), being zero in water vapour saturated air and 4.2% overestimation at Δw of 0.05. Overestimation is approximately twice as large in empirical studies quantifying stomatal O3 flux from measured leaf or canopy water flux, if neglecting both water vapour-dry air collisions (causing overestimation of leaf conductance) and collisions involving O3. Correction for transpiration-induced molecular collisions is thus relevant for both empirical research and for large-scale modelling of stomatal O3 flux across strong spatial Δw gradients.