Towards a coupled paradigm of NH 3 ‐CO 2 biosphere–atmosphere exchange modelling

Stomatal conductance, one of the major plant physiological controls within NH 3 biosphere–atmosphere exchange models, is commonly estimated from semi‐empirical multiplicative schemes or simple light‐ and temperature‐response functions. However, due to their inherent parameterization on meteorological proxy variables, instead of a direct measure of stomatal opening, they are unfit for the use in climate change scenarios and of limited value for interpreting field‐scale measurements. Alternatives based on H 2 O flux measurements suffer from uncertainties in the partitioning of evapotranspiration at humid sites, as well as a potential decoupling of transpiration from stomatal opening in the presence of hygroscopic particles on leaf surfaces. We argue that these problems may be avoided by directly deriving stomatal conductance from CO 2 fluxes instead. We reanalysed a data set of NH 3 flux measurements based on CO 2 ‐derived stomatal conductance, confirming the hypothesis that the increasing relevance of stomatal exchange with the onset of vegetation activity caused a rapid decrease of observed NH 3 deposition velocities. Finally, we argue that developing more mechanistic representations of NH 3 biosphere–atmosphere exchange can be of great benefit in many applications. These range from model‐based flux partitioning, over deposition monitoring using low‐cost samplers and inferential modelling, to a direct response of NH 3 exchange to climate change.