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... Show moreStomatal 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 H2O 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 CO2-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. Show less
The accurate representation of bidirectional ammonia (NH3) biosphere-atmosphere exchange is an important part of modern air quality models. However, the cuticular (or external leaf surface) pathway... Show moreThe accurate representation of bidirectional ammonia (NH3) biosphere-atmosphere exchange is an important part of modern air quality models. However, the cuticular (or external leaf surface) pathway, as well as other non-stomatal ecosystem surfaces, still pose a major challenge to translating our knowledge into models. Dynamic mechanistic models including complex leaf surface chemistry have been able to accurately reproduce measured bidirectional fluxes in the past, but their computational expense and challenging implementation into existing air quality models call for steady-state simplifications. Here we qualitatively compare two semi-empirical state-of-the-art parameterizations of a unidirectional non-stomatal resistance (R-w) model after Massad et al. (2010), and a quasi-bidirectional non-stomatal compensation-point (chi(w)) model after Wichink Kruit et al. (2010), with NH3 flux measurements from five European sites. In addition, we tested the feasibility of using backward-looking moving averages of air NH3 concentrations as a proxy for prior NH3 uptake and as a driver of an alternative parameterization of non-stomatal emission potentials (Gamma(w)) for bidirectional non-stomatal exchange models. Results indicate that the R-w-only model has a tendency to underestimate fluxes, while the chi(w) model mainly overestimates fluxes, although systematic underestimations can occur under certain conditions, depending on temperature and ambient NH3 concentrations at the site. The proposed Gamma(w) parameterization revealed a clear functional relationship be-tween backward-looking moving averages of air NH3 concentrations and non-stomatal emission potentials, but further reduction of uncertainty is needed for it to be useful across different sites. As an interim solution for improving flux predictions, we recommend reducing the minimum allowed R-w and the temperature response parameter in the unidirectional model and revisiting the temperature-dependent Gamma(w) parameterization of the bidirectional model. Show less
