A mechanistic, dynamic compensation point model to simulate the vegetation/atmosphere exchange of ammonia (NH₃) is described. the model is applied to long-term micrometeorological measurements of NH₃ exchange obtained over moorland in southern Scotland (1995-96). the model describes the gaseous bi-directional exchange between the atmosphere, leaf surface water films, plant stomata and apoplast. A simple chemistry module is included to simulate the exchange of water-soluble atmospheric pollutants at the air-water interface on wet plant surfaces. Initialization of the chemistry module is achieved during rain events using measured rain chemical composition. the exchange of NH₃ with stomata is based on the concept of a stomatal compensation point parametrized by means of the apoplast ammonium/hydronium I ratio. the trans-cuticular transfer of ammonium may also constitute a sink for dissolved ammonia on plant cuticular water films, and is parametrized using a trans-cuticular resistance and the concentration difference between leaf surface water and the apoplast. the leaching of base cations from the inside of the plant to foliar surfaces is simulated in a similar fashion to ammonium transfer, and the heterogeneous oxidation of sulphur dioxide (SO₂) in thin water films is also treated. Numerical iterative procedures at each time-step allow the calculation of pH and dissolved ion concentrations.

Modelled NH₃ fluxes were compared with 3259 half-hourly micrometeorological measurements over moorland, and with two existing modelling approaches, the static canopy compensation point model and the canopy resistance model. the dynamic and static canopy compensation point models both gave long-term estimates of the NH₃ dry deposition flux to moorland within 10% of actual measurements, while the canopy resistance approach overestimated deposition by about 30%. the dynamic model performed best during wet conditions for which it was designed, and performed reasonably well during dry conditions using a more empirical resistance approach.

The model was also capable of simulating SO₂ dry deposition fluxes to within 20% of measured fluxes. the model provides a tool that may also be used to simulate scenarios, whereby NH₃/SO₂ concentration ratios in the atmosphere vary, and examine ‘co-deposition’ interactions of these two species.