A dynamic chemical model of bi‐directional ammonia exchange between semi‐natural vegetation and the atmosphere

Abstract

A mechanistic, dynamic compensation point model to simulate the vegetatiodatmosphere exchange of ammonia (NH3) is described. The model is applied to long-term micrometeorological measurements of NH3 exchange obtained over moorland in southern Scotland . 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 ammoniumhydronium r 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/S02 concentration ratios in the atmosphere vary, and examine 'co-deposition' interactions of these two species.

Estimate of annual NH3 dry deposition to a fumigated ombrotrophic bog using concentration-dependent deposition velocities

“…to the outside of leaves rather than through stomata) has been shown to saturate at high concentrations, i.e. the surface resistance increases as concentrations increase (Jones, et al, 2007); and (2) the surface resistance depends upon the presence of acidic gases such as sulphur dioxide (SO 2 ) or nitric acid (HNO 3 ), which can effectively lower the surface resistance through chemical reaction with the deposited ammonia gas to form salts (Flechard, et al, 1999).…”

Ammonia Deposition Near Hot Spots: Processes, Models and Monitoring Methods

“…As an example, Figure 5 shows the effect of Nitrogen supply on C s for maize (Loubet et al, unpublished The compensation point can either be determined from the NH 4 + and H + concentration in the leaf apoplastic solution (e.g. Husted et al, 2000), or inferred from flux measurements both over vegetation surfaces (Fléchard et al, 1999) or within chambers (Hill et al, 2001). The compensation varies with plant species, nitrogen status, and growth stage as well as leaf age.…”

“…But NH 3 can also be emitted by the plants themselves, which can either act as sinks or sources of NH 3 depending on their nitrogen (N) nutrition status and the atmospheric NH 3 concentration (Farquhar et al, 1980;Sutton et al, 1995aSutton et al, ,b, 2001aSchjoerring et al, 1998;Andersen et al, 1999;Milford et al, 2001a;Hill et al, 2001). Moreover, non-stomatal adsorption of NH 3 is influenced by the load of acidic pollutants to the surface (Erisman and Wyers, 1993;Sutton et al, 1993c;Fléchard et al, 1999).…”

“…The large variability of NH x deposition near sources is known to depend critically on the canopy structure surrounding the source (roughness, side fluxes) (Klaassen, 1991;Draaijers et al, 1994;De Jong and Klaassen, 1997;Theobald et al, 2001;, the NH 3 emissions from the canopy (Schjoerring et al, 1998;Riedo et al, 2002), the litter (Nemitz et al, 2000a(Nemitz et al, , 2000b, or from the soil (Génermont and Cellier, 1997), as well as the non-stomatal NH 3 fluxes (van Hove et al, 1989;Erisman and Wyers, 1993;Sutton et al, 1995a;Fléchard et al, 1999;. Less known, are wet deposition fluxes and chemical transformations of NH x near intensive sources, as well as direct emissions of particulate NH 4 + (McCulloch et al, 1998).…”

“…(iii) the non-stomatal NH 3 deposition, which depends on leaf surface wetness but also on the ionic composition of these water films, and hence on the NH x load, but also on the interaction with other chemical compounds (Fléchard et al, 1999);…”