Modelling the atmospheric transport and deposition of ammonia and ammonium: an overview with special reference to Denmark

Requirements for NHx models are discussed. It is very important that such models can describe the dry deposition of NH3 close to sources well. A description is given of all processes that should be included as well as references to relevant literature. For the calculation of long-term average depositions, some of these processes must be parameterized in order to keep the cpu time within reasonable bounds. Which processes should be parameterized depends on the purpose of the model calculations. A Langrangian plume model using a Gaussian plume description or a K-model with many layers can be used to model NHx deposition to nature areas embedded in agricultural areas in northern Europe (spatial resolution <5×5 km2). On the other hand long-range transport of NHx can be modelled with Lagrangian or Eulerian models with correction factors to calculate dry deposition of NH3 close to the source correctly (spatial resolution of 1-layer models >50×50 km2). TREND, a statistical atmospheric transport model was applied to calculate the concentrations and depositions of NH3 and its reaction product NH4+ aerosol to Denmark and surrounding sea areas on a 5×5 km2 grid. The model is able to reproduce measured NH3 and NH4+ aerosol concentrations reasonably well, as well as NHx wet deposition. It was found that dry deposition constitutes about 66% to the NHx total deposition to the land area of Denmark. Wet deposition, however, is relatively more important at sea and constitutes 64–67% of the NHx total deposition to the Kattegat and Southern Belt sea areas.