An intercomparison of models used to simulate the short-range atmospheric dispersion of agricultural ammonia emissions

Ammonia emitted into the atmosphere from agricultural sources can have an impact on nearby sensitive ecosystems, either through elevated ambient concentrations or dry/wet deposition to vegetation and soil surfaces. Short-range atmospheric dispersion models are often used to assess these potential impacts on semi-natural ecosystems and a range of different models are used for these assessments. However, until now there has not been an intercomparison of the different models for the case of ammonia dispersion from agricultural sources and therefore it cannot be assumed that assessments are consistent. This paper presents an intercomparison of atmospheric concentration predictions made by a set of models commonly used for this type of assessment (ADMS; AERMOD; LADD and OPS-st). This intercomparison shows that there are differences between the concentration predictions of the models and some of these differences appear to be consistent and independent of the scenario modelled. The best model agreement was found for simple scenarios with area and volume sources, whereas the model agreement was worst for a scenario with elevated sources with exit velocities, for which ADMS predicted significantly smaller concentrations than the other models. The concentration predictions for the latter scenario depend strongly on the ability of the models to simulate the necessary processes, as well as the interaction of these processes with pre-processor calculations of meteorological data. When applied to two case study farms in Denmark and the USA, the performance of all of the models is judged to be 'acceptable' according to a set of objective criteria, although the LADD model version used is currently not suitable for simulations with elevated sources with exit velocities.