An intercomparison of models used to simulate the 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 depending on the location and experience of the assessors. However, until now there has not been an intercomparison of the different models, for the case of ammonia dispersion from agricultural sources. This paper presents an intercomparison of some models commonly used for this type of assessment (ADMS; AERMOD; LADD and OPS-st). This intercomparison shows that there are significant differences between the concentration predictions of the models and some of these differences appear to be consistent and independent of the scenario modelled. In addition, the agreement between the model concentration predictions is dependent on source height with the models agreeing best for ground-level sources. The level of model agreement decreases with source height although the agreement between the three Gaussian models (ADMS, AERMOD and OPS-st) improves again as the source height increases to 20 m. When applied to two case study farms in Denmark and the USA model performance is 'acceptable' (i.e. the model predictions met most of the acceptability criteria) for all of the models except for the LADD model which is probably beyond its limits of applicability for one of the case studies (elevated source with a large exit velocity).