Atmospheric dispersion and deposition models are an essential tool for assessing the environmental impact of our activities. They support environmental permitting and help policymakers evaluate measures to reduce nitrogen emissions. As these models play an increasingly important role in decision-making, it is essential that they are scientifically validated.
To answer this question, VITO participated in an international benchmark and validation study coordinated by the Dutch National Institute for Public Health and the Environment (RIVM). Eight atmospheric dispersion models used across Europe were compared using common scenarios and validated against measurements near livestock farms, motorways and industrial sites.
The benchmark showed that the models generally produced comparable results, while the validation study demonstrated that IFDM, VITO's operational dispersion model, is among the best-performing models. For all concentration datasets, IFDM met internationally recognised performance criteria and showed no significant systematic bias.
While IFDM performed very well for concentration predictions, the studies also highlighted the challenges of modelling nitrogen deposition. Moreover, deposition measurements are subject to much larger uncertainties than concentration measurements, making validation more difficult. Nevertheless, IFDM performed within the range of other European models and confirmed that the current uncertainties are driven largely by emissions and measurements rather than by the model itself.
Because ammonia emissions from open cattle stables are particularly important in Flanders, VITO also carried out a dedicated validation study using emission and concentration measurements from field campaigns by ILVO and VMM. The study confirmed that IFDM accurately predicts ammonia concentrations when emission sources are represented appropriately.
To better understand how emissions disperse around open cattle stables, VITO complemented the measurements with Computational Fluid Dynamics (CFD) simulations. These simulations showed that emissions mainly leave the stable through the side openings rather than the roof, explaining why representing open stables as point sources leads to less accurate results. They also highlighted the influence of nearby buildings and local airflow on measured concentrations.
Together, these studies confirm that IFDM provides reliable results for nitrogen assessments. Rather than modifying the model itself, the findings show that future improvements should focus on the correct representation of emission sources and on clear guidance for applying the model in environmental assessments.