The RIVM has found a partial explanation for the difference between measurements and calculations of ammonia concentrations along the coast. Based on this, improvements have been made to the OPS Operational Priority Substances (Operational Priority Substances) calculation model. The results are summarized in the now published report. With this, the RIVM concludes its research on ammonia from the sea. In the National Knowledge Program Nitrogen (NKS), the RIVM is investigating together with other research organizations how the model calculations can be further improved.
Along the Dutch coast, there is a greater difference between the calculated and measured ammonia concentration in the air than in the rest of the Netherlands. The RIVM assumed that this difference could be explained by ammonia emissions from the sea. However, this ammonia emission was overestimated. Therefore, the RIVM conducted further research on this.
Difference Better Explained
From interim reports in 2023 and 2024, it became clear that the measurements along the coast are correct and that there are no other ammonia sources that explain the difference. It also appeared that the input data, such as weather conditions, that the nitrogen model OPS Operational Priority Substances (Operational Priority Substances) uses could be improved at several points. These data have now been adjusted in the model. The effects of the adjustments will become visible throughout 2025 in the products delivered by the RIVM, such as AERIUS, the GCN Large-scale Concentration Maps Netherlands (Large-scale Concentration Maps Netherlands) report, and the Monitor nitrogen deposition in Natura 2000 areas.
Due to the adjustments, the difference between measurement and calculation along the coast has become 20% smaller. The calculations now deviate about 40% from the measurements. On average, this is 30% in the rest of the Netherlands.
Follow-up Research on Model Improvement
Every scientific model has uncertainties. To determine the extent to which uncertainties in the OPS model contribute to the difference along the coast, the RIVM compared the OPS model, together with TNO, with two other models. It turned out that all models calculate the concentrations along the coast lower than the measurements show. This suggests that the cause of the differences is the same in all models. One possible cause is that the emission of ammonia varies at different locations in the Netherlands and throughout the year is not well understood.
Together with partners from the National Knowledge Program Nitrogen, the RIVM is now investigating how the emissions of ammonia and nitrogen oxides can be better mapped. The RIVM expects that with the results, a larger part of the ammonia concentration along the coast can be explained. The RIVM will also start this year, together with the province of South Holland and drinking water company Dunea, a comprehensive measurement campaign on nitrogen deposition in the Solleveld dune area. These measurements may provide clues to further improve the deposition calculations in the dune area.