RIVM conducted two studies on how uncertain the calculations are of nitrogen deposition from a single source, such as a barn, factory, or highway. These studies confirm previous estimates of uncertainties in the Dutch nitrogen deposition model OPS Operational Priority Substances (Operational Priority Substances). It also appears that the uncertainties correspond with similar (international) calculation models. The results, and what this means for the use of OPS in policy, are summarized in a knowledge note. RIVM advises to explicitly consider uncertainties in the justification of decisions, for example in granting permissions.
OPS Operational Priority Substances (Operational Priority Substances) is used, among other things, in the AERIUS Calculator to calculate how much a specific activity contributes to nitrogen deposition in nature areas. It is important to have insight into the uncertainties, as these calculations can play a role in granting permits.
Read more about uncertainty margins in the nitrogen deposition model.
Research Method: Comparison of Different Models and Sensitivity Analysis
To determine uncertainties, calculations are often tested against measurements. However, measuring nitrogen deposition from one specific source is complicated. The deposition at one location almost always consists of contributions from multiple sources. Therefore, a different approach was chosen in these studies. One study compares the OPS model with similar nitrogen deposition models. The other study contains a sensitivity analysis of components in OPS.
Uncertainties of Individual Sources
The studies confirm previous estimates and better visualize the uncertainties in the OPS calculations. The uncertainty varies by source and distance. Therefore, there is not a single number for the uncertainty margin of an individual source. A new insight from the sensitivity analysis is that uncertainty does not necessarily increase with distance from the source.
Dry Deposition
An important component of OPS is the rate at which nitrogen falls from the air to the ground. This is called the dry deposition rate. This rate is difficult to measure and appears from the sensitivity analysis to be a significant source of uncertainty. The National Knowledge Program Nitrogen (NKS) investigates how the calculation of dry deposition can be improved.
Uncertainty in Models and Usability of OPS for Policy
Uncertainty in model calculations is unavoidable. The outcome always depends on input data, assumptions, and natural variation. But that does not mean that a calculation is arbitrary or unreliable. The calculations are always repeatable and based on physical principles.
The uncertainties are not a reason not to use the results. Previous studies have shown that OPS is suitable for national monitoring and trend analyses. The uncertainty margin in these calculations is 30 percent. For calculating the contribution of an individual source, the margins are larger, but the outcome is the most likely value based on current scientific knowledge and data. It is important to recognize that uncertainties cannot be completely eliminated. Therefore, RIVM advises to explicitly consider this in the justification of decisions, both for OPS and for its application in the AERIUS Calculator for granting permissions.
Publication of Other Knowledge Notes on Nitrogen
RIVM is also publishing three other knowledge notes on nitrogen today. One of these knowledge notes addresses a scientific discussion about the reliability of calculations for short-term projects, such as a festival, small construction projects, or a bonfire. It shows that uncertainty increases as the duration of the projects becomes shorter. Because this increase occurs continuously, there is no scientifically supported limit where the applicability of AERIUS ceases.
