How do the models behind the Flemish nitrogen deposition policy work?

The essence of the Programmatic Approach to Nitrogen (PAS) is the degree of impact of the process. Based on these impact scores and a number of other company characteristics, the reductions that the company must achieve by a certain deadline are imposed. To determine this degree of impact, within a Special Protection Zone (SBZ-H), we look for where the ratio of exploitation deposition to what nature can handle is greatest. “What nature can handle” here means the maximum permissible amount of nitrogen that the SPA can handle without being disturbed, this is called the critical deposition value (KDW).

From calculation models to impact outcomes

Calculation models are used to calculate this impact score. “There are many different models to calculate deposition, but each model follows the same path,” says nitrogen expert Wouter Lefebvre of the Flemish Institute for Technological Research (VITO). “It always starts from imported emissions, and then the amount of pollution in the air is calculated, and these are the concentrations. The model then calculates sedimentation based on this. There are several formulas behind these calculation steps.

FLOPS model

Flanders uses a combination of two models to calculate global and local precipitation. The impact score is then derived from this. On the one hand, you have the VLOPS model, where VLOPS stands for the Flemish version of the Operational Priority Materials (OPS) model, developed by RIVM (Netherlands). The model was used to calculate concentrations and deposition over the entire Flanders region with a geographical resolution of one square kilometre.

Enter data for the form They are meteorological data from 2017, data on receiving areas, emissions data from outside Flanders and Flemish and locations from the EMAV2.1 model which was updated in 2019. This model uses the annual average of livestock density in 2017 as well as the applied housing techniques. EMAV2.1 works with animal populations as defined by the manure bank. These give the average occupancy of animals throughout the year. Since there is no existing permit database, nor a database with information regarding propellers/output speed, company management, etc., EMAV cannot work with licensed animal numbers or more company-specific characteristics.

Thus, VLOPS calculates all Flemish and foreign emissions, including total nitrogen deposition in the entire Flanders region on an aggregate scale.

IFDM model

As a second model, the IFDM model is used to improve the VLOPS results so that precipitation can be examined at the small-scale project level. IFDM (Irradiation Frequency Distribution Model) is a Gaussian binary column model that starts from the emission sources and models the spread of pollution based on meteorological parameters.

Concretely, the plume from the source in question at a given time is calculated based on the wind speed and direction, and the amount of polluting emissions at that time. In short, this model calculates the deposition pattern For a particular company you want to determine the degree of its influence. The IFDM was designed by VITO and has high geographic accuracy, because it calculates all current point sources (industry, stables) and line sources (road traffic, freight) within a 20 km radius within a 20 m resolution.

The concentrations calculated using IFDM are then multiplied by the dry deposition rates arising from the VLOPS, and the wet deposition as calculated by IFDM is added. Local precipitation calculated using IFDM is then combined with large-scale VLOPS results. If the transmissions calculated using IFDM have already been included in the VLOPS calculation, a double counting correction is performed. This connection improves the large-scale deposition results from VLOPS with detailed calculation using IFDM.

This total precipitation is then placed on a single map, and it is then determined where the values ​​on the precipitation map are higher than those on the KDW map. The area there is redundant. For those exceeded areas, models are used to determine which sources contribute to the KDW exceedance and what percentage, and thus the impact score is calculated.