How does the AERIUS Calculator work?

What is the AERIUS Calculator (and why is it mandatory)?

The AERIUS Calculator calculates how much nitrogen an activity deposits on nitrogen-sensitive nature. This is a legal requirement in the Netherlands: the Dutch Omgevingsregeling (Environment and Planning Regulation) states that a calculation for a Natura 2000 permit must be made with this Calculator, and also specifies which version to use. If you calculate with an outdated version, the calculation doesn't count.

The Calculator comes into play for all kinds of projects in the Netherlands: a new housing development, the expansion of an agricultural business, the construction of a road, a factory expansion, or even a large event. AERIUS looks not only at the final situation but also at the construction itself: construction traffic and machinery also produce emissions, so that phase is calculated separately alongside the later operational phase. The end result comes back as a PDF, which you add to the permit application.

AERIUS Calculator, Connect, Check, Register, and Monitor: what's the difference?

AERIUS isn't a single tool, but a family of products, each playing its own role: from calculating an individual project to keeping track of nitrogen headroom across the Netherlands as a whole. Here's an overview of what each product does.

AERIUS Calculator is the calculation engine for project-specific calculations for a permit application. For a standard application, this is all you need.

AERIUS Connect is the engine behind Calculator: the same calculation core, but directly accessible without the web application in between. This is needed for large projects with more than 5,000 sources, or when a company wants its own software to run calculations via AERIUS without manually logging into the Calculator each time.

AERIUS Check determines whether a Dutch business is a "peak emitter" (piekbelaster): one of roughly 3,000 businesses in the Netherlands that cause the most nitrogen deposition near an overburdened Natura 2000 area. If emissions are above 2,500 mol per year, a business becomes eligible for a subsidy scheme to voluntarily close, relocate, or reduce emissions. This scheme applies only to businesses in the Netherlands.

AERIUS Register keeps the books on nitrogen headroom. When headroom becomes available somewhere, for example because a measure lowers emissions, it's recorded here. When that headroom is then allocated to a new project, that's registered here too, so that no more headroom is ever issued than actually exists. Register is only accessible to competent Dutch authorities.

AERIUS Monitor provides insight into the nitrogen situation itself, independent of any individual application. It shows where in Dutch Natura 2000 areas vulnerable nature is located, how much nitrogen ends up there, how that develops over the years, and where it's already too much. Where Calculator works out the effect of your project, Monitor shows the broader picture for an area.

For a standard permit application, Calculator is all you need. The other products are mainly relevant to competent Dutch authorities, policymakers, or specific subsidy schemes.

When is an AERIUS calculation needed?

As soon as an activity emits nitrogen oxides (NOx) or ammonia (NH3) that could be deposited on nitrogen-sensitive nature, a calculation is required. That applies to new activities, but just as much to the modification or expansion of an existing, already permitted situation.

Calculations also come into play with nitrogen offsetting ("salderen"), a Dutch legal mechanism. Say a business wants to expand but doesn't have enough nitrogen headroom of its own. One solution is to free up headroom elsewhere and use it for the new activity. If this happens on the company's own site, for example by closing or cleaning up an old part of the business, it's called internal offsetting. If it happens through another business that closes down or reduces its permitted activity and transfers that headroom, it's called external offsetting. In both cases, not all of the freed-up headroom may be used: part of it goes back to nature. This is called the skimming factor. If 100 mol becomes available and a skimming factor of 30 percent applies, the business may use 70 mol for the expansion; the remaining 30 mol is no longer available.

To see what changes on balance, all situations are calculated together: what already existed, what the headroom from elsewhere adds, and what's newly added. This keeps visible that the expansion doesn't lead to more nitrogen than was actually freed up on balance.

How does the calculation work?

From emission to deposition

AERIUS calculates in four steps. First, the emission of NOx and NH3 at the source is determined. Then comes dispersion through the air, any chemical conversion along the way, and finally deposition on soil and vegetation.

The calculation models: OPS and SRM-2

For most sources, AERIUS calculates with OPS. This model tracks how a substance spreads from its source, somewhat like smoke coming out of a chimney and dispersing: concentration is highest close to the source, and dilutes further away. OPS can calculate this over both short and long distances, covering both the effect right next to a construction site and the contribution that still lands hundreds of metres away.

For road traffic up to 5 kilometres from the road, AERIUS uses a different model: SRM-2. Road traffic behaves differently from a chimney or a fixed piece of equipment, because the emissions come from many separate, low-lying sources spread along a line (the road). SRM-2 is designed for this and first calculates how much of the substance reaches the air at a given distance from the road. That concentration is then converted into a deposition contribution, using calculation values that were previously determined with OPS.

Beyond that 5-kilometre mark, road traffic is no longer calculated with SRM-2 but simply with OPS, up to the maximum calculation distance of 25 kilometres.

A calculator, not a judgment

A key principle of the AERIUS Calculator is that the tool only calculates; it doesn't judge. The Calculator checks whether the data entered is valid, but doesn't say whether a calculated result is acceptable for a permit. That assessment lies with the competent authority: usually the Dutch province where the project is located, or the Dutch national government for larger or national projects. AERIUS always calculates using the calculation methods, emission factors, and nature data that are current at that moment, which means the same input can produce a different result in a later version.

Step by step through the AERIUS Calculator

A calculation in AERIUS Calculator goes through six steps. Picture this: a contractor is building a new business hall on an existing site in the Netherlands. Here's what the process looks like for him.

Creating situation(s)

He starts with a new situation, or builds on an earlier calculation by importing a file he saved himself. AERIUS doesn't store anything itself between sessions. A situation is a collection of emission sources and buildings that belong together. For the permit, it must be clear exactly what's changing, so the contractor records two moments: how the site looks now (the reference situation, what's already permitted) and how it will look afterwards (the proposed situation, with the new hall). AERIUS compares the two, so that only what's genuinely being added becomes visible, not what was already there.

Input per situation

For each situation, the contractor indicates where the emissions come from: an emission source. During construction, these are things like the mobile machinery moving around the site and the construction traffic coming and going. For each source, he also specifies the sector, for example Mobile machinery or Traffic. This choice determines which data he needs to enter next: for an excavator, that's fuel consumption and operating hours, for example.

Custom calculation points

AERIUS already has fixed calculation points laid out across the whole of the Netherlands, and automatically uses the points near vulnerable nature. So the contractor usually doesn't need to do anything here: his project is automatically calculated at the right locations. Only if the project could also affect an area outside the Netherlands, for example a nature area just across the border in Germany, does he add an extra calculation point himself. Such foreign areas don't automatically fall within this Dutch system of calculation points.

Assembling the calculation task

Now AERIUS actually starts calculating. The reference situation and the proposed situation are combined into one calculation task, and AERIUS works out the difference: the emissions attributable to construction alone. If a project has both a construction phase and a permanent operational phase, these are usually modelled as two separate proposed situations, each with its own project calculation: one for the mobile machinery and construction traffic during the build, and one for what remains permanently afterwards, such as traffic generated by a new housing development. AERIUS also has a specific type of calculation task, the maximum temporary effect, intended for a short-lived intermediate phase where you want AERIUS to automatically calculate the peak moment.

Assessing the results

AERIUS displays the outcome per hexagon: a hexagonal area of one hectare that, together with all the other hexagons, covers the whole of the Netherlands. The contractor can see in which area the contribution is highest. More importantly, he can see whether that contribution causes an increase anywhere on a hexagon that's already overburdened with nitrogen, or close to it. Every protected piece of nature, a habitat type, has its own critical deposition value: the threshold above which too much nitrogen damages nature. That threshold concerns the total amount of nitrogen already present, not the contribution of a single project alone. So what matters for the permit isn't whether the project's contribution stays "below" that threshold, but whether the project causes a measurable increase in places where the threshold is already reached or nearly reached. If the contribution there stays at 0.00 mol per hectare per year, the project in principle poses no obstacle to the permit.

Exporting

Finally, the contractor records the result. This can be done as a GML file, with or without the calculation results, useful for further editing or sharing, or as a PDF report, which gets added to the permit application.

How emission data for mobile machinery is entered

The outcome of an AERIUS calculation is only as good as the emission data entered. For mobile machinery, such as excavators, loaders, or generators, you first select the emission stage from a list, for example "Stage II, 2002–2005, ≤56 kW". This information is usually found on the engine's type plate, or in the machine documentation you received from the manufacturer or supplier. This single choice fixes both the build year and the engine's power rating. Next, you enter the fuel consumption in litres per year and the number of operating hours per year. Only for emission stages with an SCR system does a field for AdBlue consumption in litres per year appear. AERIUS calls this data the AUB method (a Dutch abbreviation for AdBlue, hours, and fuel), and uses it to automatically calculate the NOx and NH3 emissions, applying fixed coefficients from TNO, the Dutch applied research organisation. Source characteristics such as exit height (how high the emission is released), heat content (how warm the emission is, which determines how high it rises), and spread (the variation in exit height within the source) are already filled in for this standard method, with values matching the chosen emission stage and engine power.

If you'd rather calculate with your own figures, you can choose the "custom specification" option and enter the NOx and NH3 emissions directly in kilograms per year, along with the source characteristics exit height, heat content, and spread.

Where does reliable emission data come from?

A calculation is only as good as the figures that go into it. But how do you actually get that input if you have a new construction or infrastructure project in the Netherlands? In practice, operating hours and fuel consumption for machinery are often estimated: an estimate based on previous projects, or a rough assumption per emission stage. The problem is that these are precisely the two figures the entire emission calculation relies on.

That doesn't have to be the case if the equipment is already fitted with a system that tracks this data itself. GPS-Buddy's Flowter platform calculates NOx and NH3 per project location and period, based on the same TNO AUB method that AERIUS itself uses, and based on the actual operating hours and fuel consumption of the machine. This means the figures AERIUS asks for are already available, instead of having to be gathered afterwards.

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No rights can be derived from this text. The AERIUS Calculator is updated regularly, which means calculation methods, input fields, and the exact workings of the application may change. For the most current and official information, always visit aeriusproducten.nl (Dutch only).