Atmospheric dispersion studies: when they are essential and how to do them properly
If noise is the environmental impact most easily forgotten until it shows up, air quality is the impact hardest to defend once it is questioned. An undersized stack, an unassessed diffuse source or a dispersion model built on generic data can become the reason an environmental permit stalls, an environmental licence is refused or an activity has to change its operating regime.
The atmospheric dispersion study is the technical tool that assesses, before and during the operation of an activity, how emissions to the atmosphere behave from their source to the sensitive receptor. This guide answers the questions industries, developers and public administrations most often put to us: when it is required, what it includes, how to do it properly and which mistakes are the most common.
When a dispersion study is required: legal framework
At national level, Law 34/2007 on air quality and protection of the atmosphere and Royal Decree 102/2011 on the improvement of air quality set the ambient limit values, the monitoring obligations and the framework for atmospheric modelling. The transposition of the Industrial Emissions Directive (IED) and the integrated environmental permit (AAI) regime add specific requirements for installations subject to IPPC. In Catalonia, Law 22/1983 on the protection of the atmospheric environment and its updates complement this framework.
In consulting practice, an atmospheric dispersion study is essential in six situations:
Permitting of an AAI (integrated environmental permit) or an environmental licence for activities included in the catalogue of activities potentially polluting the atmosphere. The study is an inseparable part of the technical documentation.
Environmental Impact Statements (DIA) for projects with significant emission sources: thermal power plants, biomass plants, cement works, chemical plants, waste management, wastewater treatment plants (EDAR) with sludge treatment.
Substantial modifications to activities already in existence involving new emission sources, a change of fuel or an increase in production capacity.
Industrial odour assessment, especially in the agri-food, waste management, wastewater treatment, chemical industry and intensive livestock sectors.
Air quality studies in urban planning and in areas declared sensitive because limit values have been exceeded.
Technical defence against claims or complaints from public administrations or neighbours over perceived or documented ambient concentrations.
Types of model and when each one applies
Dispersion studies are not all alike. The type depends on the terrain, the scale of the activity and the receptor to be assessed:
Gaussian models (AERMOD). These are the regulatory reference models in Europe and Spain. Suitable for point sources, simple to moderate terrain and distances of up to 50 km. It is the model most used in AAI and DIA procedures.
Lagrangian models (CALPUFF). Recommended for cases with complex topography, variable meteorological conditions or greater distances. More accurate, but also more demanding in terms of input data.
CFD models for urban environments. For assessments in dense urban areas where buildings significantly alter dispersion. Useful in projects embedded in the urban fabric.
Odour-specific models. They combine dispersion modelling with olfactometric data (European odour units, ouE/m³). Essential in sectors with odorous emissions.
How to do it properly
A rigorous dispersion study is built on four pillars: precise characterisation of every emission source (point and diffuse, with flow rate, temperature, exit velocity and actual composition), representative meteorological data covering at least one full year for the area (preferably from a nearby station or reanalysis data), computational modelling with the software appropriate to the case, and comparison against the ambient limit values set by regulation for each pollutant.
The usual output is a map of ambient concentrations across the activity's area of influence, identifying the sensitive receptors (dwellings, schools, hospitals, protected natural areas) and explicitly assessing compliance for each regulated pollutant: NOx, SO₂, PM10, PM2.5, VOCs, heavy metals, odours.
Five common mistakes
These are the mistakes that, in our experience, most often invalidate or weaken a study:
Insufficient or unrepresentative meteorological data (a station too far away, too short a time series, poorly corrected data).
Ignoring local topography, especially in rugged terrain where actual dispersion differs substantially from the flat-terrain case.
Diffuse sources left unassessed (fugitive process emissions, storage areas, unducted equipment), which are often the ones generating the highest ambient concentrations at short range.
Modelling average conditions only, overlooking the unfavourable scenarios (calm periods, thermal inversions) that determine peak ambient concentrations.
Failing to include short peaks or intermittent emissions (start-ups, shutdowns, maintenance), which in some sectors are what generate the most complaints.
What a well-executed dispersion study delivers
A properly executed dispersion study delivers three tangible results: an environmental permit that is defensible before the authorities with full technical guarantees, optimal sizing of stacks and abatement systems (often cheaper when decided with a model than without one), and a documented technical basis for defending the activity against future claims.
At auma auma we support industries, developers and public administrations in complete atmospheric modelling, from the characterisation of emission sources through to the design of corrective measures. If you want to go deeper into the technical dimension of environmental assessment, we also recommend reading our earlier article on acoustic studies, which covers its noise equivalent.