What a dispersion study is for
A dispersion study is a professional calculation document that assesses how source emissions will manifest in surrounding air. It therefore addresses not only how much pollutant a source releases, but mainly what impact that will have in a specific locality.
In practice, a dispersion study is used especially as documentation for the regional authority, Ministry of the Environment, building proceedings, EIA, operating permit changes, or other permitting processes where the project's impact on air quality must be demonstrated.
It is typically addressed for example at:
- boiler plants and combustion sources,
- paint shops and technologies with VOC emissions,
- industrial and manufacturing plants,
- recycling centres,
- waste management facilities,
- landfills and composting facilities,
- dust sources,
- technological exhausts,
- significant related traffic,
- car parks and traffic-intensive projects.
The aim of the study is to clearly demonstrate whether the assessed project will not cause an unacceptable contribution to air pollution in the affected area.
When a dispersion study is prepared
A dispersion study is prepared especially when required by law, the regional authority, Ministry of the Environment, or another competent body within the permitting process.
It may typically involve these situations:
| Situation | Why a dispersion study is addressed |
|---|---|
| New stationary source | Authority needs to assess how emissions of the new source will manifest nearby |
| Change to existing source | Assessed whether change in output, capacity, or technology increases immission contributions |
| Capacity increase | Higher capacity may mean higher emissions, more traffic, or higher dust |
| New exhaust or change of discharge | Change in height, position, or exhaust parameters may change pollutant dispersion |
| Significant related traffic | Traffic can increase NOx, PM10, PM2.5 emissions and resuspended dust |
| EIA or screening procedure | Dispersion study is often an important annex for assessing impacts on air |
| Regional authority requirement | Authority may require the study as documentation for binding opinion or permit change |
A dispersion study is not always automatically necessary, however. What matters is the specific type of source, its classification, scope of change, type of emitted substances, location, surrounding development, existing immission situation, and requirement of the competent body.
Difference between emissions and immissions
For correct understanding of a dispersion study, it is important to distinguish emissions and immissions.
Emissions are pollutants released from a source into the air. They may come from a stack, technological exhaust, combustion equipment, paint shop, filter, material handling, or traffic.
Immissions are pollutant concentrations in surrounding air — what appears at a given location after atmospheric dispersion, for example at residential development or another assessed locality.
| Term | Meaning | Example |
|---|---|---|
| Emission | Quantity of pollutant released from a source | NOx from boiler, VOC from paint shop, dust from crushing |
| Immission | Concentration of pollutant in surrounding air | NO2 contribution at a residential building, PM10 in nearest development |
| Emission limit | Limit for source or exhaust | Substance concentration in waste gas |
| Immission limit | Limit for air quality in the area | Annual or short-term concentration of substance in air |
A dispersion study therefore usually does not start from the question of what emissions a source has, but how those emissions will manifest in the affected area after dispersion.
What immission contribution means
Immission contribution is the calculated contribution of the assessed source or project to pollutant concentration in air.
In other words: a dispersion study does not only say "there is a certain pollution level in the locality", but seeks to determine what part of that level is caused by the assessed project.
In practice, the following are mainly assessed:
- contribution of a new source,
- contribution of a change to an existing source,
- contribution of related traffic,
- contribution of handling of dusty material,
- contribution of operation as a whole,
- relationship of contribution to existing immission background.
It is important that even a relatively small source can be problematic in a locality where air quality is already burdened. Conversely, for another project contribution may be low if emissions are small, the exhaust is suitably designed, and dispersion conditions are favourable.
What inputs are needed for a dispersion study
A dispersion study is a calculation document. Quality of the result therefore depends on quality of input data. If documentation is incomplete or inconsistent, the calculation may be inaccurate or the study may need to be supplemented.
For preparation of a dispersion study, it is advisable to provide especially:
| Documentation | Why it matters |
|---|---|
| Project documentation | Describes the project, location, building solution, and links to surroundings |
| Site layout | Allows determination of position of sources, exhausts, roads, and surrounding development |
| Technology description | Helps determine where and how emissions arise |
| Equipment datasheets | Contain output, capacity, flows, temperatures, fuel, or other important parameters |
| Exhaust data | Height, diameter or cross-section, temperature, flow velocity, coordinates, and operating regime |
| Emission data | Emission limits, emission factors, measurements, balances, or manufacturer guaranteed values |
| Operating hours | Number of operating hours, shifts, seasonality, and temporal use of source |
| Plant capacity | Designed or maximum capacity is essential for emission calculation |
| Fuel or raw material data | Important for boiler plants, combustion sources, paint shops, and technological plants |
| Separator and filter data | Affect resulting emissions released to air |
| Traffic data | Vehicle numbers, routes, travel distances, vehicle composition, and temporal distribution of traffic |
| Existing operating permit | For source changes helps compare existing and new state |
| Authority requirement | Determines purpose of study and scope of assessment |
For more complex projects, it is important that data in all documentation are consistent. If the project states a different capacity than the application, a different number of exhausts than the technical report, or a different operating regime than the operating rules, this can lead to uncertainties and supplementation.
Why the wind rose matters
The wind rose is one of the key meteorological inputs for a dispersion study. It shows frequency of wind directions and speeds and also atmospheric dispersion conditions.
Put simply: the same source may have a different impact in open flat countryside, in a valley, and in built-up areas. Pollutant dispersion depends not only on emission quantity but also on meteorology, terrain, exhaust height, and surrounding development.
The wind rose helps determine for example:
- where emissions will most often spread,
- what typical dispersion conditions are in the locality,
- whether residential or other sensitive objects may be affected,
- how short-term and long-term concentrations may manifest,
- what influence source location in a valley, on a slope, or in open terrain may have.
For a quality dispersion study, it is therefore important to use a wind rose corresponding to the assessed locality. General or inappropriately chosen meteorological data can distort the result.
Why traffic must be addressed
For many projects, the main source of pollution is not only the technology itself. Related traffic can also have a significant impact.
This typically involves for example:
- delivery of raw materials,
- dispatch of products,
- dispatch of waste,
- movement of heavy goods vehicles in the site,
- car parks,
- loaders and handling equipment,
- traffic of visitors, employees, or customers,
- travel on unpaved or contaminated surfaces.
Traffic can be significant especially at waste facilities, recycling centres, landfills, logistics sites, industrial plants, car parks, and projects with a higher number of heavy goods vehicles.
For calculation, the following are especially important:
| Traffic data | Why it is needed |
|---|---|
| Number of vehicles per day | Determines total traffic load |
| Maximum hourly intensity | Important for short-term concentrations |
| Share of heavy goods vehicles | Heavy goods traffic usually has higher emission significance |
| Travel distance in site | Affects traffic emissions and dust resuspension |
| Vehicle routes | Determine where emissions will manifest |
| Road surface | Asphalt, panel surface, gravel, or unpaved road have different impacts |
| Speed and traffic flow | Affect emission factors |
| Temporal distribution of traffic | Important when assessing daily and hourly maxima |
If traffic is underestimated or not stated at all, the dispersion study may be incomplete. For some projects, traffic contribution to immissions may be more significant than contribution of the stationary source itself.
What resuspension is and why it is assessed
Resuspension means re-entrainment of dust particles from road surfaces, handling areas, or stored materials. It is therefore not only dust coming directly from an exhaust, but also dust entering the air through vehicle movement, handling, or wind.
Resuspension is important especially at:
- recycling centres,
- landfills,
- composting facilities,
- construction yards,
- quarries,
- processing of bulk materials,
- site traffic on unpaved surfaces,
- plants handling soil, waste, aggregate, or construction materials.
In practice, resuspension can be significant mainly for PM10 and PM2.5 particles. It is therefore important to address not only number of vehicles but also road surface, cleaning, wetting, speed limits, material enclosure, and other organisational or technical measures.
Dispersion study for existing and new state
For many projects, assessing only the future state is not enough. It is often necessary to compare existing, new, or proposed state and possibly variant solutions.
Typically addressed:
| Variant | What is assessed |
|---|---|
| Existing state | Impact of current operation |
| Proposed state | Impact after project or change implementation |
| Zero variant | State without project implementation |
| Variant solutions | Comparison of different technical or operational variants |
| Cumulative impacts | Combined effect of multiple sources or projects in the area |
For changes to existing operations, it is important to clearly separate what is already permitted or operated and what is newly changing. Without this distinction, it may be difficult to evaluate whether the change actually increases immission contributions.
Typical mistakes in documentation
When preparing dispersion studies, mistakes that can delay proceedings or lead to study supplementation often recur.
The most common problems are:
- it is unclear which sources are to be assessed,
- exhaust coordinates are missing,
- exhaust heights above terrain are not stated,
- exhaust diameter or cross-section is missing,
- temperature and velocity of waste air are unknown,
- designed plant capacity is not stated,
- operating hours are not described,
- emission data are missing,
- datasheets do not contain sufficient emission parameters,
- information on filtration or separation equipment is missing,
- not all related traffic movements are described,
- passenger and heavy goods vehicles are not distinguished,
- dust resuspension is not addressed,
- data in project, application, and operating rules contradict each other,
- clear purpose of study and authority requirement are missing.
A dispersion study is a calculation document, but it rests on real input data. If inputs are not well prepared, a quality and defensible output cannot be expected.
Why an equipment datasheet alone is not enough
An equipment datasheet is a useful document, but on its own it is usually insufficient.
A datasheet may state for example equipment output, filter type, air flow, or basic emission parameters. It does not, however, address all circumstances important for a dispersion study.
It usually lacks for example:
- exact source position,
- exhaust position and height,
- exhaust coordinates,
- operating hours,
- annual and hourly capacity,
- actual or designed quantity of raw materials,
- link to other sources in the site,
- related traffic,
- surrounding development,
- meteorological and terrain conditions,
- relationship to existing operating permit.
A dispersion study must therefore combine technical data on equipment with data on operation, location, exhausts, emissions, traffic, meteorology, and surrounding development.
Practical procedure for investor or designer
If you are addressing a new source, operational change, or authority requirement for a dispersion study, we recommend proceeding as follows:
- Prepare the authority requirement, if it already exists.
- Provide project documentation or technical description of the project.
- State whether it is a new source, change to existing source, or capacity increase.
- Prepare a list of emission sources and exhausts.
- For each exhaust, add height, diameter or cross-section, temperature, velocity, and air flow.
- Prepare emission data, emission limits, emission factors, or measurements.
- Add operating hours and designed capacity.
- Describe related traffic including heavy goods vehicles, routes, and travel distances.
- For dusty operations, add information on areas, handling, wetting, cleaning, and dust control measures.
- Have it verified in advance whether a dispersion study is actually necessary and in what scope it should be prepared.
This procedure helps avoid having to rework the study later due to missing or inconsistent data.
Summary
A dispersion study is a key document where it is necessary to assess the impact of a source, operation, traffic, or technology change on surrounding air quality. It does not assess emissions alone, but their dispersion in a specific locality and calculated immission contributions.
For a quality study, correct inputs are essential: technology description, emission data, exhausts, operating regime, capacities, traffic, resuspension, meteorological data, terrain, and surrounding development.
Send us basic source parameters, project documentation, or the authority requirement and we will verify the scope of the dispersion study for your project.