Typical applications include:
- assessment of modelled air pollution concentrations against air quality standards and limit values including those from WHO, EU, UK, USA and China
- planning/permitting, e.g. under Industrial Emissions Directive or Environmental Permitting Regulations,
- stack height determination,
- odour modelling,
- environmental impact assessments and
- safety and emergency planning.
The dispersion model ADMS 6 is currently used in many countries worldwide. Users of ADMS 6 include:
- over 130 individual company licence holders in the United Kingdom,
- regulatory authorities including the UK Health and Safety Executive (HSE),
- Environment Agency in England,
- Scottish Environmental Protection Agency (SEPA) in Scotland,
- Northern Ireland Environment Agency (NIEA) in Northern Ireland,
- Natural Resources Wales,
- government organisations including the Food Standards Agency (United Kingdom),
- users across Europe, Asia, Australia, North America, Africa and the Middle East.
ADMS 6 is a new generation Gaussian plume air dispersion model, which means that the atmospheric boundary layer properties are characterised by two parameters:
- the boundary layer depth, and
- the Monin-Obukhov length
rather than in terms of the single parameter Pasquill-Gifford class.
Dispersion under convective meteorological conditions uses a skewed Gaussian concentration distribution (shown by validation studies to be a better representation than a symmetrical Gaussian expression).
Model options: ADMS 6 has a number of model options including: dry and wet deposition; NOx chemistry; impacts of hills, variable roughness, buildings and coastlines; puffs; fluctuations; odours; radioactivity decay (and γ-ray dose); condensed plume visibility; time varying sources and inclusion of background concentrations.
Meteorological pre-processor: ADMS 6 has an in-built meteorological pre-processor that allows flexible input meteorological data both standard and more specialist. Hourly sequential and statistical data can be processed, and all input and output meteorological variables are written to a file after processing.
User-defined outputs: The user defines the pollutant, averaging time (which may be an annual average or a shorter period), which percentiles and exceedence values to calculate, whether a rolling average is required or not and the output units. The output options are designed to be flexible to cater for the variety of air quality limits, which can vary from country to country, and are subject to revision.
Visualisation: ADMS 6 includes the Mapper: an integrated mapping tool for displaying and editing source data, buildings and receptor locations and viewing results. The model has links to the Surfer contour-plotting package, in addition to ArcGIS and MapInfo Professional Geographical Information System (GIS) software. The GIS links can be used to enter and display input data, and display output, usually as colour contour plots.
Terrain converter: Utilities are available for creating terrain files from commonly available data formats such as SRTM and OS Terrain 50.
- Plume rise, source buoyancy and momentum
- Dispersion around buildings
- Flow over complex terrain
- Dry and wet deposition
- Time varying emissions
- NOx chemistry
- Amine chemistry
- Plume temperature and humidity output
- Plume visibility
- Impact of wind turbines on dispersion
- Radioactive decay and γ-ray dose
- Plumes or puffs
- Dispersion in coastal areas
- Dispersion in offshore areas
- Calm conditions
- Changes in surface roughness
- Link to AERMOD
Plume rise, source buoyancy and momentum
ADMS 6 uses a Runge-Kutta method to solve the conservation equations to estimate plume rise. This allows greater scope to include advanced model options than the Briggs empirical expression used in other Gaussian type models. The ADMS 6 method takes into account the effect on the plume of the source buoyancy and momentum, and includes the penetration of boundary layer inversions.
Dispersion around buildings
The building effects module in ADMS 6 includes the following features.
Up to 25 buildings can be included in each model run with a Main Building being defined for each source. For each wind direction, a single effective wind-aligned building is defined, around which the flow is modelled.
The flow field consists of a recirculating region (or cavity), with a diminishing turbulent wake downstream.
Concentrations for the cavity, CR, are uniform, and based on the fraction of the release that is entrained. The concentration at a point further downwind is the sum of contributions from two plumes: a ground-based plume from the recirculating flow region and an elevated plume from the non-entrained remainder. The concentration and deposition are set to zero within the user-defined buildings.
Flow over complex terrain
ADMS 6 uses CERC's FLOWSTAR, to calculate how the mean airflow and turbulence and hence dispersion and pollutant concentrations are changed over complex terrain.
The model predicts a three-dimensional flow and turbulence field over the region of interest, dependent on input values for the spatial variation of both the terrain height and surface roughness, as well as the local meteorological conditions.
Usually it is advisable to include terrain height effects if the gradients exceed 1:10 in the model domain. The model can typically be used for gradients up to about 1:2 but may not be reliable close to isolated slopes with higher gradients or more generally if large parts of the modelling domain have slopes greater than 1:2. The influence of the terrain will vary with the source height and position and the local meteorology.
Dry and wet deposition
The rate of dry and wet deposition to the ground can be modelled in ADMS 6. Dry deposition is assumed to be proportional to the near-surface concentration, and deposition velocities can either be entered by the user, or estimated by the model. Wet deposition is modelled through a washout coefficient; irreversible uptake is assumed, and plume strength following wet deposition decreases with downwind distance. There is also an advanced 'falling drop' option for wet deposition of SO2 and HCl which calculates the rate of dissolution and/or degassing in rain drops.
Time varying emissions
Emission rates from sources are rarely constant. The variation of the emission rate with time can be modelled in ADMS 6, in addition to corresponding variations in emission temperature, volume flow rate (or exit velocity), source diameter, and plume water content.
Odours are an important issue in areas where emission sites are located close to residential areas. The dispersion of odours can modelled using ADMS 6. Odour release rates and concentrations are specified/calculated in odour units of ouE which are a mass measure.
The chemistry scheme in ADMS 6 considers the following fast reactions involving NOx and ozone (O3) to determine the NO2 concentration in the dispersing plume(s):
NO + O3 → NO2 + O2
and a reverse reaction (day time only, hν depends on UV radiation intensity)
NO2 + hν → NO + O3
Model validation has shown the method to be more accurate than empirically based formulations.
An advanced model option allows for chemical reactions of amine to form nitramines and nitrosamines. The module has been developed as a consequence of emerging technologies for Carbon Capture and Storage (CCS) some of which are based on amine extraction of CO2.
Plume temperature and humidity output
ADMS 6 can calculate the plume-affected temperature, specific humidity and/or relative humidity at each output point.
The plume visibility module uses the initial water content of the release and the humidity of the ambient air to determine whether the plume will be visible due to condensed water droplets at each downstream distance. The effect of water on the plume density and the heating and cooling effects of condensation and evaporation are taken into account.
Impact of wind turbines on dispersion
The model can allow for the effect on dispersion of one or more horizontal-axis three-bladed wind turbines in the neighbourhood of an emission source.
ADMS 6 is the only regulatory model of its kind to model short time scale fluctuations allowing the calculation of the probability distributions of pollutant concentrations, probabilities of exceedence of specified threshold, and the range of concentration for averaging time as little as a second. The module has applications where estimates of the occurrence of peaks of concentration over short averaging times are important (e.g. odours, flammable & toxic accidental, 15 minute air quality objective for SO2). The module takes into account variations due to both turbulence and changes in meteorology.
Radioactive decay and γ-ray dose
ADMS 6 includes a radioactivity module that predicts the decay of radioactive species released from a source. Users may enter up to 10 parent isotopes in any model run, and up to 50 isotopes (parents and daughters) will be output. Half-lives of over 800 isotopes are included in the model and ADMS 6 can also calculate the associated levels of γ-ray dose.
Plumes or puffs
ADMS 6 can model both continuous releases, i.e. plumes, in addition to instantaneous and time-dependent releases, i.e. puffs.
Dispersion in coastal areas
For air dispersion modelling in coastal areas, ADMS 6 includes a coastline module to take account of increasing boundary layer height when airflow is from the sea to the land. It may be invoked when the following conditions are satisfied:
- the sea is colder than the land;
- there are convective meteorological conditions on land;
- there is an onshore wind.
Dispersion in offshore areas
ADMS 6 includes a marine boundary layer scheme for calculating surface roughness and heat fluxes over the sea, which could be used, for example, for dispersion modelling of emissions from stacks on oil platforms.
ADMS 6 includes an option to model ‘calm’ meteorological data which, in standard ADMS 6 runs, are not modelled.
The direction of the wind becomes more variable for lower mean wind speeds. The approach used for calm conditions is to calculate the concentration as a weighted average of a normal 'Gaussian' type plume and a radially symmetric plume, where the weighting depends on the wind speed. The radially symmetric plume assumes equal probability of all wind directions.
Changes in surface roughness
ADMS 6 can consider the spatial variation of surface roughness on plume dispersion.
Link to AERMOD
There is a facility to run the main options of AERMOD including the option of using the AERMET meteorological processor.
ADMS 6 can be used to assess the effect of emissions from a number of industrial source types:
- Point source e.g. emissions from a stack or vent.
- Area source e.g. evaporative emissions from a tank.
- Line source e.g.emissions from a conveyor belt at a quarry.
- Volume source e.g. fugitive emissions.
- Jet source (directional releases) e.g. emissions from a ruptured pipe.
The maximum number of sources that can be modelled in ADMS 6 is 300. Of these up to 300 may be point or jet sources, and within the limit of 300 up to 30 line sources, 30 area sources and 30 volume sources may be modelled simultaneously.
ADMS 6 uses a built-in mapper for helping to manipulate the emissions data (see the Visualisation tools page for more information).
Meteorological data is available in ADMS format from a number of suppliers. The advanced ADMS meteorological pre-processor determines the structure of the atmospheric boundary layer based on the input data. Hourly sequential or statistical meteorological data may be input.
Hourly sequential or averaged concentrations of background pollutants may be included if required.
ADMS can give output for up to 25 pollutants in one run, which may be either long-term statistical output or short-term output for each hour or meteorological condition in the input meteorological data. Rolling averages, exceedences and percentile statistics may also be calculated.
Output is given on a grid of receptor points covering a specified area and/or at specific receptor locations. Gridded results can be contoured using the ADMS Mapper or one of the included links: Surfer, ArcGIS or MapInfo (see the Visualisation tools page for more information).
Where only a single point source is modelled, ADMS 6 produces output containing detailed plume characteristics, such as plume centreline concentration, plume height, plume spread, etc., which can be graphed using the included ADMS Line Plotter.
Where hills are modelled, there is an option to output the FLOWSTAR flow field, which can be contoured using the included ADMS Vector Plotter.
Other individual model options produce specific model outputs; for full details see the model documentation.
The model interface is designed so that the user can enter the data required for the modelling in as straightforward a way as possible.
The interface consists of several main screens. To set up a model run, the user simply works through the screens entering the relevant data or referencing external data files.
The ADMS 6 model was developed by CERC, with some of the model options written in collaboration with other research organisations, such as the UK Meteorological Office, University of Surrey, and National Power (now part of E.ON). The model has been comprehensively verified. A number of verification papers are available for download from the Model validation page.
ADMS 6 is supplied with an in-depth user guide (see the User guides page) that details all user inputs and outputs, and includes a number of step-by-step worked examples.
An annual maintenance contract provides support for users; this includes:
- maintenance model upgrades,
- use of the helpdesk by e-mail, phone, fax or post,
- attendance at the annual user group meetings,
- newsletter twice a year, and
- access to the password-protected user area.