Home

ADMS Urban

ADMS Roads

ADMS Airport

ADMS 4

ADMS Screen

EMIT

GASTAR

FLOWSTAR

User Group Meetings

Visualisation Tools

Training Courses

Price List

Contact Details

ADMS 4 Industrial Air Pollution Model

WHY USE ADMS 4?

ADMS 4 is an Atmospheric Dispersion Modelling System. It is a new generation air dispersion model, which means that:

• The atmospheric boundary layer properties are described by two parameters:
  - the boundary layer depth, and
  - the Monin-Obukhov length
  rather than in terms of the single parameter Pasquill Class.
• Dispersion under convective meteorological conditions uses a skewed Gaussian concentration distribution (shown by validation studies to be a better representation than a symmetric Gaussian expression).

The ADMS 4 model includes:

MODEL OPTIONS IN ADMS 4

ADMS 4 uses a Runge-Kutta method to solve the conservation equations to estimate plume rise, rather than using empirical expressions (as used by a number of other models). The ADMS 4 method takes into account:

• the effect of plume buoyancy and momentum, and
• includes the penetration of boundary layer inversions.

Dry and Wet Deposition
The rate of dry and wet deposition to the ground can be modelled in ADMS 4. 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.

Puffs
ADMS 4 can model both continuous releases i.e. plumes, in addition to instantaneous and time-dependent releases i.e. puffs.

Time varying emission rates
Emission rates from industrial sources are rarely constant. The variation of the emission rate with time can be modelled in ADMS 4, in addition to corresponding variations in:

• emission temperature,
• volume flow rate (or exit velocity),
• source diameter, and
• plume water content.

Buildings
The dispersion of air pollution around buildings is complicated to model. The building effects module in ADMS 4 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 within the cavity, C_R, 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.

Odours
Odours are becoming an increasingly important issue in areas where industrial sites are located close to residential areas. The dispersion of odours can modelled using ADMS 4. Odour release rates and concentrations can be specified/calculated in two types of odour units: ou, which are defined in the form of a ratio, and ou_E which are a mass measure.

Fluctuations
ADMS 4 is the only model of its kind to model short time scale fluctuations. These are particularly important for the modelling of odours and for calculation of a 15 minute average for comparison with the National Air Quality Standard objective for SO2. This module takes into account variations due both to turbulence, and changes in meteorology.

Plume Visibility
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 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.

NO_x chemistry
A simple NO_x chemistry scheme is included in ADMS 4, involving the conversion of Nitrogen Dioxide (NO₂) to Nitrous Oxide (NO) and Ozone (O₃) in daylight:

3NO₂ + hv --> 3NO + O₃

(where hv = ultra-violet radiation), and a reverse reaction:

NO + O₃ --> NO₂ + O₂

that occurs both day and night.

ADMS 4 uses CERC’s complex terrain model, FLOWSTAR, to calculate the flow and turbulence fields that are then used to enhance the calculation of dispersion. The model predicts a three-dimensional flow and turbulence field over the region of interest, dependent on both input values of terrain height and roughness, as well as the local meteorological conditions. In ADMS 4, the plume is subjected to these varying flow and turbulence fields, which results in ground level concentrations that may be higher or lower than the corresponding predictions for flat terrain. It is recommended that the complex terrain option in ADMS 4 be used in regions where the gradient exceeds 1:10, but is less than 1:3.

Propagation of a plume subject to the flow and turbulence fields predicted by ADMS 4. The figure above shows a 2-D plot with arrows representing teh flow field. The figure to the right shows a 3-D close up of the terrain and the plume trajectory.

ADMS 4 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 4 can also calculate the associated levels of gamma ray dose.

Coastlines
For air dispersion modelling in coastal areas, ADMS 4 includes a coastline module that 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.

<- previous : next ->