Two fundamentally different types of atmospheric dispersion model can be identified, both based on physical dispersion equations and description of the ground surface topography: Lagrangian models and Eulerian models. 

Lagrangian models

Lagrangian models take different forms.  The most widely used are Gaussian models.  These are mechanistic models, based on turbulent eddy mixing theory, and predict for each model run a downwind ground-level concentration field of the emitted pollutant, centred on the centreline of the plume.  They are primarily designed to predict short term concentration fields downwind from a source (point, line or area), but can be extended to long term average ground level concentration fields and concentration probability distributions. In the latter case, they require extensive meteorological, topographical, source location and emission time which series data.  They are particularly suited for elevated point sources (e.g. power plant chimneys),

» Read more Lagrangian models estimate the dispersion/dilution of an atmospheric release in a coordinate system that moves from the point of release at the direction and speed of the wind.  Source emissions are given in the form of release height (z) and emission rate.  Wind direction and plume centreline (after thermal and/or kinetic plume rise) define the x-axis. For point sources, Gaussian dispersion models also require source emission flow rate, velocity and temperature. In its basic form a Gaussian dispersion model computes the progression of the concentration field on a 2-dimensional y-z plane (assuming Gaussian distribution along both y and z axes). Advanced Gaussian modelling tools predict both short-term (downwind from the source) and long term (around the source) ground level concentration fields. Other types of sources (line, area), are treated as expansions of the point source model. Physical and chemical decay processes are also incorporated in the more advanced Gaussian dispersion models.

Eulerian models

Eulerian dispersion models are mechanistic, based on fluid dynamics. They are designed to model high resolution concentration fields, which can be averaged to provide estimates of long term concentrations.  They are particularly suited for modelling concewntrations and exposures associated with complex sources, distributed over an area (e.g. road traffic).

» Read more  Eulerian models predict a ground level concentration field on a geographically fixed x,y coordinate system.  Source emissions are given in the form of release coordinates (x,y,z) and emission rates (and times). A basic Eulerian dispersion model applies fluid dynamics to compute the mixing of a point release into a turbulent air mass that moves over a terrain. The mixing is forced by characteristics of the emission, the local topography, and the wind field and thermal layering of the lower atmosphere. Other types of sources (line, area), receptors (concentration fields, mobile receptors), and times up to a year are treated as expansions of the short term point source model. Physical and chemical decay processes of the released agent are incorporated in more advanced Lagrangian dispersion models.

Limitations

Uncertainties in dispersion models need to be recognised.  These derive both from the inevitable simplification of the complex real-world processes that they entail, and from limitations of the available input data (e.g. on emission sources and meteorology).  Models tend to be most reliable in relatively open, uncluttered conditions, and for estimation of longer term (daily, annual) concentrations. 

Because they consider only ambient concentrations, dispersion models do not provide direct estimates of human exposure.  In many situations, however, ambient concentrations can be used as an indication of potential exposures and, where indoor exposures are of concern, they can be linked to indoor air pollution models.

Links and examples

Links to a selection of dispersion models, and factsheets giving further information on some models, are provided in the Model section of the Toolkit.   A number of models, including many developed on behalf of the US Environmental Protection Agency, are available in the form of freeware; to access many of these, follow this link.  Examples of using dispersion models as part of integrated environmental health assessments are also provided via the links in the panel to the left.