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7 Glossary

If the term you are looking for is not included in this glossary, further terms can be found at the California Air Resources Board, Glossary of Air Pollution Terms web page (

Transport of pollutants by the wind
An area, bounded by topographical features, within which airborne contaminants can be retained for an extended period
A mathematical process or set of rules used for calculation or problem-solving, which is usually undertaken by a computer
Assessment of environmental effects
A piece of expert advice submitted to regulators to support a claim that adverse effects will or will not occur as a result of an action, and usually developed in accordance with section 88 of the Resource Management Act 1991
Atmospheric chemistry
The chemical changes that gases and particulates undergo after they are discharged from a source
Atmospheric dispersion model
A mathematical representation of the physics governing the dispersion of pollutants in the atmosphere
Atmospheric stability
A measure of the propensity for vertical motion in the atmosphere
Building wakes
Strong turbulence and downward mixing caused by a negative pressure zone on the lee side of a building
Calm / stagnation
A period when wind speeds of less than 0.5 m/s persist
Cartesian grid
A co-ordinate system whose axes are straight lines intersecting at right angles
The relationship between cause and effect
Complex terrain
Terrain that contains features that cause deviations in direction and turbulence from larger-scale wind flows
Configuring a model
Setting the parameters within a model to perform the desired task
Vertical movement of air generated by surface heating
Convective boundary layer
The layer of the atmosphere containing convective air movements
Data assimilation
The use of observations to improve model results - commonly carried out in meteorological modelling
Default setting
The standard (sometimes recommended) operating value of a model parameter
Diagnostic wind model (DWM)
A model that extrapolates a limited amount of current wind data to a 3-D grid for the current time. It is the 'now' aspect, and makes the model 'diagnostic'.
Clean air mixing with contaminated air through the process of molecular motion. Diffusion is a very slow process compared to turbulent mixing.
The lowering of the concentration of pollutants by the combined processes of advection and diffusion
Dispersion coefficients
Variables that describe the lateral and vertical spread of a plume or a puff
Dry deposition
Removal of pollutants by deposition on the surface. Many different processes (including gravity) cause this effect.
Small-scale turbulent disturbances contained within a larger air flow
Elevated receptors
Receptors that are on the ground but above the level at which the contaminants are released
Emission factors/models
A method used to calculate the amount of emissions that a particular source will release
Emission rates
The rate at which contaminants are discharged from a particular source
Emission temperature
The temperature of the gas stream that carries the contaminants from the source
Eulerian dispersion model
The pollution distribution is described by changing concentrations at discrete points on a fixed grid
Exit velocity
The velocity at which the exhaust gases leave a stack
Far field
Locations more than about 10 km from the source region
Flagpole receptors
Receptors that are located on structures above ground level
The process whereby pollutants held above an inversion layer are transferred back to ground level during the break-up of the inversion
Gaussian plume
A plume within which the pollutants are distributed vertically and horizontally in a Gaussian (or normal) manner about the plume centreline
The situation where temperature increases with height; a highly stable condition in which vertical dispersion is suppressed and pollution is trapped
Katabatic flows
Downslope flow at night due to the air on the slope being cooler than the air at the same altitude away from the slope. The horizontal temperature gradient induces the downslope flow.
Lagrangian model
The pollution distribution is described by a set of discrete particles or puffs, which are labelled by their changing location (i.e. their trajectories are followed)
Large spatial scale, 1000 km plus
Mechanical momentum
The upward (or otherwise) force a stream of gas exerts due to the velocity with which it leaves the point of discharge
Medium spatial scale, 5 - 100 km
Small spatial scale, less than 5 km
Mixing height
The height in the atmosphere to which pollutants released at the surface can be mixed by turbulent eddy motion
Model performance
A measure of a model's ability to reliably predict pollutant concentrations
Model sensitivity
The scale to which model predictions change when the value of a particular input parameter is changed
Model validation
The process used to demonstrate that a model produces reliable output
Modelling domain
The area over which the model is making predictions
Near field
The area close to the source, usually within a few km
Orographically driven flows
Winds driven by the relief of mountains and hills
Plume depletion
The removal of pollutants from a plume by gravity or chemical reaction
Plume rise
The height to which a plume rises above its release point due to its initial momentum and thermal buoyancy
Polar grid
A receptor grid defining a group of points located on a series of concentric circles, which are usually centred on the source
Prognostic model
A meteorological model which solves fully time-dependent equations, predicting the future from a known current state
The location at which modelled concentrations need to be calculated
Instruments suspended beneath a balloon to sense and relay temperature, humidity and pressure as the balloon ascends through the (whole) atmosphere
A model run that aims to calculate the highest concentration that might occur, but gives no information on the frequency or location of the event
Screening meteorological data
A synthetic data set that contains combinations of meteorological variables which include all possible atmospheric conditions (without saying how likely each would be to occur)
Sensitivity analysis
The process of establishing the effect of changing the value of an input variable on model output
Simple terrain
Terrain that will not influence larger-scale wind flows nor has receptors at a height greater than the release height of the pollutants
Slope flows
Air flows generated up or down hillsides by surface heating or cooling
Stability classification scheme
A simplified method of categorising the amount of turbulent mixing in the atmosphere
Stack-tip downwash
The small downward movement of a plume as it leaves a stack caused by a negative pressure zone on the lee side of the stack
Steady-state dispersion model
The Gaussian-plume; a mathematical solution to the dispersion equation, which is independent of time
Surface roughness length
A parameter needed in boundary calculations. Surface roughness increases the vertical mixing of an air stream due to enhanced mechanical turbulence generated as the air moves over surface features.
Thermal buoyancy
The buoyancy of a plume generated by the temperature difference between the exhaust gas and the ambient air
Small-scale (random) atmospheric motions that tend to mix pollutants through the air
Upper air data
Meteorological data that are collected above the height of a meteorological tower
Wet deposition
Removal of pollutants through scavenging by falling raindrops
Wind direction shear
A tendency for wind direction to turn with height
Wind field
The set of vectors that describe wind speed and direction conditions over a particular modelling domain at a particular hour
Wind speed profile
A measure of the rate at which wind speed increases with height above a surface