Outline of air pollution dispersion

• The following outline is presented as an overview and topical guide to air pollution dispersion:

Air pollution emission plumes

• Buoyant plumes — Plumes which are lighter than air because they are at a higher temperature and lower density than the ambient air which surrounds them, or because they are at about the same temperature as the ambient air but have a lower molecular weight and hence lower density than the ambient air. For example, the emissions from the flue gas stacks of industrial furnaces are buoyant because they are considerably warmer and less dense than the ambient air. As another example, an emission plume of methane gas at ambient air temperatures is buoyant because methane has a lower molecular weight than the ambient air.
• Dense gas plumes — Plumes which are heavier than air because they have a higher density than the surrounding ambient air. A plume may have a higher density than air because it has a higher molecular weight than air. A plume may also have a higher density than air if the plume is at a much lower temperature than the air. For example, a plume of evaporated gaseous methane from an accidental release of liquefied natural gas may be as cold as -161 °C.
• Passive or neutral plumes — Plumes which are neither lighter or heavier than air.

  Air pollution dispersion models

• Box model — The box model is the simplest of the model types. It assumes the airshed is in the shape of a box. It also assumes that the air pollutants inside the box are homogeneously distributed and uses that assumption to estimate the average pollutant concentrations anywhere within the airshed. Although useful, this model is very limited in its ability to accurately predict dispersion of air pollutants over an airshed because the assumption of homogeneous pollutant distribution is much too simple.
• Gaussian model — The Gaussian model is perhaps the oldest and perhaps the most commonly used model type. It assumes that the air pollutant dispersion has a Gaussian distribution, meaning that the pollutant distribution has a normal probability distribution. Gaussian models are most often used for predicting the dispersion of continuous, buoyant air pollution plumes originating from ground-level or elevated sources. Gaussian models may also be used for predicting the dispersion of non-continuous air pollution plumes. The primary algorithm used in Gaussian modeling is the Generalized Dispersion Equation For A Continuous Point-Source Plume.
• Lagrangian model — a Lagrangian dispersion model mathematically follows pollution plume parcels as the parcels move in the atmosphere and they model the motion of the parcels as a random walk process. The Lagrangian model then calculates the air pollution dispersion by computing the statistics of the trajectories of a large number of the pollution plume parcels. A Lagrangian model uses a moving frame of reference as the parcels move from their initial location. It is said that an observer of a Lagrangian model follows along with the plume.
• Eulerian model — an Eulerian dispersion model is similar to a Lagrangian model in that it also tracks the movement of a large number of pollution plume parcels as they move from their initial location. The most important difference between the two models is that the Eulerian model uses a fixed three-dimensional Cartesian grid as a frame of reference rather than a moving frame of reference. It is said that an observer of an Eulerian model watches the plume go by.
• Dense gas model — Dense gas models are models that simulate the dispersion of dense gas pollution plumes. The three most commonly used dense gas models are:
• *The DEGADIS model developed by Dr. Jerry Havens and Dr. Tom Spicer at the University of Arkansas under commission by the US Coast Guard and US EPA.
• * The SLAB model developed by the Lawrence Livermore National Laboratory funded by the US Department of Energy, the US Air Force and the American Petroleum Institute.
• * The HEGADAS model developed by Shell Oil's research division.

  Air pollutant emission

• Types of air pollutant emission sources - named for their characteristics
• * Sources, by shape - there are four basic shapes which an emission source may have. They are:
• ** Point source — single, identifiable source of air pollutant emissions. Point sources are also characterized as being either elevated or at ground-level. A point source has no geometric dimensions.
• ** Line source — one-dimensional source of air pollutant emissions.
• ** Area source — two-dimensional source of diffuse air pollutant emissions.
• ** Volume source — three-dimensional source of diffuse air pollutant emissions. Essentially, it is an area source with a third dimension. Another example would be the emissions from an automobile paint shop with multiple roof vents or multiple open windows.
• * Sources, by motion
• ** Stationary source - flue gas stacks are examples of stationary sources
• ** Mobile source - buses are examples of mobile sources
• * Sources, by urbanization level - whether the source is within a city or not is relevant in that urban areas constitute a so-called heat island and the heat rising from an urban area causes the atmosphere above an urban area to be more turbulent than the atmosphere above a rural area
• ** Urban source - emission is in an urban area
• ** Rural source - emission is in a rural area
• * Sources, by elevation
• ** Surface or ground-level source
• ** Near surface source
• ** Elevated source
• * Sources, by duration
• ** Puff or intermittent source - short term sources
• ** Continuous source - long term source

  Characterization of atmospheric turbulence

The Pasquill atmospheric stability classes

• Table 1 lists the six classes
• Table 2 provides the meteorological conditions that define each class. The stability classes demonstrate a few key ideas. Solar radiation increases atmospheric instability through warming of the Earth's surface so that warm air is below cooler air promoting vertical mixing. Clear nights push conditions toward stable as the ground cools faster establishing more stable conditions and inversions. Wind increases vertical mixing, breaking down any type of stratification and pushing the stability class towards neutral.

Other parameters that can define the stability class

• Temperature gradient
• fluctuations in wind direction
• Richardson number
• Bulk Richardson number
• Monin–Obukhov length

  Advanced methods of categorizing atmospheric turbulence

• AERMOD - US EPA's most advanced model, no longer uses the Pasquill stability classes to categorize atmospheric turbulence. Instead, it uses the surface roughness length and the Monin-Obukhov length.
• ADMS 4, - United Kingdom's most advanced model, uses the Monin-Obukhov length, the boundary layer height and the windspeed to categorize the atmospheric turbulence.

  Miscellaneous other terminology

• Building effects or downwash: When an air pollution plume flows over nearby buildings or other structures, turbulent eddies are formed in the downwind side of the building. Those eddies cause a plume from a stack source located within about five times the height of a nearby building or structure to be forced down to the ground much sooner than it would if a building or structure were not present. The effect can greatly increase the resulting near-by ground-level pollutant concentrations downstream of the building or structure. If the pollutants in the plume are subject to depletion by contact with the ground, the concentration increase just downstream of the building or structure will decrease the concentrations further downstream.
• Deposition of the pollution plume components to the underlying surface can be defined as either dry or wet deposition:
• *Dry deposition is the removal of gaseous or particulate material from the pollution plume by contact with the ground surface or vegetation through transfer processes such as absorption and gravitational sedimentation. This may be calculated by means of a deposition velocity, which is related to the resistance of the underlying surface to the transfer.
• *Wet deposition is the removal of pollution plume components by the action of rain. The wet deposition of radionuclides in a pollution plume by a burst of rain often forms so called hot spots of radioactivity on the underlying surface.
• Inversion layers: Normally, the air near the Earth's surface is warmer than the air above it because the atmosphere is heated from below as solar radiation warms the Earth's surface, which in turn then warms the layer of the atmosphere directly above it. Thus, the atmospheric temperature normally decreases with increasing altitude. However, under certain meteorological conditions, atmospheric layers may form in which the temperature increases with increasing altitude. Such layers are called inversion layers. When such a layer forms at the Earth's surface, it is called a surface inversion. When an inversion layer forms at some distance above the earth, it is called an inversion aloft. The air within an inversion aloft is very stable with very little vertical motion. Any rising parcel of air within the inversion soon expands, thereby adiabatically cooling to a lower temperature than the surrounding air and the parcel stops rising. Any sinking parcel soon compresses adiabatically to a higher temperature than the surrounding air and the parcel stops sinking. Thus, any air pollution plume that enters an inversion aloft will undergo very little vertical mixing unless it has sufficient momentum to completely pass through the inversion aloft. That is one reason why an inversion aloft is sometimes called a capping inversion.
• Mixing height: When an inversion aloft is formed, the atmospheric layer between the Earth's surface and the bottom of the inversion aloft is known as the mixing layer and the distance between the Earth's surface and the bottom of inversion aloft is known as the mixing height. Any air pollution plume dispersing beneath an inversion aloft will be limited in vertical mixing to that which occurs beneath the bottom of the inversion aloft. Even if the pollution plume penetrates the inversion, it will not undergo any further significant vertical mixing. As for a pollution plume passing completely through an inversion layer aloft, that rarely occurs unless the pollution plume's source stack is very tall and the inversion lid is fairly low.

  Air pollution dispersion models

• ADMS 3 - advanced atmospheric pollution dispersion model for calculating concentrations of atmospheric pollutants emitted both continuously from point, line, volume and area sources, or intermittently from point sources.
• AUSTAL
• AERMOD
• CANARY
• CALPUFF
• DISPERSION21
• FLACS
• ISC3
• MERCURE
• NAME
• :fr:Panache |Panache
• PHAST
• PUFF-PLUME
• SIRANE

  Others

• Bibliography of atmospheric dispersion modeling
• AP 42 Compilation of Air Pollutant Emission Factors
• Atmospheric dispersion modeling
• Roadway air dispersion modeling
• Useful conversions and formulas for air dispersion modeling
• List of atmospheric dispersion models
• Yamartino method
• Air pollution forecasting