The following outline is provided as an overview of and topical guide to air pollution dispersion: In environmental science, air pollution dispersion is the distribution of air pollution into the atmosphere. Air pollution is the introduction of particulates, biological molecules, or other harmful materials into Earth's atmosphere, causing disease, death to humans, damage to other living organisms such as food crops, and the natural or built environment. Air pollution may come from anthropogenic or natural sources. Dispersion refers to what happens to the pollution during and after its introduction; understanding this may help in identifying and controlling it.
Air pollution dispersion has become the focus of environmental conservationists and governmental environmental protection agencies (local, state, province and national) of many countries (which have adopted and used much of the terminology of this field in their laws and regulations) regarding air pollution control.
Air pollution emission plumes
Air pollution emission plume – flow of pollutant in the form of vapor or smoke released into the air. Plumes are of considerable importance in the atmospheric dispersion modelling of air pollution. There are three primary types of 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 (for example, a plume of carbon dioxide). 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 (LNG) may be as cold as . Passive or neutral plumes – Plumes which are neither lighter or heavier than air.
Air pollution dispersion models
There are five types of air pollution dispersion models, as well as some hybrids of the five types:List of atmospheric dispersion models
Box model – The box model is the simplest of the model types.Air Pollution Dispersion: Ventilation Factor by Dr. Nolan Atkins, Lyndon State College It assumes the airshed (i.e., a given volume of atmospheric air in a geographical region) 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 (circa 1936)Bosanquet, C.H. and Pearson, J.L. (1936).The spread of smoke and gases from chimney, Trans. Faraday Soc., 32:1249. 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 (called puff models). The primary algorithm used in Gaussian modeling is the Generalized Dispersion Equation For A Continuous Point-Source Plume.Atmospheric Dispersion Modeling (Chapter 8, page 124) Lagrangian model – a Lagrangian dispersion model mathematically follows pollution plume parcels (also called particles) 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 referenceFeatures of Dispersion Models publication of the European Union Joint Research Centre (JRC) 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 (i.e., pollution plumes that are heavier than air). 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.DEGADIS Technical Manual and User's Guide (US EPA's download website) 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.UCRL-MA-105607, User's Manual For Slab: An Atmospheric Dispersion Model For Denser-Than-Air Releases, Donald Ermak, June 1990. 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 (for example, the emissions from a combustion furnace flue gas stack). 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 (for example, the emissions from the vehicular traffic on a roadway). Area source – two-dimensional source of diffuse air pollutant emissions (for example, the emissions from a forest fire, a landfill or the evaporated vapors from a large spill of volatile liquid). Volume source – three-dimensional source of diffuse air pollutant emissions. Essentially, it is an area source with a third (height) dimension (for example, the fugitive gaseous emissions from piping flanges, valves and other equipment at various heights within industrial facilities such as oil refineries and petrochemical plants). 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 (for example, many accidental emission releases are short term puffs) Continuous source – long term source (for example, most flue gas stack emissions are continuous)
Characterization of atmospheric turbulence
Effect of turbulence on dispersion – turbulence increases the entrainment and mixing of unpolluted air into the plume and thereby acts to reduce the concentration of pollutants in the plume (i.e., enhances the plume dispersion). It is therefore important to categorize the amount of atmospheric turbulence present at any given time. This type of dispersion is scale dependent. Such that, for flows where the cloud of pollutant is smaller than the largest eddies present, there will be mixing. There is no limit on the size on mixing motions in the atmosphere and therefore bigger clouds will experience larger and stronger mixing motions. And hence, this type of dispersion is scale dependent.
The Pasquill atmospheric stability classes
Pasquill atmospheric stability classes – oldest and, for a great many years, the most commonly used method of categorizing the amount of atmospheric turbulence present was the method developed by Pasquill in 1961.Pasquill, F. (1961). The estimation of the dispersion of windborne material, The Meteorological Magazine, vol 90, No. 1063, pp 33-49. He categorized the atmospheric turbulence into six stability classes named A, B, C, D, E and F with class A being the most unstable or most turbulent class, and class F the most stable or least turbulent class. 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 (and therefore denser) 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 (D).
Table 1: The Pasquill stability classes
Stability class Definition Stability class DefinitionAvery unstable DneutralBunstable Eslightly stableCslightly unstable Fstable
Table 2: Meteorological conditions that define the Pasquill stability classes
Surface windspeedDaytime incoming solar radiationNighttime cloud cover m/s mi/h Strong Moderate Slight> 50%< 50%< 2< 5AA – B BEF2 – 35 – 7A – B BCEF3 – 57 – 11B B – CC DE5 – 611 – 13C C – D DDD> 6> 13C D DDDNote: Class D applies to heavily overcast skies, at any windspeed day or night Incoming solar radiation is based on the following: strong (> 700 W m−2), moderate (350–700 W m−2), slight (< 350 W m−2)
Other parameters that can define the stability class The stability class can be defined also by using the
Temperature gradient fluctuations in wind direction Richardson number Bulk Richardson number Monin–Obukhov length
Advanced methods of categorizing atmospheric turbulence Advanced air pollution dispersion models – they do not categorize atmospheric turbulence by using the simple meteorological parameters commonly used in defining the six Pasquill classes as shown in Table 2 above. The more advanced models use some form of Monin–Obukhov similarity theory. Some examples include: 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 4ADMS 4 Description of the model by the developers, Cambridge Environmental Research Consultants. – 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
(Work on this section is continuously in progress)