Atmospheric stability, pollution dispersion

In another review, Hoffert discussed the social motivations for modeling air quality for predictive purposes and elucidated the components of a model. Meteorologic factors were summarized in terms of windfields and atmospheric stability as they are traditionally represented mathematically. The species-balance equation was discussed, and several solutions of the equation for constant-diffusion coefficient and concentrated sources were suggested. Gaussian plume and puff results were related to the problems of developing multiple-source urban-dispersion models. Numerical solutions and box models were then considered. The review concluded with a brief outline of the atmospheric chemical effects that influence the concentration of pollutants by transformation. 

FIGURE 4-25a Standard deviations of mass distribution in a Gaussian plume, cry and az, given as a function of both distance downwind from a point source and Pasquill stability categories. Dispersion coefficient as used in this figure means the standard deviation of the plume width or height [L] rather than a Fickian coefficient [L2/T], (From Atmospheric Chemistry and Physics of Air Pollution, by J. H. Seinfeld. Copyright 1986, John Wiley Sons, Inc. Reprinted by permission of John Wiley Sons, Inc.)... 

One commonly used suite of models that is based on Gaussian plume modeling is the Industrial Source Complex (ISC) Dispersion Models (US EPA, 1995). This suite includes both a short-term model (ISCST), which calculates the hourly air pollutant concentrations in an area surrounding a source, as well as a long-term model (ISCLT), which calculates the average air pollutant concentrations over a year or longer. ISCLT uses meteorological data summarized by frequency for 16 radial sectors (22.5° each) this data format is referred to as a stability array (STAR). Within each sector of STAR, joint frequencies of wind direction, wind speed, and atmospheric stability class are provided. 

The importance of the state of the atmosphere and the effects of stability cannot be overstated. The ease with which pollutants can disperse vertically into the atmosphere is mainly determined by the rate of change of air temperature with height (altitude) therefore, air stability is a primary factor in determining where pollutants will travel and how long they will remain aloft. Stable air discourages the dispersion and dilution of pollutants conversely, in unstable air conditions, rapid vertical mixing takes place, encouraging pollutant dispersal, which increases air quality. 

How the atmosphere behaves when air is displaced vertically is a function of atmospheric stability. A stable atmosphere resists vertical motion air that is displaced vertically in a stable atmosphere tends to return to its original position. This atmospheric characteristic determines the ability of the atmosphere to disperse pollutants emitted into it. To understand atmospheric stability and the role it plays in pollution dispersion, it is important to imderstand the mechanics of the atmosphere as they relate to vertical atmospheric motion. 

An inversion occurs when air temperature increases with altitude. Temperature inversions (extreme cases of atmospheric stability) create a virtual lid on the upward movement of atmospheric pollution. This situation occurs frequently but is generally confined to a relatively shallow layer. Plumes emitted into air layers that are experiencing an inversion (inverted layer) do not disperse very much as they are transported with the wind. Plumes that are emitted above or below an inverted layer do not penetrate that layer rather, these plumes are trapped either above or below that inverted layer. High concentrations of air pollufants are often associated with inversions, as they inhibit plume dispersions. Two types of inversions are important from an air quality standpoint radiation and subsidence inversions. 

As an example of the use of the Gaussian plume equations using the Pasquill-Gifford dispersion parameters, assume that a source releases 0.37 g s of a pollutant at an effective height of 40 m into the atmosphere with the wind blowing at 2 m s . What is the approximate distance of the maximum concentration, and what is the concentration at this point if the atmosphere is appropriately represented by Pasquill stability class B ... 

Atmospheric turbulence plays an important role in the estimation of concentrations of air pollutants at locations near the pollution source and on larger scales. This problem can be broken down into four parts estimation of plume rise, transport, dispersion, and transformation. Plume rise depends partly on meteorological variables such as wind speed and stability, partly on exit speed and temperature of the pollutants, and partly on the source diameter. Many techniques exist for estimating plume rise, and turbulent entrainment into the plume is one factor that must be considered. However, in this case, the turbulence arises from the motion of the plume itself and is not natural turbulence. Natural turbulence may eventually play a role in mixing the plume to cause it to level off. 

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