air dispersion model

8/3/2019 Air Dispersion Model

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AIR DISPERSION MODEL

Definition

Air dispersion modeling is the mathematical simulation of how air pollutants disperse in the

ambient atmosphere

Also known as Air quality models, Atmosphere dispersion models

Dispersion model

The dispersion models are used to predict the downwind concentration of an air pollutants or

toxins emitted from source such as industrial plants, vehicular traffic or accidental chemical releases.

Nowadays, it is performed with computer programs that solve the mathematical equations

and algorithms which simulate the pollutant dispersion.

The dispersion models vary depending on the mathematics used to develop the model.

The air quality modeling procedures can be categorized into four generic classes: Gaussian,

numerical, statistical or empirical and physical.

Several Levels of Model

1. Gross screening models - which require only a hand-held calculator and nomograph. Theymay treat only one source at a time.

2. Intermediate models- usually PC-based which may include varying metrology (wind speedand stability classes) and more sophisticated source information.

3. Advanced models require pc desktop or workstation. They require extensive data sets formetrology and emissions and include multiple source type.

4. Specialized model- Used for predicting dispersion of hazardous materials from chemicalindustries and power plants.

Factors required for modeling

1. Meteorological factors- wind speed, turbulence, stability, temperature, cloud cover, solarradiation.

2. Source term- emission rate, temperature of the materials, exit velocity, height of source,types of source.

3. Terrain elevation- source location, receptor location (schools, business buildings, hospitals),location, height and width of obstruction in the path of plume.


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Gaussian dispersion model

Assumptions

1. Wind speed variation in three coordinate directions is very small. Therefore it is appropriateto assume that wind speed U is constant.

2. The major concentration of pollutant is chosen to li e along the x-axis3. Steady state

Dispersion model

The general equation to calculate the steady state concentration of an air contaminant in the

ambient air resulting from a point source is given by

C,(x,y,z,H) = (Q/2Uyz) exp-(y2/2y

2){exp-((z-H)

2/2z

2)+

exp-((z+H)2/2z

2)}

Where

C,(x,y,z,)= mean concentration of diffusion substance at a point x,y,z


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X =downwind distance

Y =crosswind distance

Z =vertical distance above ground

Q =contaminant emission rate

Lateral dispersion coefficient function

Vertical dispersion coefficient function

U =mean wind velocity on downwind direction

H =effective stack height

If we consider in the ground level, then z=0

Then the equation become

C,(x,y,0) = (Q/2Uyz) exp-((y2/2y

2)+(H

2/2z

2))

If cross wind direction be zero, then y=0

C,(x,0,0) = (Q/2Uyz) exp-(H2/2z

2)

The lateral (horizontal) and vertical dispersion coefficient value is obtained from fig.1, fig.2

based on stability, temperature (Table:1) and downwind distance(x)

Table: 1

Surface wind

speed at 10 m

(m/s)

Day Night

Incoming Solar radiation Cloud Cover

Strong Moderate Slight Thinly Overcast Mostly Cloudy

< 2 A (s = 1) A-B B (s = 2)

2-3 A-B B C (s = 3) E (s = 5) F (s = 6)

3-5 B B-C C D E

5-6 C C-D D (s = 4) D D

>6 C D D D D


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Applications

1. Dispersion model is the important first step in a successful assessment of stack design.2. Predict how contaminants emitted from a stack will disperse in the outside air3. Determine the stack parameters needed to keep the contaminants from impacting on fresh air

intakes and other sensitive locations.

air dispersion model

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