L-19 and L-20Dispersion of Pollutants: Gaussian

Dispersion Model (GDM)

Air pollution and Control

(Elective -I)

Stability classes

Stability Classes

• Developed for use in dispersion models

• Stability classified into 6 classes (A – F)

• A: strongly unstable – Large lapse rates

• B: moderately unstable

• C: slightly unstable

• D: neutral- Less or zero lapse rate

• E: slightly stable – mild inversion

• F: moderately stable – moderate to severe inversion

Pasquill’s Stability table

Air Quality ModelOutput:

Concentration Distribution

Input 1:Emissions

Input 2:Meteorology

Input 3:Atmospheric

Chemistry

Input 4:Surface Properties

General Structure of Air Pollution Models

5Prof S S Jahagirdar, NKOCET

Air Pollution modeling :Parameters in Models

1. Source Parameters (Emission Characteristics)Emission rates of pollutants (mass/time)Physical location of sourceTemperature of gas releasePlume Rise

2. MeteorologyAtmospheric temp.Atmospheric stability (needed for Dipersion

coefficients) Wind velocity

3. Atmospheric ChemistryChemical Reaction in the atm.Depositions (wet or dry)

4. Surface Parameters (Properties)Surface geometry, roughness, seas, urban or rural

areas etc6

Why Use Dispersion Models?

• Predict impact from proposed and/or existing development

– NSR- new source review

– PSD- prevention of significant deterioration

• Assess air quality monitoring data

– Monitor location

• Assess air quality standards or guidelines

– Compliance and regulatory

• Evaluate AP control strategies

– Look for change after implementation7

Why Use Dispersion Models?

• Evaluate receptor

exposure

• Monitoring network

design

– Review data

– Peak locations

– Spatial

patterns

• Model Verification

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Model Assumptions• Gaussian dispersion modeling based on a

number of assumptions including

– Steady-state conditions (constant source emission strength)

– Wind speed, direction and diffusion characteristics of the plume are constant

– Mass transfer due to bulk motion in the x-direction

– Conservation of mass, i.e. no chemical transformations take place

– Wind speeds are >1 m/sec.

– Limited to predicting concentrations > 50 m downwind

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10

11

• Horizontal dispersion coefficient

12

• Vertical dispersion coefficient

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Atmospheric Stability Classes

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Dispersion Coefficients: Horizontal

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Dispersion Coefficients: Vertical

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Maximum Ground Level Concentration

Under moderately stable to near neutral conditions,

zy k ss 1

The ground level concentration at the center line is

2

2

21 2

exp0,0,zz

H

uk

QxC

ss

The maximum occurs at

2 0/

HddC zz ss

Once sz is determined, x can be known and subsequently C.

,0,0 exp 1 0.1171y z y z

Q QC x

u us s s s

---------------- Eq - A

Put in Eq - A

How to use GDM?Need to know proper orientations of both

Source and Receptor:Source at (0,0,H) and Receptor at (x,y,z) C(x,y,z;H)

Pollutant Emission Rate from source: Q (mass of pollutant/time) NOT Volume flowrate of Stack gas

Atmospheric Stability Category (A, B, C. etc.)

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Wind velocity at stack height : u

Dispersion Coefficients : σy and σz (can be determined from graphs)

Effective Stack height: H = hs + Δh Calculation of Plume rise (Δh ) by any one formula (Refer lecture on plume rise)

THEN USE GDM C(x,y,z;H) =….

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Example - 1

• A stack in an urban area is emitting80 g/s of NO. It has an effectivestack height of 100 m. The windspeed is 4 m/s at 10 m. It is a clearsummer day with the sun nearlyoverhead. Estimate the ground levelconcentration at a) 2 km downwindon the centerline and b) 2 km

downwind, 0.1 km off the centerline.20

Example

1. Determine stability class

Assume wind speed is 4 km atground surface. Descriptionsuggests strong solar radiation.

Stability class B

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Example

2. Estimate the wind speed at the effective stack height

Note: effective stack height given – no need to calculate using Holland’s formula

10

1002.0

1

212 4

p

z

zuu

22Prof S S Jahagirdar, NKOCET

p

Example

3. Determine σy and σz

σy = 290

σz = 220

290

220

23

Example

4. Determine concentration using Eq 11-12

a. x = 2000, y = 0

22

220

100

2

1exp

290

0

2

1exp

)6.5)(220)(290(

80)0,100,2000(

C

33 μg/m g/m 3.641043.6)0,2000( 5 C

24Prof S S Jahagirdar, NKOCET

Example

4. Determine concentration using Eq 11-12

a. x = 2000, y = 100

22

220

100

2

1exp

220

100

2

1exp

)6.5)(220)(290(

80)0,2000(

C

)0,100,2000(C

25Prof S S Jahagirdar, NKOCET

L-20Problems on GDM

Air Pollution and Control

Elective -I

26

Prof S S Jahagirdar, NKOCET

Example-2

• An industrial boiler is burning at 12 tons (10.9 mton) of 2.5% sulfur coal/hr with an emission rate of 151 g/s. The following exist : H = 120 m, u = 2 m/s, y = 0. It is one hour before sunrise, and the sky is clear. Determine downwind ground level concentration at 10 km.

Stability class =

sy =

sz =

C(10 km, 0, 0) =

27

Exercise-3

• If emissions are from a ground level source with H = 0, u = 4 m/s, Q = 100 g/s, and the stability class = B, what is downwind concentration at 200 m?

At 200 m:

sy =

sz =

C(200 m, 0, 0) =

28

Go to Additional problems

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Objective Questions

Q1. GDM is used for _____________________

______________________________________.

Q2. σy and σz values depend upon __________.

Q3. In GDM ‘H’ is _________ ___________.

Q4. Greater the wind speed and mixing heights _______________ will be the concentration of pollutants.

Q5. Max ground level concentration is given by _________________ .

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Theory Questions

Q1. What are assumptions made in Gaussian dispersion model?

Q2. What is Gaussian dispersion equation? Explain meaning of each and every term in it. Also give its different forms.

Q3. Write about how to use GDM.

Q4. What is use of Dispersion models?

Q5. Discuss parameters needed for air pollution modeling.

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