slide show chapter 19 may 2011 (1)

7/28/2019 Slide Show Chapter 19 May 2011 (1)

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QUESTION: What is the most important parameter that affects

dispersion of pollutants in the atmosphere????

Wind Direction

The direction of transport of pollutants emitted from sources depends on

wind direction (WD).

WD is the most important parameter affecting dispersion of

pollutants particularly from point sources.

It is also important for dispersion from mobile sources, but not asmuch as in the case of stationary point sources.


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WD should be handled very carefully in the models, because it is the onlyway to assess the impacts of emissions from more than one source in the

study domain.

In order to determine the dispersionof pollutants we must be able to

assess how wind direction changes with altitude.

Because, meteorological measurements are generally conducted at

standard 10 m altitude. But, pollutants are emitted and

subsequently transported at the top of the stack which can be up

to 300 m high.


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Wind direction change with altitude, which

is called wind shear. At the ground level,

surface friction cause the wind to turnclockwise with altitude. This process is

called veer.

Beyond certain altitude thermal

structure (horizontal temperature

variations) dominates over the friction.

And the direction of the wind is

determined by this thermal structure.

It is very common that winds that shift

clockwise due to veer, shifts

counterclockwise beyond a certain

altitude.
http://www.uao.bnl.gov/mesonet/SeaBreezePlume.html


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Wind Speed

Wind speed generally increase with height.

Most of the wind measurements are carried out at 10 m standard

altitude. But most of the emissions occur at higher altitude (exact

altitude depends on the stack height).


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Wind at the stack height can be calculated using the wind measurements

at 10 m with the following power relation.

U(z) = u(za) (z/za)p

u(z): wind speed at altitude z

u(za): measurement height (generally, but not necessarily 10 m)

p: exponent.


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The value of p is very important in wind extrapolation.

It generally takes values between 0.1 and 0.4 and depends on:

surface roughness,

stability of the atmosphere, and

depth of the layer.

The value that is most widely used for p is 1/7. (If you do not know

anything about surface roughness and stability use 1/7)


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Figure 19-1 shows measured and calculated (using the above formula and

1/7 as the p) wind profiles in different places in the USA.

The general theme of the figure is that measured and calculated

profiles do not always match well. This is generally true for most of the

calculations in the atmosphere.


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The wind speed is important in atmospheric dispersion, because it dilutes

pollutants as soon as they are emitted from the source.

Figure 19-2 is a nice example.

At wind speed of 6 m s-1 there are 1 unit of pollutant between each

line (separated by 1 m).

At wind speed of 2 m s-1 there are 3 units of pollutant.

Dilution occurs at the emission point. Because of this, in modeling wind

speeds calculated for the top of the stackare used in calculations.


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In addition to dilution, wind speed also effects:

Travel time between the source and receptor (double the wind speed

= half the time)

Plume rise (higher the wind speed lower the plume rise)


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TURBULANCE

Turbulence is the irregular motion of the wind.

Usually there is a mean wind flow and these irregularities are

superimposed onto that flow.

The irregularities which we call turbulence are usually in the form of

swirls and eddies.

Eddies are very important in the plume-dilution process, because they

move pollutants outside the plume and brings fresh air (unpolluted) intothe plume.


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Turbulence is generated by two mechanisms:

Mechanical turbulence is generated when wind passes around

objects.

Thermal turbulence is generated by the rising air parcel.

Air close to the surface of the earth heated and rise. Colder air

around these rising parcels moves down to replace them. But

usually the downward movement of cold air is slower than upward

movement of heated air parcels. Consequently, heated air parcels

move fairly fast in a slowly descending air. This generatesturbulence.


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You can feel the turbulence by gusts.

When you look at the wind records turbulence can be observed as rapid

changes in wind direction or temperature.

Eddies generated by thermal turbulence are more irregular and

larger.

Mechanical turbulance Thermal turbulance


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The most common mixing process in the atmosphere, which results in the

dilution of pollutants in a plume, is called eddy diffusion.

The swirling action in the plume removes polluted parcels from the

plume and brings unpolluted air parcels into it. The net result is

diffusion of the plume and its dilution.

Eddies are more efficient in diluting the plume if the scale of the

eddy is similar to the plume that is diluted.


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The eddies smaller than the plume, can only remove pollutants at the

edges of the plume.

The eddies that are larger than the plume can transport the plume as awhole, rather than diluting it.

As a result of the turbulence (eddies) plume widens and dispersed,

and pollutants diffuse away.


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The effect of the eddies on

the expansion of the plume

depends on the temperature

profile in the atmosphere.

The expansion and the

shape of a plume under

three different

temperature profiles and

their combinations are

given in Figure 19-4

The level of turbulence isa measure of the

dispersive capacity of the

atmosphere.


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Fanning plume

Looping plume

Lofting plume


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ESTIMATING CONCENTRATIONS FROM A POINT SOURCE

The equations, which form basis to calculate concentrations from a point

source in a 3-dimensional axis system are commonly, called Gaussian Plume

Model.

The coordinate system

x-along the plume

y-across the plume

z-height

0-at the ground

y

z

x


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The model assumes that the concentration of a pollutant at any point in

the plume is:

proportional to emission rate,

diluted by the wind at the point of emission with a rate inversely

proportional to wind speed,

concentration across the plume and vertically in the plume are

described by a Gaussian distribution.


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The standard deviations of concentrations across the plume and vertically

in the plume increase with:

Turbulence

Distance from the source

The magnitude of the standard deviation both in y and z directions

shows the expansion of the plume (diffusion of the pollutants).


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Additional assumptions in Gaussian plume model includes:

No chemical reactions of pollutants

No scavenging processes

It is assumed that when the plume touches to the ground or top of

the mixing layer it reflects back to the plume centerline.

Characteristics of the Gaussian Model are shown in Figure 19-5.


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For stable conditions or unlimited vertical mixing, concentration of a

pollutant (g m-3) at a point (x, y, z) from a point source located at (0, 0,

H) is given by

X = Q (1/u){g1/[(2)0.5y]}{g2/[(2)

0.5z]} (19-2)

X: pollutant concentration in g m-3

Q: emission rate in g s-1

u: wind speed in m s-1

y: standard deviation ofconcentration in y direction

z: standard deviation ofconcentration in z directionL: mixing height in m

h: physical stack height (actualheight of the stack)

H: effective stack height (h + plume

rise)x: downwind distance (m)

y: crosswind distance (m)

z: receptor height above ground (m)

g1 = exp(-0.5y2/y2)

g2 = exp[-0.5(H-z)2/z2] + exp[-0.5(H+z)2/2]


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For y = 0 (plume centerline)

For z = 0 (ground level)

For z and H = 0 this equation is simplified.

For unstable or neutral conditions where z > 1.6L the following equationis used (when the plume is well mixed in the vertical direction)

X = Q(1/u){g1/[(2)0.5y]}(1/L) (19-3)


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Note that:

When you use this equation

z > 1.6L. z is a measure of how much the plume is expanded in thevertical direction.

z > 1.6L means that the plume expanded so that it touches the top ofthe mixing layer and ground. Then, eddy reflection repeatedly occurs in

both both boundaries.

The net result is that plume is well mixed in the vertical direction.


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For unstable or neutral conditions where z < 1.6L (which means that theplume is fairly narrow) the following equation is used

X = Q(1/u){g1/[(2)0.5y]}{g3/[(2)

0.5z]} (19-4)

Where;

This series converges fast. Evaluation of N between 4 and +4 is

usually enough.

Computers can calculate these series fairly easily.

When you do the calculations by hand in practice it is enough to apply

equation 19-2 until z = 0.8 L


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Note that:

Eqn 19-4 is for a narrow plume which is the case close to the emission

point

Eqn 19-3 involves expanded plume and multiple reflections from the mixing

height and ground which occurs as you go away from the source.

In order to describe the whole plume you must combine the equations

describing both situations (equations 19-3 and 19-4)


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What would be the maximum concentration in the plume?

Integrate equation 19-2 and set it equal to zero

Xmax = (2Q/ueH2)(z/y)This maximum concentration occur at the distance where z = H/(2)0.5


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Alternate coordinate systems for the Gaussian equations

The coordinate system described in the previous section

0 at the bottom of the stack

z vertical

y crosswind

x downwind

The results will be identical if you put coordinate system at the bottom

of the receptor, x upwind, z vertical and y crosswind.

You can also use map coordinates or east north, or polar coordinate

systems. The results do not change.


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Determination of Dispersion parameters

Dispersion parameters in the Gaussian Plume Equation are important

as they determine how much the plume is dispersed as it travels.

True determination of dispersion parameters require measurement of

wind fluctuations, because these fluctuations determine how much theplume is dispersed.

But the measurement of fluctuations every time a modeling is

performed is not practical. Because of this usually dispersion

parameters y and z are determined from the stability of theatmosphere.


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There are various estimates of these parameters, but the most widely

used ones are based on Pasquill stability classes.

Pasquil have developed a scheme to estimate y and z if there are no

wind fluctuation measurements (which is usually the case).

Later Gifford modified these to be used in Gaussian Plume equations.

The y and z estimated from Pasquill Gifford method are fairly broadestimates


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In this method you need three parameters to determine the stability of

the atmosphere:

Wind Speed

Insolation (solar flux)

Cloudiness

These are standard parameters regularly measured in met stations.


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Six classes of stability are defined depending on wind speed and the

strength of the sunlight (insolation and cloudiness) (from class A to class F)

These are given in Table 19-3

Classes A, B and C corresponds to unstable conditions, Class D

corresponds to neutral condition and classes E and F correspond to

stable conditions of the atmosphere.

Usually for overcast conditions, neutral class D should be used no matter

what the wind speeds are.


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Once the stability class of the atmosphere is established, z and y aredetermined using charts given in Figure 19-6.

Note that units of z and y in this figure are meter and they changewith distance from the source. That is why they represent spreading

of the plume.

This type of calculation is performed for every hour by models.

z

(m)

y(m)


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Example of Dispersion Calculation:

A point source releases 0.37 g s-1 of a pollutant. (Q)

Effective height (H) = 40 m

Wind speed (u) = 2 m s-1

Stability class = B

What is the approximate distance where the maximum

concentration occurs?

What is the maximum concentration?


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The maximum concentration occurs when;

z = H/(2)1/2

z = 40/(2)1/2 = 28.3 m

for z = 28.3 m from figure 19-6.

x = 0.28 km this is where the maximum concentration occurs.

for x = 0.28

y = 49.0 m

Xmax = (2Q/ueH2)( z/y)Xmax = 1.56 x 10

-5 g m-3 = 15.6 g m-3


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Now let us see if the calculated ground level concentration is indeed the

maximum.

We have to calculate concentration (x) using equation 19-2 first

Note that this is the ground level concentration and it occurs on

the plume centerline (y = 0, z = 0)

If you set y and z to 0 in equation 19-2 you will obtain

X = [Q//uyz]exp[-0.5(H/

z)2)]

Note that the x at 0.28 km from the stack was 15.6 g m-3


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Let us calculate x at 0.26 km and at 0.30 km

First we must find yand

zfor these distances.

From figure 19-6

For 0.26 km: y

= 45.9 m and z

= 26.2 m

For 0.30 km: y

= 52.2 m and z

= 30.1 m

Plug these values into above equation

X = 1.53 x 10-5 at 0.26 km from the stack

And

X = 1.55 x 10-5 at 0.30 km from the stack

Both of these concentrations are lower than the maximum concentration

we have calculated.


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Figure 19.1. Wind variation with height- measured (solid lines) and one-seventh power law(dashed lines).


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Figure 19.2. Dilution by wind speed.


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Figure 19.3. Examples of turbulence on wind direction records: (a) mechanical, (b) thermal


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Figure 19.4. Verticalexpansion of continuousplumes related to vertical

temperature structure. Thedashed lines correspond tothe dry adiabatic lapse ratefor reference.

Fanning plume


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g p

Looping plume

Lofting plume


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Figure 19.5. Two cross sections through a Gaussian plume (total mass under curves conserved)


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Table19.3. Pasquill Stability Categories


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Figure19.6. Pasquill-Gifford y (left) and z (right)


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Figure19.6. Pasquill-Gifford y (left) and z (right)

0.28 km

49.0 m

0.28 km


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Two types of sources:

Stationary sources

Point Sources: Stacks

Area sources: Sources where emissions are distributed.Ex, emissions in a settlement area

Line Sources: Ex, Traffic emissions

Mobile Sources Emissions that moves around

Motor vehicles

Ships nowadays becoming popular

Aircraft emissions Nowadays becoming popular


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Figure X. Horozgedii istasyonu evresindeki demir elik tesisleri

Horozgedii istasyonu


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slide show chapter 19 may 2011 (1)

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