Atmospheric Dispersion of Pollutants - Part 1

Question 1

What are the three main types of release for pollutants?

Answer 1

• Continuous release of pollutant
• Short but controlled release
• Accidental release

Question 2

Why is there deformations in the contours for the concentration of pollutant near the ground?

Answer 2

There is “reflection” of the plume by the ground which then causes the concentration of the pollutants to double.

Question 3

Why are harmful pollutants released high from a stack?

Answer 3

In the hope that they will have dispersed to a safe concentration by the time they impinge at ground level.

Question 4

What are the 4 factors that the dispersion of materials depends on?

Answer 4

• The height of release, H
• The wind speed, u, and stack gas exit velocity, v
• The ambient temperature Ta and the temperature of stack gases, Ts
• The temperature gradient near the ground dT/dz

Question 5

What two factors cause the plume to transform from a straight plume to a bent over plume?

Answer 5

• higher wind speed

- lower temperature of the stack gads

Question 6

What generates a buoyancy force for the release of stack gas in ambient air?

Answer 6

The difference in density between the emitted gas and the surrounding ambient air.

Question 7

What causes the stack gas to have a different buoyancy to the ambient air?

Answer 7

• The gas being hotter or cooler than the ambient air

- A significantly different molecular weight

Question 8

What determines how high the gas initially rises?

Answer 8

• the initial momentum

- buoyancy

Question 9

What is the first assumption in the release of stack gas into air?

Answer 9

The molar mass of the emitted gas is similar to air because it will most likely be contaminated air, nitrogen and the buoyancy is then solely set by the temperature difference Ts - T at z

Question 10

How does the temperature of the stack gas equilibrate with the ambient air?

Answer 10

It will rise, expand and cool. It will transfer heat and momentum to the surroundings as it rises which causes the momentum and temperature of the stack gas to drop.

Question 11

How high do we want the initial rise to be?

Answer 11

We want the initial rise to be as high as possible. Enough height should allow enough time afterwards for effective dispersion before the plume or cloud has fallen back to the ground.

Question 12

What is the effective height?

Answer 12

This is the total height of the plume above ground level at which the temperature of the stack gas is equal to the temperature of the ambient air. Also the velocity of the stack gas will be equal to the wind speed.

Question 13

What happens to the pressure of the stack gas as it leaves the stack?

Answer 13

It instantly equilibrates to the ambient value

Question 14

What two factors mostly determine the initial rise?

Answer 14

• The buoyancy

- The exit velocity

Question 15

What is the main feature of the plume (or cloud) following the effective height?

Answer 15

Dispersion

Question 16

How is the gas molecules then dispersed?

Answer 16

By random diffusion of gas molecules. In the vertical direction this dispersion is either amplified or curtailed by buoyancy forces, depending on dT/dz.

Question 17

What is the equation for the effective height of release?

Answer 17

H + delta (H)
where H is the stack height
delta (H) is the vertical rise of the plume before temperature equilibration with ambient

Question 18

What correlation is used to estimate the effective height?

Answer 18

Holland correlation

Question 19

By which two forms of diffusion will a parcel of polluted air disperse in ambient air?

Answer 19

• pure diffusion (random, Brownian motion)

- eddy-induced diffusion (convective motion within and outside the parcel)

Question 20

In which direction is dispersion particularly enhanced or impeded by atmospheric conditions?

Answer 20

Vertical

Question 21

What is the relationship between the density and the altitude?

Answer 21

The density tends to decrease with altitude

Question 22

What happens if the density increases with altitude?

Answer 22

An instability occurs, natural convection establishes itself and mixing the air. Away from the ground this would help dispersion but it may also beat pollution back down to the ground.

Question 23

Why is the dispersion of gases affected by ambient temperatures rather than ambient pressures?

Answer 23

Pressures tend to equalise much quicker than temperatures in air (air is a poor conductor of heat)

Question 24

What is the relationship between the altitude and the temperature?

Answer 24

The temperature tends to decrease with altitude

Question 25

By what factor is the change of density with altitude largely determined by?

Answer 25

dT/dz

Question 26

What is the meaning of dispersion?

Answer 26

The stability with respect to vertical mixing

Question 27

What does the variation of ambient air temperature with altitude depend on?

Answer 27

The weather or the day

Question 28

What does the variation of stack gas temperature with altitude depend on?

Answer 28

The change in pressure with altitude

Question 29

Once the temperature of the stack gas has equilibrated what does the stability depend on?

Answer 29

dT/dz

Question 30

What is dT/dz depend on?

Answer 30

Environmental lapse rate

Question 31

What does the value of the environmental lapse rate indicate?

Answer 31

The degree of stability with respect to vertical mixing for both the column of air itself and for a parcel of polluted air within it

Question 32

Usually is dT/dz positive or negative?

Answer 32

Negative

Question 33

Give an example in which the temperature could rise with altitude.

Answer 33

On a clear night, the ground radiates heat to space and becomes cooler than the local atmosphere then cools the air just above it to below the temperature above.

Question 34

What is the concept behind the dry adiabatic lapse rate?

Answer 34

If a packet of dry gas is carried upwards its pressure and temperature falls along with the density.

Question 35

What is the fall of temperature with altitude for the dry adiabatic lapse rate?

Answer 35

The temperature falls by 1oC with a 100 m rise in altitude

Question 36

What is the concept behind the moist adiabatic lapse rate?

Answer 36

If the same packet of gas happens to contain water vapour and the temperature of the packet drops below the dew point, then condensation will occur, releasing some heat inside the packet.

Question 37

What is the fall of temperature with altitude of the moist adiabatic lapse rate?

Answer 37

The temperature falls by 0.6oC with a 100 m rise in altitude

Question 38

What is meant by a gas parcel in ‘neutral’ ambient air?

Answer 38

dT/dz is about equal to the adiabatic lapse rate

Question 39

What is meant by a gas parcel emitted in ‘unstable’ ambient air?

Answer 39

dT/dz is less than the adiabatic lapse rate

Question 40

What is meant by a gas parcel emitted in ‘stable’ ambient air?

Answer 40

dT/dz is more than the adiabatic lapse rate

Question 41

Does ambient dT/dz help to disperse emitted gas in ‘neutral’?

Answer 41

Hardly if at all, only slow diffusion

Question 42

Does ambient dT/dz help to disperse emitted gas in ‘unstable’?

Answer 42

Yes, very well

Question 43

Does ambient dT/dz help to disperse emitted gas in ‘stable’?

Answer 43

No