Atmospheric Dispersion of Pollutants - Part 1 Flashcards
What are the three main types of release for pollutants?
- Continuous release of pollutant
- Short but controlled release
- Accidental release
Why is there deformations in the contours for the concentration of pollutant near the ground?
There is “reflection” of the plume by the ground which then causes the concentration of the pollutants to double.
Why are harmful pollutants released high from a stack?
In the hope that they will have dispersed to a safe concentration by the time they impinge at ground level.
What are the 4 factors that the dispersion of materials depends on?
- 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
What two factors cause the plume to transform from a straight plume to a bent over plume?
- higher wind speed
- lower temperature of the stack gads
What generates a buoyancy force for the release of stack gas in ambient air?
The difference in density between the emitted gas and the surrounding ambient air.
What causes the stack gas to have a different buoyancy to the ambient air?
- The gas being hotter or cooler than the ambient air
- A significantly different molecular weight
What determines how high the gas initially rises?
- the initial momentum
- buoyancy
What is the first assumption in the release of stack gas into air?
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
How does the temperature of the stack gas equilibrate with the ambient air?
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.
How high do we want the initial rise to be?
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.
What is the effective height?
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.
What happens to the pressure of the stack gas as it leaves the stack?
It instantly equilibrates to the ambient value
What two factors mostly determine the initial rise?
- The buoyancy
- The exit velocity
What is the main feature of the plume (or cloud) following the effective height?
Dispersion
How is the gas molecules then dispersed?
By random diffusion of gas molecules. In the vertical direction this dispersion is either amplified or curtailed by buoyancy forces, depending on dT/dz.
What is the equation for the effective height of release?
H + delta (H)
where H is the stack height
delta (H) is the vertical rise of the plume before temperature equilibration with ambient
What correlation is used to estimate the effective height?
Holland correlation
By which two forms of diffusion will a parcel of polluted air disperse in ambient air?
- pure diffusion (random, Brownian motion)
- eddy-induced diffusion (convective motion within and outside the parcel)
In which direction is dispersion particularly enhanced or impeded by atmospheric conditions?
Vertical
What is the relationship between the density and the altitude?
The density tends to decrease with altitude
What happens if the density increases with altitude?
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.
Why is the dispersion of gases affected by ambient temperatures rather than ambient pressures?
Pressures tend to equalise much quicker than temperatures in air (air is a poor conductor of heat)
What is the relationship between the altitude and the temperature?
The temperature tends to decrease with altitude
By what factor is the change of density with altitude largely determined by?
dT/dz
What is the meaning of dispersion?
The stability with respect to vertical mixing
What does the variation of ambient air temperature with altitude depend on?
The weather or the day
What does the variation of stack gas temperature with altitude depend on?
The change in pressure with altitude
Once the temperature of the stack gas has equilibrated what does the stability depend on?
dT/dz
What is dT/dz depend on?
Environmental lapse rate
What does the value of the environmental lapse rate indicate?
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
Usually is dT/dz positive or negative?
Negative
Give an example in which the temperature could rise with altitude.
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.
What is the concept behind the dry adiabatic lapse rate?
If a packet of dry gas is carried upwards its pressure and temperature falls along with the density.
What is the fall of temperature with altitude for the dry adiabatic lapse rate?
The temperature falls by 1oC with a 100 m rise in altitude
What is the concept behind the moist adiabatic lapse rate?
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.
What is the fall of temperature with altitude of the moist adiabatic lapse rate?
The temperature falls by 0.6oC with a 100 m rise in altitude
What is meant by a gas parcel in ‘neutral’ ambient air?
dT/dz is about equal to the adiabatic lapse rate
What is meant by a gas parcel emitted in ‘unstable’ ambient air?
dT/dz is less than the adiabatic lapse rate
What is meant by a gas parcel emitted in ‘stable’ ambient air?
dT/dz is more than the adiabatic lapse rate
Does ambient dT/dz help to disperse emitted gas in ‘neutral’?
Hardly if at all, only slow diffusion
Does ambient dT/dz help to disperse emitted gas in ‘unstable’?
Yes, very well
Does ambient dT/dz help to disperse emitted gas in ‘stable’?
No
