3. Atmospheric stability

Question 1

What is a heat island

Answer 1

The term given to an area which can release or absorb more thermal outputs than the surroundings. e.g. a city

Question 2

Name an advantage and a disadvantage of a heat island.

Answer 2

+ A hot city will cause a lot of mixing that will help to dilute pollution

• Under stable conditions, plumes from stacks which travel to the city will then be mixed and transported to ground level at a higher concentration than expected given the distance

Question 3

Why might some valleys be more susceptible to atmospheric inversions

Answer 3

North-south oriented valley walls provide shade to the valley for most of the day when they might be heated by the sun, at night the valley can radiate heat as normal and so ends up very cool.

During the day the ground is not able to heat the air enough to reverse the inversion which occurred over night.

Question 4

What factors affect atmospheric dispersion

Answer 4

Emissions point characteristics
Nature of pollutant
Meteorological conditions
Terrain and anthropogenic structures

Question 5

How does wind speed affect ground level concentration of pollutants from a stack?

Answer 5

Higher wind speeds cause the pollution to bend round and reach the ground sooner however higher wind speeds also cause more turbulence and mixing which leads to lower concentration at ground level

Question 6

What causes land and sea breezes

Answer 6

Sea has a large thermal inertia (well mixed, temperature constant regardless of time of day)

Earth has a large thermal inertia, but poor conductivity (temperature changes significantly depending on time of day)

At night: Warm sea air rises and cools over land, before running down to the sea again (land to sea breeze)

At day: Warm land air rises and cools over sea before being dragged onto the land again (sea to land breeze)

Question 7

What factors effect plume height?

Answer 7

Vertical momentum (function of mass and velocity of plume outlest)
Buoyancy (dependent on density, mass and temperature)

Question 8

What factors effect the concentrations of pollutants downwind of a plume

Answer 8

Plume height (higher the plume, the more dispersion)
Amount of contaminants in plume
Downwind distance (greater the distance, more dispersion)
Wind speed and direction (higher wind speeds has more mixing and reduces contaminants
Atmospheric stability (lower stability, more dilution)

Question 9

What two types of dispersion modelling are there

Answer 9

Short term (estimate concentration levels after an accidental release)

Climatological (estimate mean concentrations over a period of time)

Question 10

What effect does inversion have on plume dispersions

Answer 10

Dispersion cannot take place through the inversion layer, and is forced to mix downwards only