# Air Pollution Control Questions and Answers – Plume Behaviour and Characteristics – Set 3

This set of Air Pollution Control Multiple Choice Questions & Answers (MCQs) focuses on “Plume Behaviour and Characteristics – Set 3”.

1. Other than the presence of a structure near the stack, which factor affects the plume dispersion?
a) Irregular terrain
b) Height of stack
c) Wind speeds
d) Temperature of atmosphere

Explanation: Both irregular terrains, alongside the presence of buildings near stacks, affect the dispersion of the plume. Due to these factors, the displacement of the plume will heavily vary from the standard plume behaviour.

2. If a stack is constructed near a building, what should its height be to ensure characteristic plume dispersion?
a) Equal to the adjacent building
b) 2.5 times the height of the surrounding building
c) It should never be built near a building
d) 10 metres

Explanation: If a stack is built adjacent to another building, its height should be about 2.5 times that of the building. This rule ensures that the plume does not get trapped in the wake zone of the building.

3. What happens when the stack height is less than 2.5 times the height of an adjacent building?
a) The pollutants disperse rapidly
b) There is no effect
c) Pollutants remain trapped near the building
d) There is too little information

Explanation: The pollutants remain in the wake zone of the building. This is because a cavity exists behind the building, where the air pollutants get trapped. They will be unable to join the mainstream wind currents immediately.

4. What happens to the plume if the stack is situated in a valley?
a) It disperses upwards
b) It disperses rapidly
c) No dispersion occurs
d) Driven to the surface of the earth

Explanation: In case of the stack built in a valley, the wind current carries the plume towards the ground. The wind coming from the top of the valley is headed downwards at an angle, and it carries the plume in the same direction.

5. What is known as downwash?
a) When the wind carries pollutant plume towards the ground
b) When water current drives pollutants downstream
c) When water washes away pollutants in the soil
d) When solid waste is washed away by water

Explanation: The downwash condition occurs in valleys. This condition occurs when an emission stack is constructed in a valley. The wind is incident in such a way that it carries the plume towards the ground.

6. When can the downwash of the plume be avoided?
a) When the stack height is short
b) When the stack is taller than the valley rim
c) When the wind speeds are low
d) When there is an inversion

Explanation: Downwash occurs in a valley, and sometimes in the vicinity of a chimney. The wind currents drive the pollutant plume towards the ground. The stack should be taller than the mouth of the valley to avoid this.

7. Building a stack behind a chimney can also cause downwash of the pollutants.
a) True
b) False

Explanation: Air pollutant plumes can also be down-washed if the stack is built near a chimney. When the wind flows towards the stack from the chimney, the plume is pushed towards the surface. This condition is undesirable.

8. What happens to the plume if its velocity is equal to the wind velocity?
a) Plume does not disperse vertically
b) Slow dispersion
c) Downwashing
d) Plume disperses horizontally

Explanation: The plume gets down-washed if its velocity is equal or less than that of the wind. When this condition occurs near a plant with tall stacks, the plume may get stuck behind it, instead of dispersing rapidly.

9. How is the behaviour of the plume determined to optimise dispersion?
a) From experience
b) Modelling
c) Calculations
d) Wind tunnel testing

Explanation: Understanding plume behaviour is necessary to optimise its dispersion. Aerodynamic disturbances are difficult to estimate, and practical testing at the small scale needs to be done. Wind-tunnel testing is helpful when mapping the behaviour of the plume in a particular area.

10. Why is it crucial to understand plume behaviour?
a) To reduce pollution
b) For pollutant emission
c) To ensure pollutant dispersion
d) For a clean environment

Explanation: Plume behaviour needs to be understood to ensure the dispersion of air pollutants. The atmosphere dissipates the air pollutants based on its meteorological characteristics. To benefit from this natural property, an understanding of its mechanics will avoid pollutant buildup.

Sanfoundry Global Education & Learning Series – Air Pollution Control.

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