Air Pollution MCQ with answer and solution

Concept

Plume dispersion patterns under stable atmospheric conditions (Fanning plume)

Formula

Fanning plume occurs under stable inversion: σ_z is very small, σ_y > σ_z

Step-by-step solution

1. Pasquill Class F indicates stable conditions with a negative or zero lapse rate (inversion). 2. Stable inversion layers suppress vertical air movements (small vertical dispersion coefficient σ_z). 3. Horizontal meandering occurs, but vertical growth is virtually zero. 4. The resulting 'fanning' plume travels long distances with very little vertical spread. Answer: C

Explanation

Class F represents moderately stable atmospheric conditions (temperature inversion with very little turbulence). Under these stable conditions, vertical air motion is strongly suppressed, so the smoke plume experiences minimal vertical dispersion while spreading slowly in the horizontal crosswind direction. This creates a thin, ribbon-like 'Fanning' plume that travels long distances downwind without reaching the ground.

Why the other options are wrong

A) Looping occurs under highly unstable Class A conditions with strong thermal eddies. B) Rapid multi-directional dispersion requires neutral or convective turbulence. D) Lofting occurs when an inversion exists below the stack while the atmosphere above is unstable.

Shortcut

Class F (Stable / Inversion) = Fanning plume = Minimal vertical spread, travels long distances.

Common mistake

Confusing fanning (Class F, stable) with looping (Class A, unstable) or fumigation.