Discussion
Which of the following best explains why the maximum ground level concentration occurs downwind rather than at the stack base?
Answer: D
Correct answer: The plume must grow before reaching the ground
The plume leaves at an effective height \( H \) and travels horizontally while turbulence spreads it. None of it reaches the surface until the vertical spread \( \sigma_z \) has grown to a size comparable with \( H \), which takes a finite distance. The ground level concentration therefore rises from nearly zero, peaks where that condition is met, and falls again as continued dilution outweighs the increasing share of plume reaching the ground.
The wind does not carry all the material away - it carries it downwind at the effective height, where it is still spreading vertically. The stack absorbs nothing, and the ground level concentration at the base is very small rather than exactly zero. How far downwind the peak lies depends on stability: close in under unstable conditions, when the plume grows quickly, and far out under stable ones.
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