Discussion
Three of the following raise ground level concentration. Which lowers it?
A greater effective stack height
A shallower mixing height
A lower wind speed
A stronger inversion
Answer: B
Correct answer: A greater effective stack height
A greater effective stack height lowers ground level concentration, and does so strongly - the maximum varies roughly as \( 1/H^2 \), so a 40 per cent increase almost halves it. The plume is released higher, so it must spread through more depth before any reaches the surface, and it is much diluted by the time it does.
The other three all raise concentration. A shallower mixing height reduces the volume available for dilution, a lower wind speed means the same emission is spread through less air per second, and a stronger inversion suppresses the vertical mixing that would otherwise disperse the plume.
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