How does increasing stack height h affect the effective height H and ground-level impact?

How does increasing stack height h affect the effective height H and ground-level impact?

• A. Decreases H.
• B. Has no effect.
• C. Increases H, which generally reduces ground-level concentrations near the stack unless Δh is large.
• D. Always increases ground-level concentrations.

Answer: (C)

The key idea is that the effective height in dispersion is the sum of the physical stack height and the plume rise: H = h + Δh. When you raise the stack, you push the release point higher, so H increases. A higher release point means the plume has more distance to mix and dilute with ambient air before it can reach ground level, which generally lowers ground-level concentrations near the stack. The caveat is that if the plume rise Δh is large, the total H is dominated by Δh, so adding more physical height h doesn’t change H by much, and the improvement in ground-level concentrations is smaller. So increasing stack height increases H and tends to reduce near-ground concentrations, unless Δh is large.

The key idea is that the effective height in dispersion is the sum of the physical stack height and the plume rise: H = h + Δh. When you raise the stack, you push the release point higher, so H increases. A higher release point means the plume has more distance to mix and dilute with ambient air before it can reach ground level, which generally lowers ground-level concentrations near the stack. The caveat is that if the plume rise Δh is large, the total H is dominated by Δh, so adding more physical height h doesn’t change H by much, and the improvement in ground-level concentrations is smaller. So increasing stack height increases H and tends to reduce near-ground concentrations, unless Δh is large.