Under which condition is downwash most likely to increase ground-level concentrations near a tall obstacle?

Under which condition is downwash most likely to increase ground-level concentrations near a tall obstacle?

• A. Very stable atmosphere with strong turbulence.
• B. Tall obstacle is present and wind is perpendicular to it, causing plume interaction and downward bending.
• C. No obstacle and light wind.
• D. Strong, aligned wind with no obstacle.

Answer: (B)

When a tall obstacle sits in the path of a plume, it disrupts the flow and creates a wake that can wrap around the obstacle. If the wind hits the obstacle perpendicularly, this disturbance is strongest and the plume interacts more with the obstacle’s wake. The resulting lee-side vortices and downward momentum pull plume air toward the ground, causing downward bending of the plume. This mechanical downwash increases ground-level concentrations in the area near the obstacle because pollutant-laden air is pushed down and concentrated close to the surface. If there were no obstacle, or the wind were aligned with the obstacle, the plume would be transported more along the flow path and mixed differently, generally producing less intense ground-level enhancement near the obstacle. The other scenarios described don’t promote this obstacle-induced downward deflection as effectively.

When a tall obstacle sits in the path of a plume, it disrupts the flow and creates a wake that can wrap around the obstacle. If the wind hits the obstacle perpendicularly, this disturbance is strongest and the plume interacts more with the obstacle’s wake. The resulting lee-side vortices and downward momentum pull plume air toward the ground, causing downward bending of the plume. This mechanical downwash increases ground-level concentrations in the area near the obstacle because pollutant-laden air is pushed down and concentrated close to the surface.

If there were no obstacle, or the wind were aligned with the obstacle, the plume would be transported more along the flow path and mixed differently, generally producing less intense ground-level enhancement near the obstacle. The other scenarios described don’t promote this obstacle-induced downward deflection as effectively.