How does urban geometry influence plume dispersion compared with open rural terrain?

How does urban geometry influence plume dispersion compared with open rural terrain?

• A. It reduces turbulence
• B. It increases mechanical turbulence, often increasing σ_y and σ_z and potentially causing downwash
• C. It has no effect
• D. It only affects ground deposition

Answer: (B)

Urban geometry adds roughness and obstacles like buildings that create mechanical turbulence in the flow, especially near streets and in wakes. This extra turbulence increases how much the plume mixes and spreads as it moves downwind. In a dispersion model, that mixing is captured by the spread parameters σ_y (lateral) and σ_z (vertical); when turbulence rises, these values grow, so the plume widens both sideways and upward/downward. The stronger mixing can also produce downward motions, or downwash, pushing portions of the plume toward the ground, which can raise ground-level concentrations in the wake of buildings or within street canyons. By contrast, open rural terrain presents smoother flow with less mechanical turbulence, so the plume spreads more slowly (smaller σ_y and σ_z) and downwash is less pronounced.

Urban geometry adds roughness and obstacles like buildings that create mechanical turbulence in the flow, especially near streets and in wakes. This extra turbulence increases how much the plume mixes and spreads as it moves downwind. In a dispersion model, that mixing is captured by the spread parameters σ_y (lateral) and σ_z (vertical); when turbulence rises, these values grow, so the plume widens both sideways and upward/downward. The stronger mixing can also produce downward motions, or downwash, pushing portions of the plume toward the ground, which can raise ground-level concentrations in the wake of buildings or within street canyons. By contrast, open rural terrain presents smoother flow with less mechanical turbulence, so the plume spreads more slowly (smaller σ_y and σ_z) and downwash is less pronounced.