How does atmospheric stability influence the dispersion parameters σ_y and σ_z in the Gaussian plume model?

How does atmospheric stability influence the dispersion parameters σ_y and σ_z in the Gaussian plume model?

• A. Higher stability generally reduces dispersion, leading to smaller σ_y and σ_z.
• B. Stability has no effect on σ_y and σ_z.
• C. Higher stability increases σ_y and σ_z.
• D. σ_y and σ_z depend only on distance, not stability.

Answer: (B)

Stability controls how much turbulent mixing takes place in the atmosphere. The plume’s spread in the crosswind (σ_y) and vertical (σ_z) directions is driven by that turbulence. Under more stable conditions, turbulence is suppressed, so mixing is weaker and the plume stays more concentrated, giving smaller σ_y and σ_z. Under more unstable conditions, stronger turbulence enhances mixing, widening the plume and increasing both σ_y and σ_z. The Gaussian plume model uses empirical relationships that tie σ_y and σ_z to downwind distance in a way that depends on stability, so these dispersion parameters grow with distance but at rates set by the stability class. Thus, higher stability leads to smaller dispersion (smaller σ_y and σ_z), while instability increases dispersion.

Stability controls how much turbulent mixing takes place in the atmosphere. The plume’s spread in the crosswind (σ_y) and vertical (σ_z) directions is driven by that turbulence. Under more stable conditions, turbulence is suppressed, so mixing is weaker and the plume stays more concentrated, giving smaller σ_y and σ_z. Under more unstable conditions, stronger turbulence enhances mixing, widening the plume and increasing both σ_y and σ_z. The Gaussian plume model uses empirical relationships that tie σ_y and σ_z to downwind distance in a way that depends on stability, so these dispersion parameters grow with distance but at rates set by the stability class. Thus, higher stability leads to smaller dispersion (smaller σ_y and σ_z), while instability increases dispersion.