In the Gaussian plume equation, what are sigma_y and sigma_z?

In the Gaussian plume equation, what are sigma_y and sigma_z?

• A. They are the initial emission rates in the along-wind and crosswind directions.
• B. They are wind shear coefficients.
• C. They are ground reflection parameters.
• D. They are the horizontal (y) and vertical (z) dispersion parameters describing plume spread due to turbulence at a given downwind distance.

Answer: (D)

In the Gaussian plume model, sigma_y and sigma_z describe how the plume spreads due to turbulence in the horizontal crosswind (y) and vertical (z) directions at a given downwind distance. They are the dispersion parameters, effectively the standard deviations of the concentration distribution in those directions, that shape the Gaussian terms in the equation. Larger sigmas mean the plume widens and the peak concentration lowers, while smaller sigmas yield a narrower, higher plume. They depend on downwind distance and atmospheric stability (and surface roughness) and are not related to initial emission rates or wind-shear or ground reflection parameters.

In the Gaussian plume model, sigma_y and sigma_z describe how the plume spreads due to turbulence in the horizontal crosswind (y) and vertical (z) directions at a given downwind distance. They are the dispersion parameters, effectively the standard deviations of the concentration distribution in those directions, that shape the Gaussian terms in the equation. Larger sigmas mean the plume widens and the peak concentration lowers, while smaller sigmas yield a narrower, higher plume. They depend on downwind distance and atmospheric stability (and surface roughness) and are not related to initial emission rates or wind-shear or ground reflection parameters.