What is meant by a 'reduction in dispersion' in stable conditions?

What is meant by a 'reduction in dispersion' in stable conditions?

• A. Higher stability increases dispersion.
• B. Stable conditions increase Gaussian width.
• C. Dispersion is independent of stability.
• D. Lower turbulence reduces sigma_y and sigma_z and concentrates plume.

Answer: (D)

In stable conditions, turbulence is suppressed, so the air mixes less effectively and the plume spreads less. In the Gaussian plume framework, the spread is described by sigma_y and sigma_z, which quantify how far the plume disperses horizontally and vertically as it travels downwind. When turbulence is reduced, these sigmas grow more slowly, meaning the plume stays narrower and closer to the centerline. That tighter, less-dispersed plume concentrates more of the pollutant near its center, leading to higher concentrations along the plume and lower concentrations off to the sides. This is why the statement about lower turbulence reducing sigma_y and sigma_z and concentrating the plume best captures what a reduction in dispersion means under stable conditions.

In stable conditions, turbulence is suppressed, so the air mixes less effectively and the plume spreads less. In the Gaussian plume framework, the spread is described by sigma_y and sigma_z, which quantify how far the plume disperses horizontally and vertically as it travels downwind. When turbulence is reduced, these sigmas grow more slowly, meaning the plume stays narrower and closer to the centerline. That tighter, less-dispersed plume concentrates more of the pollutant near its center, leading to higher concentrations along the plume and lower concentrations off to the sides. This is why the statement about lower turbulence reducing sigma_y and sigma_z and concentrating the plume best captures what a reduction in dispersion means under stable conditions.