As downwind distance increases, what happens to sigma_y and sigma_z in the Gaussian plume model?

As downwind distance increases, what happens to sigma_y and sigma_z in the Gaussian plume model?

• A. They increase with distance, reflecting greater dispersion.
• B. They decrease with distance.
• C. They stay constant with distance.
• D. They vary unpredictably with distance.

Answer: (A)

In the Gaussian plume model, the spread of the pollutant in the horizontal and vertical directions is described by sigma_y and sigma_z. As the plume travels downwind, turbulence and mixing accumulate along the path, causing the plume to spread more. That means sigma_y (lateral spread) and sigma_z (vertical spread) increase with downwind distance. This increasing spread is built into the model so that the peak concentration drops and the plume becomes wider as you move farther from the source. The exact rate of growth depends on atmospheric stability, but the general behavior is a monotonic increase with distance, not a constant, a decrease, or an unpredictable change.

In the Gaussian plume model, the spread of the pollutant in the horizontal and vertical directions is described by sigma_y and sigma_z. As the plume travels downwind, turbulence and mixing accumulate along the path, causing the plume to spread more. That means sigma_y (lateral spread) and sigma_z (vertical spread) increase with downwind distance. This increasing spread is built into the model so that the peak concentration drops and the plume becomes wider as you move farther from the source. The exact rate of growth depends on atmospheric stability, but the general behavior is a monotonic increase with distance, not a constant, a decrease, or an unpredictable change.