How does the Gaussian model treat ground-level receptors compared with elevated receptors?

How does the Gaussian model treat ground-level receptors compared with elevated receptors?

• A. Ground-level receptors are unaffected by ground reflection; elevated receptors sample same plume.
• B. Ground-level receptors sample higher portions of the plume with more ground-bound enhancement.
• C. Ground-level receptors are strongly influenced by ground reflection; elevated receptors sample higher portions with less ground-bound enhancement.
• D. Ground-level receptors sample only at ground level and do not sample the plume.

Answer: (C)

In the Gaussian plume model the ground is treated as a boundary that reflects part of the plume. This is handled by adding an image source below the ground, so the observed concentration is the sum of the real plume and its reflected image. Near the ground, this ground reflection greatly boosts concentrations because the real and reflected plumes overlap strongly, effectively increasing the plume intensity at low heights. As you move to elevated receptor heights, the influence of the reflected image diminishes. The receptor samples portions of the plume higher up where the two Gaussian components (the real plume and the image) are less coupled, so the extra enhancement from ground reflection is smaller. That means ground-level receptors feel strong ground-reflection effects, while elevated receptors sample higher parts of the plume with less ground-bound enhancement.

In the Gaussian plume model the ground is treated as a boundary that reflects part of the plume. This is handled by adding an image source below the ground, so the observed concentration is the sum of the real plume and its reflected image. Near the ground, this ground reflection greatly boosts concentrations because the real and reflected plumes overlap strongly, effectively increasing the plume intensity at low heights.

As you move to elevated receptor heights, the influence of the reflected image diminishes. The receptor samples portions of the plume higher up where the two Gaussian components (the real plume and the image) are less coupled, so the extra enhancement from ground reflection is smaller. That means ground-level receptors feel strong ground-reflection effects, while elevated receptors sample higher parts of the plume with less ground-bound enhancement.