If the ground reflection term is omitted, what is the likely effect on predicted near-ground concentrations?

If the ground reflection term is omitted, what is the likely effect on predicted near-ground concentrations?

• A. Predicted near-ground concentrations may be too low or zero.
• B. Predicted near-ground concentrations would be too high.
• C. No effect on predictions.
• D. Only affects centerline values.

Answer: (A)

The ground reflection term represents the reflected plume, like an image source below the ground that adds to the concentration at points near the surface. When you include this term, near-ground concentrations receive an extra positive contribution, so the predicted values are higher than they would be from the direct plume alone. If you omit the ground reflection, you lose that additional contribution. You’re left with only the direct plume, which tends to underestimate what is actually present right at ground level. So near-ground predictions become too low, and in some situations can be very small (even approaching zero) compared to a model that includes reflection. The impact is most pronounced for receptors close to the ground and downwind of the source.

The ground reflection term represents the reflected plume, like an image source below the ground that adds to the concentration at points near the surface. When you include this term, near-ground concentrations receive an extra positive contribution, so the predicted values are higher than they would be from the direct plume alone.

If you omit the ground reflection, you lose that additional contribution. You’re left with only the direct plume, which tends to underestimate what is actually present right at ground level. So near-ground predictions become too low, and in some situations can be very small (even approaching zero) compared to a model that includes reflection. The impact is most pronounced for receptors close to the ground and downwind of the source.