Which term in the Gaussian plume equation reflects the ground-level boundary condition?

Which term in the Gaussian plume equation reflects the ground-level boundary condition?

• A. The sum of two exponentials with H and -H
• B. The crosswind Gaussian term
• C. The first exponential term
• D. The term describing chemical decay

Answer: (A)

The ground-level boundary condition is handled by the image-source concept in the Gaussian plume model. To enforce how concentration behaves at the ground (z = 0), an imaginary source is placed at height -H, creating a mirrored contribution that, when added to the real source at height H, makes the vertical concentration profile satisfy the ground boundary. This appears mathematically as the sum of two exponential terms with heights H and -H. That combined term ensures the boundary condition is met at the ground. The crosswind Gaussian term describes lateral spread, not the boundary; the first exponential term is only part of the vertical distribution; and the decay term would represent chemical decay over time, not the ground boundary condition.

The ground-level boundary condition is handled by the image-source concept in the Gaussian plume model. To enforce how concentration behaves at the ground (z = 0), an imaginary source is placed at height -H, creating a mirrored contribution that, when added to the real source at height H, makes the vertical concentration profile satisfy the ground boundary. This appears mathematically as the sum of two exponential terms with heights H and -H. That combined term ensures the boundary condition is met at the ground. The crosswind Gaussian term describes lateral spread, not the boundary; the first exponential term is only part of the vertical distribution; and the decay term would represent chemical decay over time, not the ground boundary condition.