Which statement about the Gaussian plume centerline on the downwind axis is true?

Which statement about the Gaussian plume centerline on the downwind axis is true?

• A. It represents the peak concentration directly downwind from the source at height z.
• B. It represents the average concentration over all elevations.
• C. It is independent of σ_y and σ_z.
• D. It describes lateral spread only.

Answer: (A)

The key idea is that the Gaussian plume describes how a pollutant spreads with distance and height, with a Gaussian profile in the crosswind direction. The centerline on the downwind axis is the line where the crosswind distance is zero (y = 0). Along this line, the crosswind Gaussian factor is at its maximum (since the exponent is zero), so the concentration is the highest for that downwind distance and height. In other words, at a given height z, moving directly downwind from the source traces the peak concentration path. This centerline is not an average over elevations, and it depends on the dispersion parameters σ_y and σ_z, which control how wide the plume is crosswind and vertically. It also represents the peak along the crosswind plane, not just the lateral spread, since it follows the path of maximum concentration directly downwind.

The key idea is that the Gaussian plume describes how a pollutant spreads with distance and height, with a Gaussian profile in the crosswind direction. The centerline on the downwind axis is the line where the crosswind distance is zero (y = 0). Along this line, the crosswind Gaussian factor is at its maximum (since the exponent is zero), so the concentration is the highest for that downwind distance and height. In other words, at a given height z, moving directly downwind from the source traces the peak concentration path.

This centerline is not an average over elevations, and it depends on the dispersion parameters σ_y and σ_z, which control how wide the plume is crosswind and vertically. It also represents the peak along the crosswind plane, not just the lateral spread, since it follows the path of maximum concentration directly downwind.