Which expression represents the maximum ensemble average concentration for a continuous passive plume from a point source at a given downwind distance?

Which expression represents the maximum ensemble average concentration for a continuous passive plume from a point source at a given downwind distance?

• A. The formula tplume = E/(pi σy σz u)
• B. The formula for plume rise
• C. The formula for ground-level concentration
• D. The formula for emission rate

Answer: (A)

In a steady Gaussian plume from a continuous point source, the concentration has a peak along the plume centerline (crosswind distance zero) and at ground level when the source is at or near the ground. The centerline expression includes two symmetric contributions: the direct plume and its reflection from the ground. At ground level with the source height effectively at the ground (H ≈ 0), these two contributions add, giving a maximum concentration of Cmax = E/(π u σy σz). Here E is the emission rate, u is the mean wind speed, and σy and σz are the dispersion parameters in the crosswind and vertical directions, which grow with downwind distance and thus reduce concentration as the plume spreads. So the maximum ensemble average concentration at a given downwind distance is the centerline, ground-level value, and it takes the form E divided by (π times u times σy times σz). The other concepts referenced—plume rise, a general ground-level formula, or the emission rate alone—do not capture this peak concentration distribution for a continuous plume.

In a steady Gaussian plume from a continuous point source, the concentration has a peak along the plume centerline (crosswind distance zero) and at ground level when the source is at or near the ground. The centerline expression includes two symmetric contributions: the direct plume and its reflection from the ground. At ground level with the source height effectively at the ground (H ≈ 0), these two contributions add, giving a maximum concentration of Cmax = E/(π u σy σz). Here E is the emission rate, u is the mean wind speed, and σy and σz are the dispersion parameters in the crosswind and vertical directions, which grow with downwind distance and thus reduce concentration as the plume spreads.

So the maximum ensemble average concentration at a given downwind distance is the centerline, ground-level value, and it takes the form E divided by (π times u times σy times σz). The other concepts referenced—plume rise, a general ground-level formula, or the emission rate alone—do not capture this peak concentration distribution for a continuous plume.