Doubling wind speed in the Pasquill-Gifford puff model will reduce downwind concentration by exactly a factor of 2 only if what condition remains unchanged?

Doubling wind speed in the Pasquill-Gifford puff model will reduce downwind concentration by exactly a factor of 2 only if what condition remains unchanged?

• A. Atmospheric stability
• B. Wind direction
• C. Release rate
• D. Temperature

Answer: (B)

In the Pasquill-Gifford puff model, how concentration scales with wind speed depends on keeping the plume geometry fixed as it travels. If the wind speed is doubled but the wind continues blowing from the same direction, the puff moves down the same downwind line but faster, so it spends less time affecting the receptor on that line. Under these conditions, the downwind concentration can change in a predictable, roughly inverse way with wind speed, giving about a 2-fold decrease when speed doubles. If the wind direction changes, the receptor is no longer on the same downwind path. The plume may pass at a different angle, split, or miss the receptor altogether, so the concentration at that point won’t follow the simple 1/u scaling. The other factors—stability, release rate, and temperature—alter dispersion or mass in ways that don’t guarantee an exact 2:1 change with wind speed on their own, especially if the plume geometry relative to the receptor is not preserved. So, the precise 2-fold decrease with doubling wind speed requires the wind direction to remain unchanged, preserving the plume’s alignment with the receptor.

In the Pasquill-Gifford puff model, how concentration scales with wind speed depends on keeping the plume geometry fixed as it travels. If the wind speed is doubled but the wind continues blowing from the same direction, the puff moves down the same downwind line but faster, so it spends less time affecting the receptor on that line. Under these conditions, the downwind concentration can change in a predictable, roughly inverse way with wind speed, giving about a 2-fold decrease when speed doubles.

If the wind direction changes, the receptor is no longer on the same downwind path. The plume may pass at a different angle, split, or miss the receptor altogether, so the concentration at that point won’t follow the simple 1/u scaling. The other factors—stability, release rate, and temperature—alter dispersion or mass in ways that don’t guarantee an exact 2:1 change with wind speed on their own, especially if the plume geometry relative to the receptor is not preserved.

So, the precise 2-fold decrease with doubling wind speed requires the wind direction to remain unchanged, preserving the plume’s alignment with the receptor.