Stack Emission Plume Behavior Explained

The question relates the environmental lapse rate ($\gamma_{env}$) to the dry adiabatic lapse rate ($\gamma_d$) to determine the type of plume observed from a stack. The condition given is $\gamma_{env} \lt \gamma_d$.

Atmospheric Stability Analysis

Atmospheric stability determines how pollutants disperse vertically. The relationship between the environmental lapse rate and the dry adiabatic lapse rate indicates stability:

• Unstable: $\gamma_{env} \gt \gamma_d$ (Strong vertical mixing)
• Neutral: $\gamma_{env} = \gamma_d$ (Moderate vertical mixing)
• Stable: $\gamma_{env} \lt \gamma_d$ (Weak vertical mixing)

The condition $\gamma_{env} \lt \gamma_d$ signifies a stable atmosphere. In such conditions, vertical movement is suppressed because the surrounding air is relatively buoyant compared to the rising plume parcels, or resists downward movement.

Plume Type Identification

In a stable atmosphere, vertical dispersion is limited. Pollutants emitted from the stack spread primarily in the horizontal direction, following the wind. This results in a plume that is narrow vertically and elongated horizontally, resembling a fan.

Therefore, when $\gamma_{env} \lt \gamma_d$, the observed plume type is Fanning.