Understanding Atmospheric Dispersion Plume Types

The question describes a specific atmospheric condition impacting how pollutants disperse from a source. We need to identify the plume type characterized by an inversion layer aloft and an unstable layer below.

Analyzing the Described Atmospheric Conditions

The key features mentioned are:

• An inversion layer aloft: This acts as a lid, preventing pollutants from dispersing upwards.
• An unstable layer below: This layer promotes rapid mixing, pushing pollutants downwards towards the ground.

Evaluating Plume Dispersion Types

Let's examine the given options:

• Coning: This plume type occurs under neutral atmospheric stability. Pollutants disperse moderately in both horizontal and vertical directions. It does not involve the specific layering described.
• Fanning: This occurs under stable atmospheric conditions, often with an inversion layer present, but typically limiting vertical dispersion both upwards and downwards, resulting in a horizontally elongated plume. This contrasts with the rapid downward mixing described.
• Trapping: This occurs when a plume is released within or below a stable layer (often an inversion), which caps vertical dispersion. While an inversion is involved, the specific scenario requires downward mixing due to instability *below* the plume, which is more specific to fumigation.
• Fumigation: This type specifically occurs when a plume exists in a stable layer, and surface heating causes the layer below the plume to become unstable. The inversion aloft prevents upward dispersion, while the unstable layer below mixes the pollutants rapidly downwards, leading to increased ground-level concentrations. This precisely matches the scenario described in the question.

Identifying the Correct Plume Type

The combination of an inversion layer aloft preventing upward spread and an unstable layer below causing rapid downward mixing is the defining characteristic of the Fumigation plume type.