Atmospheric Pollutant Dispersion Dynamics

Escape the Atmosphere

In some situations, it is necessary to evacuate air contaminants, either because their uptake is impractical or because the investment is not economically justified. To carry out an evacuation of the gaseous atmosphere, it is necessary to understand certain characteristics:

1. The emitter source.
2. The behavior of the surrounding atmosphere.

Factors Determining Pollutant Dispersion

The factors determining the dispersion of pollutants are:

A) Weather Factors: Temperature and Wind

1. Temperature

This is the most important effect. When an air mass has a higher temperature than the surrounding air mass, it ascends due to lower density. As it rises, this mass expands, reducing its temperature at a rate of approximately -10°C per kilometer. As long as this gradient is negative, the air mass (and the contaminants within it) will continue to rise.

When the gradient becomes positive, due to changes in soil temperature or atmospheric conditions, the ascending air mass stagnates. This phenomenon is called temperature inversion. It prevents pollutants from dispersing, creating a sort of 'air cover' that traps the contaminant-laden air mass.

2. Wind

Speed and turbulence are the influential factors. Higher speeds and greater turbulence lead to greater dispersion of pollutants, and vice versa.

B) Emission Factors

Contaminants generally exit an emission source, typically a chimney. The mass of gases exiting the chimney before dispersing is called a plume.

The factors related to a chimney that ensure optimal dispersion of pollutants are:

1. Speed of the Emitted Current

If the emission speed is low relative to wind speed, the wind can drag pollutants down around the chimney. Conversely, high emission speeds help the plume reach a sufficient height to disperse pollutants effectively, avoiding turbulence near the chimney.

2. Temperature of the Gas

If the temperature of the emitted gases is higher than the atmospheric temperature, the pollutant gases will rise (buoyancy effect). This effect diminishes as the gases cool, reducing the temperature difference between the gases and the atmosphere.

3. Height of the Emission

The emission height is critical. It can help overcome natural barriers. Crucially, a greater chimney height allows the emitted gases to potentially cross thermal inversion layers. A serious problem occurs if the gases do not penetrate these layers, and the worst-case scenario is if the inversion negates the upward movement entirely.