2.4. Plume Shapes and Behavior

So, we have different types of plume shapes that it can take. We will go over each one of them. The first one is called as Looping, it is a super adiabatic lapse rate, which means that environmental lapse rate is greater than adiabatic.

In this section, we're discussing different shapes that a pollutant plume can take as it rises from a source. The first shape discussed is called 'Looping'. This occurs when the environmental lapse rate, which describes how temperature changes with height, is greater than the adiabatic lapse rate. Essentially, this situation leads to very turbulent and unstable air conditions, causing the plume to rise and fall significantly, resembling a loop.

Imagine a hot air balloon in very windy conditions. As the balloon tries to rise, the strong winds push it up and down unpredictably, creating a thorough mix of the air around it. Similarly, when pollutants rise in unstable atmospheric conditions, they can be pushed up or down, creating a looping shape.

The second one is called as Coning. This is neutral, which means there are thermal forces don’t play a big part in this. The environmental lapse rate is almost the same as the adiabatic lapse rate approximately.

The 'Coning' plume shape occurs under neutral conditions where the environmental lapse rate closely matches the adiabatic lapse rate. This means that thermal effects are not significantly impacting the plume's behavior. In this scenario, the plume expands outward in the direction of the wind, creating a conical shape. It signifies a balanced dispersion of pollutants, as the vertical movement due to buoyancy is minimal.

Think of a fountain with water spraying directly upward and outward. The water represents pollutants spreading into the atmosphere. When the conditions are neutral, the water sprays evenly, forming a cone shape. The fountain's ability to rise and fall is restricted, similar to how pollutants disperse under neutral atmospheric conditions.

The third condition is called Fanning and now you see here that the adiabatic lapse rate and environmental lapse rate intersect but they intersect right below the source just below the source.

Fanning occurs when the adiabatic lapse rate and environmental lapse rate intersect just below the plume's source. Here, thermal buoyancy is suppressed due to the higher temperature of the surrounding air, causing the plume to remain close to the specified mixing height without significant vertical dispersion. The pollutants spread mainly in the horizontal direction, resulting in concentrated emissions that do not rise high into the atmosphere.

Picture a candle flame that’s being blown on by a gentle wind. The flame flickers but does not move much higher; instead, it spreads its light and heat more horizontally. This represents how pollutants behave in the fanning condition – they stay low and spread out, rather than rising significantly.

This particular case where we have called as Fumigation, where it is exactly the opposite. There is inversion there.

Fumigation occurs under conditions of atmospheric inversion when a stable layer of warm air traps cooler air (and the pollutants) below it. It is hazardous because pollutants that are released below this inversion layer remain confined close to the ground, leading to higher concentrations of pollutants in the air, especially in downwind areas. This phenomenon indicates the potential for harmful exposure to emissions.

Imagine a situation where you light a campfire in a valley on a cool evening. Initially, the smoke rises, but as the night cools, a warm layer of air forms above the valley, trapping smoke closer to the ground. The smoke then spreads horizontally across the valley, creating a concentrated cloud of smoke at ground level, just as pollutants do in fumigation situations.

The last one is called as Lofting in which there are two inversions.

Lofting occurs when two layers of warm air create an enclosure above the pollutant source, preventing the pollutants from rising lower than this zone. The pollutants spread upwards into the warmer air, where conditions may allow for dispersion. This results in a higher and more dispersed plume, which can lead to lower ground-level concentrations of pollutants.

Think of a balloon filled with helium trapped between two layers of plastic wrap. The balloon tries to rise but is caught between the two layers. It can still float upwards but remains confined within a certain volume, preventing it from dropping down. Similarly, lofting allows pollutants to rise but keeps them from moving downwards toward the earth's surface.

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