2. Transport of Pollutants – Dispersion

So, last class we were discussing the basics about transport of pollutants in air, the issues of stability. So, we looked at 3 different cases unstable, neutral and stable.

In this chunk, we start with the concept of how pollutants move in the air. There are three main conditions or states under which pollutants can disperse: unstable, neutral, and stable conditions. Understanding these conditions is crucial because they affect how pollutants spread in the environment. For instance, pollutants behave differently when the air is unstable due to high turbulence compared to when the air is stable.

Imagine a crowded room filled with people. If the room is warm (unstable), people move around a lot, creating a lot of chaos as they shift in all directions. However, if the room is cold (stable), people are less active and stay in their places, making it more orderly. Similarly, pollutants behave more erratically in unstable air compared to stable air.

The unstable condition occurs in both directions, above the MMH (Mean Mixing Height) it is greater and below MMH it is smaller, so, it keeps going away from the MMH.

Under unstable atmospheric conditions, pollutant dispersion is influenced by temperature variations and high mechanical turbulence. Above the Mean Mixing Height (MMH), pollutants can disperse more widely because the air is more chaotic and mixes well. Below the MMH, dispersion is limited because the air is more stable and doesn't allow pollutants to rise effectively.

Think of boiling water. When boiling (unstable), steam (pollutants) rises vigorously, spreading throughout the kitchen. When the pot cools down (stable), the steam becomes less active and disperses less, resembling how pollutants behave under different atmospheric conditions.

In stable conditions we have inversion. Here, Tadiabatic is less than environmental which means that under stable conditions we have something like this.

Stable conditions, particularly during temperature inversion, mean that the air temperature decreases as you go up to a certain point, but then starts to warm up again. This creates a cap that traps pollutants near the ground, preventing them from mixing with cleaner air above. As a result, pollutants can accumulate, leading to poor air quality.

Imagine a glass ceiling in a room. When the air gets hot inside (pollutants), it rises, but once it hits the ceiling (inversion), it can't go up anymore and spreads sideways, creating a stagnant environment full of trapped heat and bad air.

The significance of this particular graph is essentially if you look at the relationship between adiabatic and environments it does not matter if it is lower or higher, if it gets pushed it will go to the other direction whichever direction is favorable.

The mixing height represents the altitude at which pollutants can freely disperse into the atmosphere. Below this height, pollutants are constrained due to stability in the air. Understanding this is important because it defines how high pollutants can rise before being affected by other atmospheric conditions. Factors such as the time of day can change mixing heights, impacting air quality.

Consider a balloon filled with air. As long as the balloon is sheltered (below the mixing height), it stays in one place. However, if you take the balloon outside into a strong wind (unstable conditions), it can rise high and move around freely, demonstrating how atmospheric conditions can suddenly change the behavior of pollutants.

So, keeping this in mind, there are a few cases of plume behavior that people have classified and we will just go over that a little bit.

Pollutants emitted from a source create a plume, and how that plume behaves depends on atmospheric stability. Different behaviors include looping, coning, fanning, and fumigation. Each of these behaviors represents how pollutants will spread in the air based on factors like temperature gradients and wind patterns.

Think of smoke coming from a campfire. On a windy day (unstable), the smoke may swirl and rise (looping), while on a calm day (stable) it might spread out in a pattern (coning) or remain close to the ground (fanning), showing how wind and temperature affect smoke patterns just as they affect pollutants.

The first one is called as Looping, it is a super adiabatic lapse rate, which means that environmental lapse rate is greater than adiabatic.

Looping occurs in super adiabatic conditions where the environmental lapse rate exceeds the adiabatic lapse rate. This means that the air is very unstable, which allows the plume of pollutants to oscillate vertically as it mixes with the turbulent air. High wind speeds contribute to the exaggerated movements of the pollutants in various directions.

Imagine a child on a swing. When the swing is pumped high enough (super adiabatic conditions), the child's movements are exaggerated and chaotic. Similarly, pollutants in unstable air behave energetically, moving up and down much like that swing.

This is neutral, which means there are thermal forces don’t play a big part in this.

Coning occurs under neutral conditions, where there are no significant thermal effects influencing the plume. Here, the pollutants disperse primarily due to wind-dominated turbulence. The plume takes a conical shape because it progressively widens as it moves downwind due to the mixing with ambient air, but the vertical rise is minimal.

Picture a party balloon that gets bigger as you inflate it, but doesn’t go up much. The balloon stays flat at the base but expands outward. Similarly, the coning plume broadens horizontally at a consistent height, reflecting how neutral conditions affect dispersion.

The third condition is called Fanning... the z direction dispersion is very very small.

In the Fanning condition, pollutants spread primarily in the horizontal direction (y direction) with limited vertical rise (z direction). This happens when the environmental lapse rate intersects the adiabatic lapse rate just below the emission source, making the plume stay close to the mixing height while dispersing horizontally.

Think of a wave rolling on a beach. The wave spreads outward along the shore (y direction) but barely rises above the water's surface (z direction). Fanning behavior shows how pollutants can spread widely in the air without rising significantly, similar to waves in calm conditions.

This particular case where we have called as Fumigation... this is the most dangerous of all the cases.

Fumigation occurs when pollutants are released under conditions where a stable layer traps them close to the ground, causing them to concentrate in that area. This can happen when there is inversion, preventing pollutants from dispersing upwards. As a result, they can lead to significant health risks due to high concentration levels near the surface.

Imagine smoke filling a sealed room. The smoke can’t escape, and the concentration increases, creating an unsafe environment. Fumigation acts similarly, trapping airborne pollutants and leading to dangerous conditions for people nearby.

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