1.7. Non-Idealities in Dispersion Models

There is a possible scenario that this plume will not disperse but it will come back. Right below next to the stack, so the reason for this is the same reason why particles get adsorbed with it is, the wind is here. The streamline of the wind flows around the stack when that happens there is a low pressure region that is created here right behind the stack because it’s low pressure if anything comes in this region, it gets sucked into this low pressure region and circulates there.

Why does this happen? It happens when you do not the material coming out of the stack does not have enough momentum to clear out of this region, so it happens only when the velocity of the stack gas is smaller than the wind. So what happens is as soon as it comes pushed in you get pushed to the side and it can get into this slow pressure zone and it can recirculate there. So when this happens the concentration of SO2 can increase and accumulate over a period of time, which means that for exposures of people standing here can be very high.

Stacktip downwash occurs when the plume of emissions from a stack does not disperse as expected due to the influence of wind. Typically, wind flows around the stack and creates a low-pressure area behind it. If the emissions from the stack (such as gases or particulates) do not have enough upward momentum compared to the wind speed, they can get pulled back into this low-pressure area and recirculate instead of rising away from the stack. This leads to higher concentrations of pollutants like SO2 in the area directly below the stack, which can be dangerous for people who are working or living nearby. To prevent this, the speed of emission gases must be at least 1.5 times greater than the wind speed at that site to ensure that the plume rises effectively and disperses.

Imagine you're at a party, and someone is trying to light a beautiful firework just as the wind picks up. Rather than shooting straight into the air, the wind is pushing the firework back down, causing it to sputter and stay low instead of soaring. This is similar to how emissions can get recirculated around a stack if their velocity isn't strong enough against the wind, generating a risk for those nearby.

The other type of artifacts is called the Building Downwash. This is more serious, this affects people away from the source, so there is a streamline. This dotted lines are streamlines, we see a big building in the pathway this streamline move around it and they create this regions around the building are called as wakes, the same as the stacktip downwash but this is huge stack is as a small the reason we say stack means many of these industry stacks are huge.

They are not like 20 centimeters, they are big several meters in diameter and all that, so at least at ground level. These buildings can be big, so these buildings the streamlines separation around buildings causes this low pressure regions called as the wake and now if there is a plume that is going here and this plume gets near the wake it can get sucked into this wake, so it can get pulled in and it will recirculate here.

Building downwash describes the effect buildings have on the dispersion of emissions from nearby stacks. When air flows around a large building, it creates an area of low pressure (known as a wake) behind the building. If the emissions plume from a nearby stack approaches this wake, it can be drawn into it and recirculated rather than dispersing into the atmosphere. This can lead to high concentrations of pollutants at ground level around the building, impacting people who live or work in the area. To manage this, stacks should be higher than the surrounding buildings to allow emissions to rise above the wake and disperse more effectively.

Think of a large rock in a stream of water; as water flows around it, it creates a swirling area behind the rock where the water becomes calm and stagnant. Similarly, when emissions are released near a tall building, they can be sucked into the calm air behind the building, resulting in a buildup of pollutants. To avoid this, it's like ensuring that we can create an unobstructed flow of water, or in this case, air, by raising the stack above the interference created by the building.

This can happen in a lot of building which are not planned. Can you give me an example of this? Stack on top of a building, which is not extending into the building. There a lot of examples you can go around see; Dispersion, stacktip downwash, building downwash, small sources.

Poor planning can lead to situations where emissions from exhaust stacks do not effectively disperse due to building downwash. For example, if a building has a chimney or stack that doesn't extend above its roofline, the emissions may not rise high enough to escape the low-pressure zones created by the building. This can lead to localized air quality issues, increasing the risk of exposure to harmful pollutants for those nearby. To avoid this, it's essential to keep the stack height sufficient to promote proper dispersion.

Imagine a child trying to blow bubbles with a small straw while standing under a low ceiling; the bubbles don't rise high and stay trapped in the room. In the same way, emissions need adequate height to interact with the atmosphere properly. If the stack is too short, it won’t allow the gases to rise and may result in poor air quality for people close by.

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