Smokestack Plume Distribution Review

By Melanie A. Schlosser

Introduction

        When scientists and engineers talk about air pollution, they talk about it in terms of concentration. In other words, they don't talk about how much total "bad stuff" you breath in, they talk about how much "bad stuff" per volume of air you breath in. The EPA has set different concentration limits for each harmful substance in the air. Those substances that are most harmful are regulated to have the smallest concentration, and those that are not quite so bad are allowed to have higher concentrations in the air.
        Engineers have designed several ways to reduce the concentration of these harmful chemicals. One of the oldest ways is to build a tall smokestack that will allow the pollution to disperse and be carried away before reaching the people. The higher we can build the smokestack, the lower the pollutant concentration will be by the time it sinks back into the air we breath. Unfortunately, we can only build a smokestack so high, before it is structurally unsafe. Just a simple smokestack is not enough, and the regulations the EPA sets for pollution control include regulations for what is in the smokestack in addition to regulations on the air we finally breath at ground level.
        This is not to say that smokestacks are a bad idea, in fact it is often necessary for plants to use several pollution control methods in order to meet EPA regulations. However, this web page will not go into all of these methods. Instead it will describe how the plumes from smokestacks are affected by the wind and weather, and how this in turn will affect us.

Atmospheric pressure and wind movement

The earth's atmosphere is a constantly changing environment; the air is always expanding, compressing, transferring heat, and making a mess of an otherwise stable environment. Although many people do not realize it, the atmosphere also exerts pressure on everything underneath it. Therefore, the air close to the ground is at a higher pressure than the air up in the mountains. In addition to changing vertically, the pressure of the atmosphere also changes from one point on the earth to another. It is these changes in pressure that create wind.
The air is always moving from areas of higher pressure towards areas of lower pressure, and we have named this motion of air "wind". The speed of the wind is related to the steepness of the pressure change. When there is a small pressure change over a long distance, the winds will blow very slowly. When there is a large pressure change over a small distance, the winds will blow very quickly. In addition to this motion, the wind direction is also affected by the rotation of the earth. If the earth did not rotate, the wind would flow directly across the changing pressure. Due to the rotation of the earth, an angular thrust is added to the motion of the winds. This angular motion is called the Coriolis effect.

Mechanical turbulence

Turbulence is the addition of fluctuations in the wind velocity, as compared to the average wind velocity. Mechanical turbulence is caused by fact that the atmosphere is sheared as it moves. This shearing occurs because the air actually sticks to the ground (even though we may not feel it) due to friction. Therefore the wind velocity at the earth's surface is zero. As the mass of air moves across the earth, the air on top moves faster than the air on the bottom and falls over the slower air. This "tumbling" creates a swirling motion. The faster the average wind velocity, the more tumbling and swirling is created. This mechanical turbulence is an excellent way of dispersing atmospheric pollutants.

Thermal turbulence

When the earths surface is heated by the sun, it will also heat the air directly above it. Since hot air is less dense than cool air, this heated air will rise from the earths surface to a higher elevation. This movement forces a vertical rotation of the air because the cooler air sinks to the bottom as the warm air rises. In the evening, the opposite occurs. The cold ground cools the air that is above is, causing it to become more dense. This dense air will feel heavy and will sink even closer to the ground.

Stability

        Stability is defined as the atmospheres ability to enhance or resist vertical motion. The stability of the atmosphere is affected by the wind speed and by the lapse rate (the change in air temperature with height) of the atmosphere. The atmosphere is classified as either stable, neutral, or unstable. If vertical motion is inhibited, the atmosphere is stable. If vertical motion is enhanced, the atmosphere it unstable. If vertical motion is neither enhanced or inhibited, the atmosphere is neutral.
        The atmosphere has a neutral stability when its lapse rate is equal to the adiabatic lapse rate of air, which is about -1.00 degree C per 100 meters. In other words, the temperature will drop by 1 degree C for every 100 meters we go up into the air.
        The atmosphere has a stable stability when its lapse rate is more than the dry adiabatic lapse rate of air. For example, a lapse rate of -0.6 degree C per 100 meters is stable (remember, since we are talking about negative numbers -0.6 is actually greater than -1.0).
        The atmosphere has an unstable stability when its lapse rate is less than the dry adiabatic lapse rate of air. For example, a lapse rate of -1.5 degree C per 100 meters is unstable.

But what does this have to do with air pollution?