How does this affect air pollution?

        The stability of the surrounding atmosphere can strongly affect the way the plume from a smokestack affects its surroundings. The best way to describe this is by example. Let us say we have a small cloud of polluted air, leaving a smokestack at 20 degrees C, and this polluted cloud moves at the adiabatic lapse rate. If the cloud is moved down 100 meters, it will now be 21 degrees C inside the cloud. If the cloud is moved up 100 meters, it will be 19 degrees C inside the cloud. Let us also say that the surrounding air is at 20 degrees C where the cloud leaves the smokestack.
        If the surrounding air is neutrally stable, the cloud will continue on its merry way after it is moved up or down. If the surrounding air is stable and has a lapse rate of -0.5C / 100m, then after dropping 100 meters, the surrounding air will be 20.5 degrees C. In this case the surrounding air is colder than the polluted air, and since we already know that hot air rises, this polluted air will be pushed up again. Conversely, if the polluted air is lifted 100 meters, the surrounding air will be 19.5 degrees C, but the polluted air will be 19 degrees C. Since colder air sinks, the polluted air will drop back down. In this manner, a stable atmosphere inhibits any vertical motion experienced by the polluted air.
        Sometimes there is no change in temperature with elevation and the atmospheric lapse rate is called isothermal. Sometimes the temperature of the air will actually increase with elevation, as opposed to a normal decrease with elevation. This situation is called an inversion and can cause severe pollution problems because the polluted air becomes trapped under the inversion layer and can not escape. Inversion layers are common where mountains surround an area of land, as in Los Angeles, Mexico City (Mexico), or Santiago (Chile).
        If the surrounding air is unstable and has a lapse rate of -1.5C / 100m, then after dropping 100 meters, the surrounding air will be 21.5 degrees C, but the polluted air will be 21 degrees C. In this case the surrounding air is hotter than the polluted air, and since we know cold air sinks, this polluted air will be pushed down further. Conversely, if the polluted air is lifted 100 meters, the surrounding air will be 18.5 degrees C, but the polluted air will be 19 degrees C. Since hot air rises, the polluted air will be pushed up further. In this manner, an unstable atmosphere enhances any vertical motion experienced by the polluted air.
        If we look at a special case where the land surrounding the smokestack is relatively flat, we can see how the stability of the atmosphere can affect the shape of the plume. The following pictures describe these phenomenon.

a) As you can see from the graph, the lapse rate of the surrounding air is more negative than the adiabatic lapse rate. This creates a very unstable atmosphere and any motion of the polluted air (caused by the wind) is accentuated. In this situation the pollution is mixed well within the atmosphere and is quickly dispersed, therefore reducing the concentration of pollutants that the people breathe.

b) As you can see from the graph, the lapse rate of the surrounding air is less negative than the adiabatic lapse rate. This situation is mildly stable. Pollutants from the smokestack are dispersed more slowly and remain in the air in a higher concentration.

c) As you can see from the graph, the lapse rate of the air is positive and is called an inversion. The plume from the smokestack is not allowed to move in either direction and the pollution is carried away barely reaching ground level. Although this may seem like a good situation, eventually the stability of the atmosphere will change and the pollution will affect those living downstream of the pollution source.

d) As you can see from the graph, the stability of the atmosphere switches within the elevation of the smokestack. Close to the ground the air is stable and will inhibit dispersion. Well above ground the air is unstable and will cause the pollutant to mix well within the air above. In this situation, the ground will receive very little of the pollution, it will remain in the upper portions of the atmosphere and will be dispersed above.

e) As you can see from the graph, we have another switch in the lapse rate of the atmosphere. This time the air us unstable close to the ground and stable above. In this scenario, the polluted air will be restricted from moving upward since the stable inversion acts as a barrier. The air will remain trapped, close to the ground, and will mix within this air. This situation is what causes some of the highest pollution levels we experience and describes what happens in Los Angeles, Mexico City, and Santiago.

The behavior of pollutants in the air are not only affected by the stability of the atmosphere. They are also affected by the direction the wind is coming from and the intensity at which it blows. Drafts caused by thermal and mechanical effects will blow the polluted air in that direction. All of these factors work together, and it is this motion that can be either stifled or accentuated by the stability of the air. It is important to remember that these are only simplified models of what happens in our atmosphere. They are not "the rule" nor do they perfectly describe what actually happens. They can only provide us with an idea of the actual situation and they are meant to help us understand the relationship between our actions and the actions of Mother Nature.

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