Air pollution meteorology primer
OK. Class is in session. This one is about air pollution meteorology. Why am I posting this? No reason. I just feel like explaining it. So I will.
The atmosphere is a storage and distribution medium for various gases, one of which, oxygen, we require for respiration (meaning that it is the terminal electron acceptor at the end of the cytochrome chain). Because of our requirement for oxygen we have an obligatory exposure to the air and everything in it. The average adult male breathes approximately 20,000 liters of air every day (roughly a cubic meter an hour). Quite a lot.
But we and the rest of nature often dirty the air with junk that is bad for our health. Our exposure to this bad stuff depends on how much we put into the air at a particular location, but it is also highly sensitive to meteorology. Even when the total amount and kinds of pollutants emitted are the same at two locations, pollutant concentrations (what we breathe) are often markedly different. The main reason is differences in meteorology.
Here's an analogy. Consider an individual using a toxic solvent, such as carbon tetrachloride, in a room. The concentration of carbon tet in the air will depend on the amount of ventilation and the size of the room. If the windows are open and a lot of fresh air is coming in, exposures will be different than if the room is closed up tight. In the atmosphere of a city this is called horizontal mixing, analogous to ventilation. Similarly, if you are in a tiny broom closet your exposure will be different than if you are in a huge auditorium. In a city, the "size of the room" is a form of "vertical mixing." The two determinants of dispersive capacity are horizontal and vertical mixing.
Horizontal mixing or ventilation, is largely a function of wind speed and direction. The greater the speed of the wind, the more clean air is mixed in with dirty air and the greater is the dilutional effect, hence the lower is the concentration of the pollutant. The speed and direction of wind are often extremely regular in particular locations. This regularity is one of the factors that allows emissions from plants or other sources of pollution to be modeled and points of maximum impact predicted.
It is more difficult to understand the vertical motion of the atmosphere, which corresponds in our analogy to “the size of the room.” If there is no vertical motion in the atmosphere, that is, no tendency for packets of air to move up or down, pollutants will tend to remain close to the levels at which they are emitted, which for many pollution sources is near the ground. The more vertical motion there is, the greater the volume of air through which mixing and dilution take place. Restricted vertical motion is characteristic of “small rooms”, whereas extensive vertical mixing is characteristic of a “large room”, in our analogy. The tendency for air to move vertically, called atmospheric stability, is determined by many factors, including season, local weather conditions, and time of day. While these are highly variable for any particular location, certain locations are much more likely than others to develop conditions of high atmospheric stability, leading to restricted vertical motions.
In the most extreme form of atmospheric stability, a temperature inversion, air does not move vertically at all, and pollutants can build up to high concentrations. Normally air becomes cooler as we go up. But sometimes this normal relationship is inverted (hence "temperature inversion"), with warm air lying above cooler air. Since relatively warmer air tends to rise, if warmer air is above cooler air, there will be no vertical motion (there are degrees of this, too, with cooler air above warmer air but not "enough cooler" to promote easy rising). The inversion layer prevents vertical movement of air within or through it and sits like a lid on pollution-laden air below (i.e., a cooler packet of pollution laden air has no tendency to rise through a relatively warmer layer of air). Gases and pollutants from automobiles, heating systems, and power plants become trapped under this warm inversion layer.
Some portions of the continental United States are more likely to develop such conditions than others. The southern California area is especially subject to them because of a naturally present semi-permanent high pressure system that produces a temperature inversion. In this natural setting, the LA transportation strategy of relying on internal combustion engines (a.k.a. cars) makes about as much sense as Death Valley relying on ice skates. It has been said that New York City, if it had meteorologic conditions as unfavorable as those of Los Angeles, would be uninhabitable. It is only because the dispersive capacity of the atmosphere in general is large that pollutant concentrations in the northeast do not build up to produce disastrous levels. Denver is another city with very unfavorable meteorology (and hence a bad air pollution problem). Warmer air from the coast rides up over the mountains and over lies cooler plains air in Denver, producing an instant temperature inversion. If you live in Denver or have seen the smog layer trapped under the inversion layer as you fly in, you know what I am talking about. In addition, extended periods of atmospheric stability ("stagnations") happen more frequently in certain seasons (e.g., late fall and early winter in northern climes).
So pollutant emission is one factor in air pollution, but the meteorlogical setting is another. The same amount of pollution in different locations has different impacts. I have skipped a lot of interesting details (thus driving real meteorologists crazy), but I'll quit here. Hope someone is still reading.
The atmosphere is a storage and distribution medium for various gases, one of which, oxygen, we require for respiration (meaning that it is the terminal electron acceptor at the end of the cytochrome chain). Because of our requirement for oxygen we have an obligatory exposure to the air and everything in it. The average adult male breathes approximately 20,000 liters of air every day (roughly a cubic meter an hour). Quite a lot.
But we and the rest of nature often dirty the air with junk that is bad for our health. Our exposure to this bad stuff depends on how much we put into the air at a particular location, but it is also highly sensitive to meteorology. Even when the total amount and kinds of pollutants emitted are the same at two locations, pollutant concentrations (what we breathe) are often markedly different. The main reason is differences in meteorology.
Here's an analogy. Consider an individual using a toxic solvent, such as carbon tetrachloride, in a room. The concentration of carbon tet in the air will depend on the amount of ventilation and the size of the room. If the windows are open and a lot of fresh air is coming in, exposures will be different than if the room is closed up tight. In the atmosphere of a city this is called horizontal mixing, analogous to ventilation. Similarly, if you are in a tiny broom closet your exposure will be different than if you are in a huge auditorium. In a city, the "size of the room" is a form of "vertical mixing." The two determinants of dispersive capacity are horizontal and vertical mixing.
Horizontal mixing or ventilation, is largely a function of wind speed and direction. The greater the speed of the wind, the more clean air is mixed in with dirty air and the greater is the dilutional effect, hence the lower is the concentration of the pollutant. The speed and direction of wind are often extremely regular in particular locations. This regularity is one of the factors that allows emissions from plants or other sources of pollution to be modeled and points of maximum impact predicted.
It is more difficult to understand the vertical motion of the atmosphere, which corresponds in our analogy to “the size of the room.” If there is no vertical motion in the atmosphere, that is, no tendency for packets of air to move up or down, pollutants will tend to remain close to the levels at which they are emitted, which for many pollution sources is near the ground. The more vertical motion there is, the greater the volume of air through which mixing and dilution take place. Restricted vertical motion is characteristic of “small rooms”, whereas extensive vertical mixing is characteristic of a “large room”, in our analogy. The tendency for air to move vertically, called atmospheric stability, is determined by many factors, including season, local weather conditions, and time of day. While these are highly variable for any particular location, certain locations are much more likely than others to develop conditions of high atmospheric stability, leading to restricted vertical motions.
In the most extreme form of atmospheric stability, a temperature inversion, air does not move vertically at all, and pollutants can build up to high concentrations. Normally air becomes cooler as we go up. But sometimes this normal relationship is inverted (hence "temperature inversion"), with warm air lying above cooler air. Since relatively warmer air tends to rise, if warmer air is above cooler air, there will be no vertical motion (there are degrees of this, too, with cooler air above warmer air but not "enough cooler" to promote easy rising). The inversion layer prevents vertical movement of air within or through it and sits like a lid on pollution-laden air below (i.e., a cooler packet of pollution laden air has no tendency to rise through a relatively warmer layer of air). Gases and pollutants from automobiles, heating systems, and power plants become trapped under this warm inversion layer.
Some portions of the continental United States are more likely to develop such conditions than others. The southern California area is especially subject to them because of a naturally present semi-permanent high pressure system that produces a temperature inversion. In this natural setting, the LA transportation strategy of relying on internal combustion engines (a.k.a. cars) makes about as much sense as Death Valley relying on ice skates. It has been said that New York City, if it had meteorologic conditions as unfavorable as those of Los Angeles, would be uninhabitable. It is only because the dispersive capacity of the atmosphere in general is large that pollutant concentrations in the northeast do not build up to produce disastrous levels. Denver is another city with very unfavorable meteorology (and hence a bad air pollution problem). Warmer air from the coast rides up over the mountains and over lies cooler plains air in Denver, producing an instant temperature inversion. If you live in Denver or have seen the smog layer trapped under the inversion layer as you fly in, you know what I am talking about. In addition, extended periods of atmospheric stability ("stagnations") happen more frequently in certain seasons (e.g., late fall and early winter in northern climes).
So pollutant emission is one factor in air pollution, but the meteorlogical setting is another. The same amount of pollution in different locations has different impacts. I have skipped a lot of interesting details (thus driving real meteorologists crazy), but I'll quit here. Hope someone is still reading.
posted by Revere at
6/09/2005 07:06:00 AM
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