Air Pollution Meteorology
Gases released into the atmosphere by natural and anthropogenic processes, play significant roles in various aspects of climate on small, large, and even planetary scales. In this lecture, we will discuss some of the principles associated with the branch of meteorology called Air Pollution Meteorology.
Different scales of air pollution effects
There are a countless variety of different pollutants that are released into the atmosphere. There effects on the atmosphere will depend largely on their specific chemical properties as well as their residence times. The residence time of a gas refers to how long it remains chemically active before reacting or being absorbed by another substance. Gases with high residence times are referred to as long-term gases while gases with short residence times are referred to as short-term gases.
Long-term gases can remain in the atmosphere for decades and can slowly cause significant and noticeable climate changes. Short-term gases propagate and typically will cause effects over shorter distance scales.
Pollutants come from a variety of different sources and each have different effects on the atmosphere. For instance, automobile engines create a large amount of pollutants. Automobile engines burn fossil fuels in a process called combustion.
Combustion
Combustion refers to the reaction of hydrocarbons with oxygen to produce heat, water, and carbon dioxide. A hydrocarbon is any of a number of molecules containing carbon and hydrogen. An example of a hydrocarbon is ethanol, which is beverage alcohol. Fossil fuels are hydrocarbons. When combusted in a piston cylinder by sparks from the spark plug, oxygen, water, and heat are generated, which drives the engine. The mechanical energy generated in this process drives the automobile.
Unfortunately however, fossil fuels are rarely purely hydrocarbons, in other words, hydrogen and carbon are typically not the only atoms and molecules attached to fossil fuel molecules. Sulfur and nitrogen containing molecules are often present. Also, consider that our atmosphere is not composed exclusively of oxygen. There is 77 % nitrogen. Therefore, the fossil fuel is not being combusted in pure oxygen, but is being combusted in a mixture of nitrogen and oxygen. Therefore, instead of exlusively having oxygen and water as by-products of combustion, nitrogen containing chemicals are often generated as well, which are considered air pollutants.
Even pure combustion generates carbon dioxide, which itself is considered an air pollutant as it is believed to be linked to global warming due to the Greenhouse Effect. Also, fossil fuels often contain sulfur and during combustion, this sulfur often fixates nitrogen or oxygen molecules resulting in sulfur-containing air pollutants.
A lot of the research surrounding diminishing the amount of pollution released in engine combustion centers on reducing the amount of nitrogen that is brought into the piston cylinder. Also, reducing the amount of sulfur in the pre-combusted fuel is often practiced. In addition, different types of fuels produce varying amounts of pollutants and the degree and classification of pollutants produced is often a function of combustion conditions (i.e. temperature, pressure, etc.). Therefore, manipulating the combustion conditions in the cylinder is another effective strategy in combating pollution production.
The octane number of a gasoline product is a measure of the purity of combustion that can be expected. Therefore, higher octane gasoline is generally preferred in terms of reducing pollutant production, however, its commercial price tag is much higher.
Common pollutants
Some of the more common pollutants that affect our atmosphere are as follows:
Carbon monxide - CO
This is a major pollutant of city air; colorless, odorless, poison gas; forms during incomplete combustion of fuel; over 75 % in urban areas comes from road vehicles.
Over 100 million tons of CO (the chemical formula for carbon monoxide) are spewed into the air annually in the United States alone.
Carbon monoxide is so dangerous because the human blood bonds it so much more readily than oxygen. Your brain requires oxygen to work. It obtains that oxygen from red blood cells, which themselves contain molecules called hemoglobin. Hemoglobin bonds oxygen and carries it through the bloodstream. Hemoglobin has an affinity to carbon monoxide 300 times higher than oxygen. That means it will bond carbon monoxide 300 times quicker than it will bond oxygen. Therefore, if you are in air containing carbon monoxide, the hemoglobin in your red blood cells will bond the carbon monoxide before the oxygen and your brain will shut down and you will die. This is called carbon monoxide poisoning. The concentrations of carbon monoxide in urban areas is very rarely high enough to cause death, however, it is often high enough to cause noticeable physiological symptoms such as headache, fatigue, and drowsiness.
Sulfur dioxide - SO2
This is a colorless gas that comes primarily from the burning of fossil fuels, which typically contain amounts of sulfur in them naturally. Sulfur dioxide easily transforms into other notorious sulfur-containing pollutants including sulfur trioxide (SO3) and sulfuric acid (H2SO4). Sulfuric acid plays a significant role in acid rain. Sulfuric acid will form from sulfur dioxide more easily in moist air with water vapor present.
High concentrations of sulfur dioxide cause serious respiratory problems such as bronchitis and emphysema.
Nitrogen oxides - NOx
This large class of gases is produced primarily during the combustion of fossil fuels at high temperatures. Nitrogen dioxide (NO2) and Nitrogen oxide (NO) are two examples of Nox variety gases. Nitrogen oxides react with a large number of other pollutants to form other sub-classes of pollutants. For instance, nitrogen oxide is a major player in the production of ozone (O3). Ozone, beneficial in the stratosphere, is a harmful pollutant when found in high concentrations at the surface.
Some nitrogen oxides also react with sunlight. Smog is created when sunlight interacts with certain nitrogen oxides to create a thick, brown, gas that can hover over an urban area for days in certain meteorological conditions.
Biologically, high concentrations of nitrogen oxides can lower the body's natural resistance to infection and can also cause a number of different respiratory ailments including emphysema.
Hydrocarbons
Hydrocarbons are released during incomplete combustion of fossil fuels. Suppose coal is being combusted in a furnace. Frequently, not all of the coal will be combusted and some will be carried by heat convection currents into the atmosphere. Hydrocarbons react with nitrogen oxides in the presence of sunlight to produce a variety of different types of smogs, all of which can cause respiratory ailments. Also, high concentrations of hydrocarbons in the air is believed to be carcinogenic.
Particulate matter
Particles can be released into the atmosphere by a number of different natural and anthropogenic processes. An example of a natural event that would lead to the release of tons of particles into the atmosphere would be a volcano.
Automobiles and industrial processes emit large quantities of particulate matter into the atmosphere. Particles collected in urban environments include iron, lead, copper, nickel, and carbon. Particle pollution has its most immediate influence on the human respiratory system. Lead particles, most of which enter the air from auto exhaust, are especially dangerous because they are absorbed into the body and accumulate in bone and soft tissues. High concentrations of lead in the human body can cause brain damage, convulsions, and death.
Smog
The word smog comes from the combination of the words smoke and fog. Typical fog however, often contains neither. Typical fog, the type that frequently occurs in Los Angeles, CA, is called photochemical smog. It is caused by a variety of pollutants that react with sunlight. The nitrogen oxides and ozone are heavily responsible for smog formation.
Influences of meteorology on air pollution
Weather conditions can either enhance or remove an air pollution situation. For instance, strong winds often help disperse pollutants and can help prevent concentrations of air pollutants from reaching dangerously high levels. On the other hand, in calm situations with no wind, air pollutant concentrations can reach dangerously high levels as there is no mechanism present to disperse and dilute the pollutatants.
Air pollution meteorologists represent the amount of a particular pollutant in the atmosphere in terms of its concentration. Concentration can be expressed by a number of different units, such as mass per unit volume (such as milligrams per cubic meter) or parts per million -PPM. If the concentration of ozone in the atmosphere was 9 PPM, that means that for every million molecules in the atmosphere, 9 are ozone molecules.
The stability of the atmosphere is also very important in determining the extent of an air pollution episode. In stable situations, remember that the heavier denser gases tend to nestle toward the surface and the density decreases uniformly with elevation. Whereas, during unstable conditions, the density profile (how density is changing with elevation) can be such that more dense fluids are on top of less dense fluids. Therefore, there will be vertical mixing of the air and that can disrupt an air pollution episode as the pollutants will be dispersed.
During stable conditions, the heavier pollutant molecules (which almost always have molecular weights higher than oxygen and nitrogen) nestle toward the surface and air pollutant episodes will be more dramatic.
Skew-T diagrams are very important tools in the arsenal of an air pollution meteorologist.
On Monday, we will discuss some long-term pollutants and discuss some of their implications on our climate. Some of the topics we will discuss include The Greenhouse Effect, The Ozone Hole, and Acid Rain.