Up to this point we have viewed the atmosphere in very general terms by naming and categorizing some of the various substances contained in the atmosphere. However, the atmosphere is not simply a large reservoir which merely stores these substances. Rather, the atmosphere is more like a large laboratory beaker filled with many different reactive chemicals. While it is not in the scope of this course to analyze in depth the many chemicals and reactions we find there, we will discuss a few of those substances that are central to the air quality issues we are currently facing in our communities, nation, and world.

Two terms to be aware of before we talk specifically about some of the substances in our atmosphere are aerosol and pollutant. Aerosol refers only to those liquid and solid particles (with the exception of water vapor and ice) that are suspended in the air such as pollen, dust, and smoke particles. The term pollutant, however, refers to any substance (solid, liquid, or gas) that contaminates the atmosphere and has the potential to produce adverse health effects on humans and other animals, damage plant life, or cause damage to physical structures. Some examples of pollutants include nitrogen oxides and sulfur dioxide.

As was stated in the first section of this session, the bulk of the atmosphere originated and is maintained by natural sources. Those naturally produced components are considered to be the pure atmosphere. While not all, many of the air quality issues we are currently faced with have arisen from increased concentrations of those gases and aerosols that have an anthropogenic source. Those by-products of human activities contaminate the pure atmosphere. By monitoring the concentrations of impurities in the atmosphere, knowing how they react with other gases and particles present, and modeling those emissions and reactions to predict future concentrations, legislation can be introduced to limit what we release into the air.

The term NO_xrefers to nitric oxide (NO) and nitrogen dioxide (NO₂). Nitrogen oxides have both natural and anthropogenic sources. The major anthropogenic source of nitric oxide is the high temperature combustion of fuel in automobile engines and power plants. Larger quantities of nitric oxide are released along with much smaller quantities of nitrogen dioxide as a result of a reaction between nitrogen and oxygen. This reaction is caused by the high temperatures. NO and NO₂ also occur naturally in the atmosphere as a result of bacterial action. Concentrations in urban areas range from between 10 -100 times the concentrations in non urban areas. High concentrations of nitrogen oxides can result in respiratory problems, lowering the body's resistance to infections, as well as participate in the production of photochemical smog (Ahrens, 1991).

Sulfur dioxide is another pollutant with both natural and anthropogenic origins. Sulfur dioxide is emitted into the atmosphere when coal, oil, and other sulfur containing fuels are burned. Major sources of sulfur dioxide are petroleum refineries, power plants, paper mills and smelters. Volcanic activity also releases around 10⁹ kg of sulfur per year in the form of SO₂. Sulfur dioxide can oxidize to form sulfur trioxide (SO₃) and can react with moisture in the air to form sulfuric acid (H₂SO₄). Sulfuric acid in the atmosphere can also cause respiratory problems such as bronchitis and emphysema. Sulfuric acid is also deposited in the form of acid rain which can damage plant life, destroy man-made monuments and structures, and devalue personal property.

VOC is a general term for a class of organic compounds primarily made up of hydrocarbons. Hydrocarbons contain not only carbon, as all organic compounds do, but also, hydrogen. Some examples of VOCs are benzene, formaldehyde, chlorofluorocarbons (CFC's), and methane. Methane, as a pollutant, is becoming more important due to increased concentrations in the Southeastern United States. The major source of methane in that region is livestock. Methane is one of several greenhouse gases. We will discuss the effects of greenhouse gases later in this section. With the exception of methane, the primary sources of VOCs are industry, vehicle emissions, refrigerants, and cleansers. Many VOCs are not primary pollutants and are not harmful in and of themselves. However, many VOCs do react with other chemicals to produce secondary pollutants such as ozone.

Ozone can be found in both the stratosphere and lower troposphere. In the statosphere ozone occurs naturally. In the lower troposphere however, ozone is does not occur naturally and is a pollutant. We do not release ozone into the air directly, rather, in the lower atmosphere ozone is a product of various chemical reactions involving several of the pollutants that we do emit directly into the air such as nitrogen oxides and VOCs. Ozone is an example of a secondary pollutant.

In the lower atmosphere ozone is the primary component of photochemical smog. Smog greatly reduces visibility which creates a hazard for pilots and hinders the enjoyment of viewing some of the natural beauty in our national and state parks. Not only does smog reduce visibility, which is reason enough for concern but ozone, even in small quantities can cause health problems in people and animals as well as pose a threat to vegetation. Exposure to small concentrations of ozone can result in nausea, coughing, discomfort in breathing, and pulmonary congestion. It also retards growth in vegetation and even causes serious and extensive damage to crops (Ahrens, 1994).

As stated above, aerosols, also referred to as particulate matter, include any solid or liquid particles (with the exception of water and ice) suspended in the air such as metals, dust, smoke, and tiny droplets of sulfuric acid. One major concern in terms of air quality is what air quality professionals refer to as PM-10. PM-10 stands for particulate matter with a diameter of 10 micrometers or less. Aerosols this small can easily travel deep into the lungs causing respiratory problems. A classification of aerosols even smaller than PM-10 is PM-2.5. Many aerosols that form from gases fit into this category. The immediate most obvious effect of increased concentrations of aerosols is reduced visibility. Because the term aerosol is so general, their sources are numerous and can range from automobile and industry emissions, to wind blown dirt and dust stirred up from agricultural activities, to sea spray, as well as many others.

The greenhouse gases refer to those gases in the atmosphere (most of which occur naturally) that absorb long wave radiation emitted from the earth and re-emit much of it back to the surface. This results in a warming of the atmosphere near the surface of the earth. This effect is known as the greenhouse effect. Greenhouse gases include carbon dioxide (CO₂), Methane (CH₄), Nitrous Oxide (N₂O), Ozone (O₃), Chlorofluorocarbons (CFC's), and water in the form of vapor and droplets (H₂O). Though most of these are from natural sources, human activities on earth can greatly affect the natural production of these gases, as well as, serve as a source of some of them. Three greenhouse gases with which we are most concerned in terms of air quality are CO₂, O₃, and CFC's.

Carbon dioxide levels are rising rapidly due to the burning of fossil fuels and the deforestation of the planet. In fact, from 1956 to 1992 the concentration of CO₂ increased as much as 10%. Some researchers fear that increased amounts of CO₂ will increase the greenhouse effect and cause the air temperature near the earth's surface to rise further. Global warming would result in potentially damaging climatic changes across the globe.

Ozone is also a greenhouse gas and, as discussed above, is increasing in and polluting the lower atmosphere. At the same time, ozone in the upper atmosphere, which is not a pollutant and actually protects us from the sun's harmful ultraviolet radiation, is being destroyed through chemical reactions with other greenhouse gases, the CFC's. Depletion of ozone by CFC's will result in our increased exposure to ultraviolet radiation which can cause skin cancer. CFC's are volatile organic compounds (VOCs) and just happen to been ones which have resulted totally from human activities. CFC's have been used in refrigeration coolants, aerosol repellents, and cleansers. CFC's are released at the earth's surface and slowly make their way up into the stratosphere where a single CFC molecule can reside for 100 years and destroy up to 100,000 ozone molecules. Since their destructive effects have been discovered, the use of many CFC's has been banned in these products.

At EPA's web site, you can view a table of the National Ambient Air Quality Standards (NAAQS), which includes many of the pollutants we have discussed in this session.

On to Part 2: The Vertical Structure of the Atmosphere

Back to The Variable Atmosphere

Developed by
The Shodor Education Foundation, Inc.
Copyright © 1996