Background Information on Tropospheric Ozone

  • Photochemical Smog
  • What is Ozone?
  • Ozone Precursors and Their Sources
  • Effects of Weather on Ozone
  • Effects of Ozone
  • Controlling Pollution
  • Reducing Levels of Ozone
  • Photochemical Smog

    The term "smog" originally referred to heavy layers of smoke and fog over London. Today smog refers to air pollution caused primarily by industry and motor vehicles. A dangerous form of smog is photochemical smog, which often forms a murky brown haze. It is produced, through complex photochemical reactions, when two pollutants, hydrocarbons and nitrogen oxides, react in the presence of sunlight. Photochemical smog is characterized by high concentrations of ozone (O3) and peroxyacetyl nitrate (PAN). The concentration of ground level ozone is usually used as an indicator of the overall severity of smog.

    What is Ozone?

    Ozone (O3) is a gas which is continuously being formed and destroyed in the earth's atmosphere. It is highly reactive [much more reactive than Oxygen (O2)], participating in chemical reactions with many other gases in the atmosphere, and with both living and non-living materials. Like chlorine gas, it is a strong oxidant, with the ability to decompose organic compounds.

    Ozone is both beneficial and harmful. In the stratosphere, the ozone layer protects life on earth by filtering out harmful ultraviolet rays from the sun. In the troposphere (at ground level), ozone is a harmful pollutant which affects health, vegetation, and materials. Human activities have indirectly disrupted the natural cycle of ozone formation and destruction. These disruptions of the natural cycle of ozone are of major environmental concern. While some human activities have contributed to the depletion of stratospheric ozone, others have created a build-up of tropospheric ozone. Unfortunately, there is no way to transfer the excess ozone at the surface of the earth to the stratosphere. The focus of this curriculum is tropospheric ozone, the major component of photochemical smog.

    Ozone Precursors and Their Sources

    Ozone is not emitted into the air directly. Rather, it is an indirect product of our industrial society. Ozone is formed by a series of complex chemical reactions between hydrocarbons and nitrogen oxides (such as NO2), the two ozone precursors, in the presence of sunlight. Sunlight breaks a chemical bond in nitrogen oxides, forming nitric oxide (NO) and a single oxygen atom (O). This single oxygen atom chemically combines with a molecule of atmospheric oxygen (O2) to form Ozone (O3). Sunlight also breaks chemical bonds in hydrocarbons, forming "free radicals" (substances with an "extra" electron). These free radicals then combine with atmospheric oxygen to form a new free radical product. In turn, the oxygen-containing free radical reacts with nitric oxide so that it is re-oxidized to NO2, which then reacts with sunlight and continues to lead to the formation of more Ozone.

    Sources of hydrocarbons include forest fires, decomposition of organic matter, organic solvent evaporation, industrial processes, and engine exhausts. Sources of nitrogen oxides include internal combustion engines, and furnaces fueled by coal, oil or natural gas. Ozone precursors are emitted from millions of mobile and stationary sources ranging from motor vehicles, aircraft, boats, and forest fires, to power plants, dry cleaners, and gas stations.

    Effects of Weather on Ozone

    Specific weather conditions are required to convert ozone precursors into ozone. Strong sunlight, high temperatures and stable air masses provide ideal conditions for the formation of ozone. Sunlight is needed to initiate ozone formation, and high temperatures (above 90 degrees) increase the rates of the chemical reactions that form ozone. This helps explain why smog and ozone levels peak during the summer. The number of days with maximum temperature exceeding 90 degrees Fahrenheit is sometimes used to predict the number of days with high ozone levels. Fluctuations in an area's ozone level can be attributed partially to variations in regional weather conditions.

    Atmospheric circulation usually prevents smog from settling. However, stagnant air masses keep ozone from dissipating, creating the potential for serious smog conditions. Sometimes ozone and other pollutants are trapped near the ground by a temperature inversion; a stable layer of cold air trapped under a layer of warmer, less dense air. Rain often improves air quality by both "raining out" pollutants and cooling air temperatures.

    Often, ozone and ozone precursors produced in one area are transported by winds to another area. For example, pollutants formed in New York City may show up as elevated ozone levels in rural Maine. Geographical conditions, such as mountains, valleys, and bodies of water may also have an effect on the transport of pollutants. For example, mountains often block the transport of air so that stagnant air remains in the valley (as in the Los Angeles basin). In coastal areas, land and sea breezes may transport pollutants offshore during the afternoon (when the land is warmer than the sea) and onshore during the evening (when the sea is warmer than the land).

    Effects of Ozone

    Ozone is a powerful oxidant. Oxidants cause chemical reactions in which one substance is broken down (e.g., iron rusts, silver tarnishes, fabric colors fade) -- usually by the breaking of a chemical bond and the formation of a new bond containing oxygen. When ozone oxidizes another substance, one of the oxygen atoms in ozone chemically combines with that substance, and the ozone is converted to oxygen (O2).

    Ozone has widespread harmful effects on the health of humans and animals, vegetation, and materials.


    Health Effects

    Ozone, like many other oxidants, irritates the mucous membranes of the respiratory system, causing coughing, nausea, shortness of breath, pulmonary congestion, and impaired lung function. It aggravates chronic respiratory diseases, such as asthma and bronchitis, and can cause serious health problems for people in weakened health and the elderly (see Introduction). Peroxyacetyl nitrates (PAN) and other oxidants that accompany ozone are powerful eye irritants. Exposure for 6-7 hours or more reduces lung function significantly in healthy people during periods of even moderate exercise.



    Ozone damages the leaves of plants and trees. Some plant species, such as tobacco, spinach, tomatoes and pinto beans, are especially sensitive. Damaged plants develop necrotic patterns (brown specks that turn yellow) on the upper surfaces of their leaves. The necrotic patterns on the tobacco strain Bel-W3 may be used as a indicator of ozone levels. Annually, ozone causes millions of dollars in crop loss. It also causes premature leaf drop in trees and reduced growth rates.



    Ozone reacts easily with organic materials -- causing weakness, cracking, and other chemical changes. Ozone cracks stretched rubber, reduces the strength of textiles, causes fading in fabrics and dyes (cotton, acetate, nylon, polyester), and causes premature cracking in paint.

    Controlling Pollution

    Photochemical oxidants were first noticed in the early 1940s in Los Angeles. By 1960, photochemical smog had been identified as a national problem. In 1963 the first Clean Air Act authorized the Public Health Service to study air pollution and provided training to state and local agencies to control it. The Clean Air Act of 1970 strengthened this legislation by creating a partnership between state and federal governments. State and local governments became responsible for preventing and controlling their air pollution. The Environmental Protection Agency (EPA) was established to set national pollution regulations and standards.

    In 1979, the EPA established primary and secondary standards for pollutants called "National Ambient Air Quality Standards" (NAAQS). Primary standards protect human health while secondary standards protect crops, livestock, vegetation, and materials. The primary and secondary standards for ozone are 120 parts per billion (ppb). A state is out of compliance if it exceeds the standard more than one day per year over a three-year average. The Clean Air Act of 1990 requires states to submit "State Implementation Plans" outlining how they will comply with these standards.

    Reducing Levels of Ozone

    Reduction of ground-level ozone is a complicated problem with no easy solution. It will require cooperation between government, business and individuals. Methods for controlling industrial and automotive emissions include: conservation, use of alternative fuels, and development of new technologies. Conservation measures include a shift away from the use of automobiles and toward public transportation, carpooling, bicycling, and walking. Conserving energy (electricity, heating, air conditioning) also helps, since most of our energy comes from the burning of fossil fuels (oil, coal, natural gas) that contribute ozone-producing emissions. Alternative fuels include methanol, ethanol, natural gas (methane), and hydrogen. However, their use runs the risk of solving one problem by creating another. New technologies will make it possible to reduce fuel consumption and emissions. Through new technology, the sun could provide non-polluting solar power to heat homes, power cars, or cook meals. The most promising solution to ozone reduction could be a combination of technological, social, and economic changes.

    For additional information (about how ozone is formed; its effects on people, vegetation, and materials; and actions people can take to reduce levels of ozone), you may want to refer to the following video:
    Race to Save the Planet: Do We Really Want to Live This Way?.


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