Ozone

Ozone is an allotrope (a physically or chemically different form of the same substance) of oxygen with the chemical formula O₃. This formula shows that each molecule of ozone consists of three atoms. By comparison, normal atmospheric oxygen—also known as dioxygen—consists of two atoms per molecule and has the chemical formula O₂.

Ozone is a bluish gas with a sharp odor that decomposes readily to produce dioxygen. Its normal boiling point is −112°C (−170°F), and its freezing point is −192°C (−314°F). It is more soluble in water than dioxygen and also much more reactive. Ozone occurs in the lower atmosphere in very low concentrations, but it is present in significantly higher concentrations in the upper atmosphere. The reason for this difference is that energy from the Sun causes the decomposition of oxygen molecules in the upper atmosphere:

O₂ → (solar energy) → 2O

The nascent (single-atom) oxygen formed is very reactive. It may combine with other molecules of dioxygen to form ozone:

O + O₂ → O₃

Words to Know

Allotropes: Forms of a chemical element with different physical and chemical properties.

Chlorofluorocarbons (CFCs): A family of chemical compounds consisting of carbon, fluorine, and chlorine.

Dioxygen: The name sometimes used for ordinary atmospheric oxygen with the chemical formula O₂.

Electromagnetic radiation: A form of energy carried by waves.

Nascent oxygen: Oxygen that consists of molecules made of a single oxygen atom, O.

Ozone hole: A term invented to describe a region of very low ozone concentration above the Antarctic that appears and disappears with each austral (Southern Hemisphere) summer.

Ozone layer: A region of the stratosphere in which the concentration of ozone is relatively high.

Radiation: Energy transmitted in the form of electromagnetic waves or subatomic particles.

Standard (pollution): The highest level of a harmful substance that can be present without a serious possibility of damaging plant or animal life.

Stratosphere: The region of Earth's atmosphere ranging between about 15 and 50 kilometers (9 and 30 miles) above Earth's surface.

Troposphere: The lowest layer of Earth's atmosphere, ranging to an altitude of about 15 kilometers (9 miles) above Earth's surface.

Ultraviolet radiation: A form of electromagnetic radiation with wavelengths just less than those of visible light (4 to 400 nanometers, or billionths of a meter).

Ozone layer depletion

Most of the ozone in our atmosphere is concentrated in a region of the stratosphere between 15 and 30 kilometers (9 and 18 miles) above Earth's surface. The total amount of ozone in this band is actually relatively small. If it were all transported to Earth's surface, it would form a layer no more than 3 millimeters (about 0.1 inch) thick. Yet stratospheric ozone serves an invaluable function to life on Earth.

Radiation from the Sun that reaches Earth's outer atmosphere consists of a whole range of electromagnetic radiation: cosmic rays, gamma rays, ultraviolet radiation, infrared radiation, and visible light. Various forms of radiation can have both beneficial and harmful effects. Ultraviolet radiation, for example, is known to affect the growth of certain kinds of plants, to cause eye damage in animals, to disrupt the function of DNA (the genetic material in an organism), and to cause skin cancer in humans.

Fortunately for living things on Earth, ozone molecules absorb radiation in the ultraviolet region. Thus, the ozone layer in the stratosphere protects plants and animals on Earth's surface from most of these dangerous effects.

Human effects on the ozone layer. In 1984, scientists reported that the ozone layer above the Antarctic appeared to be thinning. In fact, the amount of ozone dropped to such a low level that the term "hole" was used to describe the condition. The hole was a circular area above the Antarctic in which ozone had virtually disappeared. In succeeding years, that hole reappeared with the onset of each summer season in the Antarctic (September through December).

The potential threat to humans (and other organisms) was obvious. Increased exposure to ultraviolet radiation because of a thinner ozone layer would almost certainly mean higher rates of skin cancer. Other medical problems were also possible.

At first, scientists disagreed as to the cause of the thinning ozone layer. Eventually, however, the evidence seemed to suggest that chemicals produced and made by humans might be causing the destruction of the ozone. In particular, a group of compounds known as the chlorofluorocarbons (CFCs) were suspected. These compounds had become widely popular in the 1970s and 1980s for a number of applications, including as chemicals used in refrigeration, as propellants in aerosol sprays, as blowing agents in the manufacture of plastic foams and insulation, as drycleaning fluids, and as cleaning agents for electronic components.

One reason for the popularity of the CFCs was their stability. They normally do not break down when used on Earth's surface. In the upper atmosphere, however, the situation changes. Evidence suggests that CFCs break down to release chlorine atoms which, in turn, attack and destroy ozone molecules:

CFC → solar energy → Cl atoms

Cl + O₃ → ClO + O₂

This process is especially troublesome because one of the products of the reaction, chlorine monoxide (ClO) reacts with other molecules of the same kind to generate more chlorine atoms:

ClO + ClO → Cl + Cl + O₂

Once CFCs get into the stratosphere and break down, therefore, a continuous supply of chlorine atoms is assured. And those chlorine atoms destroy ozone molecules.

Scientists and nonscientists alike soon became concerned about the role of CFCs in the depletion of stratospheric ozone. A movement then developed to reduce and/or ban the use of these chemicals. In 1987, a conference sponsored by the United Nations Environment Programme resulted in the so-called Montreal Protocol. The Protocol set specific time limits for the phasing out of both the production and use of CFCs. Only three years later, concern had become so great that the Protocol deadlines were actually moved up. One hundred and sixty-five nations signed this agreement. Because of the Protocol, the United States, Australia, and other developed countries have completely phased out the production of CFCs. According to the Protocol, developing nations have until the year 2010 to complete their phase out.

Ozone in the troposphere

Ozone is a classic example of a chemical that is both helpful and harmful. In the stratosphere, of course, it is essential in protecting plants and animals on Earth's surface from damage by ultraviolet radiation. But in the lower regions of the atmosphere, near Earth's surface, the story is very different.

The primary source of ozone on Earth is the internal-combustion engine. Gases released from the tailpipe of a car or truck can be oxidized in the presence of sunlight to produce ozone. Ozone itself has harmful effects on both plants and animals. In humans and other animals, the gas irritates and damages membranes of the respiratory system and eyes. It can also induce asthma. Sensitive people are affected at concentrations that commonly occur on an average city street during rush-hour traffic.

Ozone exposure also brings on substantial damage to both agricultural and wild plants. Its primary effect is to produce a distinctive injury that reduces the area of foliage on which photosynthesis can occur. (Photosynthesis is a complicated process in which plants utilize light energy to form carbohydrates and release oxygen as a by-product.) Most plants are seriously injured by a two- to four-hour exposure to high levels of ozone. But long-term exposures to even low levels of the gas can cause decreases in growth. Large differences among plants exist, with tobacco, spinach, and conifer trees being especially sensitive.

Many nations, states, and cities have now set standards for maximum permissible concentrations of ozone in their air. At the present time in the United States, the standard is 120 ppb (parts per billion). That number had been raised from 80 ppb in 1979 because many urban areas could not meet the lower standard. Areas in which ozone pollution is most severe—such as Los Angeles, California—cannot meet even the higher standard. Measurements of 500 ppb for periods of one hour in Los Angeles are not uncommon.

Long-term problem

Despite a relatively rapid and effective international response to CFC emissions, the recovery of the ozone layer may take up to 50 years or more. This is because these chemicals are very persistent in the environment: CFCs already present will also be around for many decades. Moreover, there will continue to be substantial emissions of CFCs for years after their manufacture, and uses are banned because older CFC-containing equipment and products already in use continue to release these chemicals.

In studies released in late 2000, scientists said they were stunned by findings that up to 70 percent of the ozone layer over the North Pole has been lost and that the ozone hole over the South Pole grew to an expanse larger than North America. According to the National Oceanic and Atmospheric Administration (NOAA), the hole in the ozone layer over the South Pole expanded to a record 17.1 million square miles (44.3 million square kilometers).

Scientists blamed the record ozone holes on two main reasons: extreme cold and the continued use of bromine. Recent very cold winters in the two poles have slowed the recovery of the ozone layer. Cold air slows the dissipation and decay of CFCs, which allows them to destroy ozone faster. Bromine is a chemical cousin to chlorine and is used for some of the same purposes—fire fighting, infection control, and sanitation. NOAA believes bromine is 45 times more damaging to ozone in the atmosphere than chlorine. But bromine has not been regulated as strictly as chlorine because countries could not stand the loss of income if it were regulated more. Some scientists, however, believe governments will be under growing pressure in the coming years to limit the chemical.

[See also Greenhouse effect ]

Ozone

Ozone is a gas found in the atmosphere in very trace amounts. Depending on where it is located, ozone can be beneficial ("good ozone") or detrimental ("bad ozone"). On average, every ten million air molecules contains only about three molecules of ozone. Indeed, if all the ozone in the atmosphere were collected in a layer at Earth's surface, that layer would only have the thickness of three dimes. But despite its scarcity, ozone plays very significant roles in the atmosphere. In fact, ozone frequently "makes headlines" in the newspapers because its roles are of importance to humans and other life on Earth.

What Is Ozone?

Chemically, the ozone molecule consists of three atoms of oxygen arranged in the shape of a wide V. Its formula is O3 (the more familiar form of oxygen that one breathes has only two atoms of oxygen and a chemical formula of O2). Gaseous ozone is bluish in color and has a pungent, distinctive smell. In fact, the name ozone is derived from the Greek word ozein, meaning "to smell or reek." The smell of ozone can often be noticed near electrical transformers or nearby lightning strikes. It is formed in these instances when an electrical discharge breaks an oxygen molecule (O2) into free oxygen atoms (O), which then combine with O2 in the air to make O3. In addition to its roles in the atmosphere, ozone is a chemically reactive oxidizing agent that is used as an air purifier, a water sterilizer, and a bleaching agent.

Where Is Ozone Found in the Atmosphere?

Ozone is mainly found in the two regions of the atmosphere that are closest to the earth's surface. About 10 percent of the atmosphere's ozone is in the lowest-lying atmospheric region, the troposphere. This ozone is formed in a series of chemical reactions that involve the interaction of nitrogen oxides, volatile organic compounds, and sunlight. Most ozone (about 90%) resides in the next atmospheric layer, the stratosphere. The stratosphere begins between 8 and 18 kilometers (5 and 11 miles) above the earth's surface and extends up to about 50 kilometers (30 miles). The ozone in this region is commonly known as the ozone layer. Stratospheric ozone is formed when the sun's ultraviolet (UV) radiation breaks apart molecular oxygen (O2) to form O atoms, which then combine with O2 to make ozone. Note that this formation mechanism differs from the one mentioned above for ozone in the lower atmosphere.

What Roles Does Ozone Play in the Atmosphere and How Are Humans Affected?

The ozone molecules in the stratosphere and the troposphere are chemically identical. However, they have very different roles in the atmosphere and very different effects on humans and other living beings, depending on their location.

A useful statement summarizing ozone's different effects is that it is "good up high, bad nearby." In the upper atmosphere, stratospheric ozone plays a beneficial role by absorbing most of the sun's biologically damaging ultraviolet sunlight (called UV-B), allowing only a small amount to reach the earth's surface. The absorption of ultraviolet radiation by ozone creates a source of heat, which actually defines the stratosphere (a region in which the temperature rises as one goes to higher altitudes). Ozone thus plays a key role in the temperature structure of the earth's atmosphere. Without the filtering action of the ozone layer, more of the sun's UV-B radiation would penetrate the atmosphere and reach the earth's surface. Many experimental studies of plants and animals and clinical studies of humans have shown that excessive exposure to UV-B radiation has harmful effects. Serious long-term effects can include skin cancers and eye damage. The UV-absorbing role of stratospheric ozone is what lies behind the expression that ozone is "good up high."

In the troposphere, ozone comes into direct contact with life-forms. Although some amount of ozone is naturally present in the lower atmosphere, excessive amounts of this lower-atmospheric ozone are undesirable (or bad ozone). This is because ozone reacts strongly with other molecules, including molecules that make up the tissues of plants and animals. Several studies have documented the harmful effects of excessive ozone on crop production, forest growth, and human health. For example, people with asthma are particularly vulnerable to the adverse effects of ozone. Thus, ozone is "bad nearby."

What Are the Environmental Issues Associated with Ozone?

The dual role of ozone links it to two separate environmental issues often seen in the newspaper headlines. One issue relates to increases in ozone in the troposphere (the bad ozone mentioned above). Human activities that add nitrogen oxides and volatile organic compounds to that atmosphere, such as the fossil fuel burning associated with power-generating plants and vehicular exhaust, are contributing to the formation of larger amounts of ozone near the earth's surface. This ozone is a key component of photochemical smog, a familiar problem in the atmosphere of many cities around the world. Higher amounts of surface-level ozone are increasingly being observed in rural areas as well. Thus, the environmental issue is that human activities can lead to more of the bad ozone.

The second environmental issue relates to the loss of ozone in the stratosphere. Ground-based and satellite instruments have measured decreases in the amount of stratospheric ozone in our atmosphere, which is called ozone-layer depletion. The most extreme case occurs over some parts of Antarctica, where up to 60 percent of the total overhead amount of ozone (known as the column ozone) disappears during some periods of the Antarctic spring (September through November). This phenomenon, which has been occurring only since the early 1980s, is known as the Antarctic ozone hole. In the arctic polar regions, similar processes occur that have also led to significant chemical depletion of the column ozone during late winter and spring in many recent years. Arctic ozone loss from January through late March has been typically 20 to 25 percent, and shorter-period losses have been higher, depending on the meteorological conditions encountered in the Arctic stratosphere. Smaller, but nevertheless significant, stratospheric ozone decreases have been seen at other, more populated latitudes of the earth, away from the polar regions. Instruments on satellites and on the ground have detected higher amounts of UV-B radiation at the earth's surface below areas of depleted ozone.

What Human Activities Affect the Stratospheric Ozone Layer?

Initially, theories about the cause of ozone-layer depletion abounded. Many factors were suggested, from the sun to air motions to human activity. In the 1970s and 1980s, the scientific evidence showed conclusively that human-produced chemicals are responsible for the observed depletions of the ozone layer. The ozone-depleting compounds contain various combinations of carbon with the chemical elements chlorine, fluorine, bromine, and hydrogen (the halogen family in the periodic table of the elements). These are often described by the general term halocarbons. The compounds include chlorofluorocarbons (CFCs which are used as refrigerants, foam-blowing agents, electronics cleaners, and industrial solvents) as well as halons (which are used in fire extinguishers). The compounds are useful and benign in the troposphere, but when they eventually reach the stratosphere, they are broken apart by the sun's ultraviolet radiation. The chlorine and bromine atoms released from these compounds are responsible for the breakdown of stratospheric ozone. The ozone destruction cycles are catalytic, meaning that the chlorine or bromine atom enters the cycle, destroys ozone, and exits the cycle unscathed and therefore able to destroy another ozone molecule. In fact, an individual chlorine atom can destroy as many as 10,000 different ozone molecules before the chlorine atom is removed from the stratosphere by other reactions.

What Actions Have Been Taken to Protect the Ozone Layer?

Research on ozone depletion advanced very rapidly in the 1970s and 1980s, leading to the identification of CFCs and other halocarbons as the cause. Governments and industry acted quickly on the scientific information. Through a 1987 international agreement known as the Montréal Protocol on Substances That Deplete the Ozone Layer, governments decided to eventually discontinue production of CFCs (known in the United States by the industry trade name "Freons"), halons, and other halocarbons (except for a few special uses). Concurrently, industry developed more ozone-friendly substitutes for the CFCs and other ozone-depleting halocarbons. If nations adhere to international agreements, the ozone layer is expected to recover by the year 2050. The interaction of science in identifying the problem, technology in developing alternatives, and governments in devising new policies is thus an environmental "success story in the making." Indeed, the Montréal Protocol serves as a model for other environmental issues now facing the global community.

What Actions Have Been Taken to Reduce the Amount of Ozone at Ground Level?

Ozone pollution at the earth's surface is formed within the atmosphere by the interaction of sunlight with chemical precursor compounds (or starting ingredients): the nitrogen oxides (NOx) and volatile organic compounds (VOCs). In the United States, the efforts of the Environmental Protection Agency (EPA) to reduce ozone pollution are therefore focused on reducing the emissions of the precursor compounds. VOCs, a primary focus of many regulations, arise from the combustion of fossil fuel and from natural sources (emissions from forests). Increasingly, attention is turning to reducing the emissions of NOx compounds, which also arise from the combustion of fossil fuels. The use of cleaner fuels and more efficient vehicles has caused a reduction in the emission of ozone precursors in urban areas. This has led to a steady decline in the number and severity of episodes and violations of the one-hour ozone standard established by the U.S. Environmental Protection Agency (EPA) (which is 120 parts per billion or ppb, meaning that out of a billion air molecules, 120 are ozone). In 1999 there were thirty-two areas of the country that were in violation of the ozone standard, down from 101 just nine years earlier. Despite these improvements, ground-level ozone continues to be one of the most difficult pollutants to manage. An additional, more stringent ozone standard proposed by the EPA to protect public health, eighty ppb averaged over eight hours, was cleared in early 2001 for implementation in the United States. For comparison, Canada's standard is sixty-five ppb averaged over eight hours.

see also Air Pollution; Asthma; CFCs (Chlorofluorocarbons); Electric Power; Halon; MontrÉal Protocol; NO_x (Nitrogen Oxides); Smog; Vehicular Pollution; Ultraviolet Radiation; VOCs (Volatile Organic Compounds).

Bibliography

World Meteorological Organization. (2003). Scientific Assessment of Ozone Depletion: 2002. Global Ozone Research and Monitoring Project, Report No. 47. Geneva: World Meteorological Organization.

internet resources

University Corporation for Atmospheric Research. "Cycles of the Earth and Atmosphere—Module Review." Available from http://www.ucar.edu/learn/1.htm.

U.S. Environmental Protection Agency. "Automobiles and Ozone." Available from http://www.epa.gov/otaq/04-ozone.htm.

U.S. Environmental Protection Agency. "Ozone Depletion." Available from http://www.epa.gov/docs/ozone.

Christine A. Ennis

Ozone

Resources

Ozone (O₃) is a bluish gas that is relatively dense—1.6 times as heavy as air—and acts as a strong oxidant. It occurs naturally in relatively large concentrations in the stratosphere, a layer of the upper atmosphere higher than about 3.8-10.7 mi (8-17 km), depending on season and location. Ozone also occurs in the lower atmosphere (or troposphere), where it is by far the most damaging of the photochemical air pollutants. Where ozone is abundant it is a major contributor to oxidizing or photochemical smog. This condition develops in sunny places with large emissions of hydrocarbons and oxides of nitrogen from automobiles and industry, especially where atmospheric temperature inversions are common.

Photochemical air pollutants are secondary chemicals, which means they are not emitted from effluent sources, but are synthesized from primary emitted pollutants during complex photochemical reactions occurring in the presence of sunlight. In addition to ozone, important photochemical air pollutants include peroxy acetyl nitrate (PAN), hydrogen peroxide (H₂O₂), and aldehydes. These gases are the ingredients of oxidizing smogs that are harmful to people, vegetation, buildings and other structures.

Most countries have set standards for ground-level concentrations of ozone, with a goal of avoiding damage to vegetation and discomfort to humans. Until 1979, the standard for the maximum, one-hour average concentration of O₃ was 80 ppb (parts per billion) in the United States. Thereafter, the standard was raised to 120 ppb because the 80 ppb limit was so frequently exceeded. Large regions of the United States cannot meet the criterion of 120 ppb, especially in the southwestern states. In 1997, the Environmental Protection Agency (EPA) created a new eight-hour standard of 80 ppb to protect against longer exposure periods. Although contested, this standard was upheld by the Supreme Court in 2001.

In the vicinity of Los Angeles the maximum one-hour concentration of ozone can exceed 500 ppb, and it is typically greater than 100 ppb for at least 15 days per year. In other cities in North America, the annual maximum one-hour concentration is typically 150-250 ppb, and it is typically 90-180 ppb in London, England.

Humans and other animals are sensitive to ozone. This gas irritates and damages exposed membranes of the respiratory system and eyes. Ozone can also induce asthma. Sensitive people are affected at concentrations that commonly occur during oxidizing smogs. In the United States, ozone air pollution accounts for 10-20% of all summer respiratory related hospital admissions.

Ozone causes substantial damage to both agricultural and wild plants in many places, causing a distinctive, acute injury that reduces the photosynthetic area of foliage. Most plants are acutely injured by a two to four hour exposure to 200-300 ppb ozone, while longer-term exposures to about 100 ppb cause yield decreases, even in the absence of acute injuries. However, some species are relatively sensitive to ozone. In one laboratory experiment, tobacco was acutely injured by exposures to only 50-60 ppb for two to three hours, and spinach by 60-80 ppb for one to two hours. Sensitive species of conifers can be injured by 80 ppb over a 12-hour exposure.

An important field study conducted at various sites throughout the United States involved the exposure of crop plants to either ambient air at each site, or to a typical background ozone concentration of 25 ppb. Symptoms of acute ozone injuries were observed at all five of the study sites, although the damages were more frequent and severe in the southwest. On average, it was estimated that exposures to ambient ozone concentrations caused yield decreases of about 53-56% in lettuce, 14-17% in peanut, 10% in soybean, and 7% in turnip. Overall, it has been estimated that ozone causes crop losses equivalent to 2-4% of the potential yield in the United States, resulting in $3 billion in agricultural losses each year.

Trees can also be damaged by ozone, as has been well documented for conifer forests along the western slopes of the Sierra Nevada and San Bernardino Mountains of southern California. Ozone-polluted air is transported eastward from the vicinity of Los Angeles to the mountains, where forests are damaged. The most sensitive species of tree is ponderosa pine (Pinus ponderosa ), the naturally dominant species in these forests. Other species of conifers are less sensitive to ozone, and these replace the ponderosa pine when it is killed by the air pollution. The smog damage was first noticed during the 1950s, but the actual cause was not attributed to ozone until 1963. The ozone injuries to pine are diagnostic, characterized initially by a pale-green mottling of foliage, then a tissue death that spreads from the leaf tip, premature loss of foliage, and ultimately death of the tree. Ozone-stressed trees are also vulnerable to secondary damages caused by bark beetles and fungal pathogens, which often kill weakened trees.

KEY TERMS

Acute toxicity —A poisonous effect produced by a single, short-term exposure to a toxic chemical, resulting in obvious tissue damage, and even death of the organism.

Inversion —An atmospheric condition in which air temperature increases with increasing altitude, instead of the usual decrease. The occurrence of a temperature inversion causes stable atmospheric conditions beneath, which can result in an accumulation of air pollutants if emissions continue during the inversion event.

Photochemical —Refers to an enhancement of the rate of a chemical reaction by particular wavelengths of electromagnetic radiation.

Photochemical smog —Air pollution caused by complex reactions involving emitted chemicals, chemicals formed secondarily in the atmosphere, and sunlight.

Smog —An aerosol form of air pollution produced when moisture in the air combines and reacts with the products of fossil fuel combustion.

The actual mechanism by which plant damage occurs from ozone has recently been discovered. Ozone inhibits the opening of the stoma on leaves, which are the pores that allow carbon dioxide gas into the plant, and through which oxygen gas leaves. The stoma open and close by means of two guard cells, found on either side of the opening. Ozone directly affects the guard cells, inhibiting their ability to open the stoma. Current research is under way to genetically engineer plants with guard cells resistant to the effects of ozone pollution.

See also Ozone layer depletion.

Resources

BOOKS

Roshchina, Victoria V. and Valentina D. Roshchina. Ozone and Plant Cell. The Netherlands: Klewer Academic Publishers, 2003.

PERIODICALS

Environmental Protection Agency. “Ground-level Ozone (Smog) Information.” September 28th, 2006. <http://www.epa.gov/region1/airquality/index.html> (accessed October 25, 2006).

Bill Freedman

Ozone

Ozone (O3) is a bluish, relatively dense (1.6 times as heavy as air) gas and strong oxidant. Ozone occurs naturally in relatively large concentrations in the stratosphere, a layer of the upper atmosphere higher than about 3.8-10.7 mi (8-17 km), depending on season and location. Ozone also occurs in the lower atmosphere (or troposphere), where it is by far the most damaging of the photochemical air pollutants. Where these chemicals are abundant, they are known as an oxidizing or photochemical smog . This condition develops in sunny places where there are large emissions of hydrocarbons and oxides of nitrogen from automobiles and industry, especially where atmospheric temperature inversions are common.

Photochemical air pollutants are secondary chemicals, which means they themselves are not emitted, but are synthesized from primary emitted pollutants during complex photochemical reactions occurring in a sunny atmosphere. In addition to ozone, important photochemical air pollutants include peroxy acetyl nitrate (PAN), hydrogen peroxide (H2O2), and aldehydes . These secondary gases are the ingredients of oxidizing smogs that are harmful to people and vegetation.

Most countries have set standards for ground-level concentrations of ozone, with a goal of avoiding damage to vegetation and discomfort to humans. Until 1979, the standard for the maximum, one-hour average concentration of O3 was 80 ppb (parts per billion) in the United States. Thereafter, the standard was raised to 120 ppb because the 80 ppb limit was so frequently exceeded. In a practical sense, the original ozone standard could not be enforced, so it was increased. Large regions of the United States cannot meet the criterion of 120 ppb, especially in the southwestern states. In 1997, the Environmental Protection Agency (EPA) created a new eight-hour standard of 80 ppb to protect against longer exposure periods.

In the vicinity of Los Angeles the maximum one-hour concentration of ozone can exceed 500 ppb, and it is typically greater than 100 ppb for at least 15 days per year. In other cities in North America , the annual maximum one-hour concentration is typically 150-250 ppb, and it is typically 90-180 ppb in London, England.

Humans and other animals are sensitive to ozone. This gas irritates and damages exposed membranes of the respiratory system and eyes. Ozone can also induce asthma . Sensitive people are affected at concentrations that commonly occur during oxidizing smogs. In the United States, ozone air pollution accounts for 10-20% of all summer respiratory related hospital admissions.

Ozone causes substantial damage to both agricultural and wild plants in many places, causing a distinctive, acute injury that reduces the photosynthetic area of foliage. Most plants are acutely injured by a two to four hour exposure to 200-300 ppb ozone, while longer-term exposures to about 100 ppb cause yield decreases, even in the absence of acute injuries. However, some species are relatively sensitive to ozone. In one laboratory experiment, tobacco was acutely injured by exposures to only 50-60 ppb for two to three hours, and spinach by 60-80 ppb for one to two hours. Sensitive species of conifers can be injured by 80 ppb over a 12-hour exposure.

An important field study conducted at various sites throughout the United States involved the exposure of crop plants to either ambient air at each site, or to a typical "background" ozone concentration of 25 ppb. Symptoms of acute ozone injuries were observed at all five of the study sites, although the damages were more frequent and severe in the southwest. On average, it was estimated that exposures to ambient ozone concentrations caused yield decreases of about 53-56% in lettuce, 14-17% in peanut, 10% in soybean , and 7% in turnip. Overall, it has been estimated that ozone causes crop losses equivalent to 2-4% of the potential yield in the United States, resulting in $3 billion in agricultural losses each year.

Trees can also be damaged by ozone, as has been well documented for conifer forests along the western slopes of the Sierra Nevada and San Bernardino Mountains of southern California. In this case, ozone-polluted air is transported eastward from the vicinity of Los Angeles to the mountains, where forests are damaged. The most sensitive species of tree is ponderosa pine (Pinus ponderosa), the naturally dominant species in these forests. Other species of conifers are less sensitive to ozone, and these replace the ponderosa pine when it is killed by the air pollution . The smog damage was first noticed during the 1950s, but the actual cause was not attributed to ozone until 1963. The ozone injuries to pine are diagnostic, characterized initially by a pale-green mottling of foliage, then a tissue death that spreads from the leaf tip, premature loss of foliage, and ultimately death of the tree. Ozone-stressed trees are also vulnerable to secondary damages caused by bark beetles and fungal pathogens , which often kill weakened trees.

The actual mechanism by which plant damage occurs from ozone has recently been discovered. Ozone inhibits the opening of the stoma on leaves, which are the pores that allow carbon dioxide gas into the plant, and through which oxygen gas leaves. The stoma open and close by means of two guard cells, found on either side of the opening. Ozone directly affects the guard cells, inhibiting their ability to open the stoma. Current research is under way to genetically engineer plants with guard cells resistant to the effects of ozone pollution.

See also Ozone layer depletion.

Resources

books

Freedman, B. Environmental Ecology. 2nd ed. San Diego: Academic Press, 1994.

MacKenzie, J.J., and M.T. El-Ashry, eds. Air Pollution's Toll on Forests and Crops. Yale University Press, New Haven, CT: 1989.

Bill Freedman

KEY TERMS

Acute toxicity

—A poisonous effect produced by a single, short-term exposure to a toxic chemical, resulting in obvious tissue damage, and even death of the organism.

Inversion

—An atmospheric condition in which air temperature increases with increasing altitude, instead of the usual decrease. The occurrence of a temperature inversion causes stable atmospheric conditions beneath, which can result in an accumulation of air pollutants if emissions continue during the inversion event.

Photochemical

—Refers to an enhancement of the rate of a chemical reaction by particular wavelengths of electromagnetic radiation.

Photochemical smog

—Air pollution caused by complex reactions involving emitted chemicals, chemicals formed secondarily in the atmosphere, and sunlight.

Smog

—An aerosol form of air pollution produced when moisture in the air combines and reacts with the products of fossil fuel combustion.

Ozone

The name ozone comes from the Greek Ozon meaning smell. At atmospheric temperatures, ozone is a colorless gas with an odor similar to chlorine that can usually be detected at a level of about 0.01 parts per million.

High in the atmosphere, ozone plays an important protective role by diminishing the amount of potentially damaging ultraviolet radiation reaching Earth. In sufficient concentration, however, ozone is a poison that at lower atmospheric levels, is a pollutant that can be damaging to health. Ozone is also a strong oxidizing agent used in many industrial processes for bleaching and sterilization. Although ozone is often used in water treatment, the largest commercial application of ozone is in the production of pharmaceuticals, synthetic lubricants, and other commercially useful organic compounds.

In the atmosphere, ozone is formed predominantly by electric discharges (e.g., lightning ). In the laboratory, ozone can be extracted form a mixture of oxygen and ozone by fractionation.

Ozone can also be formed by ultraviolet light. Ultraviolet light is energetic, and when it strikes the atmosphere it can break down some oxygen molecules producing highly energized oxygen atoms (free radicals). These free radicals can then react with molecular oxygen to produce ozone. The absorption of energetic light radiation also triggers the decomposition of ozone. As a result, ozone is an unstable molecule that exists in a dynamic equilibrium of formation and destruction. Consequently, the protective ozone layer is also in dynamic equilibrium.

The area where ozone is formed at the fastest rate is in the atmosphere at a height of approximately 164,042 ft (50 km). At this height, the number of free radicals made by ultraviolet light and electric discharge is balanced by the concentration of diatomic oxygen, which is sufficiently high to ensure that reactive collisions occur.

The protective ozone layer is found in the upper reaches of the atmosphere (between 98,000–295,000 ft [30–90 km]) where it absorbs ultraviolet radiation that, in excess, can be harmful to biological organisms. The potential detrimental effects of increased exposure to ultraviolet light due to a lessening of atmospheric ozone are of great concern. Holes in the ozone layer, or a global breakdown of stratospheric ozone would lead to increasing doses of ultraviolet radiation at Earth's surface. Scientists fear that significant increases exposure to ultraviolet light will increase risks of cancer in animal skin, eyes, and immune systems. Studies have shown that high ultraviolet radiation doses can supply the needed energy for chemical reactions that produce highly reactive radicals that have the potential to damage DNA and other cell regulating chemicals and structures.

There are several atmospheric trace elements, including ozone, that are important in the regulation of the global climate . Although the atmosphere consists of mainly of nitrogen and oxygen, approximately one percent of Earth's atmosphere is made of small amounts of other gases. Trace gases include water vapor, carbon dioxide , nitrous oxide, methane, chlorofluorocarbons (CFCs), and ozone. Because the amount of trace gases in the atmosphere is small, human activities can significantly affect the proportions of atmospheric trace gases.

Chloroflourocarbons (CFCs) easily react with ozone, which has the effect of breaking down an already unstable molecule. Until recently, CFCs were commonly used in refrigeration and in aerosol propellants (a pressurized gas used to propel substances out of a container). After evidence indicating that the use of CFCs was tipping the ozone equilibrium toward overall ozone layer depletion , many industrialized countries opted to enforce restrictions on the use of CFCs. Consumer aerosol products in the United States have not used ozone-depleting substances such as CFCs since the late 1970s. Federal regulations, including the Clean Air Act and Environmental Protection Agency (EPA) regulations restrict the use of ozone-depleting substances.

Ozone played a critical role in the development of life on Earth. Once primitive plants evolved, oxygen started to accumulate in the atmosphere. Some of this oxygen was converted into ozone and the developing ozone layer gave needed protection from disruptively energetic ultraviolet radiation. As a consequence, complex organic molecules which would otherwise have been destroyed began to accumulate.

As well as being found high in the atmosphere, ozone can be found at ground level. At these locations it is regarded as a pollutant. Ozone at ground level can be manufactured as part of photochemical smog . This is brought about by the disassociation of oxides of nitrogen that produce oxygen free radicals. These free radicals can react with diatomic oxygen to produce ozone. Pollutant ozone can also be a by-product of the action of photocopiers and computer printers. Low level ozone is usually found at a concentration of less than 0.01 parts per million, whereas in photochemical smog, it can be encountered at levels as high as 0.5 parts per million. Levels of ozone exposure between 0.1 and 1 part per million cause headaches, burning eyes, and irritation to the respiratory passages in humans. Elderly people, asthma sufferers, and those exercising in photochemical smog suffer the greatest adverse effects.

Some plant species (e.g., the tobacco plant) are particularly sensitive to low-lying ozone. The presence of excessive ozone causes a characteristic spotting of the leaves. High ozone levels are also known to damage structural material such as rubber.

Replacing more dangerous chlorine gas, ozone is used in many waste treatment facilities to purify water. Ozone is responsible for disinfecting the water and the efficient removal of trace elements such as pesticides. Ozone kills bacteria and other small life forms and it reacts with organic compounds. During the process, the ozone is transformed to molecular oxygen.

See also Atmospheric pollution; Ozone layer and ozone hole dynamics