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Understanding Ozone

Structure of Ozone
Structure of Ozone. (Click for a larger image.)

Ozone was first discovered in 1839 by German scientist Christian Friedrich Schonbein. It is a pale blue, relatively unstable molecule made up of three oxygen atoms. The ozone molecule is angular, polar, and diamagnetic. Both oxygen bond lengths (1.28 angstroms) are identical. It is formed from molecular oxygen (O2) by ultraviolet and extreme ultraviolet photolysis followed by recombination of atomic oxygen (O) with O2.

It may also be formed by passing an electrical discharge through gaseous oxygen. It is characterized by a unique odor that is often noticed during electrical storms and in the vicinity of electrical equipment. In fact, the term ozone is derived from the Greek word ozein which means "to smell." The density of ozone is about 2.5 times that of O2. At -112°C it condenses to a deep blue liquid. It is a powerful oxidizing agent and, as a concentrated gas or a liquid, is highly explosive.

Excess oxygen atoms, also known as free radicals, oxidize materials that they contact and are associated with the aging process.

Destructive Ozone

Depending on where ozone resides, it can protect or harm life on Earth. When it is close to the planet's surface, in the air we breathe, ozone is a harmful pollutant that causes damage to lung tissue and plants, and is considered to be "bad ozone." It is a powerful photochemical oxidant that damages rubber, plastic, and all plant and animal life. It also reacts with hydrocarbons from automobile exhaust and evaporated gasoline to form secondary organic pollutants such as aldehydes and ketones. The peroxyacyl nitrates are especially damaging photochemical oxidants that are very irritating to the eyes and throat.

Ozone pollution originating in urban areas can extend into surrounding rural and forested areas that are hundreds of kilometers downwind. Episodes of elevated ozone concentrations are associated with warm, slow moving high pressure systems and contain between 30 and 50 parts of ozone per billion by volume. Concentrations 3 to 8 times greater than natural background levels have been observed. During the summer heat wave of 1988, record ozone concentrations were recorded in the United States. Even Acadia National Park in Maine and the Shenandoah mountains of Virginia were affected by dangerous levels of ozone pollution. These rural areas are far removed from industrial regions and polluted cities. The ozone pollution recorded in Acadia most likely originated in New York City. That in Virginia may have migrated from refineries on the Gulf Coast.

Photochemical oxidants are the most significant cause of agricultural loss in the United States. Their damaging effects on vegetation and crops have been confirmed in the eastern United States, adjacent areas in Canada, and much of Europe. Ozone alone, or in combination with sulfur dioxide (SO2) and nitrogen dioxide (NO2), accounts for 90% of the annual crop losses in the U.S. that are caused by air pollution.

Protective Ozone

Ozone Distribution in the Stratosphere
Ozone Distribution in the Statosphere. (Click for a larger image.)
Most ozone is concentrated in the stratosphere, at about 25 km in altitude, and is considered to be "good ozone." In this region, ozone acts as a shield to protect Earth's surface by absorbing harmful ultraviolet radiation. Without this shield, we would be more susceptible to skin cancer, cataracts, and impaired immune systems. A 1% decrease in total column ozone causes the amount of transmitted UV radiation, in the spectral region that damages deoxyribonucleic acid (DNA), to increase by about 2%. Although good ozone only represents a tiny fraction of the atmosphere, it is crucial for life on Earth.

The proportion of good and bad ozone in the atmosphere depends on the balance between processes that create ozone and those that destroy it. An upset in this balance can have serious consequences for life on Earth, and scientists are finding evidence that the balance has changed. Concentrations within the protective ozone shield are decreasing, while levels in the air we breathe are increasing.

Definition of Dobson Units
Definition of Dobson Units. (Click for a larger image.)

Ozone amounts in the stratosphere are small, rarely exceeding 10 parts per million by volume. Ozone is measured in Dobson Units. One Dobson Unit (DU) corresponds to 2.69 x 1016 molecules per square centimeter, which is equivalent to the amount of gas in one square centimeter at 1 atmosphere of pressure. Average ozone levels are 300 DU, which would be equivalent to a layer three millimeters thick if compressed to the planet's surface. Levels may range from less than 100 DU to over 500 DU globally.

Text, images and videos courtesy of Distributed Active Archive Center at NASA's Goddard Space Flight Center.

Related to chapter 4 in the print guide.
Related Materials

Learn more about ozone through the Graphic Stratospheric Ozone activity.

See the Layers of the Atmosphere to learn more about protective ozone.

Glossary Terms

Click for the definitions of the following words which are used on this page:(Definitions appear in a pop-up window.)

Dobson units
ultraviolet radiation

View the full, printable version of the glossary.

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