22 July 2009
Understanding Ozone
How governments cooperated to solve an imminent environmental crisis
Washington — In the 1980s, scientists discovered that human activity was depleting the protective layer of ozone in the Earth’s atmosphere. Governments around the world responded with an agreement to limit the production and use of nearly 100 substances that destroy ozone.
The Montreal Protocol agreement, ratified by 191 countries, helped cut production of ozone-depleting chemicals by more than 95 percent, from more than 1.8 million metric tons in 1987 to 83,000 metric tons at the end of 2005, according to the World Meteorological Organization. As a result, the ozone layer over most of the globe has not grown thinner since 1998 and is projected to return to pre-1980 levels between 2050 and 2075.
This is perhaps the first recorded case of governments around the world successfully cooperating to solve an imminent, global environmental crisis. (See “Montreal Protocol Could Be Model for Addressing Climate Change.”)
OZONE BASICS
• What is ozone?
Ozone is a naturally present gas in Earth’s atmosphere comprised of three oxygen atoms bound together (O3). Ozone is formed when ultraviolet light breaks apart an oxygen molecule (O2), producing two oxygen atoms (O). Each oxygen atom combines with an oxygen molecule to produce ozone (O + O2 = O3). Ozone production is balanced with its destruction by atmospheric gases such as bromine and chlorine.
• Where is ozone?
Ten percent of ozone is in the lower atmosphere (the troposphere), between the Earth’s surface and an altitude of between 10 and 16 kilometers. Ninety percent of ozone is in the stratosphere, from the top of the troposphere to an altitude of 50 kilometers.
The stratospheric ozone — what scientists refer to as the ozone layer — is not the same thickness all over the Earth. The ozone layer is thinnest over Antarctica, due to unique atmospheric conditions that concentrate ozone-destroying chemicals.
• What does ozone do?
Depending on its location in the atmosphere, ozone can help life on Earth or hurt it. In the troposphere, ozone acts mainly as a pollutant — a component of smog that damages animal respiratory systems and reduces plant growth. (The amount of naturally occurring ozone in the troposphere is too low to threaten human health or the environment; much of the damaging ozone in smog forms when sunlight reacts with hydrocarbons and nitrogen oxides, byproducts of automobiles and power plants that run on fossil fuels.)
In the stratosphere, ozone absorbs some of the sun’s biologically harmful ultraviolet radiation (UV-B radiation), protecting plants and animals from a range of damage, such as skin cancer and cataracts in humans.
SMALL QUANTITY, BIG IMPACT
If you were to take all of the ozone molecules in the atmosphere, bring them down to the Earth’s surface, and uniformly distribute them around the world, the resulting ozone layer would be less than one half of one centimeter thick. Yet scientists think life on land probably would not have evolved, and could not exist today, without the protective ozone layer in the stratosphere. Its depletion by human-made gases was particularly alarming for scientists, given ozone’s central importance to life on Earth.
Gases containing the chemicals chlorine and bromine are most damaging to stratospheric ozone. In 1974, Mario Molina and Sherwood Rowland discovered that some of the most destructive chlorine gases are chlorofluorocarbons (CFCs), synthetic chemicals used as propellants in spray bottles and as cooling compounds in refrigerators and air conditioners. In 1985, Joseph Farman and his colleagues found a drastic depletion of the ozone layer over the Antarctic, an ozone hole resulting from ozone reacting with chlorine and bromine derived from CFCs and other human-made gases.
CFCs were developed in the 1930s and were used in industrial, commercial and household applications because they are nontoxic and nonflammable, and do not react with other chemical compounds near the Earth’s surface.
“These were great compounds when they were first invented,” Anne Douglass, deputy project scientist for NASA’s Aura satellite, told America.gov. Aura monitors the chemical composition of the Earth’s upper and lower atmosphere.
Before the invention of CFCs, refrigerators used hazardous gases, such as ammonia, according to Douglass. CFCs, on the other hand, are relatively inert and nonreactive near the Earth’s surface. “The thing that makes these compounds dangerous to ozone, which is that they don’t get broken apart unless they’re at 30 kilometers [altitude], is the exact thing that makes them safe for people,” Douglass said.
Once in the atmosphere, CFCs drift slowly upward — it takes five or six years for gases released on the surface to reach the stratosphere. UV-B radiation in the stratosphere reacts with CFCs, releasing a chlorine atom from a CFC molecule. The chlorine atom reacts with an ozone molecule (O3), breaking it apart and forming an ordinary oxygen molecule (O2) and a chlorine monoxide molecule (Cl+O), neither of which absorb UV-B radiation.
A single oxygen atom (O) may react with the chlorine monoxide, releasing the chlorine atom and forming an ordinary oxygen molecule (O2). The chlorine atom now is free to break up another ozone molecule (O3). One chlorine atom can repeat this cycle thousands of times, eliminating thousands of ozone molecules.
THE MONTREAL PROTOCOL
On September 16, 1987, representatives of 24 nations signed the Montreal Protocol on Substances that Deplete the Ozone Layer, limiting the production and use of nearly 100 substances that destroy stratospheric ozone. The agreement has since been ratified by 191 countries. (In 1978 the United States had banned CFC propellants for virtually all U.S.-made aerosol products.)
In a 2006 follow-up report sponsored by the World Meteorological Organization, international scientific experts noted early signs of stratospheric ozone recovery.
“As a result of the Montreal Protocol, the total abundance of ozone-depleting gases in the atmosphere has begun to decrease in recent years,” according to the Scientific Assessment of Ozone Depletion. “If the nations of the world continue to follow the provisions of the Montreal Protocol, the decrease will continue throughout the 21st century.”
CFCs can remain in the atmosphere as long as 100 years, so it will take that much longer to see the full effects of decreased CFC emissions on the ozone layer’s recovery.
For more information on the Montreal Protocol, see the United Nations Web site.
For more information on the Scientific Assessment of Ozone Depletion, see the Earth System Research Laboratory Web site.
For more information on the World Meteorological Organization’s Global Ozone Research and Monitoring Project, see the organization’s Web site.
For more information on the Aura satellite, see the NASA Web site.
