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The Science of Ozone Depletion

  1. What is good ozone
  2. What is Bad ozone (ground-level ozone)
  3. The science of ozone depletion
  4. The effects of ozone depletion
  5. Ozone depletion graphics and images
  6. FAQs about ozone depletion
  7. Ozone Depletion Publications, References and Resources

 

This page contains information about the science of ozone depletion, regulations in the US designed to protect the ozone layer, information on methyl bromide, flyers about the UV index, information for the general public, and other topics.

If you're looking for information not found here, try calling the Ozone Protection Hotline toll-free at (800) 296-1996 or direct-dial at (301) 614-3396. horizontal rule

Ozone is a gas that occurs both in the Earth's upper atmosphere and at ground level. Ozone can be "good" or "bad" for people's health and for the environment, depending on its location in the atmosphere.

Ground-level Ozone
In the troposphere, the air closest to the Earth's surface, ground-level or "bad" ozone is a pollutant that is a significant health risk, especially for children with asthma. It also damages crops, trees and other vegetation. It is a main ingredient of urban smog.

Stratospheric Ozone
The stratosphere, or "good" ozone layer extends upward from about 6 to 30 miles and protects life on Earth from the sun's harmful ultraviolet (UV) rays. This natural shield has been gradually depleted by man-made chemicals like chlorofluorocarbons (CFCs). A depleted ozone shield allows more UV from the sun to reach the ground, leading to more cases of skin cancer, cataracts, and other health problems.

For an overview of both ozone issues, see Good Up High, Bad Nearby.

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The Science of Ozone Depletion

Ozone depletion is the result of a complex set of circumstances and chemistry. This page has links to articles that give overviews or broad information, shorter pieces that focus on specific aspects of ozone depletion, international organizations that issue regular updates, and images and animations of ozone levels and ozone depletion.

The Earth's ozone layer protects all life from the sun's harmful radiation, but human activities have damaged this shield. Less protection from ultraviolet light will, over time, lead to higher skin cancer and cataract rates and crop damage. The U.S., in cooperation with over 160 other countries, is phasing out the production of ozone-depleting substances in an effort to safeguard the ozone layer.

I. The Ozone Layer

The Earth's atmosphere is divided into several layers. The lowest region, the troposphere, extends from the Earth's surface up to about 10 kilometers (km) in altitude. Virtually all human activities occur in the troposphere. Mt. Everest, the tallest mountain on the planet, is only about 9 km high. The next layer, the stratosphere, continues from 10 km to about 50 km. Most commercial airline traffic occurs in the lower part of the stratosphere.

Source: World Meteorological Organization, Scientific Assessment of Ozone Depletion: 1998, WMO Global Ozone Research and Monitoring Project - Report No. 44, Geneva, 1998.

As shown in the graph, most atmospheric ozone is concentrated in a layer in the stratosphere, about 15-30 kilometers above the Earth's surface. Ozone is a molecule containing three oxygen atoms. It is blue in color and has a strong odor. Normal oxygen, which we breathe, has two oxygen atoms and is colorless and odorless. Ozone is much less common than normal oxygen. Out of each 10 million air molecules, about 2 million are normal oxygen, but only 3 are ozone.

However, even the small amount of ozone plays a key role in the atmosphere. The ozone layer absorbs a portion of the radiation from the sun, preventing it from reaching the planet's surface. Most importantly, it absorbs the portion of ultraviolet light called UVB. UVB has been linked to many harmful effects, including various types of skin cancer, cataracts, and harm to some crops, certain materials, and some forms of marine life.

At any given time, ozone molecules are constantly formed and destroyed in the stratosphere. The total amount, however, remains relatively stable. The concentration of the ozone layer can be thought of as a stream's depth at a particular location. Although water is constantly flowing in and out, the depth remains constant.

While ozone concentrations vary naturally with sunspots, the seasons, and latitude, these processes are well understood and predictable. Scientists have established records spanning several decades that detail normal ozone levels during these natural cycles. Each natural reduction in ozone levels has been followed by a recovery. Recently, however, convincing scientific evidence has shown that the ozone shield is being depleted well beyond changes due to natural processes.

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II. Ozone Depletion

For over 50 years, chlorofluorocarbons (CFCs) were thought of as miracle substances. They are stable, nonflammable, low in toxicity, and inexpensive to produce. Over time, CFCs found uses as refrigerants, solvents, foam blowing agents, and in other smaller applications. Other chlorine-containing compounds include methyl chloroform, a solvent, and carbon tetrachloride, an industrial chemical. Halons, extremely effective fire extinguishing agents, and methyl bromide, an effective produce and soil fumigant, contain bromine. All of these compounds have atmospheric lifetimes long enough to allow them to be transported by winds into the stratosphere. Because they release chlorine or bromine when they break down, they damage the protective ozone layer. The discussion of the ozone depletion process below focuses on CFCs, but the basic concepts apply to all of the ozone-depleting substances (ODS).

In the early 1970s, researchers began to investigate the effects of various chemicals on the ozone layer, particularly CFCs, which contain chlorine. They also examined the potential impacts of other chlorine sources. Chlorine from swimming pools, industrial plants, sea salt, and volcanoes does not reach the stratosphere. Chlorine compounds from these sources readily combine with water and repeated measurements show that they rain out of the troposphere very quickly. In contrast, CFCs are very stable and do not dissolve in rain. Thus, there are no natural processes that remove the CFCs from the lower atmosphere. Over time, winds drive the CFCs into the stratosphere.

The CFCs are so stable that only exposure to strong UV radiation breaks them down. When that happens, the CFC molecule releases atomic chlorine. One chlorine atom can destroy over 100,000 ozone molecules. The net effect is to destroy ozone faster than it is naturally created. To return to the analogy comparing ozone levels to a stream's depth, CFCs act as a siphon, removing water faster than normal and reducing the depth of the stream.

Large fires and certain types of marine life produce one stable form of chlorine that does reach the stratosphere. However, numerous experiments have shown that CFCs and other widely-used chemicals produce roughly 84% of the chlorine in the stratosphere, while natural sources contribute only 16%.

Large volcanic eruptions can have an indirect effect on ozone levels. Although Mt. Pinatubo's 1991 eruption did not increase stratospheric chlorine concentrations, it did produce large amounts of tiny particles called aerosols (different from consumer products also known as aerosols). These aerosols increase chlorine's effectiveness at destroying ozone. The aerosols only increased depletion because of the presence of CFC - based chlorine. In effect, the aerosols increased the efficiency of the CFC siphon, lowering ozone levels even more than would have otherwise occurred. Unlike long-term ozone depletion, however, this effect is short-lived. The aerosols from Mt. Pinatubo have already disappeared, but satellite, ground-based, and balloon data still show ozone depletion occurring closer to the historic trend.

One example of ozone depletion is the annual ozone "hole" over Antarctica  that has occurred during the Antarctic Spring since the early 1980s. Rather than being a literal hole through the layer, the ozone hole is a large area of the stratosphere with extremely low amounts of ozone. Ozone levels fall by over 60% during the worst years.

In addition, research has shown that ozone depletion occurs over the latitudes that include North America, Europe, Asia, and much of Africa, Australia, and South America. Over the U.S., ozone levels have fallen 5-10%, depending on the season. Thus, ozone depletion is a global issue and not just a problem at the South Pole.

Reductions in ozone levels will lead to higher levels of UVB reaching the Earth's surface. The sun's output of UVB does not change; rather, less ozone means less protection, and hence more UVB reaches the Earth. Studies have shown that in the Antarctic, the amount of UVB measured at the surface can double during the annual ozone hole. Another study confirmed the relationship between reduced ozone and increased UVB levels in Canada during the past several years.

Laboratory and epidemiological studies demonstrate that UVB causes nonmelanoma skin cancer and plays a major role in malignant melanoma development. In addition, UVB has been linked to cataracts. All sunlight contains some UVB, even with normal ozone levels. It is always important to limit exposure to the sun. However, ozone depletion will increase the amount of UVB, which will then increase the risk of health effects. Furthermore, UVB harms some crops, plastics and other materials, and certain types of marine life.

For more information, see the Ozone Depletion Process page.

Ozone-depleting Substances

This site includes a lists of all ozone-depleting substances (ODS) recognized by the Montreal Protocol.  The lists include ozone depletion potentials (ODP), global warming potentials (GWP), and CAS numbers for each ODS.  The lists are organized by class:

List of Class I Ozone-depleting Substances

List of Class II Ozone-depleting Substances

Numbering Scheme for Ozone-depleting Substances and Substitutes
Why chemicals are numbered certain ways.
 
Global Warming Potentials (GWPs) for ODS Substitutes
A list of the global warming potentials of ODS substitutes

 

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III. The World's Reaction

The initial concern about the ozone layer in the 1970s led to a ban on the use of CFCs as aerosol propellants in several countries, including the U.S. However, production of CFCs and other ozone-depleting substances grew rapidly afterward as new uses were discovered.

Through the 1980s, other uses expanded and the world's nations became increasingly concerned that these chemicals would further harm the ozone layer. In 1985, the Vienna Convention was adopted to formalize international cooperation on this issue. Additional efforts resulted in the signing of the Montreal Protocol in 1987. The original protocol would have reduced the production of CFCs by half by 1998.

After the original Protocol was signed, new measurements showed worse damage to the ozone layer than was originally expected. In 1992, reacting to the latest scientific assessment of the ozone layer, the Parties decided to completely end production of halons by the beginning of 1994 and of CFCs by the beginning of 1996 in developed countries.

Because of measures taken under the Protocol, emissions of ozone-depleting substances are already falling. Based on measurements of total inorganic chlorine in the atmosphere, which stopped increasing in 1997 and 1998, stratospheric chlorine levels have peaked and are no longer increasing. The good news is that the natural ozone production process will heal the ozone layer in about 50 years.

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IV. Stratospheric Protection Division

In addition to regulating the end of production of the ozone-depleting substances, the U.S. Environmental Protection Agency (EPA) implements several other programs to protect the ozone layer under Title VI of the Clean Air Act. These programs include refrigerant recycling, product labeling, banning nonessential uses of certain compounds, and reviewing substitutes.
Reports to the Nation: Our Ozone Shield
Written by the National Oceanic and Atmospheric Administration (NOAA), this booklet describes the history and science of ozone depletion.
Scientific Assessment of Ozone Depletion: 2002
This is the executive summary of the most recent World Meteorological Organization and United Nations Environmental Programme assessment. It contains the most up-to-date understanding of ozone depletion and reflects the thinking of over 250 international scientific experts who contributed to its preparation and review.

 

EPA has another web site about global warming. Please visit that site for information on global warming science, projected impacts, governmental policies, and other information.

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General Information

bulletQuestions and Answers (brief) bulletFact Sheet (detailed) bulletWhat EPA Has Done About Ozone Depletion: An Overview bulletWhat Can Individuals Do? bulletOn the Trail of the Missing Ozone
This introduction to ozone depletion first describes the causes and effects of ozone depletion, and then explains some of the solutions. Follow Farley the reporter as he learns about this issue. Links are provided along the way to more detailed information found elsewhere on the site. Three versions are available: a web-viewable set of illustrated panels, a text version, and an Adobe Acrobat version.
bulletThe Ozone Depletion Process
A broad overview of how ozone depletion occurs
bulletThe Antarctic Ozone Hole
View an animation of the 1995 hole, read why it occurs over the South Pole, and consider the many ways to measure the hole.
bulletCurrent Progress of the Antarctic Ozone Hole 
View a page from NOAA Climate Prediction Center's web site, which shows weekly updates of the hole.
bulletOzone Science Crossword Puzzle
bulletEnvironmental Indicators: Ozone Depletion
Describes the data that characterize the ozone layer and demonstrate that CFCs and other chemicals are causing ozone depletion. Many internal links allow more in-depth reading.
bulletOzone Depletion: When Less Is Not Enough 
Written by the National Safety Council's Environmental Health Center, this guide provides a well-rounded description of ozone depletion, its causes, and its impacts. It is one chapter of a book titled Reporting on Climate Change: Understanding the Science.
bulletOzone-Depleting Substances, ODPs, GWPs, and CAS Numbers
bulletMyth vs. Measurement
A series of responses to the most common misundertandings about ozone depletion.
CFCs are Heavier Than Air, So They Can't Reach the Ozone Layer
Volcanoes and the Oceans are Causing Ozone Depletion
Ozone Depletion Occurs Only In Antarctica
No Link Exists Between Ozone Depletion and Higher UV Levels
bulletWill the Ozone Layer Recover? Can We Make More Ozone?
A response to common questions about the ozone layer's recovery.
bulletThe Effects of Ozone Depletion
A basic description of how ozone depletion harms human health and the environment, with links to more detailed information.
bulletBenefits of the CFC Phaseout
This fact sheet describes specific benefits and also includes some case studies of successful use of alternatives
bulletFact sheet explaining the difference between stratospheric ozone and tropospheric ozone bulletExecutive Summary of Scientific Assessment of Ozone Depletion: 1994  
The most recent WMO/UNEP assessment contains the most up-to-date understanding of ozone depletion and reflects the thinking of 295 international scientific experts who contributed to its preparation and review. To receive a free copy of the Executive Summary in paper form, call the EPA Stratospheric Ozone Hotline at 800-296-1996. (Note: an Adobe Acrobat (PDF) version of the Executive Summary of the 1998 Assessment is available from NOAA's web site.)
bullet Information from Environment Canada 
The Stratospheric Ozone web site contains general and Canadian-focused information on ozone depletion including a primer on ozone depletion and indicators of ozone depletion, featuring many graphs and original data.

 

An EPA PDF brochure on ozone depletion and EPA's research. (PDF refers to the Adobe Acrobat format; you'll need to download the free reader from Adobe).
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Specific Aspects of Ozone Depletion

bulletNumbering Scheme for Ozone-Depleting Substances and Substitutes bulletOzone vs. Altitude
Ozone concentrations are higher in the stratosphere than in the troposphere
bulletAerosols from Mt. Pinatubo
A graph and explanation of the effects of volcanic aerosols on ozone depletion.
bulletOzone Depletion Over North America
A comparison of ozone levels in 1979 and 1994
bulletOzone Depletion over Arosa, Switzerland
A long-term graph showing a recent sharp decrease in ozone levels
bulletUGA UV Monitoring 
Information on the network of UV monitors operated by UGA and EPA
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Images and Animations

bulletAnimation of the 1995 Ozone Hole
Brief description of the causes of the ozone hole, plus an animation of the 1995 event.
bulletEPA's Ultraviolet Monitoring Program
EPA operates and maintains a network of Brewer spectrophotometers throughout the United States, measuring full-sky spectrally resolved solar radiation in the UV-B and UV-A bands. Visitors can create graphs of daily UV and ozone levels and download the data.
bulletNASA Ozone Depletion Movies, Graphics, and Original Data 
This site is full of useful data and information. Download original ozone level data and see movies and graphics of ozone depletion and the Antarctic ozone hole. The site also lets you find the ozone level over your house!
bulletNOAA Climate Monitoring and Diagnostics Laboratory 
CMDL includes several programs to measure important atmospheric characteristics.
bulletNitrous Oxide And Halocompounds (NOAH) division
NOAH measures concentrations of CFCs and other ozone-depleting substances in the atmosphere. The web site provides access to data and graphs.
bulletOzone and Water Vapor Group 
Provides links to several programs focusing on ozone measurement, including vertical profiles of ozone abundance at the South Pole during the ozone hole and total global ozone.
bulletNOAA Climate Prediction Center 
Current satellite ozone maps; UV index bulletin; TOVS data, images
bulletOzone Hole Tour 
This tour has been put together by members of the Centre for Atmospheric Science at the University of Cambridge, UK. It features text, graphics, and movies.
bulletOzone Images from the Department of Meteorology, University of Reading, UK 
Features a 90-day archive of the latest images from global, northern hemisphere, and southern hemisphere ozone measurements. Also features animations and ozonesonde measurements from Antarctica. Some of these images are also available from the NOAA Climate Prediction Center.
bulletWorld Ozone and Ultraviolet Radiation Data Centre 
Operated by Environment Canada, this Centre provides current and time series graphs of ozone and UV radiation over Canada and North America.
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International Scientific Organizations

  bulletWorld Meteorological Organization Press Releases and Bulletins
A collection of recent bulletins about the ozone hole.
bulletBritish Antarctic Survey information
Information from the group responsible for monitoring the Antarctic Ozone Hole, including regular bulletins on ozone levels.
bulletOnline papers at CIESIN
This site provides the full text of an enormous number of papers on ozone depletion.
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Links to Other Information

bulletOzone Depletion Science bulletThe Antarctic Ozone Hole bulletOzone Monitoring bulletUV Index and UV Monitoring bulletHealth Effects of Ozone Depletion

Ozone Science Frequently Asked Questions (FAQ), Answers, and References (not maintained by EPA; thanks to Robert Parson) This set of questions and answers provides carefully cited information. It goes into considerable detail and provides numerous references to original research. The FAQ is available at several sites in plain text. 

bulletUsenet Hypertext FAQ Archive bulletUniversity of Utrecht bulletOxford University bulletOhio State University
The following web sites are good starting points for information on stratospheric ozone.
bulletGregory Dubois-Felsmann's SOLIS page at NCAR. bulletThe Aeronomy and Climate Monitoring and Diagnostics Laboratories at NOAA. bulletThe HALOE project. bulletThe British Antarctic Survey. bulletThe USDA UV-B monitoring program home page at Colorado State University. bulletThe US Environmental Protection Agency's Stratospheric Ozone page. The U.S. Environmental Protection Agency and the Global Environment & Technology Foundation (a not-for profit corporation) announce the Ozone Protection Technologies Information Clearinghouse System (OPTICS). This web-based database provides information submitted by manufacturers of technologies related to recycling, identification, and destruction of ozone-depleting substances. The system is ready, and now we need help filling it with detailed information. 

If you're looking for information not found here, try calling the Ozone Protection Hotline toll-free at (800) 296-1996 or direct-dial at (301) 614-3396.