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Stratospheric Ozone

Note: EPA no longer updates this information, but it may be useful as a reference or resource.

Please see www.epa.gov/airtrends for the latest information on Air Quality Trends.

Nature and Sources of the Problem:
The stratosphere, located about 6 to 30 miles above the Earth, contains a layer of ozone gas that protects living organisms from harmful ultraviolet radiation (UV-b) from the Sun. Over the past 2 decades, however, this protective shield has been damaged. Each year, an "ozone hole" forms over the Antarctic, and ozone levels fall to 70 percent below normal. Even over the U.S., ozone levels are about 5 percent below normal in the summer and 10 percent below normal in the winter. The figure below shows ozone levels over North America in dobson units (DU) in March 1979 and March 1994. One hundred DU of ozone would form a layer 1 millimeter thick at the Earth's surface. Each color band represents an area with a similar amount of ozone overhead. Comparing the colors of the bands over a particular city, such as Seattle, shows lower ozone levels in 1994 than in 1979. This figure is a snapshot in time that shows one example of reduced ozone levels. Long-term trends are based on numerous data sets taken over several years, as opposed to single observations.

NIMBUS satellite images show reduced ozone concentration over

much of northern Canada in 1994 compared to 1979

This figure compares of satellite measurements of ozone levels over North America. Each color band represents an area with a similiar amount of ozone overhead; lower levels of dobson units indicate less protective ozone overhead. Ozone trends are based on detailed statistical analysis of large data sets, and not on simple graphs like these.

As the ozone layer thins, more UV-b radiation reaches the Earth. In 1996, scientists demonstrated for the first time that UV-b levels over most populated areas have increased. Scientists have linked several substances associated with human activities to ozone depletion, including the use of chlorofluorocarbons (CFCs), halons, carbon tetrachloride, and methyl chloroform. These chemicals are emitted from home air conditioners, foam cushions, and many other products. Strong winds carry them through the lower part of the atmosphere, called the troposphere, and into the stratosphere. There, strong solar radiation releases chlorine and bromine atoms that attack protective ozone molecules. Scientists estimate that one chlorine atom can destroy 100,000 ozone molecules.

Health and Environmental Effects:
Some UV-b reaches the Earth's surface even with normal ozone levels. However, since the ozone layer normally absorbs most UV-b radiation from the Sun, ozone depletion is expected to lead to increases in harmful effects associated with UV-b radiation. In humans, UV-b is linked to skin cancer, including melanoma, the form of skin cancer with the highest fatality rate. It also causes cataracts and suppression of the immune system.

The effects of UV-b radiation on plant and aquatic ecosystems are not well understood. However, the growth of certain food plants can be slowed by excessive UV-b radiation. In addition, some scientists suggest that marine phytoplankton, which are the base of the ocean food chain, are already under stress from UV-b radiation. This stress could have adverse consequences for human food supplies from the oceans. Because they absorb CO2 from the atmosphere, significant harm to phytoplankton populations could increase global warming (see following section on Global Warming and Climate Change).

Programs to Restore the Stratospheric Ozone Layer:
In 1987, 27 countries signed the Montreal Protocol, a landmark treaty that recognized the international nature of ozone depletion and committed the world to limiting the production of ozone-depleting substances. Today, over 150 nations have signed the protocol, which has been strengthened twice and now calls for the elimination of these chemicals.

The 1990 Clean Air Act Amendments established a U.S. regulatory program to protect the stratospheric ozone layer. In January 1996, U.S. production of many ozone-depleting substances virtually ended, including CFCs, carbon tetrachloride, and methyl chloroform. Production of halons ended in January 1994. EPA regulations control the handling and emissions of CFCs and the use of substitutes. Many new products that are either harmless or less damaging to the ozone layer are now gaining popularity. For example, computer-makers are using ozone-safe solvents to clean circuit boards, and automobile manufacturers are using HFC-134a, an ozone-safe refrigerant, in new motor vehicle air conditioners. In some sectors, the transition away from ozone-depleting substances has already been completed.

Trends in Stratospheric Ozone Depletion:
Scientific evidence shows that the approach taken under the Montreal Protocol has been effective. In 1995, measurements showed that the tropospheric concentrations of methyl chloroform had started to fall, indicating that emissions had been greatly reduced. Tropospheric concentrations of other ozone-depleting substances, like CFCs, are also beginning to decrease. It takes several years for these substances to reach the stratosphere and release chlorine and bromine. For this reason, stratospheric chlorine levels are expected to continue to rise, peak between 1997 and 1999, and then slowly decline. Because of the stability of most ozone-depleting substances, chlorine will be released into the stratosphere for many years, and the ozone layer will not fully recover until well into the next century.

In 1996, scientists developed a new technique allowing them to draw conclusions about UV-b radiation at ground level. According to satellite-based trend analyses, major populated areas have experienced increasing UV-b levels over the past 15 years. As shown by the figure below, at latitudes that cover the U.S., UV-b levels are 4 to 5 percent higher than they were 10 years ago.

A world map shows increases in surface UV-b radiation ranging from

zero at the equator to more than 10 percent at latitude 65

A 1996 study using satellite-based analyses of UV-b trends demonstrated that UV-b levels had increased at ground level. This figure shows the percent increases in average annual UV-b reaching the surface over the past 10 years. UV-b incidence is strongly dependent on latitude. At latitudes that cover the U.S., UV-b levels are 4 to 5 percent higher than they were 10 years ago.

This document is provided for historical purposes only. The most recent version can be found at AIRTrends

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