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Background on UV-B & Ozone Depletion

UV-B can damage plant and animal life on earth. More UV-B reaches the earth as ozone levels decline because stratospheric ozone is the primary absorber of UV-B. This thin blanket shields us from the sun’s harmful rays. High levels of UV-B radiation (280 to 320 nm wavelength) are responsible for many biologically harmful effects in both plants and animals. Some effects include DNA damage, eye cataracts, skin cancer, and immune system suppression in animals, as well as lower growth rates and depressed rates of leaf photosynthesis in plants. Results of calculations of the transmission of UV-B through the atmosphere indicate that a 1% decrease in ozone results in a 1.3% to 2% increase in the UV-B levels at the surface of the earth. These results have been verified by observations in the polar regions. The discovery of the ozone "hole" over Antarctica in 1985 by the British Antarctic Survey station, the link between human activity and ozone depletion, and the continuing decline of total ozone during the last few decades has highlighted the concern over the amount of UV-B reaching the surface of the earth.

UV-B is absorbed by atmospheric gases and suspended particulate matter (aerosols). The most important UV-B absorber is stratospheric ozone (O3) which is found at altitudes between 10 and 30 km, with a maximum concentration from 19 to 23 km. The total height of the ozone column above any spot on earth is quite small. At standard temperature and pressure (STP), the entire stratospheric ozone layer would have a depth of only 0.3 cm. Ozone column height at STP is usually measured in Dobson units, DU, where one DU is 0.001 cm of STP column height. While other atmospheric gases and aerosols absorb UV-B, none are as efficient in this task as ozone.

The discovery of the ozone hole over Antarctica in 1985 showed that ozone destruction was being accelerated. The discovery of the ozone "hole" over the Antarctic focused scientific attention on ozone production and destruction mechanisms. High-altitude ozone is produced (through natural processes) largely in the tropical stratosphere, where the high levels of sunlight are most effective in disassociating molecular oxygen (O2) into atomic oxygen (O + O). The unpaired oxygen atoms are then available to react with other oxygen molecules, and catalyst molecules, to form ozone. The ozone molecules are broken down naturally as they react with other chemicals in the stratosphere. Under normal conditions, the process of ozone formation and disintegration is well balanced and the average ozone layer thickness remains constant. The discovery of the ozone hole shows that the system is no longer in equilibrium—ozone is being destroyed faster than it is replaced. A theory for accelerated ozone destruction was proposed in the early 1970s.
In the 1970s, chemists Molina and Rowland showed how CFCs can break down ozone. Their theory was verified by NASA satellite data in 1987. In their 1974 research findings, chemists M. Molina and F.S. Rowland built upon the work of chemist P. Crutzen to show how human-produced chlorofluorocarbons CFCs could reduce total stratospheric ozone. In the troposphere, CFCs are stable and inert, but when they reach the stratosphere through convective air movements, the sun's ultraviolet rays cause them to decompose and release chlorine atoms. Through a series of catalytic reactions, the free chlorine atom is capable of destroying hundreds of thousands of ozone atoms. This theory was verified by the NASA Upper Atmosphere Research Satellite in 1987, when it detected high amounts CIO (a reactive chlorine species) along with decreasing ozone. In 1995, Crutzen, Molina, and Rowland shared the Nobel Prize for Chemistry for their pioneering work in the effects of man-made chemicals on the ozone layer.
In 1975, the WMO formed a group to look at the effect of human-produced chemicals on the ozone layer. Their work led to the adoption of the Montreal Protocol in 1987 and its amendments to phase out CFC production by 2000. Following Molina and Rowland’s publication, the World Meteorological Organization (WMO) assembled a group of experts and prepared a statement in 1975 entitled "Modification of the ozone layer due to human activities and some possible geophysical consequences." The group focused on effects of CFCs and supersonic transport (SST) aircraft whose exhaust contains nitrogen oxide capable of breaking down ozone molecules. Since then the WMO, working with the United Nations Environment Programme (UNEP) and scientists around the world, has issued many statements on the state of the ozone layer. Its findings were the basis for international agreements to ban the production of ozone-depleting chemicals by the year 2000.

In 1987, the Montreal Protocol was adopted with the charter to reduce CFC production by 50% by the year 2000. Since then, the Protocol has been amended (London, 1990, and Copenhagen, 1992) to completely phase out CFCs by 2000 as well as other chemicals—halons, carbon tetrachloride, methyl chloroform, methyl bromide, and HCFCs—that threaten the ozone layer.

During the 1990s, reports on the state of the ozone layer indicate that total ozone is still on the decline. The expanding ozone hole has spread to mid-latitudes and threatens more regions around the globe. In response to the concern over increasing UV-B levels in the United States and its effects on crops and animals, the United States Department of Agriculture (USDA) deployed a 30-station network around the United States to monitor UV-B levels Their data is available online at http://uvb.nrel.colostate.edu/UVB/.


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This page was last updated on Monday, September 11, 2006 .