of Ozone Depletion in the Stratosphere
FOREWORD: This is a tutorial and teacher's guide for classroom studies in both high schools and colleges. It provides a summary in easy-to-understand terms of the current scientific understanding of ozone depletion (i.e., ozone destruction in the stratosphere or 'ozone hole'). It is intended to provide a bridge between the scientific community and students on the key findings and the extent of our current understanding. Graphics are used extensively to help convey the concepts. A comprehensive glossary of terms is provided at the end.
All life on earth exists in a very thin layer called the biosphere. This consists of a layer of soil and water and atmosphere that is about 35 miles thick. To put this in perspective, the thickness of the biosphere for planet earth is comparable to the thickness of the skin on an apple.
Within the biosphere, above the highest mountain, lies a layer of the atmosphere called the stratosphere. This is also known as the "ozone layer" because of the critical role that ozone plays there in protecting the earth from ultraviolet radiation.
The stratosphere, or ozone layer, is located approximately 6 to 30 miles above the earthís surface. In contrast to the lower atmosphere where turbulence and vertical mixing occur, the stratosphere is relatively quiescent. As a consequence, it is particularly susceptible to contamination because pollutants introduced there tend to remain for long periods of time (from several years to decades). Figure 1 shows the location of the stratosphere relative to the other four principal layers of our atmosphere. Most of our atmosphereís gases are contained in the troposphere and stratosphere, as shown in Figure 2.
The Layers of Our Atmosphere
The molecules of oxygen that we need to breath are composed of two atoms of oxygen (diatomic oxygen). Ozone is a molecule that contains three atoms of oxygen (triatomic oxygen). Ozone is concentrated in the stratosphere as shown in Figure 3. However, the actual amount of ozone is very small. Scientists measure the ozone directly above us in Dobson units. A Dobson unit can be best understood as a 0.01 mm layer of ozone if all the ozone where concentrated at ground level (at 0û C). A typical amount of ozone in the stratosphere above the United States is about 350 Dobson units which is equivalent to layer of ozone only 3.5 mm thick at ground level.
In the stratosphere, ozone absorbs virtually all of the solar ultraviolet (UV) radiation with wavelengths of less than 290 nanometers (nm) and most of it in the biologically harmful wavelength region of 290 to 320 nm (which is called UVB). This prevents the radiation from reaching the surface of the Earth in quantities which could adversely affect the lives of human beings, plants, and animals. The UV radiation that is not absorbed are the same rays of the sun that cause us to sunburn and otherwise damage our skin. This UV absorption is mostly responsible for the temperature inversion (temperature increase with increasing altitude) that characterizes the upper stratosphere and produces its quiescent nature. Ozone also absorbs strongly in the infrared part of the spectrum, and this absorption plays a part in maintaining the heat balance of the globe.
Recent measurements in Antarctica, shown in Figure 4, demonstrate the effect of reduced stratospheric ozone levels on UVB radiation at the earthís surface (WMO, Ď94).
Measured Increases in UVB Radiation Due to Ozone Reductions
South Pole, Feb 1991 - Dec 1992
There is a great difference between the beneficial effects of ozone in the stratosphere and its effects in the lower troposphere. In the lower troposphere, ozone is very reactive and harmful to plants, animals and even structures. For this reason, great efforts are taken in our communities to try to reduce ozone pollution at ground level.
Stratospheric ozone has played a major role in the evolution of life on earth. Over a very long period of time, oxygen given off by primitive plant life (e.g., ocean phytoplankton) began to drift up into the atmosphere. As it did, UV radiation from the sun would strike the oxygen molecule and break it into two oxygen atoms. Some of these single oxygen atoms would combine with other oxygen molecules to form ozone. The ozone molecules were much better than oxygen at absorbing the harmful UV radiation. After millions of years, enough ozone was produced in the stratosphere that life as we know it could survive and prosper under the heat and radiation of the Sun.
Eventually, a natural balance between ozone production (as described above) and ozone destruction was reached. See Figure 5. Natural ozone destruction occurs as follows: When an ozone molecule absorbs UV radiation, the ozone molecule breaks apart into diatomic oxygen and a single oxygen atom. Further ozone destruction can occur when the free oxygen atom reacts with another ozone molecule to form two molecules of diatomic oxygen.
Because of the dynamic nature of this balance, the amount of ozone in the stratosphere isn't constant, but varies from place to place, month to month, and year to year. Variations on time-scales of up to 11 years have been observed, correlating with the solar cycle. Figure 6 shows the long-term annual variation in total ozone in the Northern Hemisphere between 1933 and 1970 to be in the range of ± 5 percent (based on measurements at Aroza, Switzerland). Natural year to year variations in the total ozone column can be as much as 1 percent, while day-to-day changes can be greater than 10 percent.
Natural Ozone Creation & Destruction Process
(Click to magnify this image)
Long-Term Annual Variation in Total Ozone in the Northern Hemisphere
In 1985, scientists from the British Antarctic Survey reported that the ozone layer over Antarctica had shrunk substantially each September and October since the late 1970s, which corresponds to the start of the Southern Hemisphere's spring season. The drop in ozone levels in the stratosphere was so dramatic that at first the scientists thought their instruments were faulty. Replacement instruments were flown out and the measurements were repeated before the ozone depletion was accepted as genuine. The decrease in the concentration of ozone over Antarctica is shown in Figure 7. This phenomenon has come to be known as the Antarctic "ozone hole." In 1994, the "hole" showed an ozone loss of about 65% and its edges reached beyond the Antarctic continent to the tip of South America. Figures 8 and 9 graphically show the growth in the size of the ozone hole from 1979 to 1994.
Ozone Depletion Over Antarctica
Average Antarctic Ozone Hole Size
October Ozone Averages, 1979 - 1994
(Click to magnify this image)
Research has shown that ozone depletion occurs over the latitudes that include North America, Europe, Asia, and much of Africa, Australia, and South America. Thus, ozone depletion is a global issue and not just a problem at the South Pole.
In 1988, an exhaustive review of NASA satellite data concluded that, averaged over the globe, ozone had decreased about 2.5 percent between 1969 and 1986. A 1991 NASA research effort revealed that the magnitude of ozone loss was bigger, that the spatial extent was larger, and that the ozone depletion was persisting for a greater part of the year than had been previously recognized (Zurer, 1993). Ozone destruction over the Northern hemisphere's mid latitudes including highly populated region such as the U.S. and Europe was two to three times as great as the scientists had previously calculated.
In 1993, data from the Total Ozone Mapping Spectrometer (TOMS) onboard the Nimbus 7 satellite showed that Global ozone levels for the winter of 1992 through spring of 1993 were 2-3 percent lower than in any previous year for these months and 4 percent lower than normal. Ozone levels for the northern mid-latitudes were about 10 percent lower than historical averages for this time of year and continued at low levels into the early summer.
Over the U.S., ozone levels have fallen 5-10%, depending on the season.
Further evidence that something has recently upset the ozone balance in the northern hemisphere was provided by the city of Aroza, Switzerland. Aroza has been making and keeping records of total ozone since 1926, longer than any other location. Figure 10 shows that the level of ozone over Aroza barely changed (a 0.1 percent increase per decade) between 1926 and 1973, but between 1973 and 1993, this changed to an average 2.9 percent decrease per decade. The year to year changes in ozone levels over the northern hemisphere are graphically shown in Figure 11.
Ozone at Aroza, Switzerland, 1926 - 1993
Total Ozone Averages, 1979 - 1994
Only recently has manís activities begun to impact the ozone layer, upsetting the natural balance of ozone creation and destruction processes. 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. The CFCs are so stable that only one process breaks them down: exposure to strong UV radiation. When a CFC molecule breaks down, it releases atomic chlorine which destroys ozone. One chlorine atom can destroy over 100,000 ozone molecules. The net effect is to destroy ozone faster than it is naturally created.
Figure 12 illustrates how the use of CFCs results in the destruction of the ozone layer and the increase in UV radiation at the earthís surface.
Ozone Depletion Process
Refer to Section III for a more detailed discussion of the sources of ozone depletion.
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