by National Science Foundation, 1993
1993 EARLY-SEASON OBSERVATIONS
Despite predictions that the total amount of ozone above Antarctica would never drop below 100 Dobson units, observations made on 8 October 1993 at Amundsen-Scott South Pole Station indicated that levels had dipped to 90 Dobson units. Furthermore, throughout the period when the ozone "hole" usually develops the total amount of ozone remained low--from 25 September to 14 October 1993 the level never rose above 105 under 105 Dobson units, once again setting a new all-time low for ozone abundance.
1992 OBSERVATIONS AND RESULTS
By late September 1992, the "ozone hole" covered an area that was an unprecedented 23 million square kilometers in size--an area about 3 million square kilometers larger than previous years. The available data also indicated that this austral spring's "ozone hole" had begun earlier and was growing at a faster pace than had been recorded in the past.
Not long after these data were recorded, however, weather patterns over the Pacific ocean caused a dramatic shift in the position of the polar vortex, the cyclonic winds that seal off the antarctic atmosphere, prevent mixing with warm ozone-rich air, and create the environment in which polar stratospheric clouds form. Unexpectedly, a large warm, ozone-rich air mass moved into the antarctic atmosphere and disrupted the track of the vortex. Some researchers believe that the warmer Pacific air mass weakened the barrier created by the polar vortex and helped slow the depletion rate.
Unlike previous year's, this year the science community disagreed about the magnitude of the depletion. Before this year's "ozone hole" began to develop, researchers predicted that volcanic debris--principally sulfuric acid droplets--present in the atmosphere as a result of the 1991 Mount Pinatubo volcanic eruption would increase the amount of ozone depleted from the antarctic stratosphere and break the records set in 1987, 1990, and 1991.
In early October at the peak of the depletion cycle, satellite data, obtained from the National Aeronautics and Space Administration's (NASA) total ozone mapping spectrometer (TOMS), indicated that this year's depletion never dropped below about 126 Dobson units. Meanwhile, researchers using balloon-borne and ground-based instruments at the geographic South Pole acquired data that contradicted the TOMS results. Their observations showed that the total amount of ozone dropped to 105 Dobson units, below the 1991 record low of 118 Dobson units.
The disparity between these data sets may relate to the method of data acquisition. The NASA TOMS system, on board the National Oceanic and Atmospheric Administration's (NOAA) Nimbus-7 satellite, measures the ultra-violet portion of the spectrum and maps the total amount of ozone within a 3,000-kilometer sector. While TOMS data provide broad view of how ozone levels vary throughout the areal extent of the "ozone hole," balloon- borne and ground-based instruments can sample ozone and other aerosols at specific altitudes.
The South Pole data show dramatic differences between the 1992 ozone loss and previous years. Researchers, supported by the National Science Foundation, believe that the high concentration of submicron aerosol particles from the Pinatubo eruption may be contributing to the lower levels of ozone abundance that their instruments recorded.
Since monitoring of the depletion cycle began, losses have been concentrated in the a 1- to 2-kilometer- thick region of the stratosphere above an altitude of 16 kilometers. This year's balloon launches showed chemical destruction of all ozone in a 4-kilometer-thick region between 14 and 18 kilometers altitude, as well as significant losses at lower altitudes (between 10 and 13 kilometers).
Usually, ozone at lower altitudes is shielded from the chemical reactions that destroy ozone in the upper altitudes because the atmosphere is too warm for polar stratospheric clouds to form. The reactions that release chlorine from chlorofluorocarbons take place on the surfaces of ice crystals in these clouds. This year, as much as one-third of the ozone appears to have been destroyed at altitudes between 10 and 13 kilometers. Early analysis of the data suggests the increased number of small particles, believed to be volcanic sulfuric-acid droplets, may have caused the depletion at these lower altitudes.
THE OZONE-DEPLETION PROCESS
Ozone, which is concentrated in a layer of the stratosphere between 15 and 30 kilometers above earth's surface, is composed of three oxygen molecules and is formed when solar ultraviolet radiation acts on atmospheric oxygen molecules. It protects life on the earth from the lethal effects of ultraviolet (UV) radiation.
Using data obtained from ground-based, balloon- borne, air-borne, and satellite-borne instruments, scientists have learned that atmospheric chemistry and climate dynamics combine to deplete ozone from the stratosphere. During the winter, a stable low-pressure system with low temperatures builds up over Antarctica. A belt of strong westerly winds--the polar vortex--seal off the atmosphere and prevent warmer, ozone-rich air from mixing with the antarctic atmosphere. This isolation coupled with temperatures lower than -80øC enables polar stratospheric clouds to form. Polar stratospheric clouds, which are made up of ice crystals, provide the environment in which the ozone-destroying chemical reactions occur.
When man-made chlorofluorocarbons are exposed to solar ultraviolet radiation in the upper atmosphere, they break down, leaving chlorine atoms free to combine with the oxygen atoms from ozone and atmospheric oxygen. Rather than recombining with atmospheric oxygen molecules, the oxygen atoms combine with the chlorine to form chlorine monoxide. As the cycle continues, more oxygen is tied up in chlorine monoxide, and ozone is depleted.
CLIMATE DYNAMICS AND OZONE DEPLETION
Since 1986, U.S. investigators have probed and monitored the changes occurring in ozone layer above Antarctica. Although they agree that chlorine, produced by man-made chlorofluorocarbons, is a key element in the chemical reactions that break down stratospheric ozone and create the ozone "hole" each austral spring, the role and impact of atmospheric phenomena, climate dynamics, and other atmospheric aerosol compounds remain unclear. These questions now are their focus--how do dynamic processes of the upper atmosphere influence the depletion, what physical properties of polar stratospheric clouds enhance the chemical reactions that destroy ozone, and what other aerosol compounds contribute to the process. Answers to these and other questions are critical not only for understanding antarctic and arctic atmosphere and climate changes but for determining these changes and process will have globally.
During the time that U.S. and other scientists have monitored the cycle of ozone destruction above Antarctica, the pattern has been one year of severe, persistent depletion followed by a year of mild, short- lived depletion. In 1990 they had anticipated that the depletion would be moderate, as 1988 had been, but by October 1990, the depletion had already surpassed the total 1988 loss. Data already collected this year suggests that 1991 will be the third consecutive year of severe ozone depletion.
The reasons for this break with meteorological predictions remain unclear. One of the methods used to predict the severity of the "ozone hole" relies a cyclic pattern of stratospheric winds above the equator known as the Quasi-Biennial Oscillation (QBO). These winds reverse direction about every 26 months. In the past, an easterly flow appeared to be linked with a less severe ozone depletion. The 1988 depletion did support this theory--the equatorial stratospheric winds blew eastward, and the depletion was less severe. This pattern was broken in 1989, however, when, despite the easterly flow, the depletion reached record-setting levels.
One suggested explanation is that concentrations of chlorofluorocarbons, the source of ozone-destroying chlorine, in Earth's stratosphere have increased so much that near-total ozone destruction could occur most years. In another theory a NOAA meteorologist proposes that the relationship between the QBO cycle and sea-surface temperatures at the equator must be considered. His data show that when stratospheric temperatures (which affect the strength and direction of equatorial winds) and sea- surface temperatures increase from one summer to the next, the depletion above the Southern Hemisphere is worse the next austral spring.
THE EFFECTS OF OZONE DEPLETION
As stratospheric ozone has become depleted, biologists working in Antarctica have become increasingly more concerned about how exposure to enhanced levels of ultraviolet radiation will affect organisms living in the oceans surrounding Antarctica. Examining these effects has become a priority for U.S. scientists.
During the 1988 austral spring, when ozone declined only about 15 percent over Antarctica, UV levels measured at Palmer Station in October were as high as those measured during December, the height of the austral summer. However, during 1990, when the ozone depletion reached the record-setting levels of 1987, researchers at Palmer Station recorded the highest levels of biologically damaging UV radiation ever recorded in this region.
During the 1993 austral spring record low levels of ozone above Antarctica, researchers using NSF's uv- irradiance monitoring network observed record increases in the amount of UV-B radiation (the part of the ultraviolet spectrum that is most harmful to life) reaching Earth's surface. They found at the U.S. stations at South Pole, McMurdo, and Palmer Stations and reported the following
Amundsen-Scott South Pole Station: a 19-percent increase in UV>B above levels recorded over the last 2 years.
McMurdo Station: a 44-percent increase in this austral spring (1993) over previous years (observations made since 1988)
Palmer Station: 55 percent higher than the same period during each of the last 6 years.
Biology, particularly at Palmer Station, is focusing on the ability of plankton and other organisms to adapt to changing levels of UV radiation. They are interested in learning what effects enhanced exposure to UV radiation has on such basic biological functions as photosynthesis and DNA replication or repair.
Between September and November 1990 during a 6-week cruise in the Bellingshausen Sea, biologists, supported by the National Science Foundation, collected data on how phytoplankton in near-surface waters are responding to increased levels of UV radiation. They found that as a result of increased UV-B radiation from the widening "ozone hole," phytoplankton production in decreases by a minimum of 6 to 12 percent during the austral spring. Phytoplankton, microscopic free-floating plants, are the foundation of the oceanic food chain and manufacture organic material that sustains virtually all other organisms in the marine ecosystem. Although they need solar radiation for photosynthesis, shorter wavelengths of solar radiation--mid-range ultraviolet (UV) radiation (280 to 320 nanometers) or UV-B--are known to be harmful to phytoplankton.
