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Two decades ago, stratospheric ozone depletion was mainly of interest to atmospheric scientists. Today, it is a worldwide environmental concern that has been addressed by several international accords. Ozone depletion epitomizes the environmental problems humans face today: it is global and the direct but unintended result of human industry. Remedying it will have direct and indirect economic consequences. Since the mid-1970s, NASA has been in the forefront of research with space-based, airborne and ground-based observing programs.
Ozone, a molecule made up of three oxygen atoms, shields life on Earth from the harmful effects of the ultraviolet radiation of the sun. The increased amounts of ultraviolet radiation that would reach the Earth's surface because of ozone depletion could increase the incidence of skin cancer and cataracts in humans, harm crops and interfere with marine life.
Because the risks of increased ultraviolet radiation are very serious, scientists around the world are working to determine how much of the ozone-related change in the atmosphere is caused by humans and how much is attributable to natural processes, such as shifts in atmospheric dynamics, volcanic activity or cyclical changes in solar energy.
Studies have shown that ozone depletion is caused by complex, coupled chemical reactions. Emissions of human-made chlorofluorocarbons (CFCs), which break down into other ozone-depleting forms of chlorine, have led to depleted atmospheric ozone. The continued build-up of CFCs, historically used in refrigeration, electronics and insulating materials, could lead to additional ozone loss worldwide. Further research is essential to provide the necessary understanding of the causes of ozone depletion.
Beginning in the 1960s, NASA pioneered the study of the atmosphere from space. The agency's commitment to environmental research continues with Mission to Planet Earth. Using the unique perspective available from space, NASA is observing, monitoring and assessing large-scale environmental processes.
Interpreted through computer models of the atmosphere, MTPE satellite data, complemented by aircraft and ground data, will allow us to better understand natural environmental changes and to distinguish natural changes from human induced changes. MTPE data, which NASA will distribute to researchers worldwide, is essential to humans making informed decisions about protecting their environment.
Within Mission to Planet Earth, NASA's ongoing commitment to ozone studies includes current and future missions. Six of the projects are detailed below:
Deployed Sept. 15, 1991, from the Space Shuttle Discovery, the Goddard Space Flight Center's Upper Atmosphere Research Satellite (UARS) made the first space-based measurements of chlorine monoxide, a principal compound in ozone-depletion chemistry. Data from UARS' 10 instruments have greatly advanced our understanding of the chemistry and dynamics of the upper atmosphere and the coupling between the upper and lower atmosphere. UARS, the first major component of Mission to Planet Earth, is also the first satellite dedicated to studying stratospheric chemistry. UARS instruments focus on the processes that control ozone distribution, complementing other satellite instruments' measurements of total ozone levels.
UARS instruments are providing the most complete data on upper atmospheric chemical composition, radiation and winds ever gathered. In its first two weeks of operation, UARS data confirmed polar ozone-depletion theories by providing three-dimensional maps of ozone and chlorine monoxide near the South Pole during development of the 1991 ozone hole. In 1992, UARS tracked record levels of chlorine monoxide in the northern hemisphere and followed the buildup of ozone depleting chemicals over Antarctica before the 1992 ozone hole developed.
The UARS observations during two recent northern hemisphere winters have provided crucial new information about the polar chemistry and its variation from year-to-year. These observations have been compared to those from the southern hemisphere, where significant ozone loss takes place. UARS also followed the evolution of the clouds of aerosols--liquid particles--produced in the atmosphere following the 1991 eruption provide a picture of the evolution of Mount Pinatubo and has provided data to evaluate the role of volcanism in ozone chemistry.
Several sounding rocket and balloon experiments, managed by NASA/Goddard's Wallops Flight Facility, Wallops Island, Va., have launched in support of the UARS program since 1991. The purpose of the flights is to validate the accuracy of UARS instrument measurements.
UARS continues to collect data on the chemistry, dynamics and radiative inputs to the upper atmosphere beyond its designed lifetime of 18 months. Eight of the 10 UARS instruments were still operating in January 1994. UARS was developed and managed by GSFC.
The Atmospheric Laboratory for Applications and Science (ATLAS), a series of Space Shuttle-Spacelab missions, carries instruments to measure ozone and other chemicals in the upper atmosphere and to measure solar energy received by the Earth system. The science objectives of the ATLAS series are to study processes in the atmosphere and climate system and to provide highly-calibrated measurements to orbiting spacecraft. ATLAS is contributing to the study of how Earth's atmosphere and climate system are affected by solar variations and by the products of industrial and agricultural activities. ATLAS is an international effort involving scientists from the U.S. and Europe.
ATLAS-1, which flew March 24-April 2, 1992, aboard the Space Shuttle Atlantis successfully conducted 12 investigations in atmospheric science, solar irradiance, space plasma physics and astronomy. ATLAS-2, flown aboard the Space Shuttle Discovery April 8-17, 1993, successfully performed observations of the atmosphere and solar irradiance. Because the ATLAS instruments are calibrated both before and after each flight, these measurements will provide valuable information to assist in the analysis of data from UARS, National Oceanic and Atmospheric Administration (NOAA) and other satellites whose instruments cannot be returned to Earth after their mission is completed. The ATLAS payload measures 30-40 trace gases in the middle and upper atmosphere, thus complementing the free-flying satellites that measure a smaller number of species.
ATLAS-3 is scheduled for launch in October of 1994. The ATLAS missions are managed by Marshall Space Flight Center, Huntsville, Ala.
Not all of NASA's ozone research is conducted from orbiting spacecraft. Aircraft expeditions flying instruments through the atmosphere have conducted detailed studies of the processes of ozone depletion. Airborne expeditions over the Arctic and Antarctic have made direct measurements of chlorine monoxide, which were essential in proving that chemical reactions involving human-produced chlorine are the main cause of ozone depletion in the upper atmosphere.
NASA, in conjunction with the National Oceanic and Atmospheric Administration, the National Science Foundation and industry groups, has conducted three airborne campaigns, over the Antarctic in 1987, over the Arctic in 1989 and the over the Arctic and northern mid-latitudes in the winter of 1991-92.
Airborne studies of the stratosphere extending from the northern mid-latitudes down through the ozone-depleted Antarctic vortex will continue in 1994 from a base in Christchurch, New Zealand. The expeditions of 1987, 1989 and 1991 relied on NASA's high-altitude ER-2 jet aircraft and the DC-8 flying laboratory. The 1994 airborne research will be conducted by the ER-2 aircraft alone. The airborne research programs are managed for NASA by the Ames Research Center, Mountain View, Calif.
NASA's most visible and best-known ozone research program is the Total Ozone Mapping Spectrometer (TOMS). Since the launch of the first TOMS aboard the Nimbus-7 polar-orbiting satellite in 1978, NASA has provided scientists with reliable, high-resolution daily maps of global ozone levels. Managed by NASA's Goddard Space Flight Center (GSFC), Greenbelt, Md., TOMS is a primary source of data on global ozone day-to-day variability and long-term trends. Ozone-depletion data from TOMS underpins several international agreements to phase out the use of CFCs and other ozone-depleting chemicals.
Analyses of TOMS data have traced in detail the annual development of the Antarctic ozone hole, a large area of intense ozone depletion that occurs between late August and early October. The ozone hole was discovered through British ground-based observations in the mid-1980s, but analysis of TOMS data indicates it has existed since at least 1979. Most recently, TOMS data showed record levels of ozone depletion over Antarctica in 1993.
TOMS data have also shown a long-term depletion of ozone in the Northern Hemisphere's mid-latitudes, reaching record lows in late 1992 and early 1993. The possibility that increased ultraviolet radiation could reach the Earth's surface during the beginning of the growing season raises questions of significant economic, environmental and health effects.
A long-term, consistent record of ozone levels is essential to understanding and predicting ozone depletion. To ensure that ozone data will be available throughout the next decade, NASA has been continuing the TOMS program using U.S. and international launches. On Aug. 15, 1991, the former Soviet Union launched a Meteor-3 satellite carrying a TOMS instrument provided by NASA. The Meteor-3/TOMS instrument ensured continuity of data when Nimbus-7/TOMS ceased operating in May 1993.
NASA will continue the TOMS program; using U.S. and foreign launches. The Japanese Advanced Earth Observations Satellite (ADEOS) will carry a fourth TOMS into orbit when it launches in 1996, and a fifth TOMS Instrument is being assembled for flight in 1998 on an undetermined satellite.
The Shuttle Solar Backscatter Ultraviolet Experiment (SSBUV) is a highly-calibrated instrument developed at GSFC for periodic flights aboard the Space Shuttle. SSBUV measures ultraviolet light from the sun and the amount of that light scattered back to the instrument by the Earth's atmosphere. The change in radiation between the two measurements is used to calculate ozone levels.
SSBUV measures the total amount and height distribution of ozone in the upper atmosphere and collects data to calibrate ozone-measuring instruments on other satellites. SSBUV measurements are checked against data from the Solar Backscatter Ultraviolet experiment aboard the NOAA-9 and NOAA-11 satellites and NASA's UARS. SSBUV has flown seven times: STS-34 (October 1989), STS-41 (October 1990), STS-43 (August 1991), STS-45/ ATLAS-1 (March 1992), STS-56/ATLAS-2 (April 1993), STS-62 (March 1994), and STS-66 (October 1994). The next planned mission is STS-72, scheduled for November 1995. Regular flights are scheduled through this decade.
The Stratospheric Aerosol and Gas Experiment (SAGE) was launched first in 1979 aboard the Applications Explorer Mission B spacecraft and provided ozone measurements using the solar occultation technique until 1981. The application of this technique represented the first global, high-vertical resolution data set for stratospheric ozone. The vertical profile measurement capability and self-calibrating nature of SAGE distinguishes its measurements from those of other spaceborne instruments.
SAGE II began operation with the launch of the Earth Radiation Budget Satellite in 1984 and is still healthy today. Its high resolution ozone, nitrogen dioxide, water vapor, stratospheric aerosol and polar stratospheric cloud observations are making important contributions to studies of the Antarctic ozone hole as well as the more subtle, but equally alarming decline of ozone over mid-latitudes.
SAGE data show that these mid-latitude ozone losses have occurred primarily in the lower stratosphere, a finding contrary to predictions of classical atmospheric chemistry models. Stratospheric aerosols such as those produced by major volcanic eruptions are thought to be important catalysts in the chemical processes leading to the observed ozone losses. In this regard, SAGE also is monitoring closely the dispersal and decay of the massive aerosol cloud formed by the 1991 eruption of Mount Pinatubo. SAGE is managed by Langley Research Center, Hampton, Va.
Once data has been gathered, scientists must process it and organize it in ways that allow them to draw conclusions about ozone's chemistry and how it is transported in the atmosphere. The most effective way to do this is to incorporate data into computer models that use mathematical formulas, called algorithms, to simulate, or model, ozone processes.
There are two critical roles for these computer models: providing quantitative tools to test our understanding of ozone processes and to predict the effect on ozone levels of changes in the atmosphere. Scientists test the model's ability to represent the observed distributions of ozone and chemicals, especially how levels of these chemicals vary across latitudes, altitudes and seasons. They also test the models to see whether the computers can simulate the evolution of the atmosphere in recent years, accurately reflecting, for example, changes in total ozone seen by the TOMS instruments.
The models are also applied to simulating the atmospheric response to predicted events, such as an increase in CFCs or emissions from a projected fleet of supersonic aircraft. These models form a critical part of internationally sponsored ozone assessments, such as those conducted for the World Meteorological Organization and the United Environment Programme.
Models can range from relatively simple one dimensional programs that follow individual air masses as they are blown through the atmosphere by the prevailing winds to three-dimensional global models. The more complex models include meteorology and chemistry in the lower and upper atmosphere. These complex models can also be used to investigate links between atmospheric chemistry and meteorology, such as what effect changes in ozone amounts may have on atmospheric temperatures (or vice versa).
NASA-sponsored ozone modelling programs are carried out at several NASA centers, most notably GSFC and the Langley Research Center, Hampton, Va., as well as at universities, other government agencies and private organizations.
NASA's ozone studies are just one element of Mission to Planet Earth. Throughout the 1990s, NASA also will undertake missions to study the Earth's land surfaces, water and ice systems, climate, tropical rainfall, ocean topography and sea-surface winds. The Earth Observing System (EOS), the centerpiece of Mission to Planet Earth, will build on the results of these missions with a series of satellites that will combine atmospheric, oceanic and land surface observations into a global environmental study, focusing on climate change.
EOS will also continue the chemical, dynamical and solar measurements important to understanding ozone processes. In particular, the EOS-Aero series (first launch in 2000) and EOS-Chem series (first launch in 2002) will provide data on atmospheric chemistry and solar radiation important to ozone studies.
As part of NASA's Mission to Planet Earth, the agency's ozone depletion studies are designed to observe the Earth with coordinated, simultaneous measurements of large environmental systems. The EOS Data and Information System will incorporate ozone and other kinds of data for the widest possible distribution to U.S. and international researchers. With this and other Mission to Planet Earth observations, scientists will be able to provide policymakers and the public with a better understanding of our changing environment. And that understanding will allow humans to make more informed decisions about the way their actions will affect their planet.