Stratospheric change Second plenary lecture at
Jekyll Island meeting
By Charles H.Jackman
The ability of humans to affect the ozone layer has been under investigation for more than 15 years. Production of odd nitrogen species by Super Sonic "ansports (SSTs) was thought to decrease ozone, and an intensive study of the ozone layer resulted in the early 1970s. By the mid-1970s it was realized that man-made chlorine- and bromine-containing compounds could have a detrimental effect on the atme spheric ozone. There now is undisputed observational evidence that atmospheric concentrations of source gases important in controlling stratospheric ozone levels (chlorofluorocarbons [CFCs], halons, methane, nitrous oxide, and carbon dioxide) continue to increase on a global scale because of human activities. It is believed that trace gas increases (CFCs and halons) combined with natural seasonal (temperature and wind) fluctuations cause the dramatic springtime Antarctic ozone decreases, which are most substantial in the lower stratosphere. between 12 and 20 km. Other substantial ozone decreases have been W1593BX1891M)W-13291.5010
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Charles Jack"
measured since 1919 with groundbased and satellite instruments at other latitudes, especially around 40 km altitude. After an intense investigation into these ozone changes, scientists involved in a recent NASA report ( I ) concluded that the ozone change is partly natural and partly anthropogenic. Because many anthropogenic chlorine- and bromine-containing compounds have long lifetimes and are still being produced, it is anticipated that even larger ozone changes will result in the future.
1989 American Chemical Society
Ozone (4)is located mainly in the stratosphere, between about 15 and 50 km. The maximum amouts are small, generally on the order of about 10 parts per million by volume @pmv). Ozone, through its absorption of ultraviolet and visible light, leads to the atmospheric temperature structure that accounts for the fist three layers of the atmosphere. The lowest layer of the atmosphere, which contains living organisms and where weather occurs, is called the troposphere. Above that, in the 15-50-km region, the absorption of light by ozone beats the atmosphere dramatically. This stratosphericregion is thus defined by a temperature increase from a minimum at the tropopause (bottom of the stratcsphere and top of the troposphere) to a maximum in temperature at the stratopause (top of the stratosphere). Because of this temperature inversion, the stratosphere is stable with respect to vertical motion. The stratcsphere. is so named because the air tends to be horizontally stratified (2). Above the stratosphere there is not enough ozone to cause substantial heating; thus the temperature decreases again until a minimum is reached at Environ. SOi.Technol., Vol.23, No. 11, 1989 1329
about 90 km. This higher region is known as the mesosphere. Because ozone does not iniluence the highest layer of the atmosphere, the thermosphere, that layer will not be discussed. Ozone in the stratosphere is important for life on Earth. Life would probably not have evolved out of the ocean onto dry land without the protective shield of ozone. Ozone shields living organisms from biologically damaging ultraviolet light. Atmospheric model predictions indicate that if human activity is not altered, then ozone in the stratosphere will decrease, leading to deleterious effects such as more skin cancer in humans. It is also likely that other organisms around the world will be adversely alhted. However, although stratospheric ozone is good for the world’s living organisms, the ozone in the tmposphere-which appears to be increasing-is a harmful oxidant in air pollution.
Prnduction and loss of ozone Ozone is produced through the photolysis of molecular oxygen (9) hy ultraviolet light into its two individual atomic oxygen, 0, COnSlituents. These atoms of oxygen react quickly with @ to form Q. Ozone is very easily photodissociated into @ and 0, hut usually the 0 will react rapidly with O2 to reform 0,.Sometimes (about 1 out of 2000 times in the middle stratosphere) the 0 will react with an O3 to form two 4, resulting in the loss of ozone. Ozone also is destroyed through catalytic reactions with nitrogen-, hydrogen-, chlorine, or bnnnine-wntaining constituents. The following is a typical ozone-loss catalytic process: c1 0, c10 + @ (1) CIO 0 c1 @ (2) Net:@+O-G+@ (3)
+
+
--
+
The chlorine atom is not consumed in reactions 1and 2, but it can !&e part in this catalytic process many hundreds of times before reacting with another molecule besides ozone. Nitrogenantaining species (NO, N@) are most important for the loss of ozone, but all constituents involved in ozone destruction must be understood before the total picture of ozone abundance is clear The WnStituents involved in ozone loss are in parts per trillion hy volume (pptv) to parts per billion hy volume (ppbv). Only through catalytic processes can these constituents regulate the abundance of ozone, which is in concentrations of ppmv in the stratosphere. Ozone is constantly being produced and destroyed. Its global level will be determined hy a balance between the production and loss. If one of its losses increases, then the global level of ozone will have to decrease. This reduction in 1324 Envlron. ScI.Technol.,VoI. 23,No. 11, 1989
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l
ozone is the liely result of the increasing atmospheric concentrations of chlorine- and hromine-xntaining species. The reactive species most responsible for the loss of ozone interact with other molecules as well as each other; thus, they can be temporarily or permanently removed from taking part in the catalytic destruction of ozone. For example, the chlorine atom, C1, can react with methane ( C h ) to form HC1, a reservoir species. Reservoir species are storage places for the reactive constituents of the atmosphere. Sometimes the reservoir species will even be lost from the atmosphere, such as when HC1 is transported down to the troposphere where it can be scavenged by water droplets and rained out. Other reservoir species result from the joining of two reactive species into a stable unreactive species. For e m ple, C10 can react with NO, to form CION@. This reservoir species is stable for days until it is photolyzed back to more reactive species. While the re active species are tied up in unreactive forms as reservoir species, they will not be able to destroy ozone. To understand the photochemistry of the strato-
sphere and the impact of humans on ozone, all these interactions as well as many others must be taken into account. Measurements ofozone
Ozone is measured by a variety of methods, including ground-based balloon-bome, sounding rocket, and satellite instruments. All are important in establishing ozone climatology and variability; however, only satellite measurements provide a consistent set of global measurements. One of the most long-lived sets of satellite ozone measurements comes from the Nimbus 7 satellite, which has both the Solar Backscatter Ultraviolet (SBUV) and the Total Ozone Mapping Spectrometer (TOMS) instruments aboard. The SBUV instrument provides altitude profile ozone data and total ozone data; the TOMS instrument provides total ozone data. The average global ozone data from the TOMS instrument for the two years 1979 and 1980 is presented in Figure 1 as a function of month and latitude. Column ozone is given in Dobson units (milli-atmosphere-cenheters). These
peculiar units for the column ozone measurement indicate the thickness of ozone if all the ozone above a point were brought to standard temperature and pressure (STP) and the height of ozone were measured. A measurement of 300 Dobson units indicates that the ozone above that point would be at a thickness of just 0.3 cm at STP. The data reveal several interesting features: Total ozone is less at low latitudes and higher at polar latitudes; Total ozone maximizes in late winter and early spring on the pole in the Northern Hemisphere and in late winter and early spring off the pole in the Southern Hemisphere; and Total ozone at low latitudes follows the sun with maximum ozone corresponding to maximum intensity of the sun.
Natural changes in ozone There are many time scales for variation of ozone. Most of these variations are solar- and wind-driven and are on the time scales of daily, several days, seasonal, annual, interannual, and about two years (quasi-biennial). There also are variations in ozone related to the solar cycle. The most important of these is that caused by the 11-year variation in ultraviolet radiation, which results in maximum ozone at times of maximum ultraviolet (solar maximum). The flux of galactic cosmic rays and solar protons also varies with the solar activity and causes changes in the production of species involved in the catalytic destruction of ozone. Other changes in ozone have been associated with natural events such as volcanic eruptions (3). Human influences on ozone Along with the natural changes in ozone, it is becoming increasingly evident that human activities can affect ozone. Atmospheric nuclear explosions in the early 1960s are believed to be responsible for some ozone depletion. In the 1970s the manufacture of the American SST plane was halted, largely because of atmospheric model predictions indicating that ozone would be reduced because of flying the SSTs in the lower stratosphere. CFCs and halons were postulated as leading to possible destruction of stratospheric ozone as early as the mid-1970s (4). CFCs (e.g., CFC13 and CF2C12)have been manufactured since the 1930s, and in the past two decades their use has increased substantially. Halons (e.g., CBrF3 and CBrC1F2) have been manufactured since the 1950s and in recent years have been used increasingly as fire extinguishers, especially for computer systems and for enclosed areas such as
those in airplanes. Both CFCs and halons are extremely useful and safe chemicals. These long-lived CFCs and halons do not react in the troposphere but work their way to the stratosphere, where they are broken down into their individual components by ultraviolet photolysis. Their chlorine and bromine by-products result in ozone destruction. Other gases are increasing in the atmosphere as a result of anthropogenic activity. These include carbon dioxide (COz), methane (CH4), carbon monoxide (CO), nitrous oxide (N20), methyl chloroform (CH3CC13),and carbon tetrachloride (CCL,). These other species along with CFCs and halons also can affect stratospheric ozone. For example, carbon dioxide and other greenhouse gases, while causing the tropospheric temperature to rise, can cause the stratospheric temperature to cool. This stratospheric cooling will decrease the rate of most photochemical reactions. These trace gases are increasing at various rates in the atmosphere from about 0.2% a year for N 2 0 to 10-15% a year for the halons.
Use of atmospheric computer models Many models of different complexities--from simple box models to general circulation models-are used to understand the behavior of ozone. These various models are useful, depending on the atmospheric problem being investigated. Nowadays the workhorses for many atmospheric prediction scenarios and other atmospheric occurrences are two-dimensional (2D) models whose dimensions are altitude and latitude. These 2D models include a transport formulation with both winds and mixing as well as a complex photochemical formulation involving about 50 atmospheric species interacting with each other through about 130 photochemical reactions. Two-dimensional models can be used to investigate the natural variations in the atmosphere as well as human influences. Such models have been used for more than 15 years and have produced a number of atmospheric predictions that can be compared with atmospheric behavior. During the last solar cycle, both 2D models and TOMS data indicated a downward trend in global total ozone between 1979 and 1986, much of which is a natural change from the de-
creasing ultraviolet flux. No model predicted the large changes observed in the Antarctic in recent years.
Antarctic ozone changes A few years ago a dramatic decrease in total ozone for the month of October was revealed in measurements taken since 1957 at Halley Bay Station, Antarctica. In that month a decrease of nearly 50% in total ozone occurred from the mid-1970s to the mid-1980s (5). This decrease was confirmed on a larger geographic scale from TOMS measurements and appears to cover the entire Antarctic continent. The global change in total ozone as observed by TOMS is given in Figure 2 for the time period between 1979-1980 and 1986-1987. Because the atmosphere has about a two-year cycle in ozone, two years of data are averaged together to obtain a more realistic change in ozone over periods of time. At polar latitudes (70-90 O S ) the ozone has changed in the September to November time period by over 20%. Nowhere at latitudes south of 50 “ S does the ozone show an increase. At other latitudes (especially in the Northern Hemisphere) the total ozone shows increases during certain times of the year. Models predict about a 2% decrease in total ozone as a result of the decrease in the solar ultraviolet output over this time period. Thus changes in the Northern Hemisphere and in the low latitudes of the Southern Hemisphere may well be natural. At high southern latitudes, however, ozone changes are outside those predicted by models to result from the solar cycle ultraviolet change. Recent campaigns to investigate these Antarctic ozone decreases have resulted in a wealth of information: Most of the ozone change is located in the 12-20-km region; a direct anticorrelation exists between C10 and O3 amounts (if C10 is large, then O3 is small, and vice versa); and polar stratospheric clouds appear to be abundant in the southern polar spring. Several theories have been postulated to explain the Antarctic “ozone hole,” including solar cycle effects, dynamic effects, and heterogeneous chemical reactions (reactions between gas and liquid or solid phases). The most plausible theory for this polar ozone destruction involves the following: The special weather conditions of the Southern Hemisphere polar winter lead to an isolation of polar air, which results in extremely low temperatures during the polar night; These low temperatures lead to polar stratospheric cloud formation; The cloud formation leads to a removal of the stratospheric reactive Environ. Sci. Technol., Vol. 23, No. 11, 1989 1331
FIGURE 2
Percentage changes by month and latitude i 1979-1980 and 1986-1987@ 90
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when compared with 1960. The CFCs and halons have lifetimes on the order of decades to more than a century, so it will he decades before the total effects of the proposed decreases will be observed. Also, model results indicate an increase in tropospheric ozone, which can be harmful to living organisms. There are no easy solutions, and if the model results are close to what may be expected in the future then the effects of any controls will not really be evident until about 2020, some 30 years away. NASA and the National Oceanic and Atmospheric Administration plan to continue to monitor ozone in the atme sphere through a series of satellites launched every couple of years amying an instrument to monitor ozone (SBUVIZ), an Upper Atmosphere R e search Satellite (scheduled for launch in 1991), and the Earth Observing System (scheduled for launch in the mid19%). Ground-based, balloon-borne, and instruments carried on space shuttles will also be used to monitor ozone. It is clear that more needs to be learned about ozone and its variability before we can definitively establish a global trend. It is also apparent, however, that ozone has decreased in the Antarctic polar stratosphere and that human activity is probably to blame.
Acknowledgments The author would like to achowledge Richard S. Stolarski of the NASNGoddard Space Flight Center and Stephen A. Klein of the University of Washington for useful comments on an earlier version of this manuscript. 0
nitrogen into nitric acid ice, and reactions on the surfaces of cloud partcles release the reactive chlorine; and The reactive chlorine leads to the observed ozone decrease when the sun comes up in the spring. Many of the details have yet to be worked out in this mechanism. It is possible that reactive bromine is also involved in some of the ozone decrease. A special issue of the Journal of Geophysical Research on the subject of Antarctic ozone destruction will be published in the fall.
Solution to ozone depletion It is becoming increasingly apparent that humans are affecting the stratosphere. Several proposals have been made recently for controlling emissions of CFCs and halons. One proposal (the Montreal Protocol) calls for a 50%cuthack from 1986 levels of CFC and halon production. This agreement resulted from an international meeting held in Montreal, Canada, in 1987 and has been signed by more than 30 nations. Since the Montreal meeting, policy makers in several nations have reaized that more stringent cutbacks are necessary in CFC and halon production to conserve stratosphericozone. 1332
Environ. Sci.Technol.,Vol.23, No. 11, I989
Western European c ~ u n t r i eas~ we' as the United states and Canada have recently called for a 100%phase-out in CFC and halon uroduction bv the vear 2OOO. These rec-ent proposal; will'undoubtedlv be discussed at the United year 2 W , because some developing countries have just ing these useful chemicals. Three scenarios were recentlv investigated with the use of the N A S A / G ~ dard Space Flight Center 2D model for a United Nations Environmental Program and World Meteorological Organization report, including no cutback in production of CFCs and Mons from 1986 levels, a 50%cutback in production of CFCs and halons from 1986 levels, and a 95 % cutback in production of CFCs and Mons from 1986 levels. All scenarios were run for a century over the years 1960 to 2060. The first scenario resulted in about an 8% decrease in global total ozone in 2060 when compared with 1960. The second scenario resulted in about a 4% decrease in global total ozone in 2060 when compared with 1960. The third scenario resulted in about a 2% decrease in global total ozone in 2060
References (1)Watson. R. T.: Prather, M. I.: Kurvlo. ~~r~~~ ,
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MD; Reference Publication 1208, 1988. (Z)Stolars!ii, R. S . 7ke American Smfisricinn 1982,36,303-11. (3)Hofmann, D. I.; Solomon, S. J. Ceophys. R ~ S i. w . % . s o z 9 - 4 1 . (4)Rowland,'F. .S.; Malina, M. 1. Nature 1574,249, 810-12. (S)Farman, 1. C.; Gardiner, B. 0.;Shanklin, I. D. Nofure 1985,315,207-10.
Charles H. J a c h is a member of the Atmospheric Chemistry and Dynamics Branch of the NASAIGoddnrd Space Flight Center in Greenbelt, MD. His research hos been on norural varkuions in atmospheric ozone and other a m spheric constiiuenrs; he hos been involved with modeling studies of the m'a2le atmosphere, including comparisons of 2 0 model results to satellite measurements. He received a B.S. degree in physics and mathematics from Nebraska Wesleyan University in 1972 anda Ph.D. in aeronomyfrom the University of Florida in 1978.
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