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PROBING PLANETARY
POLLUTION FROM SPACE
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612 Environ. Sci. Technol.. Vol. 25, No. 4. 1991
0013-936)(/91/0925-612$02.50/0 0 1991 American Chemical Society
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Jack Fishman Atmospheric Sciences Division NASA Langley Research Center Hompton, VA 23665-5225 The past decade has seen the development of spaceborne instrumentation to measure tropospheric trace gases that are pollutants. Carbon monoxide measurements have been made twice from the space shuttle, in 1981 and 1984. In the late 198Os, tropospheric ozone amounts were derived using information from two independent satellite instruments that had been originally designed to measure ozone in the stratosphere. The first measurements from space of a trace gas pollutant, carbon monoxide, were obtained over a three-day period in November 1981 when the space shuttle CoIumbio ( 1 ) used a gas cell correlation radiometer named MAPS (Measurement of Air Pollution from Satellites). MAPS was flown again in October 1984 aboard the space shuttle Challenger (2). Both sets of measurements showed high concentrations of CO over Africa, Asia, and South America, illustrating the fact that air pollution is a worldwide phenomenon, not just a problem confined to industrialized countries. A more recent set of studies 13.4) has shown that tropospheric ozone concentrations can be inferred by using satellite data sets that had initially been intended to measure ozone i n the stratosphere. From these analyses, plumes of ozone emanating from the Asian, North American, and African continents can be delineated. Subsequent analysis (5) suggests that high amounts of ozone originating from Africa may even be influencing the ozone concentrations measured over Brazil during certain times of the year. Despite the wealth of insight that these observations have provided, the future for satellite remote sensing of the composition of the troposphere is not clear. Routine measurements of air pollution on a global scale will depend on the successful development of new instrumentation that can be used from space and on a commitment by governments to provide platforms for
such instruments. One exciting new technique involves active remote sensing using DIfferential Absorption Lidar (DIAL) technology which can determine the vertical distribution of aerosols and selected trace gases with considerably more resolution than passive remote sensing techniques (6-91.Such instruments have been flown successfully in airplanes: the eventual goal is placement of these prototypes on permanent space-based platforms. This paper will review the data sets obtained from instruments that have measured carbon monoxide and tropospheric ozone from space. In addition, an example of the capability of DIAL systems will be shown and the current plans for monitoring pollution from spaceborne platforms will be discussed.
Why global-scale measurements? The scientific rationale behind the measurement of trace gases on a global scale is not the same as the reasoning behind measuring pollutants on a local scale. When the United States Environmental Protection Agency was formed in 1970, one of its primary functions was to establish National Ambient Air Quality Standards (NAAQS)for a number of pollutants to ensure that the air we breathe contains sufficiently low concentrations of certain trace gases to guarantee protection of public health (IO].Therefore, it was important to monitor the concentrations of these pollutants so that the population knew when harmful levels of trace gases (Le., violations of the NAAQS standards) were present. From a global perspective, however, it is unlikely that concentrations of pollutants will ever become high enough on a scale resolved from space that breathing such pollutants would pose a health risk. The primary purpose for understanding the global distribution of tropospheric trace gases is that continued emissions of certain trace gases will perturb the oxidizing capacity of the atmosphere. In other words, the self-cleansing ability of the atmosphere could be dramatically altered if the composition of the troposphere changes significantly over decades or centuries. The removal of most gases released to the atmosphere is Environ. Sci. Technol.. VoI. 25,No. 4, 1991 613
through reactions that are initiated by the hydroxyl radical (OH). Integrated throughout the troposphere, the amount and distribution of OH are often referred to as the oxidizing capacity of the atmosphere. The atmospheric abundance and distribution of OH, in turn, depend on the distribution and concentration of several key trace gases, including CO and tropospheric 0,. (Other important trace gases are water vapor and NOJ. Since the concentrations of these trace gases may increase because of the planet’s need to support a larger population, humankind may also be reducing the capacity of the atmosphere to cleanse itself over the next several decades. The atmospheric lifetimes of CO and 0, are sufficiently short (a few months) that observed increases in their concentrations may be quite dependent on the location of monitoring sites. Therefore, it becomes important to know the spatial behavior of these trace gases and how their distributions and abundances may be altered over the course of time. The long-term trends of trace gases such as CO, and the chlorofluorocarbons can be deduced from a network of ground-based stations around the world because their atmospheric residence time is sufficiently long (several years) to allow them to become well mixed on a planetary scale. Tropospheric ozone is also a greenhouse gas, as it absorbs infrared radiation near 9.6 pm (11. 12). Since this region of the spectrum is relatively optically thin, increases in tropospheric ozone will result in efficient warming of the atmosphere through the greenhouse effect. Therefore, from a global perspective, it is important to gain knowledge of the distribution and ahundance of carbon monoxide and tropospheric ozone because of the important role they play in determining the oxidizing capacity of the atmosphere and the greenhouse efficiency of the troposphere.
tribution of a tropospheric trace gas was obtained during the second space shuttle flight, November 1214, 1981. Although that mission lasted three days, data were obtained for only ahout 12 h because of problems with maintaining the instrument at the proper operating temperature aboard the shuttle (1). The same instrument was flown aboard the space shuttle Challenger during a nine-day mission, October 5-13, 1984. Data were obtained for approximately 85 h during this period, mostly during the first three days and the last three days of the mission. The results from this flight are shown in Figure 1. The highest concentrations of CO were found over the east coast of southern Africa, over and downwind of Asia at northern temperate latitudes, and over much of South America. Although considerably less extensive, the November 1981 MAPS data set also showed relatively high concentrations of CO over central Africa, over and downwind of China, and off the coasts of northern South America. These two auasi-global data sets ushered in a 6ew uLderstanding of CO distribution in the troposphere, in contrast to previous surveys that
The MAPS experiment The MAPS instrument works on the principle of correlation radiometry which takes advantage of carbon monoxide’s unique spectral signature of near 4.67 Fm. It is most sensitive to the amount of CO in the middle and upper troposphere and, therefore, its detection capability best describes the CO distribution found near 400 millibars (-6 km). The first global snapshot of the dis614 Environ. Sci. Technol.. Vol. 25. NO. 4, 1991
consistently indicated highest CO concentrations to be at northern middle and high latitudes (13, 14). The finding of very high CO levels in the tropics confirmed the hypothesis put forth in the late 1970s by Crutzen et a]. (15)that tropical biomass burning was a source of CO comparable to fossil fuel combustion at northern temperate latitudes. Another reason for MAPS measuring relatively high CO concentrations in the tropics is tied to the fact that boundary layer air is transported to the middle and upper troposphere more effectively at low latitudes because of more convective activity there. In many instances, the high CO concentrations found in the boundary layer at midlatitudes do not effectively get transported to higher altitudes, a necessary condition for the CO to be measured by MAPS. Distribution of tropospheric ozone The distribution of tropospheric ozone can be determined from the analyses of data sets obtained independently from two different instruments: the Total Ozone Mapping Spectrometer (TOMS) and the Stratospheric Aerosol and Gas Experiment (SAGE]. In October 1978,
TOMS was launched aboard the Nimbus 7 satellite to provide daily maps of the global distribution of total ozone. Total ozone is defined as the integrated amount of ozone between the surface and the top of the atmosphere. One of the units used to measure total ozone is a quantity known as a Dobson unit (D.U.), where 1 D.U. = 2.69 x lo1' molecules of ozone cm-'. A typical amount of total ozone found in the atmosphere is 300 D.U., and approximately 90% of this ozone is located in the stratosphere (between -15 and -55 km). At middle and high latitudes, the distribution of total ozone is primarily governed by the prevailing large-scale circulation patterns. These patterns can vary substantially on a daily basis, and intense gradients of total ozone have been observed with differences of nearly 200 D.U. at two locations less than a few thousand kilometers apart. At these higher latitudes, total ozone amounts range between -225 D.U. and -500 D.U. Only recently have values as low as 125 D.U. been observed during austral spring in conjunction with the so-called Antarctic ozone hole (16). At lower latitudes, however, the distribution patterns of total ozone behave much less dramatically than at middle and high latitudes. The intense gradients of as much as 200 D.U. found at the higher latitudes are replaced by much more subtle gradients of no more than 20-30 D.U. Figure 2 illustrates the distribution of total ozone between 50° N and 50" S averaged for the months of March and September 1985. The details of the total ozone distribution at mid-latitudes in both months cannot be discerned because of relatively high amounts of total ozone (>315 D.U.) generally present at these latitudes. In the Northern Hemisphere, the white area, representative of total ozone greater than 315 D.U., extends farther south in March than in September. Conversely, the white area in the Southern Hemisphere extends farther north in September than in March. These sharp average total ozone gradients are indicative of the mean position of the jet streams for those two particular months. The daily meandering of the jet stream and the large-scale circulation patterns that it drives are most responsible for the wide range of variability observed at middle and high latitudes in both hemispheres. In general, the location of
the jet stream extends to lowest latitudes during the late winter and early spring in both hemispheres. Because the primary intent of the measurement of total ozone was to study the distribution of stratospheric ozone, very little research was conducted using the information provided by TOMS in the tropics. When compared with the distribution of carbon monoxide shown in Figure 1, however, it is particularly noteworthy that the higher amounts of both total ozone and CO are located over Africa, the tropical Atlantic Ocean, and South America. In addition, the lowest amounts of both of these gases are found over the central equatorial Pacific Ocean. Fishman et al. (1 7)showed that the distribution of total ozone and CO revealed by the first MAPS flight in November 1 9 8 1 was also highly positively correlated at low latitudes. Therefore, the hypothesis was put forth that variations in total ozone observed by TOMS at low latitudes were likely the result of processes occurring in the troposphere, rather than in the stratosphere.
The use of TOMS for tropospheric studies was taken a substantive step further when data from SAGE were used to derive the amount of ozone in the stratosphere. Ozone measurements from the SAGE instruments (SAGE was launched in February 1979 and operated through November 1981; SAGE I1 was launched in November 1984 and is still operating) provide the vertical distribution of ozone in the stratosphere. From these profiles, the amount of ozone in the stratosphere (Le., from the tropopause to -55 km) could be integrated and then subtracted from the co-located total ozone amount derived independently from TOMS on the same day. A schematic representation of the method used to derive the tropospheric residual is shown in Figure 3. Although these two quantities are nearly equal, and the subsequent subtraction of two relatively large numbers does not appear to be capable of yielding very accurate results (both instruments claim a prec i s i o n of 3 % ) , t h e r e s u l t a n t distribution of the tropospheric re-
I
o"
Longit
I 285 I oial ozone
295
in Dobson units
*
Increasing ozone concentration
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sidual displayed remarkable patterns when examined as a function of latitude and longitude. Consequently, the global distribution of tropospheric ozone should yield important insight into the origin of this trace gas which has been the subject of substantial scientific debate since the early 1970s (IS-201. This debate centers around the relative importance of the anthropogenic and natural sources of tropospheric ozone. Although it is well established that large quantities of ozone pollution can be generated by major metropolitan areas such as Los Angeles, the importance of this source globally relative to t h e amount of ozone that is transported out of the stratosphere into the troposphere by natural processes is still not clear. Figure 4 depicts the distribution of the integrated amount of tropospheric ozone obtained from more than 32,000 coincident SAGE and TOMS measurements between 1979 and 1989. For comparison, the tropospheric residual depictions presented by Fishman et al. ( 4 ) used approximately 24,000 coincident observations through 1987. In general, however, the patterns from both studies are very similar. Of particular interest are the distinct plumes that seem to result from pollution originating in North America, Asia, Africa, and Europe. In the three northern continents, the plumes originate over the eastern portions of each landmass and are transported by the prevailing westerlies for several thousand kilometers. From this analysis, it is difficult to state definitively whether the orange area in the eastern Pacific is a continuation of the Asian plume, or an enhancement caused by lowlevel easterly winds that sometimes blow off the coast of California. Fishman et al. (4) show an example of a total ozone plume extending several thousand kilometers to the west from the southern California coast. Measurements of nitric acid (211at Mauna Loa (Hawaii) likewise indicate that pollution from southern California periodically reaches the Hawaiian Islands (22). At low latitudes, the highest concentrations of the tropospheric residual are off the west coast of Africa. At these latitudes, the prevailing low-level winds are trade winds (easterlies) which would carry the emissions from central and western Africa to the eastern south tropical Atlantic Ocean. The prevailing upper level winds, however, are west616
Environ. Sci. Technol., Vol. 25, No. 4, 1991
erlies so it is likely that once the emissions and photochemically generated ozone are transported to higher elevations, they are carried long distances to the east (23). This explanation would account for the long yellow tail that is seen to the east of Africa, almost to Australia.
Figure 5 shows how the distribution of the tropospheric residual varies seasonally. These depictions illustrate how dramatically these fields can change from season to season. During the summer (JuneAugust), the highest values of the tropospheric residual are found at
Glossary DIAL technology-Dlfferential Absorption Lidar Dobson unit (D.U.)-one of the units used to measure 1 amount of total ozone in the atmosphere is 300 D.U. EOS platform-Earth Observing System platform ESA-European Space Agency LASA platform-Lidar Atmospheric Sounder and Altimeter LIDAR-LIght Detection And Ranging MAPS-Measurement of Pollutants from Satellites MAPS experiment-flown on Columbia Nov. 13-15, 1981,and on Challena Oct. 5-13, 1984 Northern Hemisphere-0" to 90" N ppbv-arts per billion by volume SAGE-Stratospheric Aerosol and Gas Expel SCIAYACHY-German abbreviation for Scai trometer for Atmospheric Chartography Southern Hemisphere-0" to 90" S Stratosphere-90% of the ozone is located here (' TES-Tropospheric Emission Spectrometer TOMS-Total Ozone Mapping Spectrometer Tropopause-10 to 18 km (altitude at which the stratosphere begins 2 ozone concentrations begin to increase to form the so-called ozone layer) Troposphere-Lowest part of the atmosphere, generally located bet(--surface and 15 km
>
\
SAGE (limb scanner):
Isolate stratospheric component
I I (= 270 D.U.)
- - - - - - - Tropopause
northern mid-latitudes. A band in excess of 40 D.U. extends across the entire Northern Hemisphere at thi time of the year w i t h highes amounts >50 D.U. being found over Europe and downwind of Asia. In addition to the in situ generation of ozone during this time of the year, the tropospheric residual increases because the depth of the troposphere is greater in the summer than in the winter. The lowest tropospheric residuals during the summer (and during other seasons, as well) at northern mid-latitudes are found over the Himalayas and the northern Rockies, primarily because the depth of the troposphere is less at these locations than at any other northern temperate site. The June-August depiction shows a much larger hemispheric asymmetry than during the Southern Hemisphere summer (December-February). This finding indicates that the greater depth of the troposphere during the summer is not the primary cause of the much larger summertime tropospheric residual values in the Northern Hemisphere relative to the Southern Hemisphere. Figure 6 compares the seasonal cycles of the tropospheric residual at mid-latitudes (35'45') of both hemispheres. Whereas the largest values in the Northern Hemisphere are found during the summer (July), the Southern Hemisphere maximum is attained during its late winter and early spring (AugustOctober). Fishman et al. (41 suggest that input of ozone from the stratosphere in the Southern Hemisphere is strongest during this time of the year and that this source is probably the largest one in the Southern Hemisphere (rather than anthropogenic input). On the other hand, the distribution of the tropospheric residual during austral spring (SeptemberNovember) in Figure 5 suggests that ozone generated in the African plume is transported to southern mid-latitudes during this time of the year. Therefore, it is possible that the reason for the highest ozone amounts on a seasonal basis being found at southern mid-latitudes may also be a result of anthropogenic pollution. The long-term measurements of carbon monoxide and methane at Cape Point, South Africa (34' SI, also exhibit a maximum during September (24, 251. In addition, the same in-phase seasonal cycle is observed for C", and CO at Cape Grim, Australia (41's) (26,271.
The distribution of t h e tropospheric residual using annual average data, 1979-198ga 50"N
~
The observed seasonal cycle for carbon monoxide at these Southern Hemisphere sites suggests that it can be explained by the seasonal cycle of the OH radical. OH concentrations are lowest in June and July in the Southern Hemisphere (since the OH seasonal cycle should be closely tied to the amount of solar radiation reaching mid-latitudes),so CO, having an atmospheric residence time of 1-2 months, should peak with a lag of approximately 2 months, resulting in highest concentrations during August-September. The depictions in Figure 5, however, show that it is possible that the maximum observed during austral spring at southern mid-latitudes can be explained by the seasonal maximum (Augustactober) in tropical biomass burning and subsequent poleward transport by large-scale circulation processes. Such a hypothesis needs to be explored with considerably greater rigor. Examination of the tropospheric residual distribution during spring (March-May) i n t h e Northern Hemisphere also reveals considerably more tropospheric ozone in the Northern Hemisphere than in the Southern Hemisphere. Traditionally, it has been believed that outflow of ozone from the stratosphere is greatest during the spring (e.g., see Reference 281, and that this process is the primary reason for relatively high amounts of ozone being measured. The higher ozone amounts found downwind of the continents
in the Northern Hemisphere during this time of the year also suggest that there may be some photochemical production of tropospheric ozone contributing to the higher levels found during the spring. On the other hand, if there is a large source of tropospheric ozone coming from the stratosphere during this time of the year, the preferred outflow regions would be downwind of the mountain ranges, generally in locations not too dissimilar from those where the enhanced ozone amounts are found during all seasons of the year. Unfortunately, because the tropospheric residual technique cannot differentiate the vertical location of the ozone within the troposphere, either explanation is consistent with the observations. More in situ ozone measurements in the boundary layer and throughout the free troposphere are required to explain the spatial variability in the Northern Hemisphere during the spring. The most consistent feature throughout the year is the location of the lowest values of the tropospheric residual in the equatorial Pacific Ocean. Values 70-75 ppbv) are found toward the latter portions of the flight and are associated with the dense aerosol layers below 1 km and near 2 km. Another interesting feature in this depiction is the presence of relatively high ozone concentrations measured during the first part of the flight above 2 km
The Earth System Sciences Committee of the NASA Advisory Council has recommended that a platform be p r o v i d e d to make multilayer measurements of CO within the troposphere (31). It is likely that such a measurement capability will be available on the first Earth Observing System (EOS)platform, which should be launched before the turn of the century. Until that time, the MAPS instrument has been manifested for three more space shuttle flights, each spaced 18 months apart, starting in 1994. A more sophisticated instrument, the Tropospheric Emission Spectrometer (TES), has been recommended for selection on the second EOS platform, which tentatively is scheduled for launch 30 months after the first platform is in orbit. TES will have the capability of measuring quite a few tropospheric compon e n t s , including tropospheric ozone, carbon monoxide, nitrogen dioxide, some hydrocarbons, and sulfur dioxide at several layers in the troposphere. NASA also plans to launch a series of smaller satellites, called Earth Probes. It is possible that one of these will have both a SAGE and a TOMS instrument, in which case tropospheric residual ozone data can be obtained Erom the same satellite. The European Space Agency (ESA) similarly has planned to launch an EOS platform by the late 1990s. One candidate instrument for that platform, SCIAMACHY (German abbreviation for Scanning Imaging Absorption Spectrometer for Atmospheric Chartography), will measure some tropospheric trace gases directly and infer concentrations of others, such as ozone and nitrogen dioxide, by using the residual technique described above. In addition, there are plans to launch a satellite in the mid-1990s that will have instrumentation from which a tropospheric ozone residual can be computed. Farther down the road in NASA’s EOS scenario is the LASA (Lidar Atmospheric Sounder and Altimeter) platform (3.21, which will house one or more of the lidar systems like those described earlier. LASA, part Environ. Sri. Technol., VoI. 25, No. 4, 1991 619
E d
that there are no guarantees when it comes to space research. As a result of these studies, scientists now know that pollution is a widespread phenomenon that has impacted the composition of the atmosphere on continental and hemispheric scales. Such perturbations will have far-reaching consequences for global atmospheric chemistry and climate. New measurement techniques using active remote sensors have already produced exciting data sets from aircraft platforms, and should provide an entirely new capability when used from space sometime in the 21st century. Acknowledgments The author thanks Toffee Fakhruzzaman of ST Systems Corporation (STXI, Ham ton, VA, for his help in the analysis o!tropospheric residual data and the preparation of figures used io this study. Ed Browell and Hank Reichle of NASALangley supplied some of the data described io this paper and commented on an earlier version of the manuscript. The comments of Marta Fenn of STX and the previous contributions of Cathy Watson of NASA-Langley are also gratefully acknowledged. This study was supported in part by NASA's Global Tropospheric Chemistry Program.
References (1) Reichle, H. G..Jr. et al. I. Geophys. Res. 1986,91lD10), 1086547. (21 Reichle, H. G.,Jr. et al. I. Geophys. Res. 1990,95(D71,9845-56. (3) Fishman, J. et al. Clim. Appl. Meteorol. 1987,26[12),1638-54.
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of a complement of new instruments, will fly in polar orbit. The use of lidar systems from space permits derivation of remotely sensed atmospheric profiles of aerosols and several trace gases with unprecedented resolution. (Ozone and water vapor profiles have already been obtained with airborne DIAL systems; sulfur dioxide and nitrogen dioxide appear feasible.) Clearly, the realization of a platform such as LASA is more than a decade away, but technologically, the development of these sophisticated systems is within our grasp. Two major hurdles in the use of satellites for studies of global pollution are funding and the commitment of a major space agency to measure tropospheric trace species from space. Mission to Planet Earth and, in particular, the building of the EOS platforms envisioned for study of the Earth's atmosphere, land, and oceans over a 15-year period, will cost NASA more money 620 Environ. Sci. Technol.. Voi. 25. No. 4, 1991
than the agency has ever spent on a scientific observatory. The current political environment supports a scientific commitment to a comprehensive study of global change. If NASA pursues every aspect of Mission to Planet Earth, other big programs within the agency, such as the manned mission to Mars, and Space Station, most likely cannot be funded at the levels their proponents would like to see. Even within EOS, it is not clear how high the priority is to measure tropospheric trace gases. The global tropospheric pollution issue has not shared the limelight with stratospheric ozone depletion and global warming-two pressing issues to which space-based insmentation can clearly contribute meaningful measuements. Nonetheless, exciting tropospheric trace gas data sets can be obtained from spaceborne instrumentation. The prospect for more such data in the future is promising, even if not guaranteed. But history has shown
Jack Fishman received a B.A. degree in mathematics from the University of Missouri in 1971, nnd an M.S. degree and Ph.D. in meteorology from Saint Louis Universityin 1974 and 1977, respectively. Before coming to NASA's Langley Research Center in 1979, he did pastdoctorol work nt the National Center for Atmospheric Research in Boulder, CO, a n d of the Max Plonck Institute for Chemistry in Mainz, Germany. Much of his research has concentrated on the photochemistry of the background tmposphere and on the use of satellite dnta sets for tropospheric studies. He is currently a senior research scientist and the assistant head of the Chemistry nnd namics Brunch in the Atmospheric Sciences Division.
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(4) Fishman, J. et al. J . Geophys. Res. 1990, 95(D4), 3599-617. (5) Watson, C. E. et al. Presented at Chapman Conference on Biomass Burning, Williamsburg, VA, A m . Geophys. Union: Washington, DC, March 1990. (6) Browell, E.V. Proc. IEEE 1989, 77(3), 419-3 2. (7) Browell, E. V. et al. J. Geophys. Res. 1987, 92(D2), 2112-20. (8) Browell, E. V. et al. J. Geophys. Res. 1988, 93(D2), 1431-51. (9) Browell, E. V. et al. J. Geophys. Res. 1990, 95(D10),16887-901. (10) Fishman, J.; Kalish, R. Global Alert, The Ozone Pollution Crisis; Plenum: New York, 1990, pp. 227-49. (11)R a m a n a t h a n , V.; C i c e r o n e , R. J.; Singh, H. B.; Kiehl, J. T. J. Geophys. Res. 1985, 90(D3), 5547-66. (12) Fishman, 7.; Ramanathan, V.; Crutzen, P. J.; Liu, S . C. Nature 1 9 7 9 , 282 (5741), 818-20. (13) Seiler, W. Tellus 1974, 26(1-2), 117135. (14) Seiler, W.; Fishman, J. J. Geophys. Res. 1981, 86(C8),7255-65. (15) Crutzen, P. J. et al. Nature 1979, 282 (5736), 253-56.
(16) Stolarski, R. S. In The Changing Atmosphere; Rowland, F. S . ; Isaksen, I.S.A., Eds.; Wiley: Chichester, UK, 1988, pp. 105-20. (17) Fishman, 7,; Minnis, P.; Reichle, H. G., Jr. J. Geophys. Res. 1986, 92(D13), 14451-6 5. (18) Chatfield, R. B.; Harrison, H. J. Geophys. Res. 1976, 81(3), 421-23. (19) Fabian, P.; Pruchniewicz, P. G. J. Geophys. Res. 1977, 82(15),2063-73. (20) Fishman, J. In Ozone in the Free Atmosphere; Whitten, R. C.; Prasad, S. S., Eds.; Van Nostrand Reinhold: New York, 1985, pp. 161-94. (21) Galasyn, J. F.; Tschudy, K. L.; Huebert, B. J. ]. Geophys. Res. 1987, 92 (D3), 3105-13. (22) Levy, H., 11; Moxim, W. J. Nature 1989, 338(6213), 326-28. (23) Watson, C . E.; Fishman, J.; Reichle, H. G., Jr. J. Geophys. Res. 1990, 95(D10), 16443-50. (24) Scheel, H.E.; Brunke, E-G.; Seiler, W. 1.Atmos. Chem. 1990, 22(3),197-210. (25) Brunke, E-G.; Scheel, H. E.; Seiler, W. Atmos. Environ. 1990,24A(3),585-95. (26) Elsworth, C. M.; Galbally, I. E.; Paterson, R. In Baseline 86; Forgan, B. W.;
Fraser, P. J., Eds.; Australian Bureau of Meteorology, CSIRO Division of Atmospheric Research: Aspendale, Vic., 1988, p. 60. (27) Fraser, P. J,; Coram, S. In Baseline 86; Forgan, B. W; Fraser, P. J., Eds.; Australian Bureau of Meteorology, CSIRO Division of Atmospheric Research: Aspendale, Vic., 1988, pp. 65-66. (28) Danielsen, E. F.; Mohnen, V. A. J. Geophys. Res. 1977, 82(37), 5867-78. (29) Newell, R. E.; Wu, M-F. In Atmospheric Ozone, Proceedings of the Quadrennial Ozone Symposium , Halkidiki, Greece, 1984: Zerefos, C. S.; Ghazi, A., Eds.; Reidel: Hingham, MA, 1985, pp. 548-52. (30) Fishman, J. et al. J. Geophys. Res. 1987, 92(D2), 2083-94. (31) Bretherton, F. “Earth System Science: A Program for Global Change”; Report of the Earth System Sciences Committee, NASA Advisory Council: Washington, DC, 1986. (32) “LASA (Lidar Atmospheric Sounder and Altimeter)” Earth Observing System, Volume IId, Instrument Panel R e p o r t , NASA: W a s h i n g t o n , DC, 1987.
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