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Global Tropospheric Chemistry: A Plan for Action (1984)
Commission on Physical Sciences, Mathematics, and Applications (CPSMA)

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Executive Summary The lower atmosphere, or troposphere, is that part of the atmosphere in closest contact with the earth's sur- face. The weather phenomena that are experienced daily are manifestations of tropospheric processes, and many chemicals, both natural and synthetic, flow through the troposphere. These substances undergo chemical transformations as they travel. They are even- tually removed from the troposphere by rain or by up- tal~e at the earth's surface (often by vegetation) or by upward transport into the overlying stratosphere. Be- cause humans and much of the biosphere reside and respire in the lower troposphere, chemical processes and cycles in this region are a critical component ofthe atmo- sphere-ocean-soil life support system of the planet. Much of the current understanding of the earth's troposphere has arisen from a decade or more of inten- sive investigation of urban air pollution, the chemistry of the stratosphere, and the physics of the climate and from some exploratory studies of the nonurban tropo- sphere. Yet, fundamental questions remain unanswered about the basic state and dynamical response patterns of the global tropospheric chemical system. Considering the state of this science and the importance of the ques- tions, we believe the time has arrived to initiate a major international research program aimed at understanding the fundamental processes that control the chemical composition and cycles of the global troposphere and how these processes and properties affect the physical behavior of the atmosphere. Highly skilled individuals and instrumentation are available to begin the research. Accordingly, we recommend that the United States assume a major role in initiating a comprehensive investigation of the chemistry of the global tropo- sphere. The long-term goals of this Global Tropospheric Chemistry Program should tee as follows: 1. To un~lerstancl the basic chemical cycles in the troposphere through field investigations, theory aided by numerical modeling, and laboratory studies. 2. To predict tropospheric responses to perturba- tions, both natural and h~unan-inducecl, of these cy- cles. 3. To provide the information required for the maintenance and effective future management of the atmospheric component of the global life support sys- tem. The chemical composition of the unperturbed tropo- sphere is dynamically balanced; it is controlled to a large extent by the terrestrial and marine biosphere. Indeed, the present tropospheric composition could not persist in the absence of biological activity. The interaction is bidirectional, however. The productivity of the bio- sphere is dependent upon the troposphere for oxygen and carbon dioxide, for the fixation and transport of nutrients such as nitrogen and sulfur and other trace 3

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4 species, and for the maintenance of a stable climate and solar radiation environment. Changes in tropospheric composition induced by hu- man activity can alter this balance over a range of spatial and temporal scales. This realization has stimulated an explosive growth in interest in the atmosphere as an integrated chemical system. It is now known that such changes can be relatively long lasting and that they may have serious consequences for the overall function of the planetary life support system, including food produc- tion, the quality of air and water, the integrity of the natural biosphere, the global chemical cycles essential to life, and the stability of terrestrial climate. Impacts can be subtle. Release of synthetic chloro- fluorocarbon gases can alter the abundance of halogens in the stratosphere, leading to an enhanced rate of re- moval for ozone and, consequently, to greater fluxes of ultraviolet radiation at the planetary surface. A slow, long-term buildup of trace gases that absorb infrared radiation can significantly alter the earth's climate, lead to a change in the water cycle, and result in a shift in the photochemistry of the lower atmosphere that can affect the biogeochemical cycling of nutrient elements. There are also relatively direct effects of concern. Emission of nitrogen oxides and hydrocarbons in urban areas can have immediate and readily observable effects on ambi- ent levels of ozone, with potentially deleterious conse- quences for agriculture and the health of local popula- tions. The release of sulfur and nitrogen oxides by combustion and industrial processes can lead to in- creased deposition of acidifying substances on ecolog~- cally sensitive regions and the formation of climatically significant aerosol particles. Some human activities have already had a dramatic impact on certain chemical cycles in the troposphere. As a consequence of the combustion of fossil fuel and bio- mass burring, nitrogen oxides are introduced into the global troposphere at rates believed to exceed produc- tion rates in the natural environment. A similar conclu- sion holds for sulfur, primarily because of the combus- tion of sulfur-rich fuels and the smelting of ores. The manufacture of chemical fertilizers, agricultural prac- tices, and changes in land use may significantly alter the production of long-lived tropospheric and stratospheric gases such as carbon dioxide, methane, and nitrous ox- ide. Concentrations of these radiatively active gases are already increasing at a disturbing rate on a global scale. The capability for observing these changes has out- stripped the ability of scientists to understand their causes and to evaluate their impact. Estimating the ex- tent and effects of pollution on the natural global envi- ronment, including the stratosphere, is difficult, if not impossible, in the absence of a comprehensive under PART I A PLAN FOR ACTION standing that embraces the entire hierarchy of impor- tant chemical cycles in the global troposphere. Historically, the atmospheric chemistry community has operated by responding to crises. Examples include acid precipitation with its regional and hemispheric scale impacts; unexpected disturbances to the strato- spheric ozone layer resulting from ground-level emis- sions of several trace gases; and the potential effects of several trace gases, including carbon dioxide, on the earth's climate. Time and money have not been avail- able for the development of the systematic measurement and modeling programs required to anticipate possible future perturbations of the global troposphere and to respond knowledgeably to such events when they occur. If tropospheric chemical cycles are to be understood and a predictive capability is to be attained, a long-term commitment must be made to the development of com- prehensive models of the global tropospheric chemical system. This in turn requires coordinated research efforts aimed at attaining a thorough understanding of the fundamental processes controlling global tro- pospheric biogeochemical cycles. These processes in- clude the input of trace species into the troposphere, their long-range transport and distribution, their chemical transformations, and their removal from the troposphere. For these reasons we recommend that major research efforts in the Global Tropospheric Chemistry Program be undertaken with the following specific scientific objectives: 1. To evaluate biological sources of chemical sub- stances in the troposphere. Primary emphasis should be placed on investigations of temperate and tropical forests and grasslands, intensely cultivated areas, coastal waters and salt marshes, open ocean regions, tundra regions, and biomass burning. 2. To determine the global distribution of tropo- spheric trace gases and aerosol particles and to assess relevant physical properties. This program calls for field measurements and analyses coordinated with the development and validation of tropospheric chemical transport models, the development of a regional and global data base for key species in chemical cycles, and the continuation and improvement of existing monitor- ing programs for the accurate measurement of long- term trends in environmentally important trace gases and aerosol particles. 3. To test photochemical theory through field and laboratory investigations of photochemically driven transformation processes. Particularly important tests will be investigations over tropical oceans and rain for- ests with additional studies in midlatitudes. 4. To investigate wet and dry removal processes for

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EXECUTIVE SUMMARY trace gases and aerosol particles. Research should be directed not only toward evaluating chemical fluxes to land and water surfaces, but also toward a fundamental understanding of aqueous-phase reaction mechanisms and scavenging processes. 5. To develop global tropospheric chemistry sys- tems models (TCSMs) and the critical submodels re- quired for the successful application of TCSMs. A wide range of models of individual processes important for tropospheric chemistry as well as comprehensive global models that include the most important chemical and meteorological processes must be developed. Mod- eling and laboratory and field studies are necessarily symbiotic; progress in each area is dependent upon con- tributions from the others. Projects to pursue these objectives should be initiated immediately. They should proceed simultaneously throughout the development of the proposed Global Tro- pospher?c Chern~stry Program. The Global Tropospher?c Chemistry Program would build on the significant insights already achieved through pre- liminary studies of global tropospheric chemistry, re- search on urban and regional air pollution, and on the last decade of studies of the stratosphere. Many well- posed questions are to be found in the numerous subtle interactions that couple the atmosphere to the global biosphere. The proposed program would contribute to a better understanding of the functioning of the natural troposphere and how its chemistry affects its physical behavior. The program would also allow a more com- prehensive assessment of the impact of human activities on the global environment. Furthermore, it would pro- vide knowledge that would enable mankind to use the resources of the planet in a manner that is most efficient and least disruptive to the harmony of nature and that is considerate of the long-term interests of life on earth. To attain these objectives, sensitive instrumentation will be required for the measurement of chemical species in the remote troposphere. Much of this instrumenta- tion is available now, and recent advances in instrumen- tation technology convince us that the additional re- quired instrumentation can be developed. We rec- ommend that a vigorous program of instrument de- velopment, testing, and intercalibration be under- taken immediately and that it be continued through- out the Global Troposphenc Chemistry Program. The Global Tropospheric Chemistry Program will also re- quire the continuing commitment of laboratory chemi- cal kineticists and the further development of laboratory systems for investigation of the mechanisms and kinetics of gas- and liquid-phase reactions. We recommend that an increased effort be initiated on laboratory studies 5 of the rates and pathways of fundamental chemical reactions In tropospheric chemical systems. The Global Tropospheric Chern~stry Program should be international in scope. It must engage the resources and commitment of not just the national, but also the inter- national scientific and political communities. We rec- ommen`1 that the United States play a major role in a cooperative effort with other countries to attract these resources and that it do so with confidence that the International community is both ready and willing to join in this initiative. Currently available manpower in atmospheric chem- istry is adequate to initiate this program, but manpower requirements are a concern over the longer term. A number of major U.S. and European universities have begun to intensify their graduate programs in atmo- spheric chemistry. To assure that highly trained person- nel are available to conduct later portions of the Global Tropospheric Chern~stry Program, we recommend that a manpower survey for atmospheric chemistry be con- cluctecl through an appropriate scientific group. This survey should be cognizant of existing channels from the basic physical and life sciences into atmospheric chemis- try. Organizations that could be of assistance in such a survey include the American Chemical Society, the American Geophysical Union, the American Meteoro- logical Society, and the University Corporation for At- mospheric Research. The long-term vitality of tropospheric chemistry re- search and graduate education depends to a large degree on the undirected research of individuals research that does not always result in immediate applied results. Strong support for the high-quality research of individ- ual investigators at academic institutions must be main- tained. Although several federal agencies are now conduct- ing and supporting research that addresses various as- pects of global tropospheric chemistry, their current pro- grams are not adequate to achieve the goals of the recommended Global Tropospheric Chemistry Program. Fed- eral support is currently focused on urban or regional problems, such as air pollution or acid rain, or on ques- tions related to climate, such as carbon dioxide buildup. Nevertheless, many of the research tasks encompassed by existing programs will contribute substantially to the goals of the Global Tropospheric Chemistry Program and can readily serve as a strong foundation for it. We estimate that the current size of the federal program in areas recommended by our panel is $10-20 million per year. We believe that the recommended program could be launched with the addition of an increment of approxi- mately the same level in FY 1986. This increase would enable planning and exploratory observations to begin

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6 for the major field-oriented research programs and would support initial new thrusts in model development and chemical instrumentation. This funding would also be used for the improvement and required modifica- tions of the necessary field platforms for the Global Tropo- spheric Chemistry Program, particularly research aircraft, ships, and ground-based platforms. It will also be neces- sary to develop some instrumentation for the meteoro- logical measurements that are required for the chemis- try field programs. This budgetary augmentation should be sustained i subsequent years, and substantial additional incre- ments will be required as the extensive field experiments get underway. Although a detailed cost analysis is not feasible at this time, we estimate that the annual funding required to conduct the U. S. component of a fully oper- ational Global Tropospheric Chemistry Program win be in the tens of millions of dollars for a period of a decade or more. We recommend that an appropriate U.S. scientific organization exercise general scientific oversight of the U.S. component of the Global Troposphenc Chem- ~st~ Program. This group should also be responsible for providing interaction with the international scientific community. The recommended program involves a combination of small research projects involving individual principal investigators and large-scale, highly coordinated field projects. The latter will require extensive, shared facili PART I A PLAN FOR ACTION ties and innovative, effective project management. The individual investigator could be covered by the grant- support mechanism provided principally, at the present time, by the National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA) through their atmospheric chemistry pro- grams. We believe these agencies and others can work together to provide the necessary mechanisms and management to coordinate the efforts of U.S. scien- tists in universities ant! government and industrial laboratories. The participation of other government agencies such as the National Oceanic and Atmo- spheric Administration, the Department of Energy, the Environmental Protection Agency, and the Department of Defense- and of universities, private research organ- izations, and industries will also be essential for the suc- cess of the program. Because of the need for effective global observations, we suggest that a study be under- taken to examine the potential for satellite-based remote sensing and to define the role, if any, of satellite measure- ments for the Global Tropospheric Chemistry Program. The Global Tropospheric Chemistry Program represents a fundamental step toward understanding and predicting effects of human society on the planetary life support system. Benefits from this program will transcend na- tional boundaries and extend to all. The scientific com- munity is ready to meet this exciting and demanding challenge.

Representative terms from entire chapter:

tropospheric chemistry