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A Proposed Program LONG-TERM GOALS AND OBJECTIVES Over the past decade, tropospheric chemistry research has shown that the various chemical cycles in the global troposphere are interactive, complex, and of fundamental importance to the future well-being of humanity. Many essential biochemical and geochemical cycles are critically susceptible to perturbations to the global troposphere. In recognition of this, we recom- mend that the Uniter! States assume a major role in Initiating a comprehensive Investigation of the chem- istry of the global troposphere. The long-term goals of this Global Troposphenc Chemist Program should be as follows: 1. To understand the basic chemical cycles In the troposphere through field investigations, theory aided by numerical modeling, and laboratory studies. 2. To predict the tropospheric responses to pertur- bations, both natural and human-induced, to these cycles. 3. To provicle the information required for the maintenance and effective fixture management of the atmospheric component of the global life support sys- tem. Attainment of these goals will require carefully designed and complementary research programs, the development of which will involve close cooperation and . . - mteractlon among mvestlgators makmg measurements in the field, those investigating reaction rates and mech- anisms in the laboratory, and those attempting to model both the chemical systems and the meteorological proc- esses affecting the chemical distributions. Many other laboratory investigations are essential, e.g., studies of uptake of gases by plant leaves, emission of chemicals by living systems, and cloud-scavenging simulations. It is important that experiments be designed jointly by field and laboratory scientists in conjunction with modelers and that there be a continuing exchange between the developing theoretical aspects of tropospheric chemistry and the evolving laboratory and field investigations, as illustrated in Figure 3. I. It is known that such meteorological processes as transport and cloud and precipitation formation are intimately related to the chemical cycles in the tropo- sphere, and that the instrumentation and platforms required to investigate tropospheric chemistry on the global scale will be expensive. Global-scale tropospheric chemical research cannot be conducted solely by indi- vidual investigators or even by small groups of investi- gators, although the contribution of the individual investigator has been critical in the development of tro pospheric chemistry and will continue to be so in the future. Because of the complexity and diversity of the 19

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20 Provides Recommendations | for Atmospheric Measurements Provides Atmospheric Measurements for Model Input and Validation 1 Provides Mechanisms and Rates of Reaction and Exchange MODEL ~LABORATORY DEVELOF MENT l ~INVES IGATIONS AIL I Provides Recommendations 1 for Kinetic Studies Provides Standa~ dization and Calibration Methods and Sensitive Analytical Techniques Provides Atmo spheric Measurements to Validate Laboratory Reaction Mechanisms and Rates ~, , . FIELD PROGRAMS FIGURE 3.1 Schematic diagram of the essential yet interde- pendent functions served by field programs, laboratory measure- ments, and mathematical models in atmospheric chemistry research. chemical systems and because of the myriad sampling, analytical, and modeling tools required to study them, these studies will require the joint efforts of a broad spectrum of scientists: atmospheric chemists and physi- cists, marine chemists, meteorologists, ecologists, plant and soil biochemists, microbiologists, plant physiolo- gists, laboratory chemists, geochemists, engineers, and others. An effort ofthis magnitude requires the coopera- tion and participation of universities, industry, and gov- ernment agencies, both in the United States and in other countries. The four basic processes that control chemical cycles and their interactions in the troposphere production, transport and distribution, chemical transformation, and removal provide a unifying framework for the development of the Global Tropospheric Chemistry Program. Attainment of the program goals will also require the development of three-dimensional models of tropo- spheric chemical processes linked to meteorological and climatic processes, i.e., tropospheric chemistry systems models (TCSMs); these models will provide an overall synthesis of data obtained in the program, and they will provide theoretical guidance for the program's contin- ued development. For these reasons, we recommend that the Global Tropospheric Chemists Program be undertaken with the following specific scientific objectives: PART I A PLAN FOR ACTION 1. To evaluate biological sources of chemical sub- stances in the troposphere. 2. To determine the global distribution of tropo- spheric trace gases and aerosol particles and to assess relevant physical properties. 3. To test photochemical theory through field and laboratory investigations of photochemically driven transformation processes. 4. To investigate wet and dry removal processes for trace gases and aerosol particles. 5. To develop global tropospheric chemistry sys- tems models (TCSMs) and the critical submodels required for the successful application of the TCSMs. The research efforts in biological sources, photo- chemical transformations, and removal processes will require the development of individual submodels. The submodels would serve three functions: (1) to under- stand better the individual processes being investigated, (2) to extrapolate from individual observational sites to the regional and global scales, and (3) to provide compo- nents that can be used in a comprehensive three-dimen- sional meteorological model that is coupled to global tropospheric chemistry a TCSM. By contrast, the research effort in global distributions and long-range transport would serve to help validate the overall perfor- mance of a comprehensive TCSM. A tropospheric chemistry systems model would focus on meteorological transport processes that are best described by atmo- spheric general circulation models (GCMs). These GCMs would be especially designed not only to provide large-scale tracer transport in the free atmosphere, but also to parameterize transport through the planetary boundarylayer and by cloud processes. A TCSM would also require physically based cloud submodels, a good description of land surfaces, and adequate treatment of the solar radiation that drives tropospheric photochem- ~stry. Based on the five scientific objectives above, details of the specific scientific investigations proposed for the Global Tropospheric Chemistry Program are developed in the remainder of this chapter. These investigations include the Biological Sources of Atmospheric Chemicals Study; the Global Distributions arldt Lorlg-Range Transport Study; the Photochemical Trar~sformatior~s Study; the Cor~versiorz and Removal Study; and a program for the development of global Tropospheric Chemistry Systems Models (TCSMs). The discussions of these specific investigations are fol- lowed by an evaluation of instrument and platform requirements for their successful completion and a brief review of the requirement for strong international par- ticipation and cooperation in the Global Tropospheric Chemistry Program.

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A PROPOSED PROGRAM BIOLOGICAL SOURCES OF ATMOSPHERIC CHEMICALS Before an understanding of the natural or perturbed troposphere can be claimed, the flow of chemicals through it must be traced. This flow begins with entry of the chemical species into the troposphere. Although the earth's atmosphere is certainly an oxidizing environ- ment, the actual processes of chemical and physical transformation, transport, and eventual removal depend on the chemical form and other intrinsic proper- ties of the substance in question. Accordingly, the initial physical state and chemical properties of the substance at its source will affect its subsequent tropospheric fate. There are many research questions, basic and ap- plied, that require knowledge of the intensity, size, and variability of sources of tropospheric chemical species; e. g., what factors control the ambient concentration of a certain chemical, or why does the concentration in- crease or decrease in time, or how will human activity alter the source in question? As specific examples, one may ask why the global concentration of CH4 is increas- ing secularly and why it varies with season, why the observed distribution of CO varies with latitude and season, and why there is any gaseous HC1 at all in the troposphere? For these reasons, we recommend that a major re- search effort be undertaken to evaluate biological sources of chemical substances In the global tropo- sphere. The objectives of this Biological Sources of At- mosphenc Chemicals Study would be (1) to evaluate the chemical fluxes to the troposphere from critical bio- log~cal environments (biomes) and (2) to determine the factors that control these fluxes. The experimental study of the sources of atmospheric chemicals is relatively new because the nature of many of these sources has been recognized only recently and appropriate analytical instruments are just now being developed. This is especially true for biological sources. The dominant role of biological systems as sources of tropospheric trace substances is becoming widely appre- ciated; a more detailed discussion ofthe evidence for this appears in Part II, Chapters 5 through 7. In identifying topics and regions for research in the Biological Sources of Atmospheric Chemicals Study, we have employed several criteria; these are reviewed in Part II, Chapter 5, in the section by Cicerone et al. We began by focusing on key chemicals known to have important roles in atmospheric chemistry. For some of these, exist- ing data have already shown the global importance of certain blames as sources. In other cases, general princi- ples from chemistry and biology suggest that certain blames should be important. Further, we reviewed characteristics of various blames and arrived at criteria to estimate their hemispheric or global importance as 21 sources. These considerations led to the field and labora- tory investigations of biological sources of tropospheric chemicals that are proposed here. Much of the early field research in the Biological Sources of Atmospheric Chemicals Study must be exploratory in na- ture. We suggest a research strategy of preliminary in- vestigations of the various sources using relatively small research groups. When the nature and importance of specific sources are better defined, more detailed and refined field and laboratory investigations can follow. This strategy will lead to an understanding of the un- derlying physical, chemical, and biological factors reg- ulating production of the compounds of interest. In situations where sources are evident but no direct mech- anisms are apparent, investigation of indirect paths must be undertaken. Such investigations would require plausible mechanisms for the relevant kinetics and for application of the kinetics under field conditions, and coordinated measurements to verify and test proposed mechanisms. One might expect this process to involve successive iterations among field measurements, labo- ratory measurements and theory, and ultimately to re- quire studies designed to identify processes important for biochemical and microbiological production of pri- mary chemical species isoprene, for example. Certain important abiological sources (e.g., indus- trial emissions, volcanoes, and lightning) are not dis- cussed here. Also, some field studies of CO2 exchange are proposed. Even though CO2 is not important in tropospheric chemistry through its reactions, CO2 ex- change rates can provide important information on the mechanisms and rates of exchange of other gases. Investigations of Specific Sources Tundra, Taiga, and Freshwater Marshes The size of the areas covered by tundra, taiga, and freshwater marshes and some of their unique properties suggest that these regions may be important contribu- tors of CH4, N2O, NO, and other volatile species to the troposphere. On the ocean border, volatile organic halides and reduced sulfur species are also of interest. Site selection and analytical techniques for these in- vestigations are influenced by the remoteness of most representative locations. For these reasons and because so few data are available now, the initial phase of a research program probably is best accomplished by small ad hoc expeditions to acquire the basic informa- tion and exploratory data necessary to design more sys- tematic and extensive studies.

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22 PART I A PLAN FOR ACTION TABLE 3.1 Measurement Needs in Various Source Regions H2S N2O NOx CH4 DMS/RSH NMHC RX Seaboard tundra X X X X X X Taiga X X X X Marshland X X X X NOTE: RX represents organic halogen species, RSH represents mercaptans, and NMHC represents nonme- thane hydrocarbons. Determination of emission rates of CO and CO2 is also needed; CO2 fluxes could provide valuable mechanistic information. Sites to be investigated should be logistically favor- able, uncontaminated, and ecologically representative. The present understanding of these source regions suggests emphasis on the measurements indicated by an "X" in Table 3.1. Climate characteristics and local conditions will probably constrain the sampling periods narrowly, as could the availability of nearby laboratories and analytical capabilities, but data are needed from various seasons, particularly during transition times. Tropical Forests Tropical forests represent a biome of considerable im- portance for tropospheric chemistry. They include a sig- nificant fraction of the total carbon content of the global terrestrial biosphere. They may be important sources for CH4 and N2O, gases whose global concentrations have increased significantly over the past several de- cades, and tropical forests may play a significant role in the global budgets of CO, nonmethane hydrocarbons, NOx, and various forms of volatile sulfur. Studies of tropical forests will also lead to an increased understand- ing of the role of microbially mediated reactions, which should be markedly faster in the high-temperature, high-humidity tropical regions. An orderly strategy for the study of tropical forest blames might begin with careful measurements of the composition of the local troposphere to identify species whose concentrations are elevated with respect to typical ambient tropospheric background levels. An investiga- tion of a region such as the Amazon Basin might be particularly instructive. The prevailing winds are typi- cally from the east, and the concentration of key species might be expected to increase with time as air masses penetrate deeper into the basin. Measurements deter- mining the spatial and temporal change in gas concen- trations, in combination with careful meteorological analysis, could be used to derive an empirical estimate for the total net flux of selected species emanating from the local biosphere. These data, in turn, could provide an estimate of the significance of specific individual sources within the basin. The extensive measurement program should be com plemented by intensive studies of selected microenvi- ronments within the larger biome. For example, if the extensive program should establish an important dis- tributed source for CH4, the nature of the source should be clarified through more intensive investigations. Ex- ploratory studies of rivers, flood plains, and other envi- ronments should aid in the identification of important source regions. Studies of tropospheric chemistry in tropical forests should improve the understanding of the tropical biome as an integrated physical, chemical, and biological sys- tem. To this end, data relating to the transformation and redistribution of essential nutrients, such as nitrogen and sulfur, are particularly relevant and should be avail- able as a by-product of the proposed research. . Biomass Burbling Some of the most easily recognized sources of tropo- spheric chemicals can be described as point sources. Dramatic in appearance and possibly in their actual impact, these are exemplified by biomass burning, lightning, volcanoes, animal feedlots, and industrial or urban combustion and waste plumes. Both the nature and the magnitude of these sources make them impor- tant for global or hemispheric tropospheric chemistry. High-temperature processes that synthesize otherwise unnatural substances and other activities that process large amounts of raw materials, e.g., biomass burning and the refining of metals and petroleum, are particu- larly potent. We emphasize here the need for coordi- nated field investigations of biomass burning as a source of atmospheric chemicals, although much research is also needed to quantify other point sources. Biomass burning produces a variety of gases and par- ticles. The less reactive gases and the smaller particles can affect the global troposphere. Regional effects are likely from the more reactive gases and the larger parti- cles. We recommend a two-phased field investigation of biomass burning. The objectives for the early phase would be (1) to extend available methods for malting quantitative measurements of the emissions of CO2,

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A PROPOSED PROGRAM CO, N2O, CH4, NOx, nonmethane hydrocarbons, trace elements, and particles of various sizes from bio- mass burning, and (2) to obtain exploratory data (as quantitative as possible) for emission rates of COS, vari- ous cyanide compounds (RCN mostly hydrogen cya- nide (HCN) and methyl cyanide (CH3CN)), and CH3C1. Also, as in the later phase, the measurements must be made with an awareness of soil and vegetation types and conditions, and the amounts of charcoal pro- duced and areas burned must be measured because of the possible importance of charcoal in the global carbon cycle. Methods to determine charcoal production and the sizes of the affected areas need refinement to permit global assessments. An initial phase might focus on the North American continent to minimize logistical prob- lems and to take advantage of related expertise available through forest meteorologists and forest research pro- grams. There are, for example, prescribed, controlled forest fires set for research purposes in the southeastern and northwestern United States. Forest fire research laboratories could be of considerable use in exploratory programs. The goals of the early phase could be accom- plished more easily at such locations, or in Alaska, than in the tropics (where much biomass burning occurs). Some sites should be amenable to the use of instru- mented towers and to long-path spectroscopic absorp- tion methods. In the later phase, field measurements would concen- trate on tropical fires. These are extensive and frequent, and they would present greater logistical difficulties. On the basis of the results and experience already gained, it should be possible to quantify emissions of CO2, N2O, CH4, CO, and the more stable nonmethane hydrocar- bons from these large fires' and to obtain fairly reliable estimates of emissions of COS, RON, CH3C1, trace metals, particles, and nitrogen oxides. In all phases, ground sampling of gaseous and partic- ulate species should be undertaken. Aircraft should be used extensively, however, because there is rapid up- ward motion of the emissions, large areas must oe cov- ered, and sampling at representative ground locations could be difficult. Coastal Wetland and Estuar?ne Enviror~m~ts Coastal wetland and estuarine environments are hot spots for the production and emission to the troposphere of many chemically reduced gaseous species of carbon, nitrogen, and sulfur. Most natural anoxic sediments that are in close contact with the troposphere are found in these environments. For gases such as N2O, CH4, CO, COS, CS2, (CHESS, H2S, and perhaps a few oth- ers, it is reasonable to propose initiating immediately a field research program emphasizing basic processes of 23 gas production and exchange with the tropospheric boundary layer. For other species such as NO, NH3, and volatile metals, existing technology is inadequate for quantitative biosphere-atmosphere exchange stud- ies, even in relatively intense source areas. For almost all reduced gas species, measurement technology is cur- rently inadequate for flux studies of weak sources and sinks. Studies of gas fluxes in coastal wetland and estuarine environments might focus initially on subtropical and tropical salt marsh and mangrove habitats, because these typically contain extensive anoxic sediments and are characterized by high organic inputs, warm temper- atures, and high levels of microbial activity factors that enhance the production of reduced gas species. A number of characteristics (e.g., vegetation, exposed sediment surfaces, and overlying water) deserve sepa- rate study. On the basis of existing data, one can expect gas fluxes from these environments to be highly variable in both time and space on scales of minutes to hours and meters to kilometers, respectively. Critical forcing varia- bles with regular periodicity (e.g., tides, sunlight, plant physiological status, and sediment temperature) inter- act with episodic events such as severe weather to influ- ence both production and exchange rates. Both exploratory and intensive field studies of trace gas fluxes are needed in coastal wetland and estuarine environments. Exploratory measurements of fluxes from tropical mangrove and estuarine environments in South America, Africa, and Asia are needed to assess the relative magnitude of emissions from these sources compared to those from more accessible sites in the southeastern United States. Intensive, process-oriented studies of carbon, nitrogen, and sulfur emissions from salt marsh and mangrove habitats could be initiated in areas such as the southeastern U.S. Atlantic coastal re- gion, Mississippi River delta, and Florida Everglades. Together, these studies would improve the understand- ing of geographical variability in biogenic gas emissions to the troposphere and the processes that control these fluxes. A program of exploratory gas flux determinations requires research teams focused specifically on such studies, and every effort should be made to determine gas fluxes at locations where complementary meteoro- logical and ecological or biogeochemical characteriza- tions are available. Each gas flux determination should be accompanied by measurements of sediment and/or water column parameters such as water content, tem- perature, organic content, pH, and Eh (the oxidation- reduction potential). Surveys should include measure- ments at each site over vegetation, exposed sediment surfaces, and water. Intensive studies would require more continuous ef

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24 forts of research teams at sites where fluxes are deter- mined in conjunction with biological and geochemical studies of sedimentary biogeochemical processes. Agricultural Biomes Agricultural areas are enormous potential sources of tropospheric chemicals. For example, rice growing, be- cause of its large global extent and waterlogged anoxic soils, appears to be a major source of atmospheric CH4. Similarly, nitrogen-intensive grain growing (corn, cot- ton, wheat, legumes, and some rice) can emit enough volatile nitrogen compounds to influence large regions (if NH3 or NOx) and the globe (if N2O). Because of the large areas under cultivation, the growing usage of chemicals, and the inherently large turnover rates of nutrient elements with volatile compounds, we propose a program of extensive field measurements. The focus of these efforts would be on rice paddies and on heavily fertilized crops such as those in the U.S. corn-soybean belts. In Part II, Chapter 5, in the section by Cicerone et al., we discuss the potential of emissions from rice paddies to exert a strong influence on regional and global tropo- spheric chemistry. Particular attention should be paid to volatile species containing nutrient elements in reduced valence states, e.g., CH4, N2O, NH3, NO, methylated metals, isoprene, and possibly CO. For CH4, detailed and systematic field measurements are required because of the apparent importance of rice paddies and because CH4 emission rates from rice pad- dies depend on several factors. To determine the neces- sary CH4 fluxes, at least two parallel efforts would be required. The first would simply extend the current data base by using available techniques. For the second, it would be necessary to complete the development and employ a state-of-the-art meteorologically based flux- measurement technique using turbulence-correlation or gradient measurements. Methane fluxes must be de- termined over complete growing seasons, as functions of nitrogen fertilization rate and in organic-rich rice pad- dies. Soil temperature, pH, and Eh should also be mea- sured. Most of the data for other trace gases could be ob- tained while the CH4 flux investigations were being made, largely because the initially required investiga- tions would be more exploratory than systematic. If the results indicated significant emissions, then more exten- sive and systematic studies would ensue, similar to those outlined above for CH4. For NH3 and NO a similar sequence of investigations would be required. For NH3, adequate chemical trapping methods are available, but NO is more problematic; a field-compatible technique is needed. PART I A PLAN FOR ACTION Potentially large emissions from various heavily fer- tilized agricultural biomes require investigation. Mod- ern agriculture often relies on intensive management of resources, both physical and financial. Bulk quantities of nitrogen, phosphorus, and sulfur are applied com- monly, as are large quantities of trace elements. Special- ized chemicals and biochemicals, including enzymes and brominated organics, are also used to regulate proc- esses and to control insects and pests. To assess the role of these intensively managed agricultural blames as sources of atmospheric trace chemicals, some relevant research is under way in the agriculture research com- munity. We propose complementary research on (1) measurements of the emissions of species such as N2O, CH4, and CO that could have global and hemispheric tropospheric effects, and (2) f~eld measurements of shorter-lived gases that have been impractical up to now. Topic (2) could include systematic measurements of NH3 emissions from nitrogen-fertilized plots and ex- ploratory studies to detect emission of nitrogen oxides, volatile phosphorus, and metals. Gaseous emissions of NH3, along with those from animal feedlots and natural sources, could lead to NH4, (NH4~2SO4, (NH4)HSO4, and NH4NO3 in aerosol particles in remote areas of the globe. Further, NH3 lost from fertilized fields could also be a key buffer of precipitation acidity. Measurements similar to those proposed for rice paddies would address these questions. 1 ~ ., Operz Ocearts The oceans are a large-area, low-intensity source of reduced sulfur compounds to the lower troposphere. Preliminary studies have identified a number of sulfur compounds in seawater that are presumed to be bio- genic, including H2S, COS, CS2, dimethyl sulfide (DMS), dimethyl disulfide (DMDS), and dimethyl sul- foxide (DMSO). Dimethyl sulfide appears to be the most abundant species, contributing an estimated flux of approximately 40 Tg S/yr to the marine boundary layer. Once in the marine boundary layer, DMS is prob- ably oxidized by photochemical processes to produce SO2, with intermediates such as DMSO and methane sulfonic acid (CH3SO3H). Qualitatively, the concentra- tion of reduced sulfur compounds in surface seawater is correlated with indicators of algal biomass. A research effort to investigate the sulfur cycle in productive areas of the world's ocean should be initiated to elucidate sources of reduced sulfur in the ocean and their role in the global sulfur cycle and budget. Particu- larly important would be studies of in situ biogenic pro- duction of sulfur species in the water column, fluxes across the sea-air interface, and chemical processes in the marine boundary layer determining transport and

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A PROPOSED PROGRAM fate. Most of the research could be conducted from a major research vessel with periodic aircraft overflights to measure the vertical distribution of sulfur species in the troposphere and estimate rates of exchange between the boundary layer and free troposphere. Sites for such studies might include continental shelfwaters and a ma- jor upwelling area. The nitrogen cycle in the sea is strongly modulated by biological processes. Evaluation of productive oceans as a source of atmospheric N2O is particularly relevant to an improved understanding of global carbon, nitrogen, and phosphorus cycling. Capabilities should be devel- oped to explore the possibility of significant NO emis- sions and to measure sea-air N2O fluxes more directly. A program of continued oceanographic studies of N2O production and distribution should be pursued while technology is being developed for measuring low-level N2O and NO fluxes from surface and aircraft platforms. Many atmospheric halogen compounds appear to have significant oceanic sources (see Part II, Chapter 7~. The most pressing requirement is to estimate the oce- anic source strengths of CH3C1, CH3I, CH3Br, and pos- sibly other organo-bromide compounds. Estimates of the magnitude and distribution of these sources are needed before even the most rudimentary understand- ing of tropospheric halogen budgets and the role of the oceanic sources in stratospheric chemistry can be claimed. The only available experimental data on oce- anic fluxes of methyl halides (RX) are based on mea- sured supersaturation of surface waters. Much more extensive measurements such as these are strongly sug- gested. The productive oceans may be significant, albeit sec- ondary, sources of CO and CH4 to the global tropo- sphere. Again, as in the examples above, the production of these species is related to biological processes, is patchy in distribution, and often the sea-air fluxes are below the detection limits of existing measurement sys- tems. As we prepare this document, oceanographers are revising estimates of open-ocean biological productivity, upward by perhaps an order of magnitude. As major oceanographic research programs of oceanic productiv- ity are developed, every effort should tee made to include simultaneous studies of trace gas (e.g., DMS, N2O, CO, CH4, and RX) production and emissions to the troposphere. Temperate Forests Temperate forests occupy a substantial fraction of the global land area. Their annual production of dry matter represents approximately 10 percent of the total global production by the biosphere. The forests vary greatly in 25 species composition from almost pure stands of conifers to the mixed hardwoods of temperate North America and Europe, and they also vary in soil type, moisture, and climate. For these reasons, they have a broad range of properties of interest to the tropospheric chemist. Volatile emissions from forests certainly occur, but there is little quantitative information on these emis- sions. Various high-molecular-weight aromatic and aliphatic hydrocarbons and CH4 are potentially of inter- est as forest emissions. The direct emission of significant quantities of aerosol particles by trees has been sug- gested, but little quantitative information is available. To obtain the minimum necessary information on these emissions, sampling should be conducted at repre- sentative sites for the major forest types. It is desirable to design a program that is flexible both in time and space, starting with exploratory survey analyses in selected representative locations and expanding the number of sites and intensity of analysis as results dictate. Wherever possible, sites should be selected where other data are available from ongoing ecological, physi- ological, and other studies on principal forest types (con- ifers, deciduous hardwood, etc.~. The current state of knowledge of volatile emissions and reabsorption by forest ecosystems is so limited that many exploratory studies are required to guide the di- rection of more intensive research. During early studies, analysis at several sites, times of day, and seasons for CH4, CO, nonmethane hydrocarbons, and NH3 should be undertaken. Measurement methods to provide gra- dient information will be needed. Savannas and Temperate Grasslands Significant emissions of biogenic gases may occur from temperate grasslands and from savannas (tropical and subtropical). Both direct and indirect indicators point to the need to obtain estimates of emission rates for several gases. Savannas cover about 4 to 5 percent of the earth's surface, and they display high cycling rates for nutrients, i.e., high rates of gross and net primary pro- ductivity. Although they store less material in their shrubs and grasses than is found in the wood of tropical forests, the biological material of savannas has a shorter lifetime and higher nitrogen/carbon and sulfur/carbon ratios than hardwood. From the rapid turnover rates, the chemical composition of the material, and current data that suggest a large role for termites and herbivo- rous insects, one may conclude that significant volatile emissions are quite likely from certain dry tropical ar- eas. In particular, there is potential for large emissions of CH4, CO, CO2, many nonmethane hydrocarbons, N2O, and possibly methylated metals. Initial measure- ments should focus on sites and processes that concen

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26 bate nutrients (termite colonies, for example), but the large land areas involved could release significant emis- sions from lower intensity sources distributed over large areas. Although they cover only about 60 percent as much area as savannas, and their net primary productivities are only about 50 percent those of savannas, temperate grasslands mav be an important source region on the ~ J 1 ~ global scale. As with savannas, we focus attention on CH4, CO, CO2, nonmethane hydrocarbons, N2O, and possibly volatile metals. Initial measurements to quan- tify these fluxes should be similar to those for savannas, although site selection might be made so as to coincide with grassland sites that are already well-characterized ecologically. Further, consideration should be given to simultaneous measurements of downward deposition rates of O3, NOX, HNO3, SO2, and CO2 if techniques permit. Needs for Instruments and Verification of Methods In the discussion of the Biological Sources of Atmospheric Chemicals Study, we have emphasized selected blames and processes that are known or suspected to be impor- tant sources of key tropospheric chemicals. Although the proposed program is broad in scope, it is not complete nor are all means of investigation and an exact sequence of projects spelled out. For example, we have not em- phasized research on industrial or volcanic emissions, or lightning as an in situ source, nor have we emphasized sources of orimarv particles. although it is clear that . . ~ ~ J 1 ~ OCR for page 19
A PROPOSED PROGRAM validation of models, and they serve as the basis of model development. Thus a coordinated research program of tropospheric chemical measurements and transport/ chemistry model development is required. Conse- quently, we recommend a Global Distributions and Long-Range Transport Study through the establish- ment of a Global Troposphenc Chemists Sampling Net- work. The primary objective of this network would be to obtain tropospheric chemical data that can be used to identify important meteorological transport proc- esses and to validate and improve the ability of models to simulate the long-range transport, global distr~bu- tion, and variability of selected chemical species. An- cillary objectives are (1) to determine the distribution of those chemical species that play important roles in the major chemical cycles of the troposphere, and (2) to detect, quantify, and explain long-term trends in environmentally sensitive trace gases and aerosol par- ticles, especially those that are radiatively important. It is not possible at this time to define precisely the structure of this network and its sampling protocol be- cause there is insufficient knowledge of the global con- centration fields for the chemical species of interest and of the large-scale transport processes that affect these distributions. Nonetheless, one can anticipate the gen- eral features of such a network and its sampling proto- col. We stress that the design and implementation ofthis network will be, of necessity, an evolutionary process. The network and its protocol will be periodically rede- fined in response to the growing knowledge ofthe chem- ical and physical properties of the troposphere. We also anticipate that it will not be necessary for all stations to follow the same protocol. Requirements such as the spatial distribution of sampling sites and the fre- quency of sampling will vary depending on the distribu- tion of sources and sinks for a particular species, the atmospheric lifetime of that species, and the intended use of the data. Consequently, we suggest that within the framework of the overall network there exist three subsidiary networks: a Global Distributions Network, a Long-Term Trends Network, and a Surface Source/ Receptor Network. To meet the overall objectives, the networks must be well-organized, the protocols must be carefully formu- lated, and activities must be tightly coordinated. The objectives can be met by networks composited from sampling programs that are under the control of individ- ual investigators or national and international govern- ment agencies. Many current sampling activities would play a vital role in the proposed program, although in some cases the present sampling protocols might have to be modified or expanded. However, it is essential that the people involved in these programs be scientists who are active in research and who are professionally moti 27 vated to obtain good data, to maintain a high degree of quality control, to interpret the results, and to publish them promptly. Consequently, we recommend that the Global Tropospheric Chemistry Network be coordinated through an international committee of prominent at- mospheric scientists who are active in relevant areas of network studies. The Lor~g-Term Trends Network would consist of a few stations located in remote environments that are rela- tively unaffected by significant local sources, either nat- ural or anthropogenic. These stations would be dedi- cated to the long-term monitoring of environmentally important trace gases that have relatively long tropo- spheric residence times. The accurate measurements necessary for this program require skilled technical sup- port personnel and sophisticated measurement devices. The Long- Term Trends Network could be based on existing efforts such as the NOAA Global Mor~itorir~g for Climatic Charge (GMCC) Network and the Chemical Manufacturers Association (CMA) Atmospheric Lifetime Experiment (ALE) Network, with the careful intercalibration of current measurement techniques and the addition of a number of other measurement programs. The Global Distributions Network would consist of a greater number of stations operating over a relatively limited period oftime (3 to 5 years); these would provide greater spatial resolution for trace constituents with tro- pospheric lifetimes of a few months to a few years that are important both to transport studies and to chemical cycles. The Surface Source/Receptor Network would incorporate a larger number of stations that employ relatively inex- pensive and low technology techniques. They would measure the surface air concentration of selected species in airborne particles and the concentration and rate of deposition of these species in precipitation. Chemical Species and Measurement Techniques It is not now technically or logistically possible to measure the global distributions of all important chemi- cal species in the global troposphere. Furthermore, be- cause of the inadequate knowledge of the chemical proc- esses affecting these species and of their meteorological transport, it would not be possible to properly design such an experiment. Rather, we recommend that spe- cies be selected on the basis of the three objectives of the Global Troposphenc Chemists Sampling Network and with the additional requirement that the species be readily measurable with existing technology that can be implemented at relatively remote sites. Initially, the focus would be on gases having simple chemical destruction pathways, but as model develop- ment progresses, measurements would be needed of

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28 medium- and short-lived species that have a more com- plex transformation chemistry. The data on short-lived species would be used to test not only the simulation of long-range transport but also the parameterization of important subgrid scale transport processes such as con- vection and boundary layer turbulence. The species selected on the basis of these criteria are listed in Table 3. 2. A number of very important species (e.g., NO, NO2, HNO3, and SO2) are not listed at this time because the appropriate, easily transferable mea- surement technology does not now exist. When suitable techniques are developed, these species would be added to the network protocol. Each substance in Table 3.2 is classified as to its major source (anthropogenic or natu- ral), the primary application of the data (the develop- ment of transport models, importance to climatic proc- esses, or elucidation of chemical cycles), its relative lifetime in the atmosphere (short months or less, me- dium a few months to a few years, orlong-more than a decade), and the network in which it would be mea PART I A PLAN FOR ACTION sured (Global D'strtbutior~s, Surface Source/Receptor, or Lor~g- T~m Trends). The chlorofluoromethanes, e.g., CC13F and CC12F2, and other halocarbons can provide a quantitative test of long-range transport models because the distribution and strength of their sources are relatively well known. The species CO2, CH4, CO, and the aLkanes ( OCR for page 19
A PROPOSED PROGRAM radiatively active, and its concentration is increasing in the troposphere. Carbonyl sulfide and carbon disulfide play a signifi- cant role in the sulfur cycle, especially in the strato- sphere. There is considerable uncertainty regarding sources for these species, but both industrial and natural sources are believed to be significant for CS2. Most of the species above can be readily measured by collecting on-site samples with flasks or trapping tech- niques and returning the samples to a central laboratory for analysis. Many species can also be measured on-site by using automated gas chromatography. Ozone plays a critical role in the chemistry and radia- tion balance of both the troposphere and stratosphere. Because it is involved in many transformation processes in the troposphere, knowledge ofthe distribution of O3 iS essential for the development of a TCSM. Tropospheric O3 can be used as a tracer for investigating the exchange between the troposphere and the stratosphere and for studying long-range transport above the boundary layer. However, it will be necessary to make frequent measurements of the vertical profile of O3 into the strat- osphere. Ozonesondes have been used routinely with some success for many years, and a number of stations are currently in operation, most of them in the midlati- tudes in the northern hemisphere. However, improve- ments in measurement accuracy are required. New O3 sounding methods are now being developed, and when operational their implementation should be encour- aged. Water vapor is a critical species for most tropospheric chemistry processes. The global data set for water is more comprehensive than for any other trace gas in the lower troposphere. Data obtained in the standard atmo- spheric sounding programs are generally quite poor in the middle and upper troposphere where water concen- trations are low. Although there have been some excel- lent research sounding programs measuring water va- por, their spatial coverage has been sparse. Improved instruments for measuring the low-water-vapor concen- trations characteristic of the upper troposphere should be developed, and this instrumentation should be used in the O3 sounding program. The concentration and composition of aerosol parti- cles could be measured by using samples collected on filters. The species SO4-, NO3, and NH4+ are of interest because of their role in the sulfur and nitrogen cycles. The measurements of soil and sea-salt aerosol particles will provide information on the budgets of the two most massive components in the global aerosol flux. Soil aero- sol particles and Pub also serve as natural tracers for the transport of air from continents to the oceans. Vana- dium (V) serves as a tracer for emissions from many oil- fired anthropogenic processes, particularly those using . . 29 heavy residual fuel oils. The measurement of total car- bon and total organic carbon will provide data necessary for the development of global carbon budgets. Finally, the measurement of light absorption on the filters will yield data on the corresponding properties of aerosol particles (primarily that of elemental carbon or soot); these data will be vital for assessing the impact of aerosol particles on global climate. Other optical properties, e.g., the scattering component of extinction, are already being measured at GMCC sites and may be included when possible. The precipitation studies will focus on the same suite of species measured in the aerosol particle studies. In addition, analyses will be made for weak organic acids, such as formic, acetic, and oxalic; these acids are signifi . . . . . . .. cant components in precipitation in some regions. t1- nally, the samples will be analyzed for K+, Mg+ +, and Ca++. By using the concentrations of these and other dissolved species, it is possible to compute the pH of . . . . .- . . . . precipitation and check the directly measured value oy considerations of ionic balance. Experimental Design Measurement Validation A validation program should begin with an exchange of calibration standards followed by an exchange of blind standards. Past experience with N2O calibrations has demonstrated that this procedure alone will not be sufficient to establish the accuracy and precision of the techniques. A critical second step will be to carry out on- site comparisons among the candidate systems and to compare these systems with state-of-the-art instruments that are not yet ready for routine use at remote sites. Recent intercomparisons have proven to be extremely useful in a number of current programs. The validation of the absolute accuracy and precision of the measure- ments is critical for long-term trend measurements and for measurements of long-lived gases. In both cases, the detection and quantification of small concentration vari- ations are being sought. Those techniques that survive these intercomparisons should then be compared with all other state-of-the-art measurements under a variety of environmental conditions that are representative of those expected at the planned network sites. Only after successfi~1 techniques have been identified should net- work operations begin. Obselvational Protocol Concurrently with the measurement validation pro- gram, a measurement protocol should be developed. The sampling frequency and duration required to yield

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A PROPOSED PROGRAM these reasons, we recommend initiation of a major research effort to investigate the processes of conver- sion, redistribution, and removal in the troposphere. The primary objectives of this Conversion and Re- moval Study would be as follows: 1. To obtain chemical conversion and removal rates for selected cases of clear and cloudy air, includ- ing condensing and evaporating clouds, and to cle- velop methods that can be applier! to experiments investigating weather systems that are dominant In different regions. 2. To investigate the processes that control the rate of dry deposition of gases and aerosol particles to the earth's surface. At present, only limited capabilities exist to carry out detailed large-scale field studies. Constraints arise from a lack of crucial instruments and tested methods in this relatively new field of chemical meteorology. Thus a general research strategy is to call for near-term field experiments to attack specific questions with available technology and, as knowledge grows from these experi- ments, and new instruments are developed, to broaden the scope of effort accordingly, both scientifically and geographically. As progress is made toward the attain- ment ofthese objectives, a framework can be developed for major future field experiments to quantify the com- plex physical and chemical processes linking clouds, precipitation, and both wet and dry deposition with the major chemical cycles. Such experiments will be essen- tial for the verification of global- and regional-scale tro- pospheric models. We recommend a Wet Removal Expert t to assess the flow of specific chemical species into and out of selected types of clouds and subsequent redistribution of those species in the troposphere. This Wet Removal Experiment would involve cloud and precipitation chemistry re- search and a significant investment in meteorological data collection and analysis. We also recommend a Fly Removal Exper?m=tal Program that would concentrate ini- tially on development of fast-response and high-preci- sion chemical sensors and the investigation of important surface exchange processes as well as the characteristics ofthe air near the surface. Experimental Constraints The dominant tropospheric gas-phase reactions and mass transport phenomena can be described for only a few species in the troposphere. The same is true for aqueous-phase processes. The primary reason for slow progress in this area has been the lack of instruments capable of detecting species of interest with sufficient sensitivity, accuracy, or time resolution. Several poten 39 tially critical compounds have not yet been detected in the troposphere. High-relative-humidity environments (e.g., clouds) preclude operation of some current instru- ments. Detection capability for several important chem- ical species can be summarized as follows: 1. Concentrations (1 to 3 ppb) of most sulfur com- pounds can now be measured either in real time or with time resolution of minutes to hours. However, virtually all instruments for these measurements are laboratory prototypes that have become available for field testing only recently. An important goal is the development of instruments for the detection of SO2 with detection lim- its in the higher parts per trillion range and with time resolutions of about 0.1 s. 2. Many nitrogen compounds can be measured at low concentrations, either in real time or intermittently, with time resolutions of 1 s to several tens of seconds. Most ofthe instruments with the exception ofthose for NH3, NO3, and N2O5- have been or are being field tested. 3. Most carbon compounds, except CO and CO2 (gas, aqueous, and aerosol phase), cannot be measured in real time, but require sampling times of several min- utes to several hours. Considerable instrument devel . . .. . . . opment 1S requires to improve sensitivity, speclatlon determination, and time resolution for the many tro- pospheric carbon compounds. 4. Some gaseous halogen compounds of interest can be detected with time resolution of minutes. A real-time instrument for HC1 in the lower parts per trillion range is needed. 5. Field-tested instruments exist to measure O3 with the required sensitivity and time resolution. However, considerable confusion still exists as to the validity of H2O2 detection techniques. Gaseous H2O2 cannot be measured at present, although at least one method is being developed. The aqueous-phase concentration of H2O2 can be measured with sufficient sensitivity, but significant discrepancies among the various methods must be resolved. Measurement capabilities for relevant physical prop- erties ofthe troposphere can be summarized as follows: 1. Visible light scattering and absorption can be measured accurately from aircraft or on the ground. Scattering can be measured with a response time of a second, while absorption requires several minutes. In- frared optical properties are as yet much more difficult to assess. 2. Cloud condensation nuclei measurements can be made on a relatively routine basis, but measurement of ice nucleating properties may require further instru- ment development. The relationship between cloud al

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40 bedo and aerosol particles should be a target of future research. 3. Cloud water collection as a function of droplet size is important for chemical studies, as is measurement of cloud liquid water content. Further instrument develop- ment may be required in these areas. Field programs investigating conversion and removal processes in several tropospheric chemical cycles can be initiated with currently available instruments. How- ever, many fundamental questions in the sulfur, nitro- gen, halogen, carbon, and trace element cycles, and the complex relationships among the various cycles, can be answered only following further instrument develop- ment and as part of a well-integrated research strategy for field experiments. Instruments that have been devel- oped for fast-response eddy-correlation flux determina- tion for some sulfur and nitrogen compounds must be extensively field tested. The development and testing of fast-response instruments, i.e., with at least I-Hz fre- quency response for surface-based measurements and at least 10-Hz frequency response for aircraft measure- ments, or of some technique for making such direct determinations of eddy fluxes by using more slowly re- sponding instruments, is of vital importance for future field programs; see Part II, Chapter 8 for a discussion of related criteria. Recommendecl Field Experiments Wet Removal Exper?rnent The objectives of the Wet Removal Experiment are to obtain overall conversion and wet removal rates in a variety of cloud systems. While measurements should be undertaken of species in several tropospheric chemi- cal cycles, early emphasis will likely be on studies of conversion and removal rates of species in the sulfur and nitrogen cycles because measurement capabilities are generally more advanced for these substances. In Phase I of the Wet Removal Exper?rr~nt, diagnostic studies should be carried out to test, where possible, the postulated oxidation mechanisms for sulfur, nitrogen, and other elemental species and to determine the efficiency of in- cloud scavenging mechanisms. We use the term "diag- nostic study" to refer to a relatively modest research project that is carried out to answer limited research questions. A successful test of experimental concepts and newly developed instruments would subsequently enable them to be applied in Phases II and III to more comprehensive field studies, which would include more detailed studies of carbon, halogen, and trace element chemistry and increasingly complex weather systems. For studies in all phases of the Wet R~noval Experiment, PART I A PLAN FOR ACTION a thorough aerosol characterization effort should be maintained such that key chemical and physical varia- bles can be adequately studied and understood. This effort should also be made in other large-scale measure- ment programs in the Global Tropospheric Ch~rn~st7~ Pro gram. Phase I Diagnostic Studies Examples of recom- mended diagnostic studies that would provide signifi- cant insight into chemical conversion, redistribution, and wet removal are as follows: 1. The relative importance of competing oxidants for SO2 oxidation, such as molecular oxygen (02), 03, H2O2, or OH, as well as homogeneous or heteroge- neous catalysts such as transition metals, could be evalu- ated from the measurement ofthe chemical composition of cloud droplets and interstitial air between the drop- lets. Applying simple chemical models would enable an estimate to be made of the relative importance of the various proposed mechanisms. 2. Strong oxidants such as H202 are frequently present in cloud water and precipitation. The origin of this H2O2 is not clear. Possible formation pathways in- volve both gas and liquid phases. Measurements of H2O2 and other chemical species in cloud water, rain- water, and interstitial air at different elevations within a given cloud environment should aid in defining the ori- gin of H2O2 3. Fog, cloud water, and rainwater in different re- gions apparently contain different amounts of formalde- hyde (CH2O). Aldehydes react with dissolved SO2 via a nucleophilic attachment on the carbonyl carbon to form bisulfite addition complexes. With CH2O, the complex formed is hydroxymethane sulfonic acid. Formation of these adducts would increase the concentration ratio of sulfur (IV) to sulfur (VI) in cloud water in which CH2O is found. The role played by aldehydes in regulating the oxidation state of sulfur in cloud systems could be evalu- ated by analysis of cloud water. 4. A more general type of field experiment could broadly integrate several facets of in-cloud fast photo- chemistry. As discussed in Part II, Chapter 5, in the section by Davis et al., the penetration of ultraviolet radiation into a cloud may produce significant quanti- ties of OH and HOD. These reactive species couldlead to gas-phase conversion reactions In the interstitial air, and/or the reactive species could be scavenged by cloud droplets. Scavenged radicals could initiate extensive ho- mogeneous aqueous-phase chemistry involving OH, H2O2, and other ionic forms of the generalized formula HxOy as well as various halogen and trace element spe- cies. No data exist on the concentration of any of the active photochemical species in the interstitial air. Con

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A PROPOSED PROGRAM centrations of interstitial photochemically active trace gases could be contrasted to measurements in contigu- ous noncloud air. These early experiments are diagnos- tic, as it is unlikely that all key photochemical species will be measurable in the near future, and the experiments could be carried out in conjunction with the Photochemical Transformations Study. Species of potential interest include NO, NO2, 03, SO2, H2O2, CH3OOH, OH, HO2, and certain halogen species. These diagnostic studies could be carried out with one or two aircraft measuring in-cloud and simultaneously collecting cloud water and rainwater (without reference to any large-scale flow field). PhaselI When the appropriate instruments, experi- mental methods, and theoretical concepts have been developed from the Phase I diagnostic studies, it will be possible to undertake more detailed experiments in se- lected cloud systems. Such studies are essential to pro- gress in cloud chemistry and should be of great value in planning subsequent field operations in increasingly more complex meteorological systems. They also would serve as a test of laboratory kinetic studies, particularly those performed under experimental conditions differ- ent from those expected in clouds. In terms of the phys- ics of clouds, unglaciated wave clouds are perhaps the least complex, with orographic stratus, marine stratus, and isolated cumulus clouds being increasingly more complex. The choice of specific cloud systems for study should also be related to source functions for the chemi- cal species in the cycles being investigated. Cloud water, aerosol, gas, and rain sampling experi- ments would be conducted in these relatively simple cloud systems. Although the detailed experiments can- not be defined at this time, an example can be suggested on the basis of current knowledge of atmospheric cloud chemistry. Orographic clouds downwind of biologically active ocean water offer a reasonably simple cloud phys- ical situation in a potentially important source region for certain chemical species. Careful utilization of meteoro- logical information would be required to provide an accurate description of the temporal history of an air parcel as it is processed through the orographic cloud. Possible studies could include the following: 1. The oxidation of (CHESS and SO2 in the gas phase and within cloud water itself; 2. The role of organic substances in cloud water chemistry; 3. The incorporation of aerosol particles into the cloud droplets; 4. The removal of sulfur species in rain. 41 It would also be useful to attempt a mass-balance calculation for sulfur species during these studies, per- haps as part of a Lagrangian observation scheme. Phase III After completion of studies in less complex cloud systems, investigations could ultimately proceed to much more complex systems, including extratropical cyclones, either marine or continental, and their associ- ated frontal systems. A significant fraction of the precip- itation in midlatitude regions is related to such systems, and we suggest a study with particular emphasis on warm frontal precipitation. Warm frontal precipitation associated with cyclonic storms results, in principle, when warm, moist air ascends over cold air (cold sector) north of the warm front. The rising warm sector air cools until condensation occurs, effectively scavenging trace gases and aerosol particles from the warm air mass by in-cloud processes. Most of the precipitation occurs north of the surface front (in the northern hemisphere) and falls through the colder air below the cloud, from which trace gases and aerosol particles are also scav- enged. Precipitation also results from convective insta- bility, which often develops in the warm sector. Air tra- jectory analysis at several levels can help establish the general flow pattern into the complex storm system. Detailed meteorological and cloud physical back- ground information would be required in such a study. This phase ofthe Wet Removal Experiment should be devel- oped in conjunction with comprehensive meteorological research and support programs if at all possible. An example of such a program in a continental region is the proposed National Stormscale Operational and Research Mete- orology Program (STORM). ~ One emphasis of STORM is the study of the evolution of cyclonic storm systems. One of the primary requirements of a study of warm frontal precipitation would be to direct and position the participating aircraft fleet in the following general flow regimes: in dry air in the warm sector before it ascends. in nonprecipitating clouds ahead of the warm front. in precipitating clouds involving the overriding air. just below cloud base and below the cloud penetra . . ~ tlon alrcratt. in dry air west of the front (cold sector). Close coordination with forecasters would also be re- quired for mobile collection of ground-level precipita ' The National Storm Program, Frameworkfor a Plan, University Corporation for Atmospheric Research, Boulder, Colorado, 1982, 21pp.

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42 tion coinciding with the aircraft samples taken aloft in the same region. Surface-based Doppler radar would be required to document flow fields in the vicinity of the selected warm frontal system, as well as a surface-based calibrated PPI radar for documenting spatial and tem- poral variability of precipitation intensity. The success of this experiment would rest largely on the ability to follow the air as it is being cycled through the storm system, an extremely difficult task at present. Information on chemical conversion rates for species in the various chemical cycles may be deduced from measured differences in the chemical composition ofthis air before and after cloud encounter and from the chemi- cal composition of cloud and precipitation elements. The removal rate would be obtained, in principle, from the measured composition of cloud and precipitation water obtained at ground level and aloft, from the mea- sured composition of inflowing air, from the liquid water content of clouds, and from the precipitation rate. Requirements for chemical measurements would be as follows in cloud-free air and interstitial air: gases: Various species in the sulfur, nitrogen, car- bon, and halogen cycles. aerosol: SO4-, NO3, NH4+, Ca, Mg, K, Na, CI, carbonaceous material, transition metals, and acidity. Requirements in cloud water and precipitation would be as follows: SO4-, NO3, halides, H+, NH4+, Ca++, Mg++, Na+, K+, H2O2, CH2O, carbonaceous material, and transition metals. Additional variables that should be measured include aerosol particle and cloud droplet size distributions and liquid water content. Liquid sample measurements could be obtained with current instrument technology. Such a study would require approximately five me- dium-sized aircraft equipped with cloud physics and dynamics sensors, with cloud and precipitation sarn- plers, and with sufficient payload to accommodate the chemical and aerosol particle sensors and samplers. Small trailers equipped with replicate sets of chemical sensors and samplers and with several sequential precip- itation collectors would also be required. A review of existing research aircraft (see Part II, Chapter 9) con- f~rms that adequate platforms are available for this study. These platforms include well-instrumented mete- orological aircraft with adequate space for tropospheric chemistry measurements. The timetable for a Phase III warm frontal precipitation study would depend to some extent on the schedule of major meteorological field ex- perirnents on mesoscale weather systems. PART I A PLAN FOR ACTION Dry Removal Experimental Program Dry deposition includes the turbulent and diffusive transfer of trace gases and aerosol particles from the air to the underlying surface, and the gravitational settling of large particles. The processes that control dry deposi- tion are normally associated with the nature of the sur- face itself, or with characteristics of the neighboring me- dia. Detailed knowledge of the factors controlling dry deposition of key chemical species is still rudimentary. No large field effort can be offered as a panacea. The nature of the problem requires dose attention, at least initially, to small-scale factors in process-related field studies rather than large-scale integrated research pro- grams. Flux determinations would provide detailed knowl- edge of appropriate deposition velocities and their con- trolling properties. These deposition velocities could be used directly in numerical models, but they could not be used to evaluate dry deposition fluxes from field data unless suitable concentration data were available. Such data must be obtained sufficiently close to the surface so that surface-based formulae can be used to interpret them. Simple but reliable sampling methods need to be developed for this purpose. Methods analogous to high- volume filtration for airborne particles appear to offer special promise. Such methods are already in operation in some networks (e.g., in Canada, Scandinavia, and over the Pacific), but these methods need improvement to permit routine and inexpensive operation on a large scale. Special attention must be given to deposition at sea, where flux determinations are rare. Rates of exchange between the atmosphere and the ocean are major un- knowns in many geochemical cycles. For example, the atmosphere may be the primary transport path for a number of trace elements, including heavy metals, found in the open ocean, but efforts to determine deposi- tion rates for these substances are complicated by resus- pension processes at the sea-air interface. Similarly, air- sea exchange plays a major role in the biogeochemical cycle of iodine, but little information is available on the conversion rates of iodine species in the marine tropo- sphere or their exchange rates with the ocean. Existing knowledge of air-sea exchange of trace gases is based primarily on extrapolation of laboratory studies conducted over calm, clean water with some guidance derived from the use of chambers in light wind condi- tions at sea. It is suspected, however, that exchange rates are greatly accelerated in strong winds when effects of breaking waves cannot be disregarded. Recently devel- oped aircraft and surface tower methods for determin- ing dry deposition rates of specific trace gases must be

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A PROPOSED PROGRAM 43 extended to oceanic situations so that moderate- to high- tify dry deposition processes, with particular emphasis wind conditions are addressed adequately. Further- onthedevelopmentoffast-responseandhi~h-precision more, it is likely that air-sea exchange of trace gases will be moderated by biological activity in the surface wa ters. The influence of these biological factors must be . nvest~gatec . TechniquesforDeterrnir~ing Fluxes There is no generally accepted method suitable for routinely monitoring dry deposition fluxes. However, suitable techniques are available for application in intensive field studies. These techniques include micrometeorological gradient and covariance methods from towers, aircraft covariance methods, direct measurement of the accumulation of material on exposed natural surfaces (especially of leaves and snow), and mass budget studies conducted over a closely monitored research area (such as a cali brated watershed). These intensive measurements are usually intended to identify controlling processes and to quantify those that are important. Aircraft and tower studies of dry deposition fre quently make use of the covariance (or eddy-correla tion) method of flux determination in which output from a fast-response chemical sensor is multiplied by the ver tical wind component measured with fast-response co located instruments. The average of this product over a period long enough to obtain a statistically significant estimate gives the vertical flux of the chemical species through a horizontal plane at the sensor height. Aircraft covariance measurements require chemical sensors with at least 10-Hz frequency response; tower studies can be conducted with instruments of at least 1-Hz frequency response. In general, instruments that meet the requirements for aircraft eddy-correlation measurements will also satisfy the requirements for tower operation. In some cases, however, eddy-correla tion methods with their requirements for rapid response sensors can be replaced by gradient techniques. The latter place great demands on accurate measurement of mean concentration differences between several levels near the surface instead of high-frequency measure ments of concentration fluctuations. Except in a few instances, suitable instruments for flux determination by the eddy-correlation technique are not yet avanab~e. Therefore dry deposition studies remain at the mercy of sensing technology. Techniques for estimating chemical fluxes are discussed in Part II, Chapter 8, where mea surements of various terms that contribute to the large scale mass budget of a trace species are considered. Experimental Approach We recommend the continua tion and intensification of laboratory, theoretical, and experimental research programs to identify and quan v ~ chemical sensors. When the appropriate instrumenta- tion is available, we recommend consideration of a large-scale study of dry deposition at sea. Conducting experimental studies at sea is a demand- ing task that should be attempted only after appropriate new instruments have been thoroughly tested over land. Once the necessary experimental facilities are devel- oped, considerable benefit would derive from testing the various components in an integrated fashion in a large- scale "box-budget" experiment over the ocean. Such a large-scale field study would serve not only as a target for instrument development but also as a means for explor- ing the links between such interrelated aspects of oceanic geochemical cycling as the importance of flux diver- gence terms, the rate of chemical transformation in oce- anic air, and eventually the role of deep convective proc- esses and precipitation in redistributing and scavenging tropospheric trace species. However, such an experi- mental box-budget study would not be warranted before the development of accurate models for evaluating me- teorological transport and dispersion within remote areas of the troposphere, over distances on the order of at least 1000 km. We propose the following studies lead- ing to experimental investigations of surface exchange at sea. Phase I Through the use of meteorological towers at appropriate land sites, evaluate surface fluxes for trace gases, including SO2, NO, NO2, HNO3, NH3, H2O, H2O2, 03, and measurable species in the halogen and carbon cycles, as well as species present on aerosol parti- cles. Studies should be conducted over several types of surfaces at different seasons to develop confidence and experience with the measurement techniques, as well as to investigate the importance of seasonally varying bio- logical and meteorological factors. Such studies permit direct comparison of alternative methods of measure- ment and should be expanded to include research air- craft equipped with comparable sensors. Phme II Once fast-response sensors for several key species in the sulfur, nitrogen, carbon, and halogen cy- cles are available, prototype tower experiments should be conducted over the ocean, possibly by selecting a suitable island or existing tower within some selected experimental area. Fly-by missions with instrumented aircraft should be conducted both for comparison with tower-based results and to assess spatial variability. Phone III Extension of this program beyond the process-oriented and instrument development case

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44 studies would occur only after careful evaluation of the integrated box-budget experiment approach. Figure 3.3, using the sulfur cycle as an example, draws atten- tion to some ofthe physical and chemical constraints and meteorological factors that must be borne in mind. At this time, such a study cannot be discussed in detail because the required experimental capabilities do not exist. However, it is appropriate to bear in mind the wide range of phenomena to be considered in any such experiment because they are all closely interrelated, and any one could influence the interpretation of experimen- tal programs designed primarily to address another. Future Experiments Models that describe the fate of species in the various tropospheric chemical cycles incorporate parameters that must be obtained from experiments such as those described above. As new instrumentation and theoreti- cal concepts are developed, investigations of conversion, redistribution, and removal can be expanded both sci- entifically and geographically. Ultimately, we would ex- plore all chemical cycles and the relationships among them within the cloud systems embedded in the major flow regimes (global circulation) of the earth. This ex z _ SU LFU R CYCLE Wind,A MY PART I A PLAN FOR ACTION Mixing \ /' with Free \ / Troposphere y /~ FR lint)_ ~- any' ~ ___ ,/~0 x FIGURE 3.3 A simplified schematic representation of the design of a box-budget experiment involving reduced and oxidized sulfur species. Horizontal fluxes F and vertical fluxes Fz have subscripts indicating reduced sulfur species (R), SO2 (2), and SO4- (4~. H is the depth of the marine mixed layer, somewhat less than the height Z to which measurements must be made. The horizontal dimen- sion of the study volume is about 1000 km, with Z in the range of 2 to 5 km. Note that fluxes at the top and longitudinal sides of the volume are not shown. proration would yield information required for develop- ment, completion, and verification of global Tropospheric Chemistry Syster7~s Models (TCSMs). MODELING THE TROPOSPHERIC CHEMICAL SYSTEM Over the last decade, tropospheric chemistry research has progressed largely through the development of new instruments and the concomitant pioneering measure- ments of previously undetected species. It is now evolv- ing to a more mature science with the design of large- scale field programs devoted to the systematic collection of required data. Modeling has also always been a part of tropospheric chemical studies. However, in contrast to experimental chemistry, global chemical modeling has mostly been carried out by a few individuals whose primary goal has been to leap to new levels of under- standing by using simple models tailored to display the mechanistic role of the new species of interest. Regional air-quality problems, on the other hand, because oftheir more immediate practical concerns, have forced other modelers to be more empirical and to model the data at hand without going into much detail on the physical processes involved. Almost all past efforts in modeling global problems were accomplished by one or two scien- tists working together for one or two years. It is expected that such modeling will continue profit- ably not only for introducing new concepts but also for illuminating individual subprocesses within the overall global tropospheric chemical system. However, to achieve the objectives of the proposed Global Tropospheric Chemists Program, it Will also be necessary to have a more ambitious long-term perspective toward model develop- ment. That is, for the successful application of the field program data and to advance scientific understanding, it will be necessary to develop comprehensive models of the overall tropospheric chemical system. Such models should necessarily include the meterological processes that transport and in other ways interact with the chemi- cal species. Experience in closely related areas shows that the de- velopment of such models would involve considerable investments in time and computer resources over a long term. Further, it is unlikely that such model develop- ment can be achieved within existing institutional ar- rangements, such as individual short-term grant pro- grams. Thus we recommend the initiation of one or more global Tropospheric Chemistry Systems Models (TCSMs) with stable long-term support. Considerable modeling effort will also be devoted to the various individual processes related to tropospheric chemistry. These processes are required for the TCSM

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A PROPOSED PROGRAM to have a sound physical basis. The field programs that have been proposed as part of the Global Tropospheric Chemistry Program are intended in part to provide an observational basis for developing the critical models required for developing the TCSM. We recommend that additional modeling efforts be Initiated to cie- velop the critical submodels required for the TCSM. Models for Biological and Surface Sources and Sinks Biological and surface source models fall into three categories: (1) global empirical models, (2) mechanistic models of biological processes, and (3) micrometeoro- logical and oceanic models of surface transport proc- esses. The observational efforts in the Biological Sources of Atmospheric Chemicals Study will provide measurements at individual field sites. Initial exploratory efforts will iden- tify the ecological communities that provide significant emissions, but as a second stage it will be necessary to obtain sufficient observations to determine annual aver- age emissions at various sites. Variability with environ- mental parameters such as temperature, solar radiation, moisture, soil pH, and Eh will also be obtained. How- ever, due to the great variety and small-scale structure of biological systems, it will always be very difficult to col- lect sufficient data to permit straightforward numerical averaging to establish regional and global average emis- sions. Rather, more sophisticated approaches should be developed to interpolate and extrapolate the available observations to all the nonsampled areas and thereby to derive regional and global budgets. We refer to such models as global empirical models. It will also be necessary to develop models of the detailed biological mechanisms and processes responsi- ble for the measured emissions. These will range from models of soil or oceanic biochemical processes to models of whole-leaf physiology. Boundary layer and surface transport models are re- quired to describe the movement of gases and aerosols between ocean or land surfaces and the atmosphere. In the case of the oceanic processes, such models require consideration of oceanic as well as atmospheric bound- ary layers and the effects at the ocean interface of wave breaking, i.e., the movement of air bubbles on the ocean side and spray droplets on the atmospheric side. In the case of land surfaces, it is necessary to model, in conjunction with observational studies, the detailed physical mechanisms that together determine the depo- sition of a given species to a given surface. It has been found convenient to model these processes as "resist- ances" or "conductances. " A gas molecule being trans- ferred from the atmosphere to a surface first passes through the atmospheric mixed layer above the vegeta 45 lion canopy or other roughness elements. Thus it must be transferred to the air within the canopy or other roughness elements and from there to surfaces of deposi- tion. These surfaces in turn may not instantly capture the molecule but rather, as in the case of ocean surfaces, provide additional resistance to the transfer of the depos- iting species to its ultimate sink. Many gases of interest have sinks in the internal cavities of leaves so that the biomechanics of leaf stomata must be modeled, possibly in terms of leafresistance. Models for Global Distributions and Long-Range Transport Here we outline some of the research studies that one or more TCSMs would carry out in support ofthe Global Distributions arid Lor~g-Rar~ge Transport Study and, more generally, for modeling exploration of the tropospheric chemical system. Two classes of investigations would be carried out with TCSMs. First would be studies in- tended to validate and possibly develop the capability of models to simulate long-range transport and global dis- tributions and the variability of long-lived chemical spe- cies. These studies would compare model results with the data sets including measured variances obtained through the Global Tropospheric Chemists Sampling Network and, if necessary, would develop the model improve- ments required for satisfactory validation. Most of the modeling studies would be carried out in a climatologi- cal framework, but detailed event studies would also be performed in conjunction with intensive periods of field data collection. Such investigations could also be carried out with more simple chemistry if needed to simulate the sources and sinks of the long-lived species. The second class of studies would emphasize the sim- ulation of species with tropospheric lifetimes of days to about a year. At present, the sources and sinks of these species are not sufficiently well known for such studies to be used to test model transports. Rather, these studies, assuming adequate transport submodels, would explore the role of meteorological processes in determining the spatial distributions and temporal variability of these species. Such modeling studies, for example, could ad- dress the question of the importance of continental pol- lution sources of sulfur and odd nitrogen for atmo- spheric distributions at remote sites. A second extremely important example is that of O3 in the global tropo- sphere. TCSM studies are needed to compute how the marine troposphere responds to continental sources of nitrogen oxides and hydrocarbons (which, together, produce 03) and to the export of O3 itself from conti nents. The exploration of such questions would help im- prove interpretation ofthe data on many ofthe species to

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46 be monitored in the Global Distributions arid Lor~g-Range Trar~sport Study. Species of special current interest include NO, NO2, HNO3, CO, SO2, 03, sulfate-nitrate aerosol particles, and continental soil aerosol particles. The soil aerosol is of interest not only because of its optical-radia- tive effects but also as a source of Ca, which would neutralize acidity and thereby raise the pH of airborne droplets. Model s for Photo chemical lean s formations The questions of photochemical transformation in- volve the concentration and interrelationships of HxOy' NxOy' and O3. The immediate connections among these species occur so rapidly that the effects of transport are negligible; field observations can hence be tested by sim- ple box models. However, the longer-lived species, in- cluding 03, that ultimately determine the concentration of the fast-radical families must be generated by the TCSM. Global models are also needed to provide mete- orologically realistic descriptions of the radiation fluxes that drive the fast photochemistry. Models for Conversion, Reclistr~bution, and Removal Processes Conversion, redistribution, and removal involve both dry and wet processes. Dry deposition at surfaces has already been discussed. Besides the physics of sur- face removal, it is necessary to model the meteorological transport through the planetary boundary layer and the surface mixed layer. Models for the wet processes also need to be improved. Clouds and precipitation play important roles in the removal, transport, and transfor- mation of species in element cycles. For instance, wet removal is probably one of the most effective sinks for nitrogen, sulfur, and inorganic halogen compounds. Important species such as SO2, N2O5, and perhaps NO3 may go through fast aqueous transformation in cloud droplets. Furthermore, cloud convection may be an eff~- cient vertical transport mechanism for trace gases and aerosol particles. In order to evaluate these processes quantitatively, it is necessary to develop a cloud-removal model that in- cludes detailed treatments of the physical and chemical mechanisms involved. The cloud model would be a sub- model ofthe TCSM. Physical aspects ofthe cloud model would include the parameterization of radiation, con- densation, evaporation, stochastic coalescence and breakup, and precipitation development. Chemical as- pects of the model would include both homogeneous PART I A PLAN FOR ACTION gas-phase and liquid-phase reactions as well as heteroge- neous reactions. In the dean atmosphere, chemical spe- cies treated within the cloud model should include at least 03, odd-nitrogen species, hydrogen radicals, H2O2, sulfur species, CO, and CH4 and its oxidation products. In the polluted atmosphere, nonmethane hy- drocarbons and their oxidation products, metals such as Mn and Fe, and graphitic carbon should also be consid- ered. Summary Modeling should be a major component of the Global Tropospheric Chern~s~ry Program. It is clear that, in addition to current modeling efforts, there is also a need to focus on the development of tropospheric chemistry systems models. The meteorological processes in these models would best be provided through specially designed at- mospheric general circulation models (GCMs) that not only account for large-scale tracer transports in the free atmosphere but also incorporate transport through the planetary boundary layer and through cloud processes. These models also require physically based submodels, a good description of land surfaces, and adequate treat- ment of the solar radiation that drives tropospheric pho- tochemistry. With proper support, it should be feasible to develop TCSMs successfully because well-posed questions are apparent, suitable computers exist, and the substantial experience of scientists with global mete- orological models and with complex air pollution models can be tapped. The three-dimensional distribution of chemical spe- cies should be represented with a spatial and temporal resolution comparable to that ofthe meteorological vari- ables. The distribution of these species is determined on the one hand by meteorological transport and source and removal processes, and on the other hand by wet and dry chemical transformations. Besides the development of the TCSM, there should be additional new modeling programs devoted to the treatment of crucial subprocesses that will be needed for the TCSM to have a sound physical basis. We anticipate a two-way interaction with the other components of the Global Tropospheric Chemistry Program (i.e., field programs and laboratory studies of kinetics, photochemical data, and heterogeneous equilibria and reaction rates). As shown previously in Figure 3.1, these components would provide the data needed for the development and testing of the TCSM. The TCSM and the submodels would in turn provide guidance and direction for the planning of the field programs and laboratory studies.

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A PROPOSED PROGRAM INSTRUMENT AND PLATFORM REQUIREMENTS Field and Laboratory Instrumentation To attain the long-term goals and objectives of the Global Tropospheric Ch~rnistry Program, sensitive instru- mentation is required for both the measurement of chemical species and their fluxes in the remote tropo- sphere and the elucidation of critical reaction mecha- nisms and rates in the laboratory. Although currently available instrumentation is adequate to initiate some of the exploratory phases of major field studies in the pro- posed Global Tropospheric Chit sty Program, the instru mentat~on Is not adequate to carry out the detailed re- search program outlined in this report. Currently available instrumentation ranges from low-technology, low-cost sensors and collection systems with limited ac- curacy and sensitivity to high-technology, delicate, ac- curate, but costly bench-type instruments that still re- quire considerable development and intercalibration before they can be deployed in the field. In addition, there are as yet no instruments available for the mea- surement of certain critical species in the global tropo- sphere, such as HO2. A summary of currently available measurement techniques for critical species in tropo- spheric chemical cycles is presented in Chapter 9. While it is feasible today to make concentration mea- surements for most of the major species in tropospheric chemical cycles within the planetary boundary layer, many of these measurements cannot be made in the free troposphere, where concentrations are considerably lower. In addition, relatively few instruments are capa- ble of making in situ measurements with a frequency greater than one measurement per second. Absolute calibrations, instrument intercomparisons, and other quality control procedures during all research efforts in the Global Tropospheric Chern~stry Program are needed. Spe- cific requirements for instrument development have been described in the individual research program sec- tions. We recommend that a vigorous program of in- strument development, testing, and intercalibration be undertaken immediately and that it continue throughout the Global Troposphenc Chemists Pro- gram. As new and more sensitive instruments are devel- oped, the requirements for accurate calibration and standardization of these instruments will become more difficult to achieve. At the present time the National Bureau of Standards produces no Standard Reference Material gas standards with concentrations below sev- eral parts per million. The Global Tropospheric Ch~rnistry Program will require accurate analytical standards for many trace gas species in the part per thousand to part per billion range. We urge the initiation of a program to . ~ . . . . . 47 develop accurate trace gas standards in this concentra- tion range. A strong, active program of laboratory measure- ments of chemical reaction kinetics will be required in the Global Tropospheric Chernist7~ Program. The program will require the continuing commitment of chemical kineticists and the further development of laboratory instrumentation systems for the investigation of the mechanisms and rates of the gas- and liquid-phase reac- tions critical to an understanding of tropospheric chemi- cal cycles. We recommend that an increased effort be initiated on laboratory studies of the rates and path- ways of fundamental chemical reactions in tropo- spheric chemical systems. Platforms Aircraft There is a wide variety of aircraft platforms currently available in the United States from government, univer- sity, and private operators. A detailed compilation ofthe specifications and characteristics of these aircraft is now being developed by the National Center for Atmo- spheric Research. A brief summary of research aircraft platforms in the United States is presented in Part II, Chapter 9. Available platforms range from single- and two-engine aircraft with limited range and space for scientific equipment (more than 20 such aircraft) to long-range, four-engine turbojet and turbo-prop trans- ports. Currently, three jet aircraft and five turbo-prop aircraft are being used in various aspects of tropospheric chemistry research. In some cases, the aircraft platform is available as an unmodified vehicle and in others as a complete aircraft measuring system, often dedicated for extensive periods of time to meteorological and atmo- spheric chemistry studies. We believe that the current aircraft fleet number and type is adequate to undertake the Global Tropospheric Chern?st7~ Program, assuming ready access to these aircraft. Certain improvements and modifications to some aircraft and the meteorological support equipment aboard them will undoubtedly be necessary. Ships A large number of dedicated oceanographic ships are now active in the United States registry. They are oper- ated by academic institutions through coordination with the University National Oceanographic Laboratory System (UNOLS), the Navy, NOAA, the Coast Guard, and other institutions. There are currently more than 30

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48 active ships from academic institutions and 35 operated by the federal government. These ships vary widely in capability and in the geographic areas they cover. A summary of the specifications of academic oceano- graphic ships in the United States is given in Part II, Chapter 9. There is no oceanographic vessel designed specifically to carry out tropospheric chemistry research or dedicated to this area of research. Therefore, oceano- graphic sampling platforms will be a compromise be- tween the needs of the atmospheric chemistry commu- nity and the missions for which the ships were designed. Sufficient ships are available to undertake the proposed field research at sea in the Global Tropospheric Chemistry Program. Satellites Ideally, spaceborne remote sensors could provide near-global measurements and thus could satisfy the ultimate goal of obtaining a three-dimensional distribu- tion of certain atmospheric trace constituents. The im- portance and future potential of spaceborne instrumen- tation for investigating the chemistry of the troposphere are highlighted in a forthcoming report of the National Research Council.2 Eventually, this approach should provide the tropospheric chemistry community with the opportunity to iterate a variety of distribution measure- ments with evolving mathematical models of the tropo- sphere. An assessment of the capability of current re- mote sensor technology for performing measurements 2A Strateg~for Earth Scier~cesfrom Space irz the 1980 's arid 1990 's. Part II: Atmosphere and Interactions with the Solid Earth, Oceans, arid Biota. Committee on Earth Sciences, Space Science Board, National Academy Press, Washington, D.C., 1984 (in press). PART I A PLAN FOR ACTION in the global troposphere reveals that three classes of remote sensors have demonstrated unique capabilities in meeting some of the measurement needs. The first class includes imaging spectroradiometers currently be- ing used in earth observation satellite systems. A second class includes passive remote sensors that measure spec- tral emission or absorption of atmospheric molecules by using external sources of radiation. A third class in- cludes active remote sensors in which lasers are used in a manner similar to an active radar system. Through a combination of scattering by aerosol particles and mole- cules in the atmosphere and selective absorption by at- mospheric molecules, these sensors should ultimately be able to provide range-resolved measurements of aerosol particles and many tropospheric molecular species. However, significant technological advances, relative to both the species that can be detected and spatial resolu- tion, are necessary to satisfy the foreseeable needs of the Global Tropospheric Chemists Program. A review of current spaceborne sensor capabilities is presented in Part II, Chapter 9. We suggest that a study be undertaken to lef~ne the role of satellite measurements in the Global Tropospheric Chemistry Program. Regardless of the technology developed for the re- mote detection of tropospheric constituents, it will al- ways be necessary to perform complementary point measurements using in situ sensors from surface-based or airborne platforms (ground truth). Thus it will be necessary to maintain in-depth research and develop- ment programs for both types of sensors.

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A PROPOSED PROGRAM INTERNATIONAL COOPERATION The Global Tropospheric Chemistry Program win be inter- national in scope. The research plans outlined on the preceding pages recognize the necessity for investigating these critical biogeochemical cycles throughout the many and varied physical and chemical regimes found in the global troposphere. To attain a detailed and com- prehensive understanding of global tropospheric chem- istry will require cooperative efforts by many nations in addition to the United States. The resources and com- mitment ofthe international scientific and political com- munity will be vital to the success of the Global Tropo- sph~c Chemistry Program. For this reason, we recom- mend that the United States join in a cooperative international effort to commit these resources and that it do so with confidence that the international commu- nity is both ready and willing to join in this initiative. Many members of the international community of atmospheric chemists were contacted as this report was being developed, and many provided thoughtful and valuable comments. Copies of the report will be distrib- uted widely abroad. It is clear that to achieve the maxi- mum benefit from international cooperation in such a global-scale study, any plan of action must be discussed and developed in an open international forum. The program proposed in this document is only a start to- ward the development of a truly international effort. Although we have attempted to outline the major types of investigations necessary to achieve an in-depth un- derstanding of global tropospheric chemistry, many ad- ditional studies will be required in the future. We suggest that a forum for discussion of an inter- national Global Tropospheric Chemistry Program could be provided for the international scientific community through the International Council of Scientific Unions (ICSU) and some of its member bodies. The three most appropriate organizations in ICSU would include these: 1. The International Union of Geodesy and Geophysics (IUGG) and its member organization, the International Association of Meteorology and Atmo- spheric Physics (IAMAP); 2. The International Union of Pure and Applied Chemistry (IUPAC); and 3. The Scientific Committee on Problems of the En- vironment (SCOPE). 49 We further suggest that one possible focal point for this forum within ICSU could be the IAMAP Commis- sion on Atmospheric Chemistry and Global Pollution. Commission members are active atmospheric chemists from many nations who are concerned about global- scale problems. A second possibility is to form an Interunion Com- mission with representation from IUGG (IAMAP), IUPAC, and SCOPE. Within IUPAC, the appropriate committee would probably be the Commission on At- mospheric Chemistry of the Applied Chemistry Divi- sion. SCOPE has wide-ranging interests and concerns relative to biogeochemical cycles in the atmosphere and other compartments of the environment. Any or all three of these groups could initiate discussions and for- mulate programs relative to the international science aspects of a Global Tropospheric Chemistry Program. A third possible approach to the development of inter- national cooperation in a Global Tropospheric Chemistry Program would be the formation of a special committee by the International Council of Scientific Unions with appropriate representation from the nations and scien- tific unions concerned with tropospheric chemistry re- search. The strength and ultimate success of any interna- tional Global Tropospheric Chemists Program win depend upon the quality of science and scientists involved. The best research groups in the world will be required. How- ever, routine global observations of meteorological data will also be needed. Some ofthese meteorological obser- vations can be obtained by the international scientific research groups involved in the Global Tropospheric Chem- istry Program, and others can be obtained from currently existing observation stations. The capability to provide this type of data has been ably demonstrated by the member countries of the World Meteorological Organi- zation. It is expected that maximum use of these facili- ties will be made in appropriate geographical regions by the various investigations undertaken as part of a Global Tropospheric Chemistry Program. Close cooperation be- tween chemists and meteorologists will be a critical fac- tor in the success of such a global-scale program.