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~ AFr rk To date, much of the research in tropospheric chemis- try has focused on isolated questions or on one of the many elemental cycles, for example, carbon, nitrogen, or sulfur. Because of the complexity of tropospheric chemistry and the poor state of knowledge at that time, narrowly focused studies and exploratory programs were justified. Indeed, such studies were successful in that they advanced the field of tropospheric chemistry to its present status where one can begin to discern the overall structure and interaction of the chemical systems in the troposphere. Because of these complex interac- tions and the dynamic nature of the chemistry and phys- ics of the troposphere, we believe that future advances in many areas of tropospheric chemistry can be best achieved by fostering research within a unifying concep- tual framework based on atmospheric chemical proc- esses. This framework is that of geochemical and bio- geochemical cycles. Four major categories of processes dominate chemi- cal cycles in the troposphere: those related to sources, to chemical reactions and transformations, to transport, and to removal. Within the context of tropospheric chemistry, a chemical cycle begins when a substance is emitted into the troposphere. Consequently, a knowl- edge of the strength and distribution of sources is criti- cal. Materials injected into the troposphere can undergo chemical reactions, some of which are cyclic and some of which produce a wide range of species that can have chemical and physical properties very different from those of the reactants; such transformations can effec- tively remove the species from a specific chemical cycle. The distribution of a species in the troposphere will be dependent on source characteristics and on the control- ling chemical reactions; however, distributions are also greatly affected by a variety of transport processes that range in scale from that of boundary layer turbulence to that of planetary flow. Often, physical interactions oc- cur in which the composition can influence the radiative or physical properties of the atmosphere or the underly- ing surface. Finally, the tropospheric cycle is terminated by removal of the species from the reacting system, usu- ally through deposition at the earth's surface. In this chapter, we outline a research strategy for tro- pospheric chemistry that encompasses these four funda- mental processes and their roles in mediating atmo- spheric physical processes. A major effort would also be directed toward the development of global tropospheric chemistry models that can satisfactorily incorporate and describe these processes. A process-oriented discussion of tropospheric chemistry cycles has heuristic advan- tages over one that focuses on individual cycles in that it incorporates in a coherent manner many interrelated aspects of tropospheric chemistry. The disadvantage of this approach is that important aspects of tropospheric chemistry might be ignored if they do not fit into the framework ofthe discussion. The fact that a specific area 11

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12 of research is not mentioned in this program is not meant to imply that such work necessarily has a low priority. Similarly, the fact that the program calls for an increased level of cooperation and coordination does not mean that it must be highly regimented. Indeed, throughout the report, we stress that this effort can suc- ceed only if independent active research scientists partic- ipate fully in its design, implementation, and manage PART I A PLAN FOR ACTION meet. Such participation is essential because the program is an evolutionary one in which there must be . . . . . an ~nterc range among scientists wor ring on various processes and a feedback between experimental scien- tists, theoreticians, and modelers. Finally, the research activities of independent individual scientists, unaff~i- ated with the proposed program, will continue to be needed for the health of this scientific field. SOURCES Primary sources for tropospheric substances fall into three major categories: surface emissions, in situ forma- tion from other species, and, in some cases, injection from the stratosphere. In each of these categories there are natural and anthropogenic components. Natural surface sources are affected by a wide range of factors such as season, temperature, nutrient level, organic content, and pH; consequently, source strengths are highly variable in space and time. The terrestrial and marine biospheres are of particular im- portance as natural sources for chemical species in all the element cycles, but very little quantitative information is available on these sources. Some measurements of local fluxes from biological sources have been made for methane (CH4) and a few reduced sulfur compounds. Estimates of local source strengths in the ocean, as well as in rivers and lakes, have been made for methyl chlo- ride (CH3C1), N2O, and reduced sulfur compounds by measuring their degree of supersaturation in surface waters, a quantity that varies greatly. To estimate fluxes from measured supersaturations, theoretical models of gas exchange across the water surface are required. At this time, it is not possible to make accurate global esti- mates of the natural surface sources for such globally important trace gases as CH4, N2O, ammonia (NH3), carbonyl sulfide (COS), carbon disulf~de (CS2), hydro- gen sulfide (H2S), dimethyl sulfide ((CHINS), and CH3C1. It is even more difficult to determine the natural source strengths of the many reactive hydrocarbons that are released by vegetation. Future research must not only develop equipment and experimental protocols that are capable of accurately determining the local fluxes from the biosphere, but it must also concentrate on understanding the biological, chemical, and mete- orological factors that control these fluxes. For the United States and Europe, there are relatively accurate inventories of the anthropogenic sources of combustion products, such as nitric oxide (NO), nitro- gen dioxide (NO2), carbon monoxide (CO), and sulfur dioxide (SOL; considerably less accurate estimates are available for the rest of the globe. The same is true of a wide range of industrial emissions. Emission rates from low-technology combustion, such as wood fires and slash-and-burn agriculture, are much less accurately known, even on local or regional scales. There is grow- ing evidence that biomass burning, particularly in the tropics, may be a significant source of many trace spe- cies in the troposphere. In general, existing estimates of fluxes to the troposphere from anthropogenic surface sources are much more accurate than those available for natural sources. We emphasize here the need for re- search on the latter, and indeed on a subset biological natural sources. Trace substances are introduced above the lowest lay- ers of the troposphere by in situ sources, both human (e. g., aircraft and tall stacks) and natural (e. g., lightning and volcanoes). While in situ sources are much smaller than surface sources on a global scale, they are more effective than surface sources because they inject water- soluble and surface-reactive gases into the middle and upper troposphere, where gas lifetimes are considerably longer than near the surface. Lightning is possibly an important source of nitric oxide and nitrogen dioxide (both hereafter referred to as NOX>, and volcanoes can be major episodic sources of sulfur compounds and other materials. Aircraft and tall stacks are important in situ sources of anthropogenic NOx and SO2. Many spe- cies are produced in situ by various tropospheric photo- chemical processes; these processes are considered to be secondary sources and will be discussed in the sections dealing with transformations. Stratospheric sources are the result of high-energy ultraviolet radiation that can dissociate species such as N2O and oxygen, which are normally nonreactive in the troposphere. Materials are transported from the strato- sphere through the tropopause into the troposphere; such stratospheric injections serve as a relatively uni- form source of many species compared to emissions of the same species from surface sources. However, cur- rent theories, observational data, and dynamical model calculations all show that injections into the troposphere occur predominantly at midlatitudes and more in the northern than the southern hemisphere. Stratospheric injection of ozone (03), originally produced in the mid

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.A FRAMEWORK die stratosphere, is a major and perhaps dominant source of tropospheric O3. There is considerable contro- versy about the importance of stratospheric injection of O3 relative to in situ photochemical production; this controversy extends to some other substances as well. 13 Although the stratospheric injection of NOX and nitric acid (HNO3) is much smaller than the combustion sur- face source, it could be the dominant source for these substances in the upper troposphere. TRANSPORT AND DISTRIBUTION Transport processes involve a wide range of space and time scales. However, the discussion of the troposphere can be greatly simplified by separating it, conceptually, into three layers: the planetary boundary layer, the free troposphere, and the tropopause region. The planetary boundary layer (PBL) is the layer of the atmosphere that interacts directly (on the order of hours) with the earth's surface. The PBL plays a critical role in transport and distribution processes because most surface sources, whether natural or anthropo- genic, emit directly into this layer. The structure of the PBL is strongly dependent upon surface properties such as roughness, temperature, and quantity and type of vegetation. Typically, the daytime continental bound- ary layer is connectively mixed as a result of solar heat- ing at the surface. Atmospheric constituents in the PBL are efficiently mixed throughout its depth, which can extend up to several kilometers. Because of the cooling of the earth's surface at night, a stably stratified bound- ary layer (a few hundred meters in depth) is formed over land; the internal mixing within this layer is less efficient than that in the daytime PBL. Chemicals emitted into the stably stratified nighttime boundary layer may be transported long distances horizontally without exten- sive vertical mixing. Over the ocean, the PBL is well mixed to a height ranging from 0.5 to 2.0 km. Where the surface layer of water is warm, e.g., in the tropics and the summertime midlatitudes, this mixing is primarily convective and driven by evaporation. Where the water is colder than the air, the mixing is caused by mechanical turbulence, and a much shallower layer is formed. In both cases there is no strong diurnal cycle similar to that observed over land. The winds in the free troposphere, i.e., above the PBL, have a strong latitudinal component that is east- erly in the tropics and westerly in midlatitudes. Conse- quently, materials that have a residence time in the at- mosphere of greater than about one month will have a distribution that reflects the spatial distribution of sources, and there will be a stronger concentration gra- dient in the north-south direction than around the latitude circle. However, superimposed on the mean east-west horizontal flow are large-scale, wave-like oscil- lations, particularly in midlatitudes, and strong vertical and north-south, thermally driven regional flows such as the Indian monsoon in the tropics. Thus the distribu- tion of relatively long-lived trace species in the atmo- sphere will depend primarily on the location of the source of the injected material relative to the features of the general circulation. The distribution of shorter-lived species will be more complicated because the transport patterns will be determined to a considerable degree by the nature of the local weather systems occurring at the time. An especially distinctive feature of the global tropo- spheric circulation is the relatively restricted interhemi- spheric flow across the equatorial regions of the oceans. As a result, the transfer of materials between the north- ern and southern hemispheres proceeds at a relatively slow rate, leading to differences in the interhemispheric concentration distribution of some species. These differ- ences are especially noticeable for certain anthropogenic materials. Their concentrations are significantly greater in the northern hemisphere where industrialization and energy utilization are greater than in the southern hemi- sphere. The tropopause separates the relatively turbulent and well-mixed free troposphere from the relatively stable and stratified stratosphere. Net upward transport through the tropopause occurs predominantly in the tropics, and net downward transport occurs mostly at midlatitudes. Vertical transport processes within the troposphere range in scale from that of deep cumulus convection through that of cyclone-scale interactions in the polar and subtropical jets to that of the global-scale Hadley circulation. These processes mix trace gases and parti- cles throughout the free troposphere and provide a link- age between the PBL and lower stratosphere. Local convection is particularly important in tapping the PBL and vertically mixing the free troposphere. Synoptic- scale cyclones also mix the free troposphere vertically. Though less intense than individual convective clouds, they tap much larger regions of the PBL. Cyclones, in combination with upper tropospheric jet streams, also provide an effective mechanism for transporting materi- als downward from the upper troposphere and lower stratosphere. On the largest scale, thermally driven up- ward transport in the tropics mixes the troposphere as a whole in that region and transports trace gases from the

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14 subtropical and tropical PBL to the tropical lower strato- sphere. As a general rule, the smaller the ratio of the input (or removal) rate of any substance to the total mass of that substance in the troposphere, the more uniform is its tro- pospheric distribution. Long-lived gases- such as N2O, COS, and chlorofluoromethanes, which have atmo- spheric lifetimes of decades or more are well mixed throughout the troposphere. Gases such as CH4 and CH3C1, with somewhat shorter tropospheric lifetimes of a few years, are generally well-mixed vertically, and there are only small hemispheric differences. Gases with tropospheric lifetimes of a few months or less (CO, O3, SO2, NO, NO2, HNO3, end reactive hydrocarbons, for A key process in all the biogeochemical cycles is the chemical transformation oftropospheric trace gases into species that are either nonreactive in the troposphere or easily removed by rain or surface deposition. The oxida- tion of CO to CO2 by the OH radical is an example of the former, while the oxidation of NO by O3 to NO2 followed by further oxidation to HNO3 by the OH radi- cal is an example of the latter. All the chemical transfor- mations can be grouped into three basic classes: homo- geneous gas-phase reactions (reaction of one gaseous species with another), homogeneous aqueous-phase re- actions (reaction of one dissolved species with another), and heterogeneous reactions (reactions of species at a phase interface). Homogeneous Gas-Phase Transformations Ozone plays a significant role in tropospheric gas- phase chemistry. It reacts directly with some compounds such as NOX and unsaturated hydrocarbons, and, more importantly, the photodissociation of O3 leads to the formation of OH radicals. Hydroxyl-radical-initiated oxidation is the major pathway for the transformation of a large variety of tropospheric compounds and deter- mines their chemical lifetimes. The oxidation of NO2 leads directly to HNO3 vapor. In the case of CH4 and more complex hydrocarbons, numerous reaction inter- mediates, induding free radicals, are produced; ulti- mately, these are either converted to stable nonreactive products, such as CO2 and water, or removed from the gas phase by heterogeneous processes. Chemical trans- formations of a number of trace gases are interrelated by reactions involving common reactive species. This leads to a strong chemical coupling of the various element cycles and, in many cases, to a chemistry that is cyclic in PART I A PLAN FOR ACTION example) are not well mixed, and their distributions show large vertical and latitudinal gradients that are generated by source and sink distributions, chemical transformations, and removal processes. As the tropo- spheric lifetime of a species decreases, transport has less influence on the distribution on the hemispheric and global scale. Highly reactive species, such as the hy- droxyl (OH) and hydroperoxyl (HO2) radicals, have very short lifetimes; thus their concentration distribu- tions do not depend directly on transport. However, because of reaction pathways involving other cycles, their distributions may depend on other species whose distributions are influenced by transport processes (e. g., water, 03, and CO). TRANSFORMATION nature. At present, there is considerable uncertainty as to the identity and fate of compounds produced in situ in the troposphere during the oxidation processes. Many of the reaction steps following the initial chemical attack of the OH radical on CH4, NH3, SO2, and the more complex hydrocarbons are not known for certain. A number of potential intermediate products such as alde- hydes, peroxides, and organic nitrates have been found in the troposphere, but conclusive laboratory confirma- tion of the mechanisms leading to these products and their subsequent reactions is still needed. Furthermore, there is a need to determine the reaction rates under conditions similar to those found in the troposphere (1 percent water, 20 percent oxygen, and atmospheric pressure in the range 0.1 to 1.0 atm). Homogeneous Aqueous-Phase liransformations Homogeneous reactions in water droplets appear to have a significant impact on the cycles of sulfur, nitro- gen, and perhaps other elements. This chemistry takes place in both the submicrometer aqueous aerosol parti- cles and the larger, 2- to 40-,um droplets found in convec- tive and stable stratiform clouds and in fog. The reac- tions that occur in these two aqueous environments can be quite complex because of the presence of many inter- acting species: neutral free radicals, free radical ions, nonfree radical ions, as well as neutral semistable spe- cies. Representative of this type of system is the HxO'/ sulfur/halogen system shown in Figure 2. ~ . This system illustrates the important oxidative capacity of HxO' spe- cies (both in their ionic and neutral forms) in the aque- ous phase. There is still much to learn about aqueous-phase processes in the troposphere. A major question is the

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A FRAMEWORK ~ 3 -I Gig ~ Am: :~1 1 - 1 Mclultlstep | I} ~ rso4 ~ ~ I H O I ~ ~ of_ IHO2 ~3 _~ FIGURE 2.1 The proposed chemical pathways for (OH)aq and SHOUT in a cloud droplet. S(OH ) and S(HO2 ) represent the scav- enging sources of these species to the droplet. The open double arrow indicates rapid chemical equilibrium, and the closed single arrow indicates an aqueous-phase reaction. End products are circled. degree to which laboratory reaction rate constants apply to the actual environment of a water droplet. In addi- tion, there are still many key reactions for which there The final stage of any tropospheric elemental cycle is removal from the troposphere. The process can involve the episodic collection of particles and gases in water drops and ice crystals that fall as rain and snow (wet deposition), the more continuous direct deposition of gases and particles at the earth's surface (dry deposi- tion), or the chemical conversion of trace gases to inert forms. Aerosol particles act as nuclei for the condensation of water vapor in warm clouds and fog and for the genera- tion of ice crystals in supercooled clouds. These nuclea- tion mechanisms are the basis of most cloud-forming processes. As these drops grow in size, they serve as sites for the conversion of SO2 to sulfate and adsorb soluble gases such as HNO3, hydrogen chloride (HC1), hydro- gen peroxide (H2O2), and formaldehyde (CH2O). A small percentage ofthe droplets or ice particles will grow sufficiently large that they fall toward the ground as snow and rain. While falling, they collect other particles and soluble gases. Drops that evaporate before reaching the ground release gases to the atmosphere and the resi 15 are no data. Finally, one must recognize that there are few data on the concentrations of many critical species within aqueous aerosol particles and cloud droplets at different tropospheric locations and as a function of time. Heterogeneous Transformations Normally, a heterogeneous reaction occurs at an in- terface between two phases, although several interfaces can be involved in an overall process. Most reactions that occur on such surfaces are thought to be noncata- lytic. These include chemical reactions in which both phases participate as consumable reactants and physical processes that involve either transport or growth, or both. Adsorption or absorption are, of course, heteroge- neous processes. In heterogeneous catalytic processes the interracial material or a species adsorbed on it is conserved. A special class of heterogeneous process, gas . . . to-part~cle conversion, converts a trace gas to a particle or a liquid droplet suspended in the atmosphere. Important heterogeneous reactions in tropospheric chemistry include the conversion of gas-phase ammonia and nitric acid to ammonium nitrate and the conversion of gas-phase sulfur dioxide to sulfate in cloud droplets. There is still not much known about the role of heteroge- neous processes in many trace gas and trace element cycles. However, it is clear that such processes are im-- portant in removal by deposition to surfaces. REMOVAL due forms a particle; in effect, such droplets transport gases and particles to lower levels in the troposphere. This also occurs for nonprecipitating clouds, although in this case the transport may be upward. Those drops that reach the ground are an intermittent but highly efficient means of converting and removing soluble trace gases and particles from the troposphere. The key issue here is their chemical composition and deposition rate since these provide essential information on the sources and sinks of various species. It is difficult to develop esti- mates of global removal rates from local data sets be- cause of the great spatial variability of precipitation events and of the concentration of particles and soluble trace gases. In addition, it is difficult to measure accu- rately species that are often present at trace levels. At this time there are no reliable data on global precipitation removal rates for any of the chemical cycles. The dry deposition of particles larger than about 20- ~m diameter is largely controlled by gravity. Submi- crometer particles behave more like a gas; their deposi- tion is controlled by factors such as their diffusivity and

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16 their rate of turbulent transport through the lowest lay- ers of the atmosphere. After transport to the immediate vicinity of the surface by turbulence, trace gases and small particles are deposited on the surfaces of vegeta- tion, soils, the ocean, and so on. For species like SO2 and 03, the flux to vegetation is frequently governed by biological factors, such as stomata! resistance. Reactive gases, such as HNO3 and 03, are removed rapidly by most surfaces, although O3 is removed quite slowly from air over wafer. Dry deposition is a much slower but more continuous process than wet deposition. For gases with surface sources such as NO2 and SO2, dry removal is, however, even more difficult to evaluate globally be- cause local concentration measurements are extremely dependent on the distance of the sampling site from the sources. Deposition velocities, i.e., the ratio ofthe flux of a substance to its mean air concentration at some refer- ence height near the surface, have been measured for O3 over a range of surfaces, and there are some similar data for HNO3, NO2, and SO2. At the moment there exist only highly uncertain estimates of global dry removal rates for some ofthese trace gases and particles. A possi- ble exception is O3. A transformation reaction that converts a trace gas PART I A PLAN FOR ACTION into a form that no longer interacts in its elemental cycle may be classed as an in situ removal reaction. Excellent examples of such conversion reactions are the radical- radical reaction between the OH and HO2 radicals lead- ing to molecular oxygen and water, and the oxidation of CH4 to CO2 and water. Aqueous and heterogeneous reactions that convert gaseous species to dissolved salts e. g., NH3, NO2, and SO2 to ammonium (NH4 ), nitrate (NO3 ), and sulfate (SO4-) can also be consid- ered in situ removal reactions. Some species such as unneutralized HNO3 would return to the gas phase if the droplet evaporated. For long-lived compounds, such as N2O and certain chlorofluoromethanes (e.g., CF2C12 and CFC13), the principal sinks are in the stratosphere, where high-en- ergy ultraviolet photons, excited atomic species, and radicals are available to dissociate them. The products of dissociation, such as NO and the chlorine atom, un- dergo transformations and are eventually transported back into the troposphere, where they continue to react in elemental cycles until deposited at the surface. For trace gases from surface sources, the longer the tropo- spheric lifetime the greater is the fraction that will be destroyed in the stratosphere. PHYSICAL EFFECTS OF TRACE SUBSTANCES IN THE TROPOSPHERE Returning to the broader picture of the integrated chemical systems ofthe troposphere, we see that another rationale exists for the study of tropospheric composi- tion. Throughout the cycle of source/transport/transfor- mation/removal, trace substances in the troposphere have effects on important physical processes. Some spe- cies (e. g., O3 and SO2) absorb incoming solar ultraviolet radiation; some (e.g., elemental carbon and NO2) ab- sorb visible light; some (e.g., CO2, N2O, certain chlorofluoromethanes, and many others) absorb and emit infrared radiation; some substances in particles act as condensation or freezing nuclei in clouds and, in doing so, may alter the cloud radiative properties. The source/transformation/removal cycles of trace atmo- spheric materials are depicted in Figure 2. 2; the primary meteorological effects in the troposphere are related to the respective categories oftrace substances: gases, aero- sol particles, hydrated aerosol particles, and clouds. The absorption and emission of radiation by gases is probably the best understood of the mechanisms by which chemical species affect physical processes in the atmosphere; an assessment of these effects requires mainly the measurement of the concentrations of the relevant gases in the atmosphere. The fundamental as- pects of the scattering and absorption of radiation by aerosol particles are reasonably well understood, but direct measurements must be made to determine the magnitude of the relevant parameters and their depend- ence on chemical processes and composition. For example, aerosol particles can produce either a heating or a cooling of the earth-atmosphere system, the end result depending on the relative magnitude of the scattering and absorption coeth~cients for visible light and on the total optical extinction. Scattering is often controlled by submicrometer sulfate aerosol particles, whereas absorption is usually due to mineral compo- nents and elemental carbon, especially the latter. The fundamental nature of the nucleation process and of the freezing of cloud droplets is also understood, but a good understanding is lacking ofthe dependence of these processes on particle composition and the conse- quent impact on the removal of materials from the at- mosphere. It is known that aerosol particles play a major role in cloud processes. Some aerosol substances act as condensation nuclei for clouds, whereas others serve as freezing nuclei; these nucleation characteristics are strongly influenced by the chemical composition of the particles. In turn, as previously discussed, clouds are a major factor in controlling atmospheric composition, because precipitation is the dominant mechanism for the removal of gases and particles from the atmosphere. Again, direct measurements are needed both to estab

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A FRAMEWORK FIGURE 2.2 Phase transitions within the atmospheric chemical systems and their consequences. Rectangles denote chemical entities or physical environ- ments in the atmosphere. The right side of the figure lists atmospheric processes affected by the species or environment identified in the rectangles. Triangles denote processes where material flows primarily in one direction; diamonds represent reversible processes: a, sources; b, sinks; c, gas-to-particle con- version; d, sorption; e, deliquescence; f, efflorescence; g, Raoult's equilibrium; h, reaction in concentrated solution droplet; i, nucleation and condensation of water; j, evaporation; k, capture of aerosol by cloud drops; l, reaction in dilute solution; m, rain; n, freezing of supercooled drop by ice nucleus; o, melting; p, direct sublimation of ice on ice nucleus; q, precipitation. (From Atmospheric Chemistry: Pro bleats and Scope, National Academy of Sciences, Wash- ington, D. C ., 1975 ). 1 { - Gaseous, Nonaerosol Precursors ~ CO, CO2, CH4, /._ Gaseous Aerosol Precursors - _ SO2, H2S, NO, NO2, HC'NH3, (H201 l ._: ~ { - Low R H Aerosol RH OCR for page 11
18 As discussed above, investigation of the atmospheric chemical processes related to biogeochemical cycles in the troposphere provides a unifying conceptual frame- work for the study of global tropospheric chemistry. These processes include the sources, chemical reactions and transformations, transport, and removal ofthe vari- ous species within any chemical cycle in the tropo- sphere. A quantitative understanding of these funda- mental processes will enable predictive models of the PART I A PLAN FOR ACTION SUMMARY tropospheric chemical system to be developed and the physical effects of trace substances in the troposphere to be determined. Predictive models will allow the effects of future perturbations ofthe global troposphere to be eval- uated. In the chapter that follows, the specific programs proposed in the areas of sources, transformations, trans- port, and removal of chemical species in the troposphere are presented in detail, as is the need for the develop- ment of global tropospheric chemistry systems models.