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APPENDIXES

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APPENDIX A Current Tropospheric Chemistr y Research In the Unitec! States There are programs under way in the United States and elsewhere that are already addressing certain aspects of global tropospheric chemistry. In the United States, these programs, active in several agencies are formally coordinated through the Subcommittee on Atmospheric Research of the Committee on Atmo- spheres and Oceans, which is established under the Fed- eral Coordinating Council for Sciences, Engineering, and Technology. None of the current programs includes all of the Global Tropospheric Chemistry Program ele- ments recommended in this report, and all of them together wig not, without significant augmentation, achieve the goals of the recommended global program. Relatively few of the existing programs have global tropospheric chemistry issues as their focus. Most of the larger programs are focused on urban or regional prob- lems such as air pollution or acid rain. Nevertheless, many of the research tasks encompassed by the existing federal programs will contribute substantially to solving tropospheric chemical problems encountered on the global scale. A brief summary of the various programs follows. NSF's ATMOSPHERIC CHEMISTRY PROGRAM The Atmospheric Chemistry Program of the Atmo- spheric Sciences Division of the National Science Foun- dation (NSF) supports a wide range of laboratory, field, and modeling investigations of the troposphere, the stratosphere, and planetary atmospheres. The ultimate goal of understanding the complex interactions of hun- dreds of chemical reactions with transport and radiation phenomena is supported by studies of the pathways and kinetics of molecular-level processes, of the global cycling of chemical elements, and of new approaches to the measurement of trace species, including free radi- cals. Trace gases and aerosols are investigated in both clean and polluted atmospheres. The program lends support to advances in methodol- ogies for the identification of individual aerosol particles and to the study of the interaction of these particles with their gaseous environment. The chemistry of the parti- cle-size domain between molecular dimensions and fil- terable particles receives added emphasis. The Atmospheric Sciences Division of NSF, through the University Corporation for Atmospheric Research, supports the National Center for Atmospheric Research (NCAR) to initiate, coordinate, and carry out atmo- spheric research that requires long-term cooperative efforts among scientists at NCAR and at universities and government laboratories and to provide and develop facilities and related services for the atmo- spheric research community. Research within the Atmospheric Chemistry and Aeronomy Division at NCAR has recently focused on the role of CO, NO, CH4, and nonmethane hydrocarbons in the tropo 171

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172 - 1 ~ spheric O3 budget, on biospheric processes as they influ- ence the atmosphere, on biomass burning as a source of atmospheric trace gases, and on cloud chemistry and . . . . . ace precipitation anc its causes. NASA's GLOBAL TROPOSPHERIC EXPERIMENT A research effort named the Global Tropospheric Experiment (GTE) has been initiated by the National Aeronautics and Space Administration (NASA) to study the chemistry of the global troposphere and its interaction with the stratosphere. The first phase of the project, aimed at developing and validating measure- ment techniques for HxOy and NOx trace species in tropospheric chemical cycles, is designed to lead to the development and implementation of a cooperative global tropospheric chemistry research program with the goal of understanding the chemical cycles that con- trol the composition of the global troposphere and its charlges. The immediate emphasis of the GTE is on the devel- opment, testing, and evaluation of measurement tech- niques that can achieve, under field conditions, the extreme sensitivity required to determine accurately atmospheric concentrations of key chemical species. A later phase ofthe GTE will focus on widespread, system- atic measurements supported by modeling and labora- tory studies to understand the principal processes that govern key chemical cycles in the troposphere. The role of the global troposphere as the source and sink for the stratosphere, the details of the troposphere- stratosphere interchange, the processes that control global tropospheric O3, and the atmospheric role in bio- geochemical cycles are of particular interest to NASA, as is the eventual development of an enhanced capability to study the troposphere and its composition from space. Instrument development in the initial phase of the GTE will involve a three-step test and evaluation pro- gram comprising a ground-based intercomparison, an airborne intercomparison in the tropical troposphere with particular attention to the boundary layer over the ocean and over tropical forests, and an airborne inter- comparison in the upper troposphere. This strategy will systematically expose the measurement systems under current development and evaluation to conditions that will be encountered in later global tropospheric chemis- try field experiments. NOAA's GEOPHYSICAL MONITORING FOR CLIMATIC CHANGE PROGRAM The NOAA Air Resources Laboratory's Geophysical Monitoring for Climatic Change (GMCC) program operates four baseline observatories at which measure APPENDIX A meets are made of atmospheric trace constituents important for climatic change. These observatories are located in remote clean-air sites where the measured values are representative of background concentrations of trace gases and particles in the atmosphere. The observatories are located at Barrow, Alaska; Hilo (Mauna Loa), Hawaii; American Samoa; and South Pole, Antarctica; and are also components of the WMO Background Air Pollution Monitoring Network (BAP- MoN) Program. The objectives ofthe GMCC program are as follows: I. To determine concentrations, their variations with time and space, and properties of atmospheric trace gases and aerosols that can potentially have an impact on climate; 2. To understand the sources, sinks, transport, mod- ification, and budgets ofthose trace constituents; and 3. To apply (in collaboration with others) those mea- surement data to determine their effect on global weather and climate. The GMCC monitoring effort at the baseline observatories is primarily for long-term sur- veillance of atmospheric trace species concentrations. Additional flask sampling measurements of selected atmospheric trace constituents are made near Niwot Ridge, Colorado; at Boulder, Colorado; at a global net- work of CO2 flask sampling stations; at a number oftotal O3 monitoring stations located mainly in the contiguous United States; and occasionally at other locations in support of particular research objectives. GMCC research primarily focuses on analyses of these data sets. Programs for the measurement of O3 (surface, verti- cal profiles, and total column), atmospheric aerosols, halocarbons (Freons 1 1 and 12), N2O, water vapor, and precipitation chemistry are currently under way by NOAA at one or more of the GMCC program sites. A primary objective for the GMCC program is to make measurements of trace species in a monitoring mode to document long-term trends. As such, most programs are continuing, in contrast to expeditionary. In the near future, programs will begin in the automa- tion of Dobson spectrometers, and on the measurement of radiatively active trace gases, and of gases and aero- sols in the Arctic. NSF's SEAREX PROGRAM The Ocean Science Division of NSF has been spon- soring a coordinated research effort investigating the atmospheric transport of material from continental regions to the ocean. The Sea-Air Exchange (SEAREX) program was initiated in 1977 and directly involves 11 universities and laboratories from the United States, France, and England, and cooperative

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APPENDIX A ancillary programs with investigators from New Zealand, Australia, Japan, and the People's Republic of China. The SEAREX program has concentrated its efforts on investigating air-sea exchange in the westerly and trade wind regimes of the North and South Pacific Ocean. The objectives of SEAREX are as follows: 1. The measurement of the rate of exchange of selected trace elements and organic compounds across the sea-air interface, 2. The investigation of the mechanisms of exchange of these substances, and 3. The identification of the sources for these sub- stances in the marine atmosphere. Chemical substances being investigated in SEAREX include selected heavy metals (e.g., Pb, Cd, Hg, Zn, Se, Fe, Mn, V, and Cr), soil dust, sea salt, halogens, 2~0Pb and typo, sulfate, nitrate, phosphate, particulate organic carbon, and such organic species as PCB, DDT, HCB, aliphatic hydrocarbons, phthalate plasticizers, fatty acids, fatty and polycyclic alcohols, and low-molec- ular-weight ketones and aldehydes. Extensive field pro- grams have been carried out at Enewetak Atoll, Mar- shall Islands in 1979, at American Samoa at the NOAA GMCC station site in 1981, and in northern New Zealand in 1983. A final SEAREX field program is planned from an oceanographic research vessel located at about 40 N. 1 70 W in 1986. THE NATIONAL ACID PRECIPITATION ASSESSMENT PROGRAM The Acid Precipitation Act of 1980 established the Interagency Task Force on Acid Precipitation to develop and implement a comprehensive locational Acid Precipi- tation Assessment Program. The act required the task force to produce a national plan for a 10-year research program. The purpose of the National Acid Precipita- tion Assessment Program is to increase understanding of the causes and effects of acid precipitation. The national program includes research, monitoring, and assessment activities that emphasize the timely develop- ment of a firmer scientific basis for decision making. The National Acid Precipitation Assessment Program is co-chaired by the Environmental Protection Agency (EPA), NOAA, and the Department of Agriculture, and it includes a major involvement by the national labora- tories of the Department of Energy (DOE). Research is proposed in the National Plan in nine categories. Each of the research tasks described focuses on a specific area and generally involves the coordinated participation of several agencies. Focusing on research needs and tasks relevant to atmospheric sciences in general and atmospheric chem 173 istry in particular, the plan pursues the following . . O Electives: 1. Identify the natural emissions that can influence precipitation chemistry and develop an experimental data base that sufficiently characterizes the source strengths of these species. 2. Quantify pollutant emissions of interest from man-made sources with supporting energy-use, eco- nomic, and technical data for appropriate sources and regions for certain time periods. 3. Develop and maintain quality-assured emission models and methods to support other task groups assess- ing control strategies for acid deposition. 4. Conduct special research projects into economic, energy-use, technological, and other factors that affect pollutant emissions from major man-made sources. 5. Determine the important aspects of meteorologi- cal transport of acidic substances and their precursors on spatial scales ranging from local to global. 6. Determine the important overall physical and chemical pathways and specific reaction processes regu- lating the formation of acid substances in the atmo- sphere through laboratory and field measurements and theoretical interpretation. 7. Determine the relative importance of wet and dry removal processes for acid substances and their precur- sors within the atmosphere. 8. Develop state-of-the-science modeling frame- works using advanced products resulting from the atmospheric processes research program. The models will serve as the media for integrating the full spectrum of phenomenological research in acid deposition and as the primary assessment tools in identifying future con- trol strategies for mitigation. 9. Develop a comprehensive data base for evaluation and verification of acidic deposition models and associ- ated process component modules. 10. Determine the spatial and temporal variations in the composition of atmospheric deposition within the United States for a period measured in decades through a National Trends Network. The time schedules contained in this federal plan call for preliminary assessment of the research results by 1985- 1986 and a more complete assessment by 1988-1989. OTHER RELATED PROGRAMS In addition to the above programs that have regional- to global-scale tropospheric chemistry as their major thrust, several mission agencies support work that is applicable to the research goals of a global tropospheric chemistry program. The DOE has supported for many years a program

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174 designed to yield understanding of the relationship between energy production activities and effects on the atmosphere, and, from work of its predecessor agencies, to study the impact of nuclear explosions in the atmo- sphere. Chemistry program elements involve labora- tory and field studies of the mechanisms and kinetics of the production and transformation of emissions related to use of energy. A major research area is the study ofthe physics and chemistry ofthose processes that control the removal from and reinsertion into the atmosphere of gases and aerosols. Recent emphasis has also been placed on problems related to emerging energy technol- ogies. Since 1978, the DOE has been engaged in a major effort to analyze the causes and climatic consequences of CO2 buildup in the atmosphere, an activity that requires measurement of fluxes and involves study of the carbon cycle as a whole in the biosphere, including the atmosphere. 1 APPENDIX A The EPA, as the regulatory agency with responsibility for establishing and enforcing environmental standards within the limits of various statutory authorities, con- ducts a major atmospheric research and monitoring program. A major fraction of this activity is directed toward understanding and characterizing problems within urban and regional air sheds with focus on crite- ria pollutants, hazardous pollutants, long-range trans- port, transformations, particles, vehicular emissions, and large-scale and long-term effects of air pollution on the biosphere. An analysis ofthis program is beyond the scope ofthis report, but the program has produced, and continues to produce, results of immediate application to global tropospheric chemistry investigations. Partic- ularly notable examples are the studies of the long-range transport and fate of atmospheric pollutants and the extensive transport and fate models designed for appli- cations on scales up to the regional.

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APPENDIX B Remote Sensor Technology The following overview will provide a perspective of the ultimate role of spaceborne remote sensor tech- niques in providing global measurements to improve understanding of the processes involved in the chemis- tr.y, dynamics, arid transport phenomena in the global troposphere. To provide this perspective, this overview will highlight two spaceborne sensors that are currently providing measurements from space, provide a survey of potential instrument techniques that can provide key measurements important to tropospheric science, and project some technological developments that need to be implemented to realize the hill potential of remote sen- sor techniques from space. Ultimately, remote sensor techniques should be utilized from orbiting satellites for long-duration missions to exploit the potential to mea- sure and detect long-term trends related to changes in the global balance of the troposphere due to anthropo- genic and nonanthropogenic processes.) However, the path to developing research instruments for long-dura- tion satellites should capitalize on other airborne and spaceborne platforms, including high-flying aircraft, the opportunity to iterate a variety of measurements with evolving mathematical models of the troposphere, and provide the instrument scientific community with the opportunity to develop and evaluate advanced sen- sor technology to optimize the measurement base required to validate the mathematical models devel- oped. PRESENT SPACEBORNE SENSOR MEASUREMENTS Three classes of remote sensors have demonstrated unique capabilities in meeting some ofthe measurement needs in the global troposphere. The first class includes imaging spectroradiometers currently being used in Earth Observation Satellite Systems for meteorological and earth resource measurements. These sensors have recently shown the ability to detect regions of elevated haze layers and aerosol loading in the troposphere. A second class of instruments includes passive remote sen sors that measure spectral emission or absorption of the Space Shuttle, and possibly spaceborne pallets or atmospheric molecules with external sources of radia free flyers of shorter duration missions. This approach lion. Vertical distributions of molecular species, pres sure, and temperature can be inferred through the use of inversion algorithms. A third class of instruments includes active remote sensors in which lasers ire the ultraviolet, visible, and infrared portion ofthe spectrum are used in a similar mode as an active radar system. will provide the atmospheric science community with Report of the NASA Working Group on Tropospheric Program Plaurling.~. H. Seinfeld. NASA RP 1062. 1981. 175

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176 Through a combination of scattering by aerosols and molecules in the atmosphere and selective absorption by atmospheric molecules, these sensors can provide range-resolved measurements of tropospheric mole- cules and aerosol loading. HAZY AIR MASSES (Jurrent research objectives and approaches for spec- troradiometer systems encompass the following ele- ments. Overall, the current objective is to evaluate capa- bilities of existing satellite and aircraft systems in . . . . . . . ~ conjunction Wlt n exlstmg image processing systems tor monitoring air pollution episodes Outline the extent of the pollution area as seen on the visible imagery. Attempt to measure the threshold quantitatively over land as well as water. Note the condition, if any, under which the area is seen on the infrared imagery. Track the motion of the pollution area by noting the motion of its "center of gravity" or the motion of any discrete edge or feature. Compare any apparent motion with wind informa- tion obtained from conventional meteorological sources. Delineate the radiance values associated with indi- vidual pixels within the area and draw isopleths of selected radiances. Measure the variation of the radiance, i.e., the change of the isopleth values, as `` function of the time of day. Make a correction for wind-related changes in the apparent density of the haze to obtain the variation in radiance related primarily to the solar zenith angle. Follow up university efforts to calibrate the SMS/ GOES satellite visible sensors, and continue this effort by comparing the radiances as seen by the GOES sen- sors over a region of uniform brightness, with radiances expected from that surface through a normal haze-free atmosphere, as predicted by the Fraser model. In this model, the solar relationship to the surface is included. Adjust the radiances measured by the SMS/GOES sensors according to the calibration, and with the adjusted values, calculate the optical depth and mass loading at various locations of the smog area by using the Fraser model. ~. (correlate any computed smog density with various APPENDIX B . . . height of inversionlayers, relative humidity, and ground visibility. The ground-based smog measurements are optical thickness and particulate count. Attempt to generate atleast two sets of correlations, one based on the model-calculated mass loading, and another based on the basic SMS/GOES radiances. Under the modeling phase of the study, the approach involves an investigation of the scattered sunlight radi ance. The radiance of the sunlight scattered toward a satellite is being computed for models of the air pollution for two purposes: (1) To determine the response of the radiance to the important physical parameters; (2) To estimate the accuracy with which air pollution parame ters such as aerosol optical thickness, sulfate mass, and visibility can be derived from satellite observations. The important variable parameters for study in specifying the model are the bidirectional reflectivity ofthe ground, the aerosol optical thickness, the relative humidity and amount of water vapor, the composition of the aerosols, and their size distribution. The computed radiation characteristics will be com pared with experimental results. Even more impor tantly, the aerosol parameters derived from satellite observations will be verified with experimental results. A large body of good experimental data on the physical and chemical characteristics of dense air pollution was obtained during the Persistent Elevated Pollution Epi sodes (PEPE/NEROS) Field Measurement Program during the summer of 1980. The basic program does not provide for measurements of the aerosol optical thick ness, which is the most important aerosol parameter that will be derived from satellite observations. Solar transmission observations at about 12 stations obtained during the PEPE/NEROS experiment will be utilized. The transmission observations will be analyzed to derive values of the aerosol optical thickness. The satellite observations expected during the PEPE/ NEROS experiment include the visible and infrared spin and scan radiometer (VISSR) on GOES. In addi - lion, more precise radiometer data will be used from the coastal zone color scanner (CZCS) on Nimbus 7. MEASUREMENT OF AIR POLLUTION FROM SATELLITES (MAPS) An earth-orbiting experiment flown on the Space Shuttle can provide scientists with data to accurately map changes within the earth's atmosphere. The exper meteoro~og~ca~ parameters, as weft as with various iment, Measurement of Air Pollution from Satellites ground-based measurements. The most interesting (MAPS),charted concentrations ofCO gas around the meteorological parameters for initial investigation world over a range oflatitudes extending from30Sto appear to be wind at various levels, the existence and 38N during the second Space Shuttle flight. The per

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APPENDIX B formance of MAPS on this flight proved the system to be a faster, more efficient method of mapping trace gases in the earth's atmosphere than chromatograph devices. MAPS used a gas filter radiometer to obtain mea- surements of the CO mixing ratio in the middle tropo- sphere and stratosphere. The radiometer method is sim- pler and much less expensive than previous gas chromatograph devices. The experiment produced con- centration maps of CO. Early analysis of MAPS data concentrated on mea- surements performed during orbital Pass 15 on Novem- ber 13, 1981 . This orbital pass of the Shuttle began over Central America, continued east over the Mediterra- nean Sea, turned southeast over the Persian Gulf, the Arabian Sea, and extended to the southern tip of India. The concentration of CO within this extended area ranged from 70 ppb over the eastern Atlantic Ocean and 140 ppb in the Mediterranean area. These measure- ments were performed in less than 20 min from the vantage point of the Shuttle. Similar measurements from aircraft platforms would have required observa- tions over many hours. Analysis of experiment data so far indicates signifi- cant concentrations of middle troposphere CO mixing with both north-south and east-west variation over the North Atlantic and the Mediterranean Sea and the Middle East. Accuracy of the measurement has been determined to be within 15 percent with a repeatability of about 5 percent from orbit to orbit. CO gas is pro- duced by natural processes (e.g., oxidation of CH4 in wetland areas and forest fires) and man's activities (e.g., slash-and-burn agriculture and automobile emissions). Man's contribution to the global atmospheric budget of CO has grown significantly during this century, result- ing in a large asymmetry between the CO concentration in the northern and southern hemispheres. The major atmospheric sink for CO is a complex sequence of photochemical reactions that oxidize CO molecules into CO2. One ofthe initial steps in this oxida- tion process is the combination of CO with the hydroxyl radical OH. OH plays a major role in a variety of other atmospheric processes, notably reactions involving SO2, nitrogen oxides, and chlorofluoromethanes. CO oxidation can potentially divert OH from reac- tions with these other gaseous species and alter the over- al1 chemical balance within the earth's atmosphere. NASA plans to refly MAPS on the seventh Shuttle mission, scheduled for summer 1984, to study seasonal variations in the total abundance and regional distribu- tion of CO within the earth's atmosphere. Although the second Shuttle flight was abbreviated, the experiment collected data for about 42 hr. and the investigators were able to corroborate the sampled areas 177 with the instrument readings taken with underflying aircraft and with other surface information. Reduction of MAPS data is continuing at the present time, and the first global map of CO concentrations at low-latitudes to midlatitudes is now available. FUTURE INSTRUMENT TECHNIQUES Passive Remote Sensors In general, remote sensors for atmospheric applica- tions can be developed to measure changes in the three basic properties of electromagnetic waves including energy (absorption or emission), wavelength (frequency shifts), and polarization. Passive spectroscopic remote sensors, summarized in this section, correspond to the class of sensors that measures one of these properties (absorption or emission) with an external source of radi- ation. For tropospheric measurements from space or aircraft observing platforms, the downward-viewing modes (nadir) provide the widest vertical and horizontal coverage. Solar occultation measurements from space and airborne platforms have been used extensively in stratospheric applications, but due to the extent of global cloud cover, geographical coverage of the troposphere is severely limited in this operational mode. External sources of radiation for nadir-viewing experiments include upwelling thermal radiance of the earth-atmo- sphere system, reflected radiation from the surface ofthe earth, and scattered radiation from molecules and aero- sols in the atmosphere. Compared to detection of direct solar radiation through the atmosphere, as in strato- spheric solar occultation measurements, these sources of radiation are relatively weak and require more sensitive detection instruments. However, in addressing some of the tropospheric scientific questions, sensors developed for stratospheric observations in solar occultation can be used from ground-based platforms to provide some ver- tical layering of tropospheric constituents, which are important in establishing global concentrations of well- mixed gases, or those gases showing seasonal and tem- poral variability. In addition, with improvements in sen- sitivity and instrument optimization to a 300K source and broader spectral response functions, many sensors developed for stratospheric applications should become viable tropospheric sounders from space platforms. Before addressing the question of the available pas- sive remote sensing techniques, some basic properties of atmospheric radiation should be considered for the vari- ous external sources of radiation. In viewing the upwell- ing thermal radiance of the earth-atmosphere system, passive instruments detect radiation that is a composite of energy transmitted through the atmosphere, that is

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178 absorbed and reemitted by atmospheric molecules at all altitudes between the source and the sensor. The ther- mal emission of this radiation is primarily governed by the temperature lapse rate of the lower atmosphere, which ranges from approximately 300 K near the ground to 220K in the lower stratosphere. Since the Planck function of a blackbody radiator peaks at approximately 10 ,um for a 300K blackbody, the desired wavelength region varies from 4.5 to 15.0 ,um. Also, in this wavelength range, contributions to atmo- spheric radiance from scattered solar radiation are small and can be considered of a second order. Because of the pressure broadening of the spectroscopic absorption or emission lines, the energy received in the wings of the atmospheric emission lines reflects the presence of mole- cules at high pressures (i.e., lower altitudes), while energy received near line centers is representative of molecules at lower pressures (i.e., higher altitudes). This gives rise to the possibility that by selectively mea- suring radiation at various spectral regions from line center, one could, in principle, generate vertical profiles of gas concentrations. This is analogous to the inference of temperature profiles by remote sensing methods using thermal infrared wavelengths where wavelengths in various parts ofthe emission line of a uniformly mixed gas (e.g., CO2 and N2O) are used to obtain altitude discrimination. In order to invert radiances detected at the top of the atmosphere to concentration profiles, inversion algorithms must be developed that take into account emission of the earth-atmosphere system, gov- erned by the temperature lapse rate. In inverting the radiance to obtain useful concentration accuracies, tem- perature profiles must be simultaneously measured with the radiance to a desired accuracy of approximately + 2K. For measurements of minor trace gases in the troposphere, thermal emissions are weak due to a com- bination of low concentrations and low reservoir tem- peratures. Also, the temperature lapse rate and the rela- tively low-pressure gradient in the atmosphere limit the degree of vertical discrimination possible, independent of the spectral resolving power of the instrument beyond a resolution of approximately 0.01 cm- ~ . Furthermore, radiances emitted and absorbed near the ground (within the first 3 km) are faced with the limitation of a small temperature gradient between the earth and the layer of atmosphere near the earth, thus making fine discrimi- nation of layers near the ground difficult. In the free troposphere, however, broad layers of the atmosphere can be identified when simulating the inver- sion of the earth-atmosphere radiance in the thermal infrared, and weighted averages of gas concentrations at specific altitudes can be obtained. Therefore, these tech- niques provide the synoptic coverage and spatial resolu- tion required to study the global transport of minor trace gases such as CO (refer to previous section). Such obser APPENDIX B vations are required to address those scientific questions related to global budgets and distributions of gas con- centrations and to the relative changes that occur over time of some ofthe well-mixed gases such as CH4, N2O, and CO2, especially in the free troposphere. The other two external sources of radiation that can be used in tropospheric sounding of atmospheric trace gases include reflected radiation from the earth's surface and scattered radiation from the earth's atmosphere. In the latter mode, scattered radiation from aerosols and molecules is predominant in the ultraviolet and visible portion of the spectrum and can be used to infer inte- grated molecular concentrations through direct absorp- tion by atmospheric spectra. In the ultraviolet and visi- ble portion of the spectrum, the dependence of line width on pressure has a smaller functional dependence than in the infrared, and vertical layering of the atmo- sphere using this physical process is more difficult to infer than in the thermal infrared. However, inversion of radiance data in the ultraviolet and visible is less sensitive than the thermal infrared to the knowledge of atmospheric temperature profiles. Relevant tropo- spheric molecules in this spectral region include 03, SO2, and NO2. In viewing reflected solar radiation from the earth's surface, one should restrict observations to approxi- mately the 1.0- to 3.5- ,um region, since in this range the radiance at the top of the atmosphere is primarily due to reflected solar radiation, and absorption can be used as the physical process to infer molecular concentrations. Some vertical layering of molecular concentrations can be achieved through pressure broadening ofthe molecu- lar absorption lines in the atmosphere, and integrated measurements to the ground are possible. The accuracy of the retrieved concentrations has a smaller functional dependence on the temperature profile than in the upwelling thermal infrared region. In the intermediate spectral band (i.e., 3.5 to 4.5 ,um), the radiance at the top of the atmosphere is composed of comparable values of the upwelling thermal radiance and reflected solar radiation. Although interesting absorption and emis- sion lines of major atmospheric molecules lie in this region, inversion of the radiance measurement to molecular concentrations is complicated by the com- plexity ofthe radiative transfer equation, which makes it difficult to quantitatively invert radiances to obtain molecular concentrations. A summary of the current passive remote sensors developed under the NASA remote sensing program has been reviewed by Levine and Allario2 and will not be discussed further. 2"The global troposphere: biogeochemical cycles, chemistry, and remote sensing." J. S. Levine and F. Allario. Environmental Monitoring and Assessment 1: 263-306. ~ 982.

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APPENDIX B Active Remote Sensors The use of laser techniques for measuring range- resolved concentrations of aerosols, molecular constitu- ents, and meteorological parameters in the stratosphere and troposphere has represented a major research activ- ity in the NASA research and applications programs over the last decade. In considering some of the funda- mental limitations in using external sources of radiation for measuring tropospheric molecules, it should be obvi- ous that the use of powerful and tunable, monochroma- tic sources in the ultraviolet, visible, and infrared por- tions ofthe spectrum has the potential to remove some of the limitations of passive remote sensing imposed by the physical processes of the atmosphere. For example, in probing the troposphere from the top of the atmosphere in the nadir mode, the laser beam has the potential to probe down to the surface of the atmosphere, and through the process of molecular absorption and range gating (to be discussed later), the vertical distribution of molecular concentrations, aerosols, and meteorological parameters can, in principle, be obtained to a spatial resolution of c 0.1 km. The ability to meet these mea- surement needs is dependent on the energy of the laser and the magnitude of the differential absorption cross section of the species to be measured. In general, the ability of lidar systems to obtain individual measure- ment parameters to a given accuracy depends on the magnitude of optical scattering coefficients and molecu- lar absorption cross sections, if one assumes sufficient flexibility in system parameters, such as telescope size, detector quantum efficiency, and energy of the laser transmitter. Research in the NASA program has evolved from initial studies with h~xed-wavelength lasers to measure optical backscatter from aerosols, to the use of tunable and narrow bandwidth lidar systems for remote mea- surements of atmospheric gases and aerosols in the troposphere. Investigations have been conducted by using Raman scattering techniques for remote measure- ments of water vapor and SO2, but this technique was found to be limited to high gas concentrations, short ranges, and nighttime operation as a result of the small Rarnan scattering coefficients. The differential absorp- tion lidar (DIAL) technique can overcome some of these limitations because absorption cross sections can be 6 to 8 orders of magnitude larger than the Raman cross sec- tions. This technique has recently been demonstrated as viable for detecting O3 and aerosol layering in the tropo- sphere from an aircraft platform. In general, successful demonstrations of the feasibility of DIAL techniques have evolved as the technology of the tunable laser sources has improved in energy output, spectral purity, and amplitude and frequency stability. DIAL tech- niques, to date, have been applied from fixed and ~. . . 179 mobile ground stations to detect aerosols and SO2 emit- ted by stack plumes and to measure the vertical distribu- tion of tropospheric aerosols, 03, and water vapor from aircraft platforms. The principle of the DIAL techniques is discussed below. In general, the DIAL technique depends on the existence of a molecular feature (absorption line) that is specific to a gas molecule, whose spectroscopic charac- teristics are well known in the atmosphere (i.e., line intensity, line position, and line broadening as a func- tion of pressure), and which is relatively free of spectro- scopic interference from other molecules in the atmo- sphere. In order to detect the molecular feature, the wavelength of the laser is tuned to overlap the absorp- tion feature, preferably at the center ofthe line. A second laser wavelength is required whose wavelength is removed from the molecular feature but sufficiently close by in wavelength to detect the same atmospheric scattering properties of the atmosphere at the two wave- lengths. The two laser wavelengths are pulsed and can be fired simultaneously or within a time spacing that essentially freezes atmospheric dynamics during the measurement period (i.e., c 100 ns). The two laser pulses are backscattered to the receiver telescope by atmospheric aerosols and molecules, so that the return signal represents a time history of the atmospheric scat- tering and absorption properties of the atmosphere for each laser pulse. This time history is related to a spatial profile of the atmospheric scattering and absorption properties of the atmosphere, through the equation (cAt)12 = AR, where At represents a time gating interval that can be selectable in time to correspond to a range gate interval, AR. In the date processing mode, the ratio of return signals at the "on" wavelength, Pon, to the "off' wavelength, Poff' iS measured and, through the lidar equation, can be related to the molecular concen- tration of the gas in the atmosphere as a function of range from the transmitting telescope. Another useful mode for a DIAL experiment is to employ a continuous wave (COO) laser that uses reflection from the ground to return the two laser wavelengths back to the receiving telescope. In the infrared, where pressure broadening of atmospheric spectral lines as a function of altitude is larger than in the ultraviolet and visible portion of the spectrum, one can selectively probe various segments of the absorption line by tuning the wavelength of the transmitting laser from the central peak into the wings to obtain vertical layering of the selected tropospheric mol- ecule. Conceptually, this is similar to the passive solar reflected technique discussed earlier. In this case, how- ever, the source of radiation is monochromatic, allowing a single absorption line to be probed. Furthermore, the radiative transfer equation for the transmitted and reflected signals through the atmosphere can be described by straightforward atmospheric processes.

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180 A summary of the current active remote sensors developed under the NASA remote sensing program is given by Levine and Allario3 and will not be discussed further here. FUTURE THRUSTS A Workshop on Passive Remote Sensors for perform- ing tropospheric measurements was conducted in Vir- ginia Beach, Virginia, July 20-23, 1981. The purpose of this workshop was to define the long-range role of pas- sive remote sensors in tropospheric research and to iden- tify the technology advances necessary to implement that prescribed role. Recommendations and conclu- sions from the two panels of the workshop attendees are given below. Details leading to these conclusions are given in NASA CP 22374 and are abstracted below. RECOMMENDATIONS AND CONCLUSIONS Sensor Systems Panel Recommendations Following are the conclusions and recommendations from a systems point of view for passive remote sensing oftropospheric constituents: 1. Passive remote systems exhibit promise and should be developed for two-layer measurement of some of the more abundant tropospheric species (e.g., O3, CO, CH4, CO2, HNO3, H2O, and NO). 2. A measurement scenario consisting of a combina- tion of nadir viewing and solar occultation should be considered for measurement of gases such as O3 and HNO3. Measurement of these gases in the troposphere presents a unique challenge in that well over 90 percent of the total burden of the gas resides in the stratosphere. 3. For multilayer (i.e., more than two) measure- ments of a wide range of species, a nadir-viewing instru- ment capable of obtaining continuous spectra in the 3- to 15- Em spectral region with a spectral resolution of less than 0.1 cm- ~ is desired. 4. Gas filter radiometer instruments (e.g., MAPS and HALOE) should be developed concurrently with a scanning instrument (see previous recommendation). Such systems may provide near-term two-layer tropo- spheric measurements of gases such as CO and CH4 with only modest improvements in system perfor- mance. For gases such as O3 and NO, major problems may be encountered with the gas cell technology and resolving more than one atmospheric layer. Child. 4Tropospher?c Passive Remote Sensing. L. S. Keafer, Ir. (ed.~. NASA CP 2237. APPENDIX B 5. Further development of aerosol retrieval algorithms is required for obtaining aerosol thickness and size distributions on a global scale. Although the technology currently exists for obtaining aerosol infor- mation over water, additional channels extending from the visible to near infrared would be desirable for future measurements. 6. The prospect and the possibility of initiating a feasibility study to determine if polarization measure- ments of scattered solar radiation can yield the refractive index (i.e., composition) of aerosols should be reas- sessed. 7. Existing spectrally scanning radiometers (e.g., interferometers) and/or gas filter systems should be employed from balloon and Shuttle platforms to study the effects-of instrument noise and background fluctua- tions on inversion techniques. Short-term nadir-view- ing Shuttle missions should be coordinated with existing solar occultation missions to study the feasibility of uti- lizing simultaneous occultation and nadir-viewing data. 8. Feasibility studies for both gas filter and spectrally scanning instruments should be initiated to study (a) the extent to which nadir-viewing systems can obtain pro- f~les oftwo ormorelayers within the troposphere, (b) the accuracy requirements on molecular line parameters, meteorological parameters (i.e., temperature and pres- sure), radiance data, and background effects, and (c) the extent to which solar scattering can be used to obtain lower-level tropospheric data. Sensing Technology Panel Recommendations The technology needs presented earlier are restated in Table B. 1 as critical needs and are recommended as technology development thrusts for elements of several passive sensing techniques for tropospheric research. In one sense, this table indicates the needs once a system is chosen. In a larger sense, the technology requirements and their apparent difficulty and cost should be a critical part of the instrument evaluation studies. Certain tech- nology thrusts, however, are needed regardless of the sensing system choice. Examples from Table B. 1 are as follows: (~) detector arrays, (2) cryogenic cooling (of sensors and optics), (3) sophisticated optical elements, and (4) data processing as an integral part of the sensor. Although not explicit in Table B.1, calibration tech- niques and equipment should be added to the list of needed technology thrusts. In response to the continu- ing need for more sensitive and accurate measurements over the full globe for long periods of time, producing great volumes of data, the workshop participants felt that the application of technology advances in these five areas would yield the greatest benefit in passive remote sensing of the troposphere.

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APPENDIX B TABLE B. 1 Technology Thrusts Sensor Technology Needs Gas filter radiometry All types Broadband spectrometry All types Grating type Interferometer Narrow-band spectrometry Laser heterodyne type Fabry-Perot Gas filter test cells Linear, high dynamic range detectors Highly uniform optical elements Onboard smart processing Cryogenics/cooling 103 element arrays Large gratings Mitigation of background fluctuations Multiaperture, multiband interferometer In-flight alignment verification Tunable lasers and heterodyne arrays Improved coatings at long wavelengths SUMMARY In the current state of development of remote sensors, two sensor systems are being used from spaceborne plat- forms, and several active and passive systems have pro- gressed to the stage where they are being used or pro- posed in field measurement programs from airborne and ground-based platforms to measure gaseous species (e.g., CO, 03, SO2, H2O, NH3, and NO2), tropo- spheric aerosols, mixing heights, and optical extinction coefficients. Future developments of remote sensing sys- tems are aimed toward extending this capability to other molecular species with improved sensitivity and vertical resolution. Passive remote sensors are categorized generically into four categories: (1) gas filter correlation techniques, (2) interferometry, (3) infrared heterodyne radiometry, and (4) spectroradiometers. Technological improve- ments in the sensitivity of passive remote sensors should expand the number of instruments within each generic class as potential payloads for airborne and space plat- forms. Despite the potential improvements in the sensi- tivity of passive remote sensors through technological improvements in systems and detector technology, sev- eral fundamental limitations in passive sensors will restrict the use of these instruments for some of the scientific experiments envisioned in tropospheric research. Passive sensors in the thermal infrared have difficulty measuring molecules in the biosphere and are limited to measuring in broad vertical layers in the mid 181 die and upper troposphere. The relatively long integra- tion times required to measure low-concentration spe- cies will restrict the horizontal resolution of the measurement to broad geographic areas and will proba- bly require the use of geostationary satellite platforms to investigate regional areas, such as the northeastern U.S. corridor. Instruments using earth-scattered sunlight are capable of measuring total integrated burdens of molec- ular species, but have inherently limited temporal and geographic coverage. Despite these limitations, passive remote sensors do exhibit several attractive characteris- tics for global measurements. Their systems and tech- nology are the most advanced for satellite missions. Pas- sive instruments using upwelling thermal radiance have potential for measuring a large number of interesting tropospheric molecules simultaneously and should be effective in studying the chemistry of selected chemical systems. Passive instruments using earth-scattered sun- light have the capability of measuring the total burden of several major tropospheric gases. A satellite mission incorporating both types of instruments should be inves- tigated in light of the scientific requirements for an early dedicated satellite for the lower atmosphere. Considerable effort has been expended by NASA and other organizations during the past decade to develop active remote sensing techniques using lasers. The active remote sensing systems in the NASA Air Quality Program can be classified into fixed and tunable wave- length systems. The former is important primarily in measurements of tropospheric and stratospheric aero- sols, aerosol extinction, and measurements of inversion layer heights. Tunable wavelength lidar systems cur- rently have the highest potential for measuring a variety of trace gases in the troposphere simultaneously, with vertical range extending to the ground and with vertical resolution approaching 1 krn. Active remote sensors under development, however, have high potential to meet some of the major scientific requirements for mea- surements in the troposphere from an airborne plat- form, including vertical resolution of approximately 1 km for major species, vertical range extending to the ground, day/night operation, true column-content measurements, and inherent high spectral resolution and tunability to measure tropospheric species simulta- neously and uniquely in a background of interfering gases. For applications to space platforms, however, major technological developments must be made in the sources themselves, including high power and effi- ciency, improved collimation and spectral purity, wider tunability, and higher frequency and amplitude stabil- ity. In order to perform many of the scientific investiga- tions of the troposphere, it will be necessary to develop appropriate sources for active remote sensing systems. Further, existing instruments must be tested under

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182 flight conditions not only to demonstrate their opera- tional reliability and sensitivities, but also to provide data that can be interpreted and analyzed under condi- tions as dose to an actual mission as possible. The time scale for utilization of laser systems and research satellite investigations is currently difficult to estimate because of APPENDIX ~ the rapidly emerging technology of laser systems. Therefore, in generating scientific requirements for sat- ellite missions, the science requirements must be tem- pered somewhat by the status of laser technology, and, in some cases, priorities for missions must be dictated by availability of laser technology. l

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APPENDIX ~ Element Cycle Matrices A matrix approach was used in an effort to systemati- cally, but simply, indicate what is currently known about the primary species involved in the sulfur, carbon, halo- gen, nitrogen, and trace element cycles, as well as the importance of the species in each cycle to an overall understanding of that cycle. Individual species were rated to indicate current knowledge and their impor- tance in each cycle relative to their major sources, removal and transformation processes, and tropo- spheric distribution. In the "knowledge" category, rat- ings range from one (very low, or no, knowledge) to four (high knowledge level almost all is known that it is necessary to know). In the "importance" category, rat- ings range from one (the factor is very important in understanding the complete element cycle) to four (the factor has little or no importance in understanding the element cycle). The combination of the "knowledge" and "impor tance factors tor each component of each matrix leads to an "urgency" factor for that component. An urgency factor of A indicates a very important component of an element cycle about which very little is known. Low "urgency" factors (e.g., B or C) flag areas that require research emphasis in each element cycle. A "D" urgency factor indicates a relatively unimportant com- ponent of a cycle about which quite alot is known. These areas would require relatively little research emphasis in that cycle. Two matrices are presented for each element cycle in the following tables (Tables C . 1 through C. 5~. The first is the "knowledge" and "importance" matrix. Both the "knowledge" and "importance" ratings are indicated in each matrix element, with the "importance" rating in parentheses. The second matrix indicates the "urgency" factor ratings for each cycle. 183

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184 TABLE C.1 Sulfur Cycle APPENDIX C H2S so2 DMS COS Sulfate Other Knowledge and (Importanceja Source Anthropogenic 4~2) 4~1) 2 (2) 2 (2) 3 (l) 2 (2) Biological ~ (2) l (l) 1 (2) 1 (2) 2 (1) ~ (2) Other natural 1~3) l (2) ~ (3) l (3) ~ (2) 1~3) Stratospheric l (2) l (3) Removal Wet (land) 2 (2) 3 (l) 1 (2) l (2) 3 (1) 3 (2) Wet~ocean) 1~2) 1~1) l (2) l (2) l (l) l (2) Dry~land) 2 (3) 2 (1) l (2) l (2) 2 (1) 1 (2) Dry (ocean) l (2) 2 (l) l (2) l (2) l (l) l (2) Transformation Homogeneous aq.phase 2(3) 2(2) '(3) 1(3) 3(2) 1(3) Homogeneous gas phase 2(3) 2(1) 2(2) 2(2) - 1(2) Heterogeneous 2(2) 2(1) 1(2) 1(2) 3(1) 1(2) Distribution 2 (2) 2 (2) 1(2) 1(2) 2 (2) 2 (3) Urgency Factor6 Source Anthropogenic C B B B B B Biological A A A A A A Other natural B A B B A B Stratospheric - - - - A B Removal Wet(land) B B A A B C Wet (ocean) A A A A A A Dry (land) B A A A A A Dry (ocean) A A A A A A Transformation Homogeneous aq.phase C B B B C B Homogeneous gas phase B A B B A Heterogeneous B A A A B A Distribution B B A A B C Interaction with other HrO' C C C C C cycles N N H. Oy H.~Oy N H;Oy H.rO' Aerosol H~O Aerosol aKnowledge: 1 = low, 4 = high; Importance: 1 = high, 4 = low. 6Urgency factor: A = extremely urgent for that cycle, B = considerably urgent, C = moderately urgent, D = of little urgency.

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APPENDIX C TABLE C.2 Carbon Cycle 185 Gas Phase Methane and Organ~c Compounas Carbonaceous Its React~on Aerosol Products C2-Cs c5 Knowledge and (Importance) Anthropoge ic 2(3) 2(3) 2(3) 2(,3) Biolog c 1 ( ) ( ) ( ) ~ Other natural - - (2) (2 Stratospheric - - _ Wet (1 md) 1 (2) - 1 (2) 1 (2) Transformation 1(2) 1(2) ~ (2) Homogeneous aq.phase - 2 1 1(1) 2(1) Homogeneous gas phase 1 1 ( ) 1 (1) 1 (1) Heterogeneous ( ) Distributlon 1(1) 3(1) 1(1) 1(1) Urger~cy Factorb Source C Anthropogenic C C A A Biological A _ A A Other natural Stratospheric Removal Wet (land) A _ BA BA Wet(ocean) A A Dry(land) A _ B B Dry (ocean) B Transformation Homogeneous aq.phase - A A A Homogeneous gas phase - _ A A Heterogeneous A B A A ~Distribtution ith th S H'`Oy HxOy H~Oy cycles N N N N C C Aerosol Aerosol aKnowledge: 1 = low, 4 = high; Importance: 1 = high, 4 = low. bUrgency factor: A = extremely urgent for that cycle, B = considerably urgent, C = moderately urgent, D = of little urgency.

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186 APPENDIX C TABLE C.3 Trace Element Cycles Hg Se As B P Pb - Knowledge and (Importanceja Source Anthropogenic 3 (1) 3 (1) 3 (1) 2 (2) 3 (2) 4 (1) Biological 2 (1) 2 (1) 2 (2) 2 (1) 1 (2) 1 (2) Other natural 2 (2) 1 (2) 2 (2) 2 (1) 2 (2) 2 (2) Stratospheric - - Removal Wet (land) 2 (1) 2 (1) 2 (1) 2 (1) 3 (1) 3 (1J Wet (ocean) 2 (1) 1 (1) 2 (1) 2 (1) 2 (1) 3 (1) Dry (land) 1 (3) 1 (2) 1 (2) 1 (4) 2 (2) 2 (3) Dry (ocean) 1 (2) 1 (3) 1 (3) 1 (3) 2 (2) 2 (4) Transformation Homogeneous aq. phase 1 (3) 2 (3) 2 (3) 2 (2) 2 (4) 1 (4) Homogeneous gas phase 2 (1) 1 (3) 1 (3) 1 (2) 1 (4) 1 (4) Heterogeneous 1~2) 1~1) 1~1) 1~2) 1~4) 1~3) Distribution 2 (1) 1 (1) 2 (1) 1 (1) 2 (1) 3 (1) IJraent:v Factorb , 4~, , _ , Source Anthropogenic B B B B C B Biological A A B A B B Other natural B A B A B B Stratospheric Removal Wet (land) A A A A B B Wet (ocean) - A A A A A B Dry (land) B A A B B C Dry (ocean) A B B B B C Transformation Homogeneous aq. phase B C C B C B Homogeneous gas phase A B B A B B Heterogeneous A A A A B B Distribution A A A A A B Interactions with HxO' HxO' HxO' HxO other cycles C C C aKnowledge: 1 = low, 4 = high; Importance: 1 = high, 4 = low. bUrgency factor: A = extremely urgent for that cycle, B = considerably urgent, C = moderately urgent, D = of little urgency.

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APPENDIX C TABLE C.4 Halogen Cycles 187 Fo F.; F- Clo Cli Cl- BrO Bri Br- Io Ii I Knowledge and (Importancef Source Anthropogenic 3~2) 2~2) 2~2) 3~1) 3~2) 3~2) 2~1) 2~2) 2~2) 2~2) 2~2) 2~2) Biological 2~2) 1~3) 2~2) 2~1) 1~3) 2~2) 1~1) 1~3) 2~2) 2~1) 1~3) 2~2) Other natural 3~3) 3~3) 2~3) 3~3) 3~3) 1~3) 2~3) 2~3) 2~3) 2~3) 2~3) Stratospheric 3~4) 4~4) 4~4) 3~4) 3~4) 3~4) 2~4) 2~4) 2~4) 2~4) 2~4) 2~4) Removal Wet (land) 3~2) 2~2) 3~2) 2~2) 3~2) 3~2) 2~2) 2~2) 2~2) 2~2) 2~2) 2~2) Wet (ocean) 2~2) 2~2) 2~2) 2~2) 3~2) 3~2) 2~2) 2~2) 3~2) 2~2) 2~2) 2~2) Dry (land) 2~3) 3~3) 3~3) 2~3) 2~3) 2~3) 2~3) 2~3) 2~3) 2~3) 2~3) 2~3) Dry~ocean) 2~3) 2~3) 2~3) 2~3) 2~3) 2~3) 2~3) 2~3) 2~3) 2~3) 2~3) 2~3) Transformation Homogeneous aq. phase 2~2) 3~2) 2~2) 3~2) - 2~2) 2~2) - 2~2) 2~2) Homogeneous gas phase 2~2) 2~2) - 2~2) 3~2) 2~1) 2~2) 2~1) 2~2) Heterogeneous 1~2) 1~2) 2~2) 2(1) - 1(2) 1(1) 1(2) 2(1) Distribution 3(2) 2(2) 2(2) 3(2) 2(2) 3(2) 1(2) 1(2) 1(2) 2(2) 1(2) 1(2) Urgency Factor6 Source Anthropogenic Biological Other natural Stratospheric Removal Wet (land) Wet (ocean) Dry (land) Dry (ocean) Transformation Homogeneous aq. phase Homogeneous gas phase Heterogeneous Distribution Interactions with other cycles C B B B B B C C D D D B C C B C B B C C C C B C B B B - B A A - B C B B C Aerosol B C C A B B C C C D D D B C C B C C C C C C C C C C A B S N HxOy Aerosol A B B A B B B C C C C C B B C C B C B B C C C C B B B A B B C C C C C C B B B B B B C C C C C C B B B B A B - A B A A A A B A A A A HxOy Aerosol HxOy Aerosol aKnowledge: 1 = low, 4 = high; Importance: 1 = high, 4 = low. 6Urgency factor: A = extremely urgent for that cycle, B = considerably urgent, C = moderately urgent, D = of little urgency. NOTES: XO = organic gaseous compounds; X; = inorganic gaseous compounds; X~ = halides in droplets and aerosols; I- includes IO3 .

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188 TABLE C.5 Nitrogen Cycle APPENDIX C Odd Nitrogen Species NH3/NH4 HCN N~O Knowledge and (Importancef Source Anthropogenic 2(2) 3(3) 1(2) 2(1) Biological 1(1) 1(1) 1(3) 2(1) Other natural 2(4) 2(3) - 2(3) Stratospheric 3(3) 3(4) 1(3) 3(4) Removal Wet(land) 2(~) 2(1) 2(2) 2(4) Wet(ocean) 1(1) 1(1) 2(2) 2(4) Dry(land) I (~) 1 (1) 2 (3) 3 (4) Dry(ocean) 1(1) 1(~) 2(3) 3(4) Transformation Homogeneous aq.phase 2(2) 2(3) 2(2) Homogeneous gas phase 3(1) 2(2) - 2(2) 4(2) Heterogeneous 2(2) 2(3) 2(2) Distribution 2(1) 1(1) 2(3) 4(2) Urgency Facto7i Source Anthropogenic B C A A Biological A A B A Other natural C C B C Stratospheric . C D B D Removal Wet(land) A A B C Wet(ocean) A A B C Dry(land) A A C D Dry(ocean) A A C D Transformation Homogeneous aq.phase B C B Homogeneous gas phase B B B C Heterogeneous B C B Distribution A A C C Interactions with H `O' S H':Oy other cycles S Aerosol C Aerosol aKnowledge: 1 = low, 4 = high; Importance: 1 = high, 4 = low. bUrgency factor: A = extremely urgent for that cycle, B = considerably urgent, C = moderately urgent, D = of little urgency.