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The Role of Modeling in Unclerstanding 6 Tropospheric Chemical Processes BY R. DICKINSON AND S. LTU PRINCIPLES OF MODELING We have just discussed the various processes entering into tropospheric chemical cycles. These processes are studied individually to better understand them and to improve our descriptions of the quantitative relation- ships between the different variables entering into a process. These relationships are never exact representa- tions of reality. They are subject to continuing improve- ment using better measurements and new insights. These mathematical relationships between various components of a process are referred to as "process models. " It is useful to distinguish between those variables of a process system that are internal to that system, that is, calculable from the process model, and those that are external, that is, prescribed from observation. In partic- ular, the linkages to other processes are external varia- bles in the formulation of an individual process. If all the linked processes of the tropospheric chemical system are considered together with their linkages in- cluded as internal variables, we have a "system model. " A system model still contains some external parameters, but if the model is sufficiently comprehensive, such pa- rameters can be reliably prescribed for current condi- tions. The system model has two basic functions. First, be- cause it has maximized the number of variables it calcu- lates, it provides a good opportunity to carry out exten 94 sive comparisons between the model calculated versus observed variables of the system; such comparisons help identify weaknesses in the individual process models. Second, it allows projections as to the future state of the tropospheric chemical system as various external pa- rameters change with time. Of special interest in this context are external changes imposed by human activ- ity, but also of interest is long-period natural variability in external conditions. Because of the complexity of system models, they are generally integrated by means of computer programs. One important aspect of such integration is the develop- ment and use of numerically accurate procedures for solving the differential equations that are used to define the process models and, hence, system models. Tropo- spheric chemistry shares with meteorology a concern for a wide range of interacting scales, beginning on the scale of individual microeddies, e.g., within a smoke plume from a power plant, and ending in the global scale. Satisfactory parameterizations of the role of smaller scale processes should be one of the objectives in devel- oping and improving system models of global tropo- spheric chemistry. EXISTING MODELS The generality and detail possible in a complete sys- tem model are limited by difficulties of interpretation as
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THE ROLE OF MODELING a result of its extreme inherent complexity and the large demands placed on computer and programming re- sources. Up to now, the necessary staff and institutional support to pursue a complete system model of tropo- spheric chemistry have not been developed. Further- more, there have been considerable uncertainties in the individual processes. Consequently, the research tools that have been used in tropospheric chemistry studies for synthesis and interpretation lie between the concept of a process model and a complete system model. We shall refer to these tools simply as tropospheric chemistry models. There has been developed a wide range ofthese models, with their content depending on the interests and objectives of their developers as well as their access to computing resources. Basically, what distinguishes the models now used from a complete system model is that they attempt to model accurately only some of the processes of the system, the others being included only in a simplified or ad hoc fashion. Existing models can be best classified by the issues they address. Usually these models consist of two major parts: the chemistry and the transport. Depending on the characteristics of the subject being studied, each model employs different degrees of sophistication in the treatments of chemistry and transport. At one extreme are meteorologically oriented models that obtain the motions ofthe atmosphere and temperature structure as three-dimensional time-varying fields by solution of the continuum equations of hydrodynamics and therrnody- namics in response to realistic boundary conditions; however, these models have until recently approximated tropospheric chemistry by ignoring all species except water. Some studies are now under way using the winds generated by some regional and global meteorological models to provide transport for simplified chemical models. At the other extreme are the one-dimensional or box chemical models. As a consequence of their ex- tremely complex processes of chemical species transfor- mations, their transport consists essentially of empiri- cally derived time scales for movement of species from one box or level of the model to another. At the current state of development, one ofthe promi- nently distinguishing features of chemical models is their dimensionality. Thus there are zero-, one-, two-, and three-dimensional models. The zero-dimensional box models simulate laboratory chemical reaction mea- surements such as in smog chambers. Reactants in the reaction chamber are assumed to be completely mixed so that transport can be neglected. Chemistry is treated in detail by including all relevant elementary reactions. Usually Gear's code with small integration time steps is used to study the time-dependent behaviors of all reac- tants. Multidimensional models can be viewed as alarge number of zero-dimensional models, coupled together 95 by transport and radiation submodels, and each differ- ent because of different transport source-sinks and dif- fering environmental conditions. In the modeling of the global tropospheric photochemistry, emphasis has been on the problems of 03, CO, CO2 (or carbon cycle), nitrogen cycle, and sulfur cycle. Box models and one-, two-, and three-dimensional models have been devel- oped to study the natural chemistry and possible effects of anthropogenic activities on these species. Because of the computer resource requirements for three-dimen- sional modeling, full-scale chemistry has not been in- cluded. On the other hand, simpler models with full- scale chemistry usually do not successfully parameterize the important transport processes and hydrological cy- cle ofthe atmosphere. Besides dimensionality, treatment of model time structure is also notable. For example, is the model steady state or capable of following transient changes? How does it treat diurnal and seasonal variations? Some models only calculate fast chemical transformations but prescribe as external, slowly changing species. Such models avoid the need for transport submodels because transport is primarily important for determining the distribution of the slowly changing species. Other models prescribe the species with fast chemistry, in par- ticular OH, as external, and concentrate on the interac- lion among source, sinks, and transport of slow species. One important distinction with regard to model objective is the difference between climatological and event models. This distinction arises because ofthe large day-to-day variability of meteorological processes, including transport. Thus a detailed case study of the processes of tropospheric chemistry over several days or less must recognize the actual transport occurring over that interval, either by explicit measurement of it over the interval, or by measuring enough initial meteorolog- ical data to allow integration of a weather forecast model for the time and space domains of interest. On the other hand, if a study is more concerned with the average behavior of the atmosphere as described by means and higher statistical moments, then there is less demand on temporal accuracy in providing the meteorological transport terms. The most effective tools in this instance are the general circulation models that obtain from first principles the statistical properties of the atmosphere by direct numerical simulation. That is, they calculate day- to-day weather variations over a long period oftime that do not correspond to any particular time period but are supposed to have the same statistics as actual weather systems. In other words, they model the climate of the atmosphere system. Much of the work on three-dimensional modeling of tropospheric chemistry up to now has been on the urban and, more recently, regional scale. The chemical models
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96 developed for these studies should be considered in developing models for global tropospheric chemistry. Dispersion models have been developed to study the dispersion of air pollutants from power plants as well as cities. These models are concerned with urban-scale transport but usually ignore chemical transformations of air pollutants. Models that do include air pollution chemistry but employ simpler transport pararneteriza- tions are called air quality models. Most of these models are developed for metropolitan and suburban areas; occasionally regions as large as the eastern United States have been included. Because air quality models emphasize the oxidant problem, the chemistry usually includes that of 03, NOx, and hydrocarbons. Studies using smog chambers have led to the development of detailed mechanisms for specific hydrocarbons. Unfortunately, the chemistry in these mechanisms is far too extensive to be incorporated into air quality computer models. In order to circum- vent this problem, at least two approaches have been utilized. These involved "lumping" the hydrocarbons by classes and using a generalized reaction mechanism for these classes, or by using a carbon-bond approach, which partitions the chemical species on the basis of the similarity of their chemical bonding. These chemical models have been tested against and tuned to a variety of smog chamber data. Usually good agreement is achieved between measured and predicted concentra- tion-time profiles for all measured species. When these reaction mechanisms are incorporated into air quality models and compared to field measurements, the agree- ment becomes much poorer. Discrepancies could be due to poor transport pararneterizations, but there is little doubt that lack of understanding of the chemistry in the real atmosphere also contributes to the discrepancies. In particular, the chemistry of aged and diluted air pollu- tants may be poorly understood because it cannot be effectively tested in smog chambers. Furthermore, het- erogeneous reactions are either not included or treated by oversimplified parameterizations. Acid deposition models are used to study wet and dry deposition of acid material such as sulfur and nitrogen compounds. Acid deposition models have been devel- oped for Europe and the eastern half of North America. The major objective of these models is to establish the source-receptor relationship of acid deposition. So far, very little chemistry is included in the acid deposition models. Constant, linear conversion rates of SO2 to SO4- and NO2 to NO3 have been used. Wet scavenging and dry deposition are assumed to be independent of cloud types or topography. Most of the modeling effort is focused on the development ofthe meteorological aspect of the model. There is a clear need to incorporate into these models the full-scale fundamental chemistry and PART II ASSESSMENTS OF CURRENT UNDERSTANDING kinetics involved in the transformation of sulfur and nitrogen compounds. In conclusion, as the field of tropospheric chemistry matures, the various kinds of models should tend to converge more toward ideal system models. This occurs, on the one hand, as modelers learn to treat more elaborate model systems and, on the other hand, as they are able to understand the error implied by various con- venient approximations and hencejustify these approxi- mations when their implied error is acceptable. Global modelers and regional modelers should collaborate on those aspects of their models that are of common interest. MODELING IN SUPPORT OF THE PROPOSED RESEARCH PROGRAMS We discuss here the modeling programs required to provide guidance to and help synthesize the results ofthe research programs proposed in Part I ofthis report. The programs in biological sources, photochemical proc- esses, and removal processes require the development of submodels describing the individual processes involved. These would serve three purposes: (1) help to under- stand better the individual process, (2) help to extrapo- late from individual observational sites to regional and global averages, and (3) provide submodels to be used in a comprehensive three-dimensional meteorological model coupled to global tropospheric chemistry. By con- trast, the global distributions and long-range transport program would be used to help validate the overall per- formance of the chemical aspects of comprehensive three-dimensional models of tropospheric chemical processes. As submodels are developed for the various subprograms, they will be incorporated into the com- prehensive models. Biological and Surface Source Models The models required to support the biological and surface source subprogram fall into three categories: (1) global empirical models, (2) mechanistic models of bio- logical processes, and (3) micrometeorological and oce- anic models of surface transport processes. The observational efforts in the biological source sub- prograrn will provide measurements at individual field sites. Initial exploratory efforts will identify the ecologi- cal communities that provide significant emissions, but as a second stage, it will be necessary to obtain sufficient observations to determine annual average emissions at various sites. Variability with environmental parame- ters such as temperature, solar radiation, and moisture will also be obtained. However, due to the great variety and small-scale structure of biological systems, it will
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THE ROLE OF MODELING always be very difficult to collect sufficient data to permit straightforward numerical averaging to establish regional and global average emissions. Rather, more sophisticated approaches will be required to interpolate and extrapolate the available observations to all the non- sampled areas. Exactly the same problem occurs in summarizing other types of data obtained from ecologi- cal communities. Ecologists, in particular, have been forced to resort to empirical procedures for obtaining such parameters as net primary productivity and bio- mass carbon (e. g., Table 5. 1) from a ratherlimited num- ber of field sites (less than 100~. One systematic proce- dure has been to correlate ecological data with readily available climatic parameters pertaining to the sampled sites, in particular rainfall and temperature. The corre- lations so obtained are used to transform global maps of climatic parameters into global maps of ecological parameters. Such a procedure will be used to develop empirical models (i.e., maps) for the global distributions of the various measured biological emissions. Complementary to the development of global distri- butions of biological emissions will be the development of models of the detailed biological mechanisms and processes responsible for the measured emissions. These will range from models of soil or oceanic biochemical processes to models of whole leaf physiology. Their development will require intensive collaboration with experts in other biological and chemical areas outside the atmospheric chemistry community. These efforts will, however, differ from current and past modeling in these other disciplines in the following aspects. First, they will be focused on the processes responsible for providing atmospheric emissions. Because these emis- sions have for the most part been recognized only recently, or in some cases not yet, the other disciplines have only begun to consider how such emissions could be provided from their existing submodels. Second, this modeling effort will be focused on the whole biological - system (plant-soil-microorganisms, etc.) as it interacts with the atmospheric environment. Because of the great complexity of the processes involved, a model of the whole biological system will undoubtedly require sim- plifications in the descriptions of biochemical processes and the treatments of differences between species of organisms. Modeling the effects of soil microorganisms would require modeling the environment where the processes occur. For example, the question of methane production requires a model of the diffusion of CO2, H2, and CH4 from the production site to the atmosphere to address the question as to whether increasinglevels of CO2 could increase methane production. Boundary layer and surface transport models are required to describe the movement between ocean or 97 land surfaces and the atmosphere. In the case of oceanic processes, such models require consideration of oceanic as well as atmospheric boundary layers and the effects at the ocean interface of wave breaking, i.e., the move- ment of air bubbles on the ocean side and spray droplets on the atmospheric side. The transfer of gases from and to a surface generally involves near-stationary diffusion-like transport proc- esses that are represented in terms of effective resistances or conductances. That is, if ca represents the concentra- tion of a gas in the atmospheric mixed layer, and this gas is maintained at some concentration cs at some surface, then the rate of flux of the species to the mixed layer from the surface is modeled as given by (cs-ca)/r`, where rat is the total resistance of the diffusion processes between the atmospheric mixed layer and the surface. The most thoroughly studied gas transfer process, for example, is that of water vapor from soil and foliage. The water vapor concentration next to the mesophyllic cells inside a leaf is that of saturation at the temperature ofthe leaf. To reach the mixed layer, the water must pass through leaf stomata, the leaf boundary layer, the leaf canopy, and a roughness sublayer above the canopy before reaching the atmospheric mixed layer. Each of these barriers is modeled by one or more resistance in series or parallel. This description in terms of resistance, although somewhat simplistic, provides the maximum level of detail that can be practicably matched to models of atmospheric transport above the mixed layer and validated by micrometeorological observations. In most cases, the surface boundary conditions are not as easily modeled as that of water vapor. Surface boundary conditions for species of interest would be one of the practical outputs of the modeling in the biological source subprogram. For example, rather detailed models are now available for leaf photosynthesis that provide the concentrations of CO2 within the leaf cavity. Plants exert physiological control over water losses through stomata! closure; the stomata! resistance is sig- nificant not only for leaf exchange of water and CO2 but also for SO2, NO2, 03, and NH3, and is modeled in terms of soil moisture and root resistances. Detailed boundary conditions for other gases, in particular SO2, NO2, 03, and NH3, and biologically emitted sources need to be established. Field programs, together with continuation of labora- tory (i.e., wind tunnel) studies should provide the data needed to develop and refine models of oceanic gas transfer processes, in particular for those species where surface boundary layers within the ocean provide addi- tional resistance to their flux between ocean and atmosphere. The micrometeorological processes of gaseous and particulate exchange within complex vegetated cano
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98 pies are still poorly understood in detail, and need to be further studied through combined observational and theoretical approaches to generate improved models to be used in modeling gas and particle exchange. A special effort should also be made to develop pa- rameterizations for the suspension into the atmosphere of continental soil aerosols, so that generation of these aerosols can be simulated in meteorological models. The Monin-Obukhov similarity theory gives an ade- quate basis for modeling fluxes through the surface mixed layer over horizontally homogeneous terrain. Its adequacy over more complex terrain, for example, with only sparse trees, is still an open question. This inade- quate understanding of the micrometeorology above complex terrain introduces uncertainty into the model- ing of gaseous and particulate fluxes between the atmo- sphere and these surfaces, and so into the average re- gional and global fluxes. Modeling Noncyclic Transformation and Removal Processes In Clouds Clouds and precipitation play important roles in the removal, transport, and transformation of species in element cycles. For instance, wet removal is probably one of the most effective sinks for nitrogen and sulfur compounds. Important species such as SO2, N2O5, and perhaps NO3 may go through fast aqueous transforma- tion in cloud droplets. Furthermore, cloud convection may be an efficient vertical transport mechanism for trace gases and aerosol particles. In order to evaluate these processes quantitatively, a cloud-removal model should include detailed treat- ments of the physical and chemical mechanisms involved. The cloud model should be a submodel of a meteorological model that includes self-consistent and realistic treatments of heat, moisture, and momentum transports. Physical aspects of the model include the parameterization of radiation, condensation and evapo- ration, and stochastic coalescence and breakup. Chemi- cal aspects of the model include both homogeneous gas-phase and liquid-phase reactions as well as heteroge- neous reactions. In the clean 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 hydrocarbons and their oxidation products, metals such as Mn and Fe, and graphitic carbon should also be considered. Many fundamental parameters of gas-to-particle reactions and kinetics need to be studied in the labora- tory and by modeling. For instance, sticking coefficients for above-mentioned gases on various types of aerosols PART II ASSESSMENTS OF CURRENT UNDERSTANDING under atmospheric conditions need to be measured and investigated theoretically. Currently, cloud models exist that include various degrees of sophistication in treating the physical proc- esses mentioned above. Their incorporation into a self- consistent meteorological mesoscale model is in prog- ress. On the other hand, photochemical cloud-removal modeling is at a rudimental stage. Some advances have been made recently, due primarily to the worldwide interest in the acid deposition problem. Because these processes are subscale for global models and sometimes for regional models, proper parameterization of their effects will be a crucial step in modeling these processes. A program that consists of well-coordinated field and laboratory measurements and model studies is needed to gain an adequate understanding ofthe cloud-removal processes. Each important mechanism involved, both physical and chemical, needs to be tested through itera- tive intercomparisons among modeling, field, and labo- ratory studies. Only then can a realistic cloud-removal model be developed based on the parameterization of these mechanisms. Modeling Fast-Photochemical Cycles and Transformations Some of the outstanding problems confronting the modeling of fast-photochemical transformations are (1 ) the prediction of the concentrations of OH and HO2 in the ambient atmosphere and their dependence on the concentrations of NOx and hydrocarbons; (2) the bud- get of 03; (3) the mechanisms and rates of the oxidation of NOx and SO2 to nitrate and sulfate, involving both homogeneous and heterogeneous reactions; and (4) the nature and rates of fast transformations involving atmospheric hydrocarbons and their oxidation prod- ucts. Unlike the modeling of cloud-removal processes, mathematical techniques of modeling fast gas-phase photochemical transformations are well developed. Computer models dealing with fast-photochemical transformations exist for polluted urban air as well as for clean background troposphere. The major deficiency in the understanding of fast-photochemical transforma- tions lies in the lack of laboratory and field data. For instance, concentrations of key species such as OH, HO2, and H2O2 have not been reliably measured, and kinetic data of many key reaction rate constants have not been determined under tropospheric conditions. Recent developments in field and laboratory meas- urement techniques have made it possible to measure some of the above-mentioned key species and reaction rate constants. Further major advances in the under- standing of fast-photochemical transformations can be
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THE ROLE OF MODELING made by coordinating modeling with the field and labo- ratory studies. Heterogeneous processes may play some important roles in the fast photochemistry. As discussed in the pre- vious section, major advances in measurement technol- ogy and theoretical treatment are needed in this area. Modeling Global Distributions and Long-Range Transport with a Three-Dimensional Meteorological Model A three-dimensional model of tropospheric chemical processes linked to meteorological and climatic proc- esses or, in brief, a Tropospheric Chemistry System Model (TCSM) is an important tool for the overall syn- thesis and theoretical guidance of the GTCP. A recom- mended institutional framework for the development and operation of such a TCSM is given in the next subsection. Here we outline some ofthe research studies that one or more such TCSMs would carry out in sup- port of the global distributions and long-range transport subprogram and, more generally, for modeling explora- tion of the tropospheric chemical system. Two classes of studies would be carried out with TCSMs. First would be studies intended to validate and possibly develop the ability of models to simulate long-range transport and global distributions and variability of long-lived chemi- cal species. These studies would compare model results with the data sets obtained through the global distribu- tion and long-range transport subprogram of GTCP and, if neccessary, develop the model improvements required for satisfactory validation. Most of the model- ing studies would be carried out in a climatological framework, but one or more detailed event studies would be performed in conjunction with intensive pe- riods of field data collection. Such studies could also be carried out with more simple chemistry as needed to simulate the sources and sinks of the long-lived species. The second class of studies would emphasize the sim- ulation of medium-lived species. 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 model trans- port submodels, would explore the role of meteorologi- cal processes in determining the spatial distributions and temporal variability of the medium-lived species. Such modeling studies, for example, could address the question as to how important are continental pollution sources of sulfur and odd nitrogen for atmospheric dis . . . try buttons at remote sites. The exploration of such questions would help im- prove interpretation ofthe data on many ofthe species to be monitored in the global distribution and long-range transport subprogram. Medium-lived species of special 99 current interest include NO-NO2-HNO3, NH3, SO2, sulfate-nitrate aerosol, and continental soil aerosol. The soil aerosol is of interest not only because of its optical- radiative effects, but also as a source of Ca, which can raise the pH of droplet aerosols. Institutional Framework for Development and Application of a Three-Dimensional Tropospheric Chemistry System Model (TCSM) The study of tropospheric chemistry, as it has devel- oped over the past decade, has largely been in an explor- atory phase of study with emphasis primarily on the development of new instruments and concomitant pio- neering measurements of previously unseen species. It is the thesis of our report that tropospheric chemistry as ,. . . . . . . a dISCIp Ine Is ripe to become a more mature science wit. large-scale field programs devoted to the systematic col- lection of required data. Essential to the successful appli- cation of these data to advance scientific understanding are not only submodels of the various processes studied but also system models of the overall tropospheric chem- ical system. Such models necessarily indude the mete- orological processes that transport and in other ways interact with the chemical species. The meteorological processes are best provided through versions of atmo- spheric general circulation models (GCMs) that have been especially designed to satisfactorily provide not only large-scale tracer transports in the free atmosphere but also transport through the planetary boundary layer across the tropopause and through cloud processes. These models also require physically based cloud sub- models, a good description of land surfaces, and ade- quate treatment of the solar radiation driving tropo- spheric photochemistry. The three-dimensional distribution of chemical spe- cies should be represented with spatial and temporal resolution comparable to that of the meteorological vari- ables. The distribution of these species is determined by meteorological transport and source and removal proc- esses and also by wet and dry chemical transformations. It is recommended that one or more research groups be established, building on current modeling strengths, to develop such models in the time frame of the Global Tropospheric Chemistry Program. These groups should be strongly committed to carrying out the sys- tems modeling studies required by the Global Tropo- spheric Chemistry Program, as well as efforts in devel- oping the critical submodels required for successful application of the TCSM. They should contain exper- tise in both chemical and meteorological modeling and maintain close contacts with the observational subpro- grams of the Global Tropospheric Chemistry Program. The computer and programming resources necessary
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100 for successfully carrying out the task should be made available. BIBLIOGRAPHY Anthes, R. A. (1983~. Regional models ofthe atmosphere in middle latitudes (a review). Mon. Weather Rev. 111:1306-1335. Bass, A. (1980~. Modeling long-range transport and diffusion, in Conference Papers, Second Joint Conference on Applications of Air Pollu- tionMeteorology, New Orleans, La., pp. 193-215. PART II ASSESSMENTS OF CURRENT UNDERSTANDING Demerjian, K. L. (1976~. PhotochemicalDiffusionModelsforAirQual- itySimulation: Current Status. Assessing TranspoTtation-RelatedImpacts. Special Report 167. Transportation Research Board, National Research Council, Washington, D.C., pp. 21-33. Lerman,A.(1979~. GeochemicalProcesses.WaterandSedimentEnviron- ments. Wiley, New York, 481 pp. Logan, I. A., M. I. Prather, S. C. Wofsy, and M. B. McElroy (1981~. Tropospheric chemistry and a global perspective. J. Geophys. Res. 86: 7210-7254. Mahlman, I. D., end W. I. Moxim(1978~. Tracer simulation using a global general circulation model: results from a midlatitude instantaneous source experiment. i. Atmos. Sci. 35: 1 340- 1374. Turner, D. B. (19793. Atmospheric dispersion modeling, a critical review. J. Air Pollut. Control. Assoc. 29:502-519.
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