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S Research Needs In light of existing uncertainties in knowledge about the relationships between emissions and the deposition of acid-forming materials, it is appropriate to consider the research that may help to clarify understanding. In this chapter we describe research that we believe is needed to answer the most central questions in the shortest amount of time. A considerable amount of research is currently being performed with funding from both governmental and private sources in the United States and Canada as well as in Europe. We have not attempted to review these research programs. As a consequence, some of the research activities that we recommend may be--indeed we know they are--included in current programs. Others, to the best of our knowledge, are not. On the basis of our review of current knowledge as described in this report, we are convinced that the research that we recommend forms essential elements in an integrated effort to improve understanding of the phenomenon of acid deposition. We do not know whether incorporation into a model of all the chemical and physical processes currently thought to be important would account for the concentrations of sulfate observed in precipitation during both winter and summer. No quantitative determination of the relative contributions to the production of sulfate and nitrate by gas- and liquid-phase processes has been attempted to date; such attempts are probably several years in the future. The details of dispersion of pollutants from sources, their chemical transformation and transport over long distances, and their contribution to acids in precipi- tation systems are sufficiently unknown and complex that a great deal of research will probably be required before 148
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149 they can be described with precision by models. Further- more, many years of precipitation measurements will be needed to establish a reliable data set from which tests of models can be made. We believe that extensive laboratory, field, and modeling studies should be continued, to establish the physical and chemical mechanisms governing acid depo- sition. However, it appears to us that useful informa- tion about the delivery of acids to rural areas by transport and transformation processes can be determined fairly quickly by direct empirical observation in the field. Although the results of such field studies may not yield complete detailed descriptions of the inter- actions of all the processes involved, they are likely to provide basic phenomenological evidence with sufficient reliability to form the basis for improving the near-term strategy for dealing with the problem of acid deposition. The data are essential for enhancing theoretical under- standing and developing improved deposition models. In the long term, the ultimate strategy for dealing with acid deposition will depend on the application of realistic, validated models. F TELD STUDIES Field research capable of testing the possibility of a nonlinear relationship between emissions and wet deposition in North America is needed. A limitation of oxidant in winter is believed to be a principal cause of such nonlinearity. Because the nature and availability of oxidants is sensitively dependent on the general composition of polluted air, field studies should be conducted in regions particularly involved in the problem of acid precipitation. Of primary importance are those experiments directed at discovering the rates of production of SO4 and Ned in clouds and the detection of possible limitations of those rates. Supplemental information, such as the availability of oxidants under varying conditions, is also important. We describe below some of the more important issues currently amenable to study in the field. Cloud Processes As described in Chapter 2 and Appendix A, the conversion of SO2 to H2SO4 in certain cloud systems may be
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150 relatively complete and rapid, but we lack conclusive information on either oxidation rate constants or the completeness of conversion in acidified clouds typical of the eastern United States and Canada. Most of the mass of sulfuric acid already existing in aerosols in dry air is efficiently incorporated into cloud droplets as the air is cooled and the cloud forms. Models indicate that nitric acid vapor is also incorporated with high effici- ency into cloud droplets. If, during the warm months, when the rate of wet deposition of acid is highest, complete in-cloud conversion of SO2 and NO2 occurs, and if the rainout of HNO3 vapor and sulfate aerosol is highly efficient, then the assumption that the wet deposition of sulfuric and nitric acid is linearly related to ambient concentrations of SO2 and NOx would be justified. This could be readily checked experimentally with available technology in appropriate storm systems, such as warm fronts and summertime convective storms. Studies of Chemical Mechanisms A number of key problems relating to the linearity of the dependence of acid production rates on precursor concentrations provide the focus for studies of chemical mechanisms. (1) Tile efficiency and seasonal dependence of transformation processes are no doubt different for gasphase and aqueous-phase processes. What are the relative contributions of these two pathways? (2) What are the lifetimes of NC~ and SO2 in clouds? Do rates of acid production differ markedly in various cloud types? Are the production rates inhibited by increasing acidity or limited by availability of oxidants? t3) what are the dominant reaction paths for SO2 oxidation in clouds? (4) Do competing reactions with other atmospheric constituents (e.g., formaldehyde), especially in polluted air, seriously inhibit or lower the effectiveness of H202 reactions with SC: in clouds? (5) what processes govern the apparently significant rate of production of HNO3 in clouds? (6) What are the relative rates of production of HNC~ and H2SO4 in clouds? (7) What is the role of ultraviolet light in the production of free radicals and H202 in the gas phase in clouds?
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151 Dry Deposition We noted in Chapter 3 that dry deposition of SCt and NO2 may account for about one half of the total deposition of sulfur and nitrogen oxides and acids in eastern North America. Accurate evaluations of the extent of dry deposition of these pollutants is required to obtain a quantitative measure of their fates in the environment. The eddy-correlation technique, in which simultaneous measurements of the vertical wind speeds and specific pollutant concentrations are measured with rapid-response instrumentation, should prove to be a valuable tool in the study of the deposition of S02, NO2, HNO3, and other pollutant gases. The develop- ment, refinement, and application of this method and possibly other new and accurate methods of measuring dry deposition, which are suitable for monitoring applicar Lions, are important research objectives. The challenge of developing accurate methods is great, adapting as they must to the great variety of surfaces that cover the Earth and to the effects of humidity, temperature, sunlight intensity, and other factors. Tracers To develop the most cost-effective strategy for ameliorating the problem of acid deposition, it is necessary to know the relative impacts of specific source regions on specific sensitive receptor regions. In view of the uncertainties inherent in the calculation of trajectories, especially during storm conditions, it seems important to develop tracer techniques that can yield experimental tracking of air parcels. Such tech- niques could be applied especially during the meteoro- logical conditions that are currently believed to provide the greatest opportunity for long-range transport of acidic materials. The use of insoluble and chemically inert gaseous tracers, as is currently planned, provides a promising approach. Admixtures of insoluble and unique materials that are subject to reaction with oxidants such as the HO radical would in addition provide information on possible limitations of the oxidation of SO2 and NOx. In light of the potential advantages of the method of elemental tracers for understanding the relationships between source and receptor regions, a considerable
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152 amount of research and development to establish and test the method on a regional scale is warranted. The first priority in this work should be to measure the detailed patterns of elemental composition of particulate matter collected at various rural sites in both source and receptor regions in eastern North America. The particles should be segregated by size, and a large number of elements and species should be measured for each size range. Other data should also be collected concurrently, including ambient concentrations of pollutant gases, wind speed and direction, relative humidity, precipitation, and optical qualities of the atmosphere. High-quality back trajectories should be calculated for the air masses sampled for each sampling period to aid in interpretation of the results. Few of these data now exist. Research using elemental tracers would attempt to determine if particles coming from major source regions have distinctive chemical signatures that can be used to identify the origins of polluted air masses at long distances from source regions. For example, are there elemental tracers that can be used as clear indicators of coal combustion, in the same way as vanadium apparently can be used to indicate oil combustion? Can observed patterns of elemental composition in rural areas be resolved into linear combinations of known composition released by certain types of sources? Does the behavior of fine particles, particularly those bearing soluble species, provide insight into the behavior of sulfate and nitrate species that are of primary concern in acid deposition? Building on the first stage of research, it may also be necessary to conduct studies of the elemental composition of particles in clear air and clouds as well as on the ground during specific episodes, such as the dusting of a polluted air mass from the midwestern United States toward the northeast along a southward-moving warm front. Changes in elemental composition as a specific parcel of air moves on a regional scale should help to identify the types of changes in the characteristics of the air during transport and as it passes over additional sources. Much of this work could be conducted in con- junction with the field studies described in the previous section.
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153 Meteorological Studies The climatology of storm movements in North America is well developed. The development of a quantitative relationship between storm type and acid deposition over eastern North America would help to estimate the long- range transport of pollutants. Despite the associated uncertainties, statistical studies of air parcel trajec- tories associated with the types of storm systems responsible for depositing acid precipitation would also be extremely valuable. An important requirement in the experimental deter- mination of rates of SO2 and NOx oxidation in clouds is a realistic evaluation of the airflow in the vicinity of clouds to establish the quantity of materials nroo-~.~a by and the time resident in the cloud. All of these field studies bear directly on the critical question of the zone of influence of sources on receptor regions. Our suggestion that these field measurements be a first priority is not meant to imply that pertinent laboratory and modeling studies should not also proceed. However, in view of the complexities in these systems, we believe that well-conceived field studies may answer many of the outstanding questions in a shorter time than that required for a complete molecular and dynamic description of the phenomenon. _ _ _ ,= ~ LABORATORY STUDIES The direct measurement of elementary rate constants for the many apparently important reactions related to the chemistry of acid deposition has been restricted largely to conditions that are not typical of the lower tropo- sphere. The direct determination of the rate constants for reactions of the HO radical with SC2 and NO2 should be made at pressures (near 1 atm of air) and temperatures characteristic of the troposphere, as should other measurements. Many aspects of cloud chemistry can and should be examined auantitativelv under ~ontr~1 1 ~ 1 Herr conditions. For example, the mechanisms of the develop ment Of H2O2 from O3 and other unidentified reactants should be established. Other important parameters amenable to laboratory measurement that are required for the quantitative description of cloud processes are the
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154 sticking coefficients of the gas-phase reactants such as HO, HO2, and NO3 on collision with cloud droplets. In general, laboratory studies are needed to establish the mechanistic detail that is required in the develop- ment of the chemically and physically sound models of acid precipitation. DEVELOPMENT OF I HEORETICAL MODELS Models of the development, transport, and deposition of acids and acid-forming materials based on physical and chemical principles are under development. These models will serve as a useful framework with which the latest data from field and laboratory studies can be combined to provide a suitable test of theory and improved planning for further field studies. State-of-the-art theoretical models that treat quantitatively the complicated gas- phase chemistry, cloud processes, transport, and deposition will require many years for development. The ultimate test of our understanding of the chemistry and physics of the processes of acid rain is the successful development and use of such models.
Representative terms from entire chapter: