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Summary During the past 25 years in Europe and the past 10 years in North America, scientific evidence has accumulated suggesting that air pollution resulting from emissions of oxides of sulfur and nitrogen may have significant adverse effects on ecosystems even when the pollutants or their reaction products are deposited from the air in locations remote from the major sources of the pollution. Some constituents of air pollution are acids or become acidic when they reach the Earth's surface and interact with water, soil, or plant life. Several studies have docu- mented the potentially harmful effects of the deposition of acids on ecosystems, which are of particular concern in areas with low geochemical capacities for neutralizing the acidic inputs (such as parts of the northeastern quadrant of North America, the Appalachian Mountains, and some of the mountainous areas of western North America). Although the pollutants may be deposited in dry form or in rain, snow, or fog, the deposition phenomenon is often called acid rain or acid precipitation. In this report we use the term acid deposition to encompass both wet and dry processes. The question of what, if anything, to do about acid deposition is a complex one, involving generation and interpretation of scientific evidence, assessment of risks, costs, and benefits, and political considerations, both domestic and international. This report deals with a small, but important, part of the analysis that cur- rently is being conducted to answer the question--the scientific evidence concerning the relationships between emissions of acid-forming precursor gases and deposition of potentially harmful pollutants. Our purpose is to assess the current state of scientific information that can be marshaled to describe those relationships in the 1
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2 hope that our assessment will be valuable to decision makers in government and in the private sector. We focus on conditions in portions of eastern North America, for which more information is available than for other areas of the continent. The central issues of concern in this report are the adequacy of current scientific understanding about the relationships between emissions and deposition, the extent to which the relationships are strongly nonlinear, and the extent to which distant sources contribute to deposition in ecologically sensitive, remote areas. We have reviewed the available scientific evidence that pertains to the issues of nonlinearity in the relation- ships between emissions and deposition and long-range transport. In the report we describe the current state of understanding about atmospheric processes (Chapter 2 and Appendixes A, B. and C), review the development of theoretical models (Chapter 3), and analyze the available observational evidence for source-receptor relationships (Chapter 4). Much remains to be learned about the detailed mechanisms involved and their relative impor- tance for the relationships between emissions and deposition. As scientists, our training leads us to be concerned about the current limits of our understanding of the relevant processes and the uncertainties asso- ciated with assessing cause and effect. Much of our report, therefore, has been devoted to exploring the areas of uncertainty in understanding of the phenomena. Continuing research on acid deposition is needed to resolve or reduce the uncertainties and thereby to provide information useful in making more informed public-policy decisions regarding acid deposition (Chapter 5). Our findings and conclusions are summarized below. STATUS OF SCIENTIFIC KNOWLEDGE Current scientific understanding of the relationships between emissions of precursor gases, such as sulfur dioxide (SO2) and the oxides of nitrogen (NOk), and deposition of acids or acid-forming substances, such as sulfuric acid (H2SO4), nitric acid (HNO3), the anions sulfate (SOi) and nitrate (NO5), and the cation ammonium (Nut), is based on theoretical considerations, the results of modeling exercises, and analysis of observa- tional data.
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3 Data Data are limited that can be used to characterize air quality, meteorological conditions, and emissions from which relationships between patterns of emissions and air quality in rural areas might be discerned in North America. Most of the historical data on air quality describe urban conditions. The most reliable information on rural conditions comes mainly from a single study in the northeastern United States, the Sulfate Regional Experiment (SURE), performed in 1977-1978. A long-term record (18 years) of reasonably reliable data on depo- sition chemistry is available at only one site in North America. Reliable data on regional precipitation chemistry have been collected only over the past 4 or 5 years through monitoring networks set up in the United States and Canada. There are no regional data from observations of dry deposition. Available data on precipitation chemistry and on annual average ambient concentrations of SC2, NOk, sulfate, ammonium, and nitric acid indicate elevated levels of pollutants in the air and acidic substances in precipitation over much of eastern North America. Ambient concentrations are much higher than can be accounted for by emissions from natural sources on a regional scale. The geographical distributions of SO2 and sulfate differ somewhat: SO2 concentrations are more localized in the regions around major concentrations of sources, and ambient sulfate aerosols appear to be more widely distributed. The distributions of sulfate in precipita- tion are similar to those of sulfate in the ambient air. Currently the molar concentrations of nitrate and sulfate in precipitation are roughly comparable over much of the eastern United States. Ambient concentrations of air pollutants are highly variable over time, whereas rates of emissions of the precursor gases SC2 and NOx are less variable. Differ- ences in temporal behavior are due in large measure to the variability of meteorological conditions and, for secondary pollutants such as sulfate and ozone, the chemical reactivity of the atmosphere and the amount of solar radiation. Concentrations of sulfate in both the air and precipitation tend to reach their maximum values in summer in the northeastern United States; seasonal variations are less evident in the Midwest and Southeast. In many areas, ambient SC2 and NOk concentrations are highest in the winter, although no measurements of these
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4 parameters in ecologically sensitive areas have been reported. Nitrate in precipitation shows much less seasonal variation in the northeastern United States. Both ambient sulfate concentrations and sulfate in precipitation have been shown to be statistically related to aerometric parameters. For example, variations in ambient sulfate concentrations (measured at ground level) are related to variations in SO2 and ozone concentra- tions, relative humidity, winds, and ventilation, whereas sulfate in precipitation is related to winds, the types and rates of precipitation, and ambient concentrations of sulfur oxides. Ambient nitrate data are not amenable to similar analyses because of uncertainties in the analyti- cal chemical methods. Not all sulfates and nitrates in the air or in precipitation contribute to the acidity of the air or precipitation. Acidity in solution is a function of the concentration of hydrogen ions. Some sulfate and nitrate in the air and in precipitation may be associated with cations other than hydrogen, such as ions of calcium or ammonium. Thus the acidity of deposition is the result of influences of the variety of cations and anions that may be present and in general cannot be identified with one or two anions. However, once deposited, sulfate and nitrate associated with cations other than hydrogen, such as ammonium, may still result in acidification of eco- systems as a result of biological and chemical inter- actions in soils and water. Meteorological Processes One of the greatest difficulties in establishing relationships between sources of pollution and conditions in ecologically sensitive areas is that of accounting for the influences of atmospheric processes on the behavior of pollutants. These processes include the large-scale transport of air masses, atmospheric mixing near the Earth's surface, physical and chemical reactions among pollutants and naturally present species, deposition of gases and suspended particles, and cloud processes leading to precipitation. Transport, mixing, physical and chemical reactions, and cloud processes are respon- sible directly or indirectly for the distribution and rate of deposition of pollutants to the ground. Our empirical and theoretical understanding of the processes is strong in some aspects and weak in others.
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5 Meteorological processes control the transport and dispersion of pollutants from their sources. Regions of northeastern North America that are considered to be sensitive to acid deposition are subjected to highly variable meteorological conditions, with the result that, in addition to local sources, a number of geographically widespread source regions are likely to contribute to the total deposition of acid-forming chemicals. Nonetheless, empirical analyses suggest that many of the precipitating air masses--and therefore most of the pollution-related ions dissolved in precipitation--reaching several sensi- tive, remote areas of the northeastern United States and southeastern Canada have their origins in unwind regions to the south and southwest. Because of the high variabil- ity in synoptic-scale meteorological phenomena affecting sensitive areas, however, all sources in eastern North America must be considered as contributing in one degree or another to the phenomenon of acid deposition. Evidence exists for long-range transport of pollutants leading to acid deposition, but the relative contributions of specific source regions to specific receptor sites currently remain unknown. Models Methods are available for estimating the effects of emissions of SC2 and Nod on regional distributions of ambient concentrations of these gases. The methods include statistical analysis of observational data and theoretical calculations using deterministic models of the chemical and physical processes involved. The methods employed to date, however, do not produce reliable esti- mates of spatial and temporal distributions of acidity, partly because of in~omnl~t" i nu~nP^r i "= off ban i ~ mom_ ~ - ]r ~ ~ _ ~ ~ _ _ %~ ~ ~ ~ . . . pounds in the air that neutralize some of the acidity. The actual limitations of predictive models for calculating the effects of sources on sulfate deposition are not well defined because of deficiencies in knowledge of atmospheric processes and the lack of coherent regional data on air quality and deposition. No studies of the validity or limitations of models for nitrate concentrations have been performed. No model for wet deposition of acidity has been developed that takes account of sources and distributions of all the important ions in precipitation. The deterministic models that are available employ by
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6 necessity approximations to the atmospheric processes that are known or hypothesized to be important to acid deposition. All deterministic models are limited in their usefulness by the sparseness of meteorological data compared with that needed as input to project reliably the movement and mixing of air masses. ISSUES Applicability of Models to Decisions on Control Strategy Results from current air-quality models applied to regional-scale processes have provided guidance on the significance of dynamic processes influencing sulfur deposition. The results of the models are qualitatively consistent with observations, thus demonstrating impor- tant temporal and spatial scales of the source-receptor relationships. Qualitatively, the models have pointed to the importance of certain geographical groupings of (SO2) sources and the potential influence of the sources on certain receptor areas. However, current models have not provided results that enable us to have confidence in their ability to translate SO2 emissions from specific sources or localized groupings of sources to influences on specific sensitive receptors. Little has been done in modeling to translate NOx emissions into nitrate deposition or to link sulfate and nitrate to acid (H+) deposition. A predictive capability that includes accounting for important cations is considered an essential requirement where long-range transport processes are involved. Because of the simplifying assumptions made in order to develop practical, economical regional-scale air- quality models and because data are not available to validate or verify the models, workers in the field generally have only limited confidence in current results. The models and their results are useful research tools; but given the state of knowledge of the physics and chemistry of the atmosphere in the context of long-range transport in air pollution, we advise caution in using deterministic models to project chances in patterns of deposition on the basis of changes in patterns of emissions of precursor gases. For practical purposes, deterministic models have been and will continue to be used for research on atmospheric
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7 transport and transformation chemistry, and this use may lead to improvements in our understanding of deposition of acidifying substances in eastern North America. Before models can be relied on for development of refined strategies for dealing with acid deposition that take into account the specific locations of sources and receptors, however, we will need a greatly improved base of meteorological data and more precise treatment in the models of both chemical and meteorological aspects such as vertical distributions of meteorological variables, including clouds. In the near term, we believe that collection and analysis of field data are likely to lead to improved understanding more quickly than refinement of deterministic models. In fact, such data are needed to improve the models themselves. Confidence in control strategies will be strengthened to the extent that they are founded on scientifically sound, verified models. Laboratory evidence suggests that an alternative model of the chemical processes involved in acid deposition may be postulated that represents the gas-phase reactions leading to the oxidation of SO2 more correctly than the model of Rodhe, Crutzen, and Vanderpol, which has been widely used for this purpose. In keeping with results of laboratory experiments, the alternative model employs a series of reactions that results in oxidation of SO2 without net consumption of a major oxidant, the hydroxy radical. When these gas-phase reactions are incorporated into the model, the previously reported nonlinearity in the relationship between changes in ambient concentra- tions of SO2 and changes in ambient concentrations of sulfate aerosol is greatly reduced (see Chapter 3). Nonlinearity There is, admittedly, much to be learned about the relationships between emissions and deposition. However, on the basis of analysis of currently available data in eastern North America and within the limits of uncer- tainty associated with errors in the data and in estimating emissions, we conclude that there is no evidence for a strong nonlinearity in the relationships between long-term average emissions and deposition. This conclusion is based on analysis of available data on historical trends (mainly at the Hubbard Brook Experi- mental Forest in New Hampshire), the ratios of pollutants
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8 in emissions and deposition, and comparison of the percentages of emissions of SO2 and NOk that are deposited as sulfate and nitrate in precipitation (see Chapter 4); it is also supported by theoretical calculations taking account of the latest results of realistic laboratory studies (see Chapter 3). The only available direct evidence of strong non- linearity between average emissions and average depo- sition of sulfur compounds is found in the extensive data taken in Europe over the past 25 years. Analysis of the European data has focused on historical trends in bulk sulfate deposition and on the spatial distribution of the ratio of sulfate to nitrate in monthly bulk samples. The analysis has also employed the original Rodhe-Crutzen- Vanderpol model. The observed trends in Europe are somewhat uncertain because of changes in sampling and analytical techniques and analytical laboratories throughout the period (see Chapter 4). Reasonably reliable historical data indicating trends in North America are available only from the Hubbard Brook site (see Chapter 4). These data, from 13 years of weekly bulk samples, show no evidence of strong nonlinearity. Analysis of the spatial distributions of the molar ratio of SO2 to NOx in emissions and the molar ratio of sulfate to nitrate in precipitation provides addi- tional though indirect evidence that there is no strong nonlinearity in the relationships between long-term average emissions and deposition in eastern North America (see Chapter 4). The molar ratio of sulfate to nitrate in precipitation does not vary substantially over a large region in eastern North America; and for annual average data, it is similar to the average molar ratio of SO2 to NOx in emissions. Analysis of the data from the Midwestern and northeastern United States also ndicates that the percentage of emitted SO2 that is deposited as sulfate in precipitation is approximately equal to the percentage of emitted NOk deposited as wet nitrate in that region. Since the conversion of NC2 to HNog and its subsequent incorporation into cloud water are believed to be relatively rapid and efficient processes, the data suggest that the combined gaseous and aqueous conversions of SC2 to sulfate are similarly efficient. It is therefore improbable that the oxidation of SO2 is sufficiently hindered by a lack of oxidant to cause a disproportionately small reduction in sulfate concentra
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9 Lions in precipitation as a result of a given reduction in S0: emissions. The North American data therefore suggest that (a) whatever atmospheric processes are taking place, pollutants are being thoroughly mixed over a region with linear dimensions up to 1,000 km, and (b) the formation of sulfate is neither enhanced nor retarded relative to the formation of nitrate. The conclusion is clouded by three types of uncertainty: the limited amount and uncertain quality of the data, the natural variability of atmospheric processes, and the lack of firm understanding of the physical and chemical processes involved. If improved measurements indicate that the relationships between emissions and deposition in Europe and eastern North America are different, the differences between the two regions in meteorology, or latitude, or other factors, such as the spatial distribution of sources, may be responsible. Influences of Local and Distant Sources Theoretical and observational evidence exists for the long-range transport of air pollutants leading to acid deposition (see Chapters 3 and 4). However, the relative importance for deposition at specific sites of long-range transport from distant sources as compared with more direct influences of local sources cannot be determined from currently available data (see Chapter 4) or reliably estimated using currently available models (see Chapter 3). Trends in the historical data at the Hubbard Brook Experimental Forest appear to reflect general trends in emissions (see Chapter 4). Available meteorological analyses of trajectories of precipitating systems at three locations in the Northeast (Whiteface Mountain and Ithaca in New York and south central Ontario) indicate that much of the acidity in precipitation--as well as much of the precipitation--comes from air masses arriving from the South and Southwest. Based on the analysis of spatial distributions of the annual average molar ratios of pollutants in emissions and deposition, it appears that the atmospheric processes in eastern North America lead to a thorough mixing of pollutants, making it difficult to distinguish between effects of distant and local sources (see Chapter 4).
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10 IMPLICATIONS FOR EMISSION-CONTROL STRATEGIE S The implications of our findings and conclusions for choosing among possible emission-control strategies, should they be deemed necessary, are limited. We do not believe it is practical at this time to rely upon cur- rently available models to distinguish among alternative strategies. In the absence of other methods, analysis of observational data provides guidance for assessing the consequences of changing SO2 emissions for wet deposition of sulfate. If we assume that all other factors, including meteorology, remain unchanged, the annual average concentration of sulfate in precipitation at a given site should be reduced in proportion to a reduction in SO2 and sulfate transported to that site from a source or region of sources. If ambient concentrations of NOk, nonmethane hydrocarbons, and basic substances (such as ammonia and calcium carbonate) remain unchanged, a reduction in sulfate deposition will result in at least as great a reduction in the deposition of hydrogen ion. It can be stated as a rule of thumb that the farther a source is from a given receptor site, the smaller its influence on that site will be per unit mass emitted. Analysis of air-mass trajectories and modeling may provide insight into the relative contributions of sub- regional groupings of sources to sulfate deposition in ecologically sensitive areas. Interpretation of this information, however, is subjective, and it will entail considerable judgment in assigning zones of influence of sources, even for long-term averages. This subjectivity has been a source of major differences in expert opinion, and it will continue to be until scientific knowledge improves considerably. On the basis of analysis of the spatial distributions of the molar ratios of pollutants in emissions and deposition and assuming that all other emissions and conditions remain unchanged, we would expect that if the molar ratio in emissions in eastern North America were changed by changing SO2 emissions, a similar change would occur in the ratio of sulfate to nitrate in wet deposition. If, as described in Chapter 4, dry deposition is linearly proportional to emissions, then the average annual ratio in total deposition in the region should also respond to changes in the emission ratio. Because the analysis is based on spatial distributions, its applicability is limited to
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11 circumstances in which the spatial distribution of emissions is not changed. Because we cannot rely on current models or analyses of air-mass trajectories, we cannot objectively predict the consequences for deposition in ecologically sensitive areas of changing the spatial pattern of emissions in eastern North America, such as by reducing emissions in one area by a larger percentage than in other areas. RESEARCH NEEDS We believe that extensive laboratory, field, and modeling studies should be continued if we are to establish the physical and chemical mechanisms governing acid deposition (see Chapter 5). It appears to us, however, that useful information about the delivery of acids to ecologically sensitive areas by transport and transformation processes can be determined more quickly by direct empirical observation in the field than by other means. Although the results of such field studies may not yield complete detailed descriptions of the interactions of all the processes involved, the studies are likely to provide basic phenomenological evidence with sufficient reliabil- ity to form a basis for improving the near-term strategy for dealing with the problem of acid deposition in eastern North America. Indeed, the data are essential to enhance theoretical understanding and to develop improved deposition models. In the long term, however, the ultimate strategy for dealing with acid deposition will depend on the application of realistic, validated models.
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