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Introduction 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 hydrocarbons and 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 (National Research Council 1981, Environment '82 Committee 1982). 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 documented the potentially harmful effects of the deposition of acids on ecosystems (NRC 1981, National Research Council of Canada 1981, Overrein et al. 1980, Drablos and Tollen 1980). 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. DEPOSITION ACIDITY An acid is a chemical substance that, in water, provides an excess of hydrogen ions (H+) to the solution. In solutions, the electrical charges of positively charged ions (cations) balance those of negatively charged ions (anions). In acid precipitation, excess hydrogen ions are usually balanced by sulfate (SO4), nitrate (N05), and to a lesser extent chloride (C1-) ions. There may, in general, be other cations in addition to H+ present in precipitation. Organic acids, for example, are 12

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13 found in all areas, but they are important as donors of hydrogen ions only in remote areas where concentrations of sulfate and nitrate are lower (Galloway et al. 1982). Acid deposition in dry form consists of gases such as sulfur dioxide (SO2), nitrogen oxides (NOX), and nitric acid vapor (HNO3) as well as particles containing sulfates, nitrates, and chlorides. Acids occur naturally in the atmosphere because, for example, of the dissolution of carbon dioxide (CO2) in water or the oxidation of naturally occurring compounds of sulfur and nitrogen. The "natural" acidity of rainwater, measured as pH, 2 is often assumed to be pH 5.6, which is an idealized value calculated for pure water in equilibrium with atmospheric concentrations of CO2. However, the presence of other naturally occurring species, such as Sol , ammonia, organic compounds, and windblown dust, can lead to "natural" values of pH between 4.9 and 6.5 (Charlson and Rodhe 1982, Galloway et al. 1982). The presence of compounds of sulfur and nitrogen of anthropogenic origin tends to increase the acidity (lower the pH) of precipitation. More than half the acidity of precipitation averaged over the globe may be due to natural sources, but anthropogenic sources may dominate in some regions. For example, in eastern North America (i.e., east of the Mississippi River) 90 to 95 percent of precipitation acidity may be the result of human activities, although natural sources may also be important at times in specific locations (U.S./Canada Work Group #2 1982). Figure 1.1 shows the mean value of pH in precipitation weighted by the amount of precipitation in the United States and Canada in 1980. There are no known natural causes that can account for either the distribution or the value of acidity in eastern North America. The region of highest acidity does, however, correspond to the regions of heavy industrialization and urbanization along the Ohio River Valley and the Eastern Seaboard, where anthropogenic emissions of sulfur dioxide (Figure 1.2), nitrogen oxides (Figure 1.3), and hydrocarbons are high. Figures 1.4 and 1.5 indicate the spatial distributions of sulfate and nitrate, respectively, weighted by the amount of precipi- tation that was deposited in North America in 1980. The data were obtained from several monitoring networks in the United States and Canada. Trends in acid deposition in North America have been difficult to discern, and data with which to assess them are sparse. Comparisons of historical data (for example, Cogbill and Likens 1974) have been questioned because of

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14 / ~ ; ant-- ''l: ;.7~/' i.. ~ 4~:,~ lo. _~__J I FIGURE 1.1 Annual mean value of pH in precipitation weighted by the amount of precipitation in the United States and Canada for 1980. SOURCE: U.S./Canada Work Group #2 (1982~. difficulties associated with comparing data obtained by means of different experimental methods of uncertain comparability at different sites at different times and because of difficulties in taking into account the influence of neutralizing substances on the data (Hanson and Hidy 1982, Stensland and Semonin 1982). A long-term (18-year) record of reasonably reliable data on deposition chemistry is available at only one site in North America (see Chapter 4). The relationship between emissions and deposition in North America is complicated by changes that are not reflected in data on aggregate emissions. For example,

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53. tHI) 15 because of concern about urban air pollution in the 1960s, there has been a tendency since then to build large new facilities away from urban centers and to use tall stacks to eject emissions at higher altitudes, hence promoting dispersal and dilution of the pollutants. Pollution control equipment installed during this period also changed the chemical and physical characteristics of the emissions, substantially reducing direct emissions of sulfates and neutralizing substances in fly ash. Thus, while total emissions of SO2 in the United States increased between 1960 and 1970, urban concentrations of SO2 decreased (Altshuller 1980). Almost all available data on air quality reflect urban conditions. Only recently have extensive networks of monitors been established in rural areas. so2 ~- ~ 220 \\ 193 I'm! ( ~ , Sit ~3 ~ 247 ~ ~ / ~ J 55 OCR for page 12
16 NOx \~\'~ ~ ~ 174 ~ 113 231 -___ 'I L _ >_ _ _ _ l_ _ 76 1 131 251 _ ~_ _ ~W] - .D o FIGURE 1.3 Representative values of NOX emmissions in the United States and Canada in 1980 (thousands of metric tonnes). SOURCE: U.S./Canada Work Group x3B (1982~. ENVIRONMENTAL EFFECTS Atmospheric deposition involves three components: emissions, deposition, and effects on receptors. Certain aspects of the effects of atmospheric deposition are of particular importance for the development of effective policies for emission control. They concern the sig- nificance for receptors of (1) physical and chemical states of deposited materials and (2) rates and reversibility of acidification. In discussing these issues, it is helpful to distinguish between primary, secondary, and tertiary receptors accord- ing to their proximity to the initially deposited material. Primary receptors experience direct contact with atmospheric pollutants. Examples are the surfaces of

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1/ . A ~ . _~ _ :.4 3-44~ , ~ .7.89-06' I 5. I ~ ~ 10 1 i~ ; -~z'~--~.2~'',e,~// J ~- ~I 1~66-3 5.2- l 1 4 ~ Of - - - - 1 t- - - 7_ t_ _ ~ - 1 4.8 ( \ I V L _ ~ .__-~ l- 5.3 \ ~+ 2.2 \~_ \ rV 1 - ~ ? ~3.7 \ ~52.7 ~: it'\ 1 m mole/m2 = 0.961 kg/ha FIGURE 1.4 Spatial distribution of mean annual wet deposition of sulfate weighted by the amount of precipitation in North America in 1980 (mmoles/m2~. SOURCE: U.S./Canada Work Group #2 (19823. structures and materials, the outer foliage of vegetative canopies, and the surfaces of soils that are not protected by vegetative canopies. Secondary receptors are subject to wet and dry deposition indirectly and only after the pol- lutants have been in contact with other materials. Examples include the inner foliage of vegetative canopies, soil underneath vegetation, and subsurface layers of exposed soils. Tertiary receptors are even further removed from the point of initial contact with deposition. Examples are subsoil, underlying rock formations, streams and lakes that receive most of their water from runoff from the watershed, and lake and stream sediments. The rates of transfer and mixing of materials are affected by the proximity of the receptors to the point of initial deposition and by their mass and other physical and chemical properties.

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18 ~ ~0.5 \` 4.3 / _ 1 d _ - ~7~ ~ ~( . 0.91 ~I_ - --- ~\ a/ ~ 3.5 ~ / i4.~m ~ 0~ // \~N 8.9 ~w I" is r /.- 4-5 7.5 ~ (4,4- ~\~ 4. -_ it- 7- - -1_ 5 . . ~N ~ F___ _ _ _1 ~ _ _ T ~-- at_ 14.3 8.7 - 1 7.2 1 :; _~__J 1 m mole/m2= 0.62 kg/ha ;' ;,,:: 2-0~"^": LO 14-~ _~e d~^ 19. w_ ~ it" b0 / FIGURE 1.5 Spatial distribution of mean annual wet deposition of nitrate weighted by the amount of precipitation in North America in 1980 (mmoles/m2 ). SOURCE: U.S./Canada Work Group #2 (1982~. Physical and Chemical States of Deposited Materials The effects of atmospheric deposition on primary receptors depend on the physical state (solid, liquid, or gaseous) and the chemical state (e.g., sulfur or nitrogen species) of the deposited materials (NRC 1981). The physical form of deposited material determines its availability for reaction, whereas its chemical form determines its reac- tivity. Tertiary receptors are less responsive to the physical and chemical form of atmospheric deposition than primary and secondary receptors because of dilution. The effects of acid and acidifying ions (hydrogen, sulfate, nitrate, and ammonium) are dependent in part on the accompanying rates of deposition of neutralizing cations

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19 (calcium). Hydrogen ions are harmful to the extent that the receptors cannot prevent or compensate for changes in acidity or the consequences of acidification. Many chemical compounds of both sulfur and nitrogen are naturally present in soils and are involved in chemical and biological transformations in soils and vegetation. Sulfur and nitrogen are essential nutrients required for plant growth. There are many differences in the properties and biological action of the compounds of the two elements, and there are differences in the types of transformations they undergo in the environment. Biological processes (e.g., metabolic action, decomposition) have a great influence on nitrogen transformations, while both geological processes (e.g., weathering) and microbial transformations strongly affect the sulfur cycle. There is a larger pool of endogenous nitrogen than of sulfur in organisms, and larger amounts of nitrogen than sulfur are required for plant growth. The two nutrients are closely related, so that addition of one element to an ecosystem allows greater biological utilization of the other (Turner and Lambert 1980). The optimum molar ratio of sulfur to nitrogen in terrestrial ecosystems is approximately 0.03. Nitrogen usually is efficiently metabolized in undisturbed ecosystems (Likens et al. 1977), while sulfur frequently is not retained by forest soils (Abrahamsen 1980). For aquatic ecosystems, therefore, sulfur is more important for acidification than nitrogen. Alkaline as well as acidic cations accompany the movement of sulfate from soils to aquatic systems; consequently, acidification of soil is more likely to occur from excessive sulfate deposition than from excessive nitrate deposition. The spring flush of acids into aquatic systems may, however, be closely associated with the accumulation of nitrate and sulfate in snowpack (Galloway and Dillon 1982, McLean 1981). One of the important effects of acidification is the potential mobilization of elements from soils due to increased Volubility and subsequent uptake by vegetation or movement to aquatic systems. Aluminum is present in bound form in many soils, and it can be dissolved and become available for accumulation by organisms to which it can be toxic. Dissolution of aluminum or other metals depends on the amount of water passing over a surface; solubility generally is enhanced in an acidic solution. Thus heavy rainfall exceeding surface evaporation--even with low acid content--can mobilize ions over time. This mobilization from the soil may be enhanced when acid-forming materials also are deposited from the atmosphere and washed away.

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20 However, insufficient understanding of the interactions between soils and groundwater currently precludes estimation of the rate of mobilization of metals. Major factors determining effects of atmospheric pollutants on forests are (1) the chemical nature and loading of deposited elements, (2) the ion exchange characteristics of soils, (3) the residence times and hydrological pathways of water through the watershed, (4) the nature and extent of existing vegetation, and (5) the geochemical activity of bedrock and soils (Evans et al. 1981, Zinke 1980). All forest ecosystems are not expected to respond to acid deposition in the same way. Effects are likely to be site-specific and dependent on the relative contributions of external and internal sources of acidity. Major factors determining effects of atmospheric pol- lutants on lakes and streams are (1) the total loadings of particular compounds, (2) hydrological pathways through the terrestrial systems upstream of the water body, (3) the ion-exchange characteristics of soils in terrestrial systems upstream, (4) the residence times of water in the terrestrial systems, and (5) the geochemical reactivity of the bedrock and soils of the terrestrial systems. Reversibility and Irreversibility Ecosystems are repeatedly stressed by natural disasters, extreme climatic and meteorological events, and human influences such as changes in land use and pollution. Responses may be reversible or irreversible, depending on the stress, the receptor, and the time span of interest. For example, a river may carve a new channel after a flood, an effect that may be considered irreversible except by human intervention. Over a period of hundreds of years, however, the channel may fill with silt, so even this effect can be "reversed." A lake may become turbid with sediment and organic matter after a heavy rain, an effect that usually is reversed rather rapidly by natural processes. So the consequences of extreme events often are reversed by natural processes over time; as a result, considerations of the reversibility or irreversibility of effects of acid deposition should take account of the time span of interest. The most common effect of stress on an ecosystem, such as may be caused by exposure to pollutants, is retrogression to conditions typical of an earlier stage of ecological succession. Reduction in species diversity and simplifica

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21 tion of ecosystem structure are typical responses to pollutants (Whittaker 1975). These changes often are accompanied by alterations in productivity. When exposures to pollutants are reduced or eliminated, natural processes may return systems either to their previous pathways of succession or to different successional pathways. The extent to which effects of acid deposition are reversible~depends on the receptor and the type of effect. Although there have been no clear demonstrations of effects on terrestrial systems to date, it is reasonable to believe that adverse effects on primary receptors are more readily reversed than those on tertiary receptors. For example, the yield of one annual crop might be reduced by the contact of acidic deposition with foliage or flowers, but a sub- sequent crop may be less severely affected if less acid is deposited. Damage to trees and perennial plants, particu- larly those that retain foliage for several years (most conifers), may be less easily reversed because of the long period required for regeneration and recovery of most woody plants. When both the aquatic and the terrestrial eco- systems are acidified in an area in which rates of mineral- ization and decomposition of organic matter are low, reversibility is unlikely. Acid deposition results in net accumulations of certain elements and net losses of others in ecosystems over long time scales; the identity of the elements in each category and their rates of change vary with the ecosystem and rates of deposition. The effects of slow but persistent changes may not be apparent for many generations. Signs of these changes may be observed, but the time scales for occurrence of irreversible changes are difficult to predict, because the processes that produce and consume hydrogen ions and the reactions that affect the accumulation and loss of elements are complex and poorly understood. Extensive regions of North America (Figure 1.6) and northern Europe have little geochemical acid-neutralizing capacity. Perhaps only in retrospect will we know with certainty that systems have changed, and the reversibility of these effects by natural processes might require far more time than the period initially required to cause the changes due to anthropogenic acidification. OTHER RELATED REGIONAL AIR POLLUTION PHENOMENA In addition to the atmospheric processes affecting acid deposition, there are other regional air pollution

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22 ~R4~ art\ /' ~ ~-'Q W! I' FIGURE 1.6 Regions of North America with low geochemical capacity for neutral- izing acid deposition. SOURCE: Galloway and Cowling (1978). phenomena of consequence for environmental quality that are related to acid deposition in that they are the result of similar chemical and physical processes acting on the same pollutants. One is the occurrence of elevated concentrations of ozone (03) in polluted air masses extending over several hundred to a thousand kilometers (Vukovich et al. 1977, Wolff et al. 1977). Episodes of elevated ozone occur in summer under conditions that also lead to high atmospheric concentrations of sulfate aerosol, which is eventually removed by precipitation. The events are believed to be associated with increased concentrations of precursor gases, such as nitrogen oxides and hydrocarbons, that undergo reactions to form oxidants under conditions of largerscale atmospheric stagnation.

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23 A second related phenomenon is the impairment of visibility by optically dense haze extending over large geographic areas (Trijonis and Shapland 1979). The phenomenon, which has been recognized in the eastern United States for some time, occurs during episodes of high relative humidity. Degradation of visibility has also been observed in the West (Macias et al. 1981, Trijonis 1979) and in the Arctic (Rahn and McCaffrey 1980). Although the optical characteristics of the atmosphere are linked to natural climatic factors, such as relative humidity, Husar and Patterson (1980) found an apparent association in the historical record of changes in visibility with changes in the combustion of fossil fuels. There seems little doubt that sulfate aerosols and other fine particles play a significant role in regional haze (NRC 1980). Haze in the Arctic in winter has been attributed to long-range trans- port of air masses polluted with sulfates and particulate carbon from sources in northern Europe (Rahn and McCaffrey 1980). PURPOSE OF THE STUDY 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 both domestic and international political considerations. This report deals with a small but important part of the analysis that is currently being conducted to answer the question--the scientific evidence concerning the relationships between emissions of 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 hope that our assessment will be useful to decisionmakers in government and in the private sector. The impetus for our work has been proposals for the adoption of policies to control emissions of sulfur dioxide and nitrogen oxides (beyond current limitations on emissions from new facilities) as a means of reducing acid deposition and hence alleviating reported and anticipated damage from that deposition. The operators of sources of the pollution (mostly electric utilities, industrial boilers, and motor vehicles) reasonably wish to ensure that the costs they--and their customers--would bear as a result of control policies are commensurate with any benefits to

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maintaining the status quo. porate uniform reductions (rollback) 24 be obtained. A critical link in the evaluation of benefits is the estimation of the reduction in the deposition of acids that would accompany a reduction in emissions. A recent report by the National Research Council (NRC 1981) concluded that current rates of deposition of hydrogen ions should be reduced by about 50 percent (corresponding to an increase in pa of 0.3 unit) if sensitive regions of eastern North America are to be protected from adverse effects of the deposition. If this goal were adopted, by how much would emissions have to be reduced? Conversely, by how much would deposition rates be reduced if there were specific reductions in emission rates? Our committee was organized to assess current scientific understanding about atmospheric processes that might be applied to answering these questions. Our objective was to determine what conclusions can be drawn from the state of knowledge late in 1982 about the relationships between emissions and deposition. Essential facts we faced in our work are that the subject under study is complex, the scientific evidence is evolving, and uncertainties in current understanding remain. Nevertheless, our goal required that we take account of both the theoretical understanding and observational evidence that are available today and make our best scientific judgment about their meaning. If national policy on acid deposition is to be made on the basis of the scientific information currently avail- able, that policy could take several forms, including Other policies might incor in emissions, might be designed to achieve the maximum possible environmental benefit, or might be carefully engineered to bring risks, costs, and benefits into optimal balance. The different options require scientific and technical information in different degrees of detail. In addition, they all, to one degree or another, must account for uncertainties in understanding. Decisions on almost all issues of public policy--including military affairs, the economy, and social welfare no less than environmental issues--are routinely made in light of uncertainties in knowledge. Provided uncertainties are taken into account, sufficient informa- tion is avail able for deciding what, if anything, to do about acid deposition. Recognizing that uncertainties in scientific under- standing about acid deposition currently exist and that uncertainties are likely to exist to some degree into the future, we believe that, whatever the near-term decision on

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25 acid-deposition policy, research and development should proceed with the goal of supporting more advanced and sophisticated future policies. Laboratory and field research on atmospheric processes will be extremely important in this effort. In the meantime, it seems prudent to adopt policies that are flexible and adaptable to the changing base of scientific understanding. As research continues, it can be hoped that our ability to design carefully constructed optimal strategies will continually improve. ORGANI ZATION OF THE REPORT This report describes the state of knowledge as of the end of 1982 regarding the atmospheric processes relating emissions of precursor gases and acid deposition. It does not include a detailed examination of the effects of acidic or acidifying substances on ecosystems once deposited. For such a treatment, see NRC (1981). Chapter 2 is a general review of the current theoretical understanding of the major atmospheric processes involved in acid deposition: transport and dispersion, chemical transformation, and deposition. More complete reviews are contained in the appendixes. Chapter 3 is a general review of the theoretical models currently used or proposed for assessing the relationships between sources and receptors. Chapter 4 reviews and analyzes field data on deposition in order to develop a phenomenological understanding of acid deposition in North America. Needed research is described in Chapter 5. The focus of the report is on conditions in portions of eastern North America, for which more informa- tion is available than for regions elsewhere on the continent. NOTES 1. More precisely, acidity in aqueous solutions is a function of the concentration of the hydrated hydrogen ion . For convenience, we adopt the conventional notation, referring to H3O+ as H+. In solutions, the product of the molar concentration of HE with that of the hydroxide ion (OH-) is approximately constant (about 10 14 at 25C). As acid is added to water, the concentration of H+ increases and that of OH- decreases so that the product remains (BOO ), which is also called the hYdronium ion.

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26 constant. By an excess concentration of H+, we mea n that the concentration of H+ is greater than that of OH . 2. The acidity or alkalinity of a solution is measured on the pH scale. A solution that is neutra 1 (neither acidic or alkaline) has pH 7.0. Decreasing pH indicates increasing acidity. The pH scale is logarithm) (pH equals the negative logarithm to the base 10 of the hydrogen ion concentration), so a solution of pH 4.0 is 10 times more acidic than one of pH 5.0. REFERENCES Abrahamsen, F. 1980. Acid precipitation, plant nutrients and forest growth. Pp. 58-63 in Proceedings of the International Conference on Ecological Impact of Acid Precipitation, D. Drablos and A. Tollan, eds. Oslo: SNSF Project. Norwegian Council for Scientific and Industrial Research. Altshuller, A.P. 1980. Seasonal and episodic trends in sulfate concentrations (1963-1978) in the easter n United States. Environ. Sci. Technol. 14:1337-1348. Charlson, R.J., and H. Rodhe. 1982. Factors controlling the acidity of natural rainwater, Nature 295:683-685. Cogbill, C.V., and G.E. Likens. 1974. Acid precipitation in the northeastern United States. Water Resources Res. 10:1133-1137. Cronan, C.S. 1980. Consequences of sulfuric acid inputs to a forest soil. Pp. 336-343 in Atmospheric Sulfur Deposition: Envirorunent Impact and Health Effects, D.S. Shriner, C.R. Richard, and S.E. Lindberg, eds. Ann Arbor, Mich.: Ann Arbor Science Publishers. Drablos, D., and A. Tollan, eds. 1980. Proceedings of the International Conference on Ecological Impact of Acid Precipitation, Sandefjord, Norway, March 11-14, 1980. Oslo: SNSF Project. Norwegian Council for Scientific and Industrial Research. Environment '82 Committee. 1982. Acidification Today and Tomorrow. Translated by S. Harper. Stockholm: Swedish Ministry of Agriculture. Evans, L.S., G.R. Hendry, D.J. Stensland, D.W. Johnson, and A.J. Francis. 1980. Acid precipitation: considerations for an air quality standard. Water, Air, Soil Pollut. 16:469-509. Galloway, J.N., and E.B. Cowling. 1978. The effects of precipitation on aquatic and terrestrial ecosystems: c

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27 a proposed precipitation chemistry network. J. Air Poll. Control Assoc. 28:229-235. Galloway, J.N., and P.J. Dillon. 1982. Effects of acidic deposition: the importance of nitrogen. Stockholm Conference on Acidification of the Environment. Stockholm: Swedish Ministry of Agriculture. Galloway, J.N., G.E. Likens, W.C. Keene, and J.M. Miller 1982. The composition of precipitation in remote areas of the world. J. Geophys. Res. 11:8771-8786. Hansen, D.A., and G.M. Hidy. 1982. Review of questions regarding rain acidity data. Atmos. Environ. 15:1597-1604. Husar, R.B., and D.E. Patterson. 1980. Regional scale air pollution: source and effects. Ann. N.Y. Acad. Sci. 338:399-417. Last, F.T., G.E. Likens, B. Ulrich, and L. Walloe. 1980. Acid precipitation--progress and problems. Pp. 10-12 in Proceedings of the International Conference on Ecological Impact of Acid Precipitation, D. Drablos and A. Tollan, eds. Oslo: SNSF Project. Norwegian Council for Scientific Research. Likens, G.E., F.H. Bormann, R.S. Pierce, J.S. Eaton, and N.M. Johnson. 1977. Biogeochemistry of a Forested Ecosystem. New York: Springer-Verlag. Macias, E., J. Zwicker, and W.W. White. 1981. Regional haze case studies in the southwestern U.S. II. Source contributions. Atmos. Environ. 15:1987-1999. McLean, R.A.N. 1981. The relative contribution of sulfuric and nitric acids in acid rain to the acidification of the ecosystem: implications for control strategies. J. Air Pollut. Control Assoc. 31:1184-1187. National Research Council. 1980. Controlling Airborne . Particles. Washington, D.C.: National Academy Press. National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, D.C.: National Academy Press. National Research Council of Canada. 1981. Acidification in the Canadian aquatic environment: scientific criteria for assessing the effects of acid deposition on aquatic ecosystems. NRCC No. 18475. Ottawa: National Research Council of Canada. Overrein, L.H., N.M. Seip, and A. Tollan. 1980. Acid Precipitation--Effects on Forest and Fish. Final report of the SNSF Project 1972-1980. Oslo: Norwegian Council for Scientific and Industrial Research.

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28 Rahn, R., and R. McCaffrey. 1980. On the origin and transport of the winter arctic aerosol. Ann. N.Y. Acad. Sci. 338:486-S03. Stensland, G.J., and R.G. Semonin. 1982. Another interpretation of the pH trend in the United States. Bull. Am. Meteorol. Soc. 63:1277-1284. Trijonis, J. 1979. Visibility in the Southwest--an explanation of the historical data base. Atmos. Environ. 13:833-844. Trijonis, J., and R. Shapland. 1979. Existing Visibility in the U.S. Report EPA 450/5-79-010. Research Triangle Park, N.C.: U.S. Environmental Protection Agency. Turner, J., and M.J. Lambert. 1980. Sulfur nutrition of forests. Pp. 321-333 in Atmospheric Sulfur Deposition: Environment Impact and Health Effects, D.S. Shriner, C.R. Richard, and S.E. Lindberg, eds. Ann Arbor, Mich.: Ann Arbor Science Publishers. U.S./Canada Work Group #2. 1982. Atmospheric Science and Analysis. Final Report. H.L. Ferguson and L. Machta, cochairmen. Washington, D.C.: U.S. Environmental Protection Agency. U.S./Canada Work Group #3B. 1982. Emissions, Costs and Engineering Assessment. Final Report. M.E. Rivers and K.W. Riegel, cochairmen. Washington, D.C.: U.S. Environmental Protection Agency. Ulrich, B. 1980. Production and consumption of hydrogen ions in the ecosphere. Pp. 225-282 in Effects of Acid Precipitation on Terrestrial Ecosystems, T.C. Hutchinson and M. Havas, eds. New York: Plenum Press. Vukovich, F.M., W. Bach, B. Crissman, and W. King. 1977. On the relationship between high ozone in the rural surface layer and high pressure systems. Atmos. Environ. 11:967-984. Whittaker, R.H. 1975. Communities and Ecosystems. 2nd ed. New York: Macmillan Publishing Company. Wolff, G., P.J. Lioy, G. Wright, R. Meyers, and R. Cederwall. 1977. An investigation of long range transport of ozone across the midwestern and eastern U.S. Atmos. Environ. 11:709-802. Zinke, P.J. 1980. Influence of chronic air pollution on mineral cycling in forests. Pp. 88-99 in Proceedings of the Symposium on Effects of Air Pollutants on Mediterranean and Temperate Forest Ecosystems. General Technical Report PSW-43. Pacific Southwest Forest and Range Experiment Station, U.S. Forest Service. Washington, D.C.: U.S. Department of Agriculture.