CHAPTER 7
SULFATES AND ACIDITY IN PRECIPITATION: THEIR RELATIONSHIP TO EMISSIONS AND REGIONAL TRANSPORT OF SULFUR OXIDES

(Chapter 7 was written by Ian Nisbet under the general supervision of the committee, which reviewed the work at several stages and suggested modifications which have been incorporated. While every committee member has not necessarily read and agreed to every detailed statement contained within, the committee believes that the material is of sufficient merit and relevance to be included in this report.)

INTRODUCTION

The principal route by which sulfates are returned to earth is in precipitation (Kellogg et al. 1972, Rodhe 1972, Robinson and Robbins 1968, Eriksson 1963, Bolin et al. 1971). Hence—with certain important limitations to be discussed below—the occurrence of sulfates in precipitation can provide useful information about the distribution and deposition of sulfates. Such information is needed to complement the somewhat limited data provided by direct measurements of ambient concentrations of suspended sulfates. This chapter of the report summarizes observations of sulfates in precipitation, and uses them to amplify the evidence given in Chapter 6 for regional transport of sulfur oxides and a secular increase in sulfate concentrations. This section also summarizes measurements of the acidity of precipitation, which has attracted



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Air Quality and Stationary Source Emission Control CHAPTER 7 SULFATES AND ACIDITY IN PRECIPITATION: THEIR RELATIONSHIP TO EMISSIONS AND REGIONAL TRANSPORT OF SULFUR OXIDES (Chapter 7 was written by Ian Nisbet under the general supervision of the committee, which reviewed the work at several stages and suggested modifications which have been incorporated. While every committee member has not necessarily read and agreed to every detailed statement contained within, the committee believes that the material is of sufficient merit and relevance to be included in this report.) INTRODUCTION The principal route by which sulfates are returned to earth is in precipitation (Kellogg et al. 1972, Rodhe 1972, Robinson and Robbins 1968, Eriksson 1963, Bolin et al. 1971). Hence—with certain important limitations to be discussed below—the occurrence of sulfates in precipitation can provide useful information about the distribution and deposition of sulfates. Such information is needed to complement the somewhat limited data provided by direct measurements of ambient concentrations of suspended sulfates. This chapter of the report summarizes observations of sulfates in precipitation, and uses them to amplify the evidence given in Chapter 6 for regional transport of sulfur oxides and a secular increase in sulfate concentrations. This section also summarizes measurements of the acidity of precipitation, which has attracted

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Air Quality and Stationary Source Emission Control much attention in recent years, both in North America (Likens et al. 1972, Likens and Bormann 1974, Cogbill and Likens 1974, Likens 1972) and in Europe (Bolin et al, 1971, Oden 1968, Brosset 1973).1 The acidity of rainfall is not only of importance in itself, because of its known or suspected effects on biological systems (Chapter 5), but can give indirect information—otherwise lacking-about the acidity of the suspended sulfates. THE SULFUR CYCLE AND SULFATE DEPOSITION The occurrence of sulfates in rain and snow must be interpreted in the context of the sulfur cycle in nature (Kellogg et al, 1972, Rodhe 1973, Robinson and Robbins 1968, Eriksson 1963, Bolin et al. 1971). Sulfur is introduced into the atmosphere by three principal routes: in spray from ocean waves (primarily in the form of neutral sulfates); by decomposition of biological materials (primarily in the form of hydrogen sulfide); and by combustion of fossil fuels (primarily in the form of sulfur dioxide). After transport and oxidation, it is returned to earth by four principal routes: absorption of gaseous sulfur dioxide by the soil or vegetation; deposition of sulfur dioxide in rain or snow; deposition of sulfates (including sulfuric acid) in rain or snow; dry deposition of particles containing sulfates. Studies of the sulfur cycle (Kellogg et al. 1972, Rodhe 1972, Robinson and Robbins 1968, Eriksson 1963, Bolin et al. 1971) suggest that (a) and (c)—direct absorption of sulfur dioxide and deposition of sulfates in precipitation—are the most important routes of deposition. Concentrations of sulfur dioxide in precipitation appear to be generally low in comparison with those of sulfates (Miller and de Pena 1972, Dana et al. 1973), and suspended sulfates are generally on small particles which are deposited very slowly (Garland 1974).

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Air Quality and Stationary Source Emission Control On a global basis, deposition of sulfates in precipitation is estimated to account for 40–80 percent of the sulfur deposited on land; direct absorption of sulfur dioxide accounts for much of the rest (Kellogg et al. 1972, Rodhe 1972, Robinson and Robbins 1968, Eriksson 1963, Bolin et al. 1971). According to the models of sulfur transport and deposition outlined in Chapter 6 and in Chapter 13 (Appendix A), direct absorption will be relatively important in circumstances where the oxidation rate of sulfur dioxide to sulfates is low (especially in dry, unpolluted air). Deposition of sulfates will be more important where the oxidation rate of sulfur dioxide is high, or far downwind from sources where ambient concentrations of sulfur dioxide are low. Two principal processes lead to the occurrence of sulfates, including sulfuric acid, in precipitation: (a) the absorption of sulfur dioxide into cloud droplets, with subsequent oxidation (Miller and de Pena 1972, Dana et al. 1973, Beilke and Georgii 1968, Petrenchuk and Drozdova 1966)2; (b) the uptake of suspended particulate sulfates into raindrops (Kellogg et al. 1972, Miller and de Pena 1972). Where the ambient concentration of sulfure dioxide is very high, the former process may be more important (Kellogg et al. 1972, Beilke and Georgii 1968, Petrenchuk and Drozdova 1966). However, where the ambient sulfur dioxide level is moderate or low, scavenging of suspended particulates (“washout”) is probably the dominant source of sulfates in precipitation (Likens 1972). Hence, close to major sources, the concentrations of sulfates and acidity in precipitation may be determined primarily by local oxidation of sulfur dioxide; far from major sources they are likely to reflect suspended sulfate levels. Thus, with due attention to the likely influence of local sources, regional patterns of deposition of sulfates can provide indirect information about the distribution of airborne sulfates.

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Air Quality and Stationary Source Emission Control SULFATES IN PRECIPITATION IN EASTERN NORTH AMERICA Large-scale surveys of the chemical composition of precipitation in the United States were conducted in 1955–56 (Junge 1958, Junge and Werby 1958) and in 1965–66 (Pearson and Fisher 1971, Lodge et al. 1968, Gambell and Fisher 1966). By multiplying the average concentration of sulfates in precipitation by the observed rainfall for the year, it is possible to calculate the total amount of sulfur deposited in precipitation per unit area at each station. This has been done for the 1955–56 survey by Eriksson (1960), whose map of “excess”3 sulfur deposition is reproduced here as Figure 7–1. It will be seen that the rates of sulfur deposition were relatively high throughout the northeastern United States; the highest rates [up to 13 kilograms of sulfur per hectare per year (kg/ha/yr)] were observed in western Pennsylvania and western New York. (Eriksson’s map was based on data from the United States alone: in view of subsequent observations of high rates of deposition in southeast Ontario (Shiomi and Kuntz 1974)4 it is likely that the area of maximum deposition should be extended somewhat further to the north). The data from the 1965–66 survey (Pearson and Fisher 1971, Lodge et al. 1968, Gambell and Fisher 1966), when treated in the same way, show a very similar pattern, but with consistently higher rates of deposition. At nine stations that were in operation during both surveys, the average rate of sulfur deposition was higher by 63 percent in 1965–66 than in 1955–56. Within the area outlined by the contour line for 9 kg/ha/yr in 1955–56, all stations operated in 1965–66 reported deposition rates between 13 and 20 kg/ha/yr. Similarly the contour line for 6 kg/ha/yr in 1955–56 corresponds closely to that for 10 kg/ha/yr in 1965–66. Thus there appears to have been an increase in total deposition of sulfur by 60–65 percent during the decade, with little change in the geographical pattern of distribution. In addition, the 1965–66 survey indicated especially high rates of deposition at three urban stations: Chicago (32–62 kg/ha/yr),

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Air Quality and Stationary Source Emission Control FIGURE 7–1: Excess Sulfur in Precipitation Over the U.S. in 1955–56 (After Eriksson 1960, Fig. 7.5: Original Data From Junge and Werby 1958). Units: KG Sulfur per Hectare per Year. Sulfates Associated With Sea Salt Have Been Subtracted From the Observed Sulfate Deposition Before Compiling the Map.

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Air Quality and Stationary Source Emission Control Albany (21 kg/ha/yr), and Philadelphia (19 kg/ha/yr). No comprehensive survey has been conducted since 1965–66. Data obtained at Hubbard Brook, New Hampshire (Table 7–1) and in central New York (Likens 1972) suggest a levelling-off or even a decline in sulfur deposition rates between 1965–66 and 1970–71. However, relatively high deposition rates (average 17.2 kg/ha/yr) were recorded at seven stations in southern Ontario in 1970–71 (Shiomi and Kuntz 1974, see fn. 4); deposition at the Hubbard Brook station increased again to reach its highest level in 1972–73 (Table 7–1), and relatively high deposition rates were recorded at five other widely scattered stations operated by NOAA/EPA in 1972–73.5 Thus the available data are consistent with a continued, if irregular, increase in sulfur deposition in eastern North America since 1965–66. By integrating the observed deposition rates in the areas marked on the maps in Figure 7–1, it is possible to estimate the total amount of sulfur that fell out as sulfates in precipitation in eastern North America in 1955–56. This estimate, together with corresponding estimates for 1965–66 and 1972–73, is given in Table 7–2 and compared to estimates of emissions in the same area. According to these estimates, deposition of “excess” sulfates in precipitation accounted for about one-third of the sulfur emitted into the air in eastern North America. This estimate is a minimal figure for the fraction of sulfur dioxide converted to sulfates, because some airborne sulfates must be carried eastwards over the North Atlantic Ocean. During the 17-year period under review, total deposition of sulfates in eastern North America has roughly doubled (Table 7–2). This is probably less rapid than the increase in emissions from power plants (about 200 percent), but more rapid than the increase in total emissions in the area (about 50 percent). This suggests that power plants are somewhat more important than other sources in leading to sulfate formation—perhaps because sulfur dioxide emitted from stacks is mixed better with the atmosphere than sulfur dioxide emitted from

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Air Quality and Stationary Source Emission Control TABLE 7–1. Sulfur Deposition in Rain and Snow at Hubbard Brook, New Hampshire, 1965–73 (Likens and Bormann 1974, Cogbill and Likens 1974, Likens et al. 1971, Cogbill, personal communication) Year 1965–66 '66–'67 '67–'68 '68–'69 '69–'70 '70–'71 '71–'72 '72–'73 Mean sulfate concentration in bulk precipitation (mg/l) 3.3 3.1 3.3 2.4 2.4 2.8 2.7 2.9 Total precipitation (mm) 132 142 128 130 126 123 151 186 Calculated sulfur deposition (kg/ ha/yr) 14.5 14.7 14.1 10.4 10.1 11.5 13.6 18.0

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Air Quality and Stationary Source Emission Control TABLE 7–2. Comparison of SO2 Emissions in Eastern North America1 with Total Deposition of Sulfates in Precipitation (Units: Million tons sulfur)   1955–56 1965–66 1972–73 Estimated emissions       Canada2 1.3 1.8 1.3 U.S.: Electric power plants3 2.9 5.6 8.9 U.S.: Other emissions3 3.6 3.4 3.6 Natural sources4 1.8 1.8 1.8 Total 9.6 12.6 15.6 Estimated deposition in precipitation 2.9 4.7 5.75 Deposition as % of emissions 30% 38% 37% Notes: 1. The area considered is that from the Mississippi River east to the Atlantic Coast and Nova Scotia (60E), north of central Alabama (33N) and south of 50N (Winnipeg to the Gulf of St. Lawrence). 2. Canadian emissions for 1970 are given in Rennie and Halstead (1973): these were dominated by the emissions from the Sudbury smelters, which have probably been relatively constant during the period considered (Beamish and Harvey 1972, Balsillie, personal communication). 3. U.S. emissions were estimated by interpolating from the data for 1950, 1960, and 1970 in EPA (1973), assuming that 80 of power plant emissions and 50 of other emissions were in the area considered (cf. 13–10 and Figure 6–2). 4. Natural emissions were estimated by scaling the global estimate of ref. 1 in proportion to the area under consideration (about 2 percent of the earth’s land surface) (cf. Rodhe 1972). 5. An increase of 3 percent per year is assumed for the period 1965–1972: this is consistent with Table 7–1 and other data quoted in the text, and also with 6–9b.

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Air Quality and Stationary Source Emission Control low-level sources, and therefore has a longer period in which it can be oxidized to sulfates before it can be absorbed by the ground. If emissions from tall stacks and low-level sources are weighted in the ratio 2:1, the observed increase in sulfate deposition would be roughly parallel to that in the weighted emissions. Very few data are available on sulfates in precipitation prio to the 1955 survey.6 ACIDITY OF PRECIPITATION IN EASTERN NORTH AMERICA Acid rain and snow have been reported in a number of localities in the eastern and northeastern United States (Likens et al. 1972, Likens and Bormann 1974, Cogbill and Likens 1974, Likens 1972, Johnson et al. 1972). The only large-scale synoptic survey is a student-conducted study (Anon 1974) carried out between 15–31 March 1973.7 The results of this study, reproduced here as Figure 7–2, indicated that rain below the “normal” pH of 5.58 was falling throughout most of the eastern United States; the pH level was actually below 5.0 over extensive areas, especially in the Northeast. In addition there were small areas with precipitation pH below 4 near several cities, including Los Angeles, Chicago, Evansville, Louisville, Birmingham, Philadelphia, New York, Providence, and Boston. No direct measurements of the pH of precipitation in the United States prior to 1959 appear to be available (Likens and Bormann 1974, Likens 1972). However, Granat (1972) and Cogbill and Likens (1974) have shown that is is possible to compute the pH of chemically analyzed rainwater samples with reasonable accuracy, by means of a stoichiometric balance between cations and anions. The principle of the method is shown in Figure 7–3: tests have shown that it can predict the pH of precipitation samples within ±0.1 unit (Cogbill and Likens 1974; Figure 7–2).9 Accordingly Cogbill and Likens (1974) have constructed maps of the average pH of precipitation,10 based upon the precipitation chemistry data obtained in the surveys conducted in 1955–56 (Junge 1958, Junge and Werby 1958)

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Air Quality and Stationary Source Emission Control FIGURE 7–2: Acidity of Rainfall in the United States in the Period 17–31 March 1973 (ANON 1974). The map is based on a survey conducted by 16,000 High School Students at 1,100 Stations, Using a Method Accurate to ±0.5 Units or Better. See footnote 7 for Data on Rainfall in this Period.

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Air Quality and Stationary Source Emission Control FIGURE 7–3: Chemical Composition of Rainfall at Hubbard Brook, New Hampshire (from Cogbill and Likens, in press, 1975)

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Air Quality and Stationary Source Emission Control SUMMARY AND CONCLUSIONS Deposition of sulfates in rain and snow results both from oxidation of sulfur dioxide absorbed into water droplets, and from washout of suspended sulfate particulates. With due regard for the local importance of the former process near major sources of sulfur dioxide, the occurrence of sulfates in precipitation gives information about the regional distribution of sulfates. Concentrations of sulfates in precipitation are highest in the northeastern United States: their distribution parallels that of suspended sulfate particulates. About one-third of the sulfur dioxide emitted to the atmosphere in the eastern United States is subsequently deposited as sulfates in precipitation. The rate of deposition of sulfates in precipitation has increased in the past 20 years. The increase has probably been more rapid than the increase in total emissions of sulfur dioxide; it agrees better with the increase in high level emissions (from power plants and industrial processes). Concentrations of nitrates in precipitation have increased even more rapidly than those of sulfates. The increase in the rate of deposition of nitrates is similar to the increase in high level emissions of nitrogen oxides. The acidity of precipitation in the northeastern United States has increased rapidly in recent years. The areas affected by acid precipitation now covers most of eastern North America, including parts of southeast Canada. In some small areas in the Northeast the average pH of precipitation is near or below 4.0. The distribution of acidity in precipitation is generally similar to that of dissolved sulfates. However, the fraction of the acidity attributable to nitric acid has increased and now amounts to roughly 24 percent. Acid rain in the northeastern United States is associated with air parcels that have traveled through the major emitting areas in the mid-western and east-central states one or two

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Air Quality and Stationary Source Emission Control days earlier. This supports the hypothesis of regional transport of sulfur oxides and nitrogen oxides. Closely similar phenomena have been reported from western Europe. Even if sulfur oxide emissions are held constant, a small increase in the acidity of precipitation is likely by 1980, as a consequence of increased nitrogen oxide emissions. If sulfur oxide emissions are allowed to double between 1970 and 1980, the average acidity of precipitation in the northeastern United States and in southeastern Canada is likely to increase by a factor of 2–3. The area affected may be enlarged also. Acid precipitation is partly neutralized when it falls on vegetation and further neutralized in soil. The process of neutralization removes cations, including important nutrients. The processes of neutralization in foliage and soil are usually incomplete, so that run-off water is often significantly acidic. If the acidity of precipitation is maintained or increased, soils and surface waters are likely to become progressively acidified. The rate of acidification depends on the buffering capacity of the soil or water and is difficult to predict in individual cases. FOOTNOTES 1   A cooperative study is in progress under the auspices of the OECD, and some preliminary results are now available (Garland 1974, Nord 1974). 2   The mechanisms of absorption and oxidation of sulfur dioxide in water droplets are complex and cannot be reviewed fully here (see Miller and de Pena 1972, Dana et al. 1973 for discussion). Uptake of sulfur dioxide is strongly limited by the acidity of the raindrops (Dana et al. 1973). The rate of uptake and oxidation is increased in the presence of ammonia (Scott and Hobbs 1967) or other catalysts. Observations of

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Air Quality and Stationary Source Emission Control     precipitation around point sources of sulfur dioxide indicate marked variations in the rate of oxidation. In a plume from a heating plant at Uppsala, Sweden, much of the sulfur dioxide emitted during precipitation was deposited as sulfate within 50–100 km (i.e., within 2–3 hours) (Hogstrom 1973). Studies around a sulfur extraction gas plant in Alberta Canada, showed similarly rapid oxidation of sulfur dioxide during rain (32–46 percent deposited as sulfate within 40 km), but much slower oxidation in snow (Summers and Hitchon 1973, Summers 1974). However, studies around a smelter at Sudbury, Ontario, showed very slow oxidation of sulfur dioxide (less than 1 percent deposited as sulfate within 50–100 km) even when emitted during rain (Muller and Kramer 1974, Wiebe and Whelpdale 1974). The low rate of oxidation in the Sudbury area may be related to the peculiar chemistry of the plume, which has high concentrations of iron, nickel, and copper, but low concentrations of vanadium and manganese which are effective catalysts for oxidation of sulfur dioxide to sulfates (A. Wiebe: personal communication). 3   Dissolved sulfates in precipitation include a neutral fraction derived from ocean spray: this can be estimated from concentrations of sodium and chloride and is customarily subtracted out to derive an “excess” sulfate concentration (Junge and Werby 1958, Eriksson 1960). For details see notes to Figure 7–3. 4   The deposition rates recorded in Shiomi and Kuntz (1974) were probably slightly low because the collectors did not sample snow efficiently. An earlier study of precipitation in southeastern Ontario in 1965–66 (Rutherford, Can. J. Earth Sci. 4:1151–1160, 1967) had shown anomalously high rates of sulfur deposition (average 39 kg/ha/yr), but the presence of silicates and aluminum in these samples suggests the possibility of contamination by wind-blown dust. For discussion of other technical problems see Likens (1972) and Pearson and

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Air Quality and Stationary Source Emission Control     Fisher (1971). In particular, most of the studies cited in this section were of bulk precipitation (which includes dry fallout), whereas those reported in Junge and Werby (1958) and Lodge et al. (1968) were designed to exclude dry fallout. However, there is indirect evidence that dry fallout of sulfates is only a small component of deposition (Gambell and Fisher 1966). Where the surveys of Lodge et al. (1968) and Gambell and Fisher (1966) overlap there is no discernible discrepancy in calculated deposition rates. 5   Sulfate concentrations supplied by J.Miller per C.v.Cogbill; precipitation data from NOAA Environmental Data Service. 6   At two stations in central New York, sulfate levels in precipitation were much higher between 1915 and 1950 than after 1952 (Likens and Bormann 1974, Likens 1972). However, both stations were close to sources burning fossil fuels; the occurrence of higher concentrations in winter is consistent with a local origin (Likens 1972). 7   The period 15–31 March, 1973 was unusually wet in much of the eastern United States. The total rainfall during this 17-day period at selected stations was as follows: Boston 1.08"; New York City 2.17"; Philadelphia 2.49"; Chicago 2.78"; Louisville 3.88"; Birmingham 6.84" (from Daily Weather Maps, NOAA Environmental Data Service). Hence, except perhaps for Boston, the high acidity recorded near these stations cannot be explained by low dilution factors. 8   pH is the symbol for the negative logarithm of the concentration of hydrogen ion [H+ solution in grams per liter. The more acid a solution, the lower the pH. Doubling the [H+] concentration reduces the pH by 0.301 units. Unpolluted rain water is slightly acid because it contains dissolved carbon dioxide, which dissociates to form [H+] and bicarbonate ions. If in equilibrium with atmospheric carbon dioxide, rain water would have a pH of 5.5–5.7. As strong acids are added, carbon dioxide is displaced and the pH falls below 5.5

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Air Quality and Stationary Source Emission Control 9   Several reviewers have questioned the use of bulk precipitation samples or monthly samples for determining pH, because of the possibility of chemical changes prior to analysis. However, pH estimates obtained by the methods of Cogbill and Likens (1974) appear to agree well with those obtained from weekly or monthly samples (Cogbill and Likens 1974, Likens 1972). In this survey both single-storm and monthly samples are used indiscriminately, except where there was evidence of contamination by airborne dust or insects (for discussion see Likens 1972). 10   “Average” pH’s quoted in this section are derived from weighted averages of hydrogen ion concentrations throughout a year. They thus represent the pH that corresponds to the average chemical composition of precipitation. 11   Unpublished data from the National Precipitation Network, supplied by R.A.McCormick (EPA). 12   Unpublished data of Canada Center for Inland Waters: personal communication from F.C. Elder. 13   Unpublished data of Massachusetts Division of Fisheries and Game. 14   Published measurements of bicarbonates in rainwater in the eastern United States prior to 1932 indicate that the pH of precipitation was uniformly above 5.7 at that period (Likens and Bormann 1974, Likens 1972). 15   A similar conclusion holds a fortiori for nitrates, because the stoichiometric analysis summarized in Figure 7–3 assigns hydrogen ions to sulfates and nitrates in proportion to their ratio in the precipitation sample. 16   This hypothesis does not necessarily conflict with that of Trijonis (Chapter 6), that oxidation rates of sulfur dioxide to sulfates are limited by the acidity of particles or water droplets when neutralizing materials are depleted. This mechanism would limit the local rate of oxidation in places where ambient levels are

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Air Quality and Stationary Source Emission Control     high. It would not prevent oxidation of the sulfur dioxide after further dispersal and dilution. 17   The figures in Table 7–3 can be used to estimate the total amount of nitrates deposited in precipitation in eastern North America. For 1972–73 this turns out to be about 7 million tons expressed as nitrate, equivalent to 3.5 million tons NO. Although this estimate is indirect and subject to some error, it is clearly much smaller than the total NOx emissions in the area (about 17 million tons) or even those derived from stationary fuel combustion (9 million tons). Evidently deposition in precipitation is a substantial, but not the major sink for nitrogen oxides emitted into the atmosphere. 18   Use of the high projection is based on the consensus of opinion in the Committee that a massive increase in nuclear generation of electric power is unlikely to take place by 1980. However, even the low projection involves an increase in NOx emissions about three-quarters as large as that considered here.

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Air Quality and Stationary Source Emission Control LITERATURE CITED Almer, G., W.Dickson, C.Ekstrom, and E.Hornstrom (1974) Effects of acidification on Swedish Lakes. Ambio 3:30–36. Anon (1974) Acid rain in the United States. Current Science 59(14):1–5. Reprinted in Scientific American 230:122–127. Barrett, E. and G.Brodin (1955) The acidity of Scandinavian precipitation. Tellus 7:251–257. Beamish, R.J. and H.H.Harvey (1972) Acidification of the La Cloche Mountain Lakes, Ontario, and the resulting fish mortalities. J. Fish. Res. Bd. Canada 29:1131–1143. Beilke, S. and H.W.Georgii (1968) Investigations on the incorporation of sulfur dioxide into fog- and rain-droplets. Tellus 20:435–442. Bolin, B. et al. (1971) Air pollution across national boundaries. The impact on the environment of sulfur in air and precipitation. Sweden’s case study for the United Nations conference on the human environment. Stockholm, Royal Ministry of Foreign Affairs and Royal Ministry of Agriculture. Brosset, C. (1973) Air-borne acid. Ambio 2:1–9. Cogbill, C.V. (19) personal communication. Cogbill, C.V. and G.E.Likens (1974) Acid precipitation in the northeastern United States. Water Resources Research: in press. Dana, M.T., J.M.Hales, W.G.N.Slinn, and M.A.Wolf (1973) Natural precipitation washout of sulfur compounds from plumes. U.S. Environmental Protection Agency: Ecological Research Series, Report EPA-R3–73–047. Eaton, J.S., G.E.Likens, and F.H.Bormann (1973) Throughfall and stemflow chemistry in a northern hardwood forest. J. Ecol. 61:495–508. Eriksson, E. (1960) The yearly circulation of chloride and sulfur in nature; meteorological, geochemical and pedological implications. Part II. Tellus 12:63–109. Eriksson, E. (1963) The yearly circulation of sulfur in nature. J. Geophys. Res. 68:4001–4008. Frohliger, J. (19) personal communication.

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Air Quality and Stationary Source Emission Control Gambell, A.W. and D.W.Fisher (1966) Chemical composition of rainfall in eastern North Carolina and southeastern Virginia. Geol. Survey Water Supply Paper 1535-K, 41 pp. Garland, J.A. (1974) Progress Report. U.K. Proposal No. 2: sorption of sulfur dioxide at land surfaces. MS. Gordon, C.C. (1972) Mount Storm study. Report to Environmental Protection Agency under contract no. 68–02–0229. Granat, L. (1972) On the relation between pH and the chemical composition in atmospheric precipitation. Tellus 24:550–560. Hagen, A. and A.Langeland (1973) Polluted snow in Southern Norway and the effect of the meltwater on freshwater and aquatic organisms. Environmental Pollution 5:45–57. Herman, F.A and E.Gorham (1957) Total mineral material, acidity, sulfur, and nitrogen in rain and snow at Kentville, Nova Scotia. Tellus 9:180–183. Hogstrom, U. (1973) Residence time of sulfurous air pollutants from a local source during precipitation. Ambio 2:37–41. Johnson, N.M., R.C.Reynolds, and G.E.Likens (1972) Atmospheric sulfur; its effect on the chemical weathering of New England. Science 177:514–516. Junge, C.E. (1958) The distribution of ammonia and nitrate in rainwater over the United States. Amer. Geophys. Union Trans. 39:241–248. Junge, C.E. and R.T.Werby (1958) The concentration of chloride, sodium, potassium, calcium, and sulfate in rainwater over the United States. J. Meteor. 15:417–425. Kellogg, W.W., R.D.Cadle, E.R.Allen, A.L.Lazrus, and E.A.Martell (1972) The sulfur cycle. Science 175:587–596. Likens, G.E. (19) personal communication. Likens, G.E. (1972) The chemistry of precipitation in the central Finger Lakes region. Cornell Univ. Water Resources and Marine Science Center, Tech Rep. 50.

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Air Quality and Stationary Source Emission Control Likens, G.E. and F.H.Bormann (1974) Acid rain: a serious regional environmental problem. Science 184:1176–1179. Likens, G.E., F.H.Bormann and N.M.Johnson (1972) Acid rain. Environment 14:33–40. Likens, G.E., F.H.Bormann, R.S.Pierce, and D.W.Fisher (1971) Nutrient-hydrologic cycle interaction in small forested watershed ecosystems. In Productivity of Forest Ecosystems (P.Duvigneaud, Ed.), pp. 553–563. Proc. Brussels Symposium, 1969. UNESCO, Paris. Lodge, J.P., Jr., K.C.Hill, J.B.Pate, E.Lorange, W.Basbergille, A.L.Lazrus, and G.E.Swanson (1968) Chemistry of United States precipitation. National Center for Atmospheric Research, Boulder, 66 pp. Malmer, N. (1973) On the effects of water, soil, and vegetation from an increasing atmospheric supply of sulfur. Statens Naturvardsverk PM 402:1–8. Miller, J.M. and R.G.de Pena (1972) Contribution of scavenged sulfur dioxide to the sulfate content of rain water. J. Geophys. Res. 77:5905–5916. Muller, E.F. and J.R.Kramer (1974) Precipitation scavenging in central and northern Ontario. Paper presented at Precipitation Scavenging Symposium, USAEC Symposium Series, Champaign, Illinois. Munn, R.E. and H.Rodhe (1971) On the meteorological interpretation of monthly precipitation samples. Tellus 23:1–13. Nord, J. (1974) Sulphur pollution arising from distant emission sources. MS. Oden, S. (1968) The acidification of air and precipitation and its consequences on the natural environment. Natural Research Council of Sweden, Ecology Committee, Bull. 1, 86 pp. Translated by Translation Consultants, Ltd., Arlington, Va. (TR-1172). Overrein, L.N. (1972) Sulphur pollution patterns observed; leaching of calcium in forest soil determined. Ambio 1:145–147. Pearson, F.J., Jr. and D.W.Fisher (1971) Chemical composition of atmospheric precipitation in the northeastern United States. Geol. Survey Water Supply Paper 1535-P. 23 pp.

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