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AIR POLLUTANT DISTRIBUTION AND TRENDS Rudolf B. Husar Center for Air Pollution Impact and Trend Analysis (CAPITA), Washington University, Box 1124, St. Louis, MO 63130 ABSTRACT Forest health is influenced by the soil, physical climate, and its chemical climate. Historically, among the environmental factors, the role of chemical climate has received the least attention. This paper constitutes an annotated chemical atlas of the atmosphere over North America and to a lesser extent Europe. It covers the emissions for sulfur and nitrogen oxides, wet deposition of sulfate, nitrate, and ammonia, airborne concentrations of ozone and sulfate, and the spatial temporal pattern of atmospheric haziness. INTRODUCTION Forest growth is determined by the environmental conditions that include soil and soil and physical climate several centuries. Much chemical specific physical and chemical climate of the atmosphere. The role of has been dealt with extensively in the literature for the past less is known about the long-term effect of atmospheric chemicals, i.e., climate of the forests. Recognizing the severe uncertainties as to what atmospheric chemicals may be responsible for potential forest damage, it seems beneficial to assemble an annotated atlas of atmospheric chemicals that may be relevant to forests. An examination of such a chemical-climatic atlas and trends in conjunction with physical climate and observed forest change may reveal the chemical that could be significant to the damage. MAN-MADE SOX AND NOX EMISSION PATTERN AND TRENDS The North American emission densities for SO and NOX for 1977-78 are displayed in Fig. 1. The contour lines have units of grams of sulfur or nitrogen per square meter per year. Currently, the sulfur emissions in the subcontinent are estimated to range between 1 1 and 15 million tons/year (22-30 million tons SO2/year). It is evident that the highest sulfur emission density occurs over the Ohio River region. The highest NO emission density is over the states north of the Ohio and east of the Mississippi rivers. There are substantial differences in the emission trends of smaller regions within eastern North America. Comparisons of the sulfur emissions north and south of Ohio River are given in Fig. 2, expressed as emission per unit area (g S/m2/yr). The emissions north of the Ohio River have increased about 25% since the 1 920s. In contrast, the emissions south of the Ohio River show a threefold increase since the 1 930s. Currently, the sulfur emissions density is comparable for the regions north and south of the Ohio River. 29

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30 SULFUR EM I SS I ON F I ELD 'it ~ 977-78 ~0 1-~ `1~: ', . _ . . , ~ ~ G. /M~ ~ ~ . ~ ~ J NRH 0. 59 J.73 ENa EUS SURE U. 57 3.39 NOX-N EM I SS I ON F I ELD 'a ~ 1 977-78 1~ OOO —7.e El,.~-~.e 3.0-6.0. >6.0 19 ~ j ~ ~~: END EUS 2.27 SURE 3.05 O0.25-0.s0 ~0,50-1 .50 ~3 1 ~ 50-3 00 - ~ >3.00 US - Fig. la' lb. Emission densities, g/m2/yr for the year 1977-78 of man-made SOx and NOx over North America (Husar and Holloway, 1983).

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31 5 - . 3 — >` C. ,.) e. a o ~ 2 — - - 1 Izr or >~ o it, , l8BO 1900 ~ 1~_ l92O 1~0 North Bomb 1960 1~0 ~000 Fig. 2. Sulfur emission density, g/m2/yr for regions north and south of the Ohio River (Husar, 1985~. 12 ~ ~ ~ _ 10 - 9 — L 8- Z 7 - C B - ~ 5 - - ~ ~ _ 3 - 2 — O ~ 1 i l:) _~ ~~ 1 ~- 1 880 1 900 ........................ , 1 920 1 940 of OMo Riv. 1 960 1 980 2000 Fig. 3. NOx emission trend for regions north and south of the Ohio River (Husar, 1985~.

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32 The overwhelming fraction of NOX emissions arises from the combustion of fossil fuels (coal, oil, and gas). Fuel consumption data constitute the most important input for the estimation of NOX emissions. However, the estimation of the current and historical emission trends is more difficult than for sulfur compounds. Since most of the nitrogen oxides are formed by the fixation of air nitrogen at high temperatures rather than oxidation of fuel nitrogen. Thus, NO emission depends primarily on the combustion process and to a lesser degree on the duel properties. The fuel combustion processes in internal combustion engines and boilers have changed since the turn of the century. It is thus likely that the NO emission per fuel consumption has also changed historically. Hence, the following ~Ox emission trends have substantially higher uncertainty than those for sulfur oxides. The NOX emissions since 1940 (Fig. 3) have been estimated to have increased by threefold by the 1 970s. The sharp rise in the 1 960s is attributed to the rise of the emissions from industrial and electric utility sources. Evidently, the NOX increase was more pronouncer! north of the Ohio River compared to the south. WET DEPOSITION DATA FOR SULFATE, NITRATE, AND AMMONIA Motivated by increasing concern about "acid rain," at least five major precipitation chemistry networks have operated over North America since 1978. In Europe, most of the monitoring is conducted in the framework of the Economic Community of Europe (ECE), Co-operative Programme for Monitoring and Evaluation of the Long Range Transport (EMEP) network. Sulfate The yearly average sulfur deposition and the precipitation- weighted concentration fields arising from the five network data sets for the time period 1977-1980 are given in Fig. 4. Some of the sulfur in precipitation is due to sea salt and its contribution may be estimated either from Na or C1 data. Taking the sea salt sulfate to be 0.047 x C1, the marine sulfate deposition over the continent was calculated to be only about 6% of the sulfur wet deposition over North America. For the sites more than 100 km from the coastline the marine sulfur deposition was insignificant. In the following discussion S refers to the excess beyond the sea salt sulfur. For the North American continent (NAM), excluding Mexico, the average sulfur wet deposition is 0.34 g S/m2/yr. The area considered is 18 x 1012 m2 and the total wet S deposition is thus 6.2 Tg S/yr. The highest sulfate deposition rate and concentration in precipitation occurs in the region surrounding the eastern Great Lakes. The sulfate wet deposition there exceeds 1 g S/m2/yr. For the region east of the Mississippi River and south of the James Bay (defining here eastern North America ENA area 5.8 x 1012 ma) the sulfate wet deposition is about 0.63 g S/m2/yr. The lowest sulfate deposition rate of about 0.1 g/m2/yr is measured in northwestern Canada and southwestern U.S. Both of these regions have less than 0.5 m/yr of rainfall. The weighted average sulfate concentration in precipitation (Fig. 4) ranges between 15 ,ueq/1 in remote U.S. and Canadian regions to about 70 peq/1 in the vicinity of the Great Lakes. Hence, while the average deposition - rate varies tenfold over the continent, the average precipitation sulfate concentration increases only fivefold from remote regions to industrial hot spots. There are few areas of North America for which it can be safely assumed that the sulfate deposition values represent the "natural background." Considering the wind pattern and the anthropogenic S emission fields over the continent, a possible area uninfluenced by man may be northwestern Canada, inland from the Pacific Coast. The

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b SOq-S DEPOSITION ~~ J~N77-OEC80 nor ~ Ad; f4:> J:> .. :. : A:::: : ::: ::. ~ ~ - A; G/M2. YR / ENR 0.6S EUS 0, 73 ~ - \ ~ SURE 0.82 33 5 On -5 ~ONCENTRRT I ON 'A ~ JRN77-OEC80 K~1~2 ........ .... ,, , _% ~~, ~ 22' `~9''22 -I Elos-oso ~ ~ ~25-50 > 1 . 00 ~ / END 41.26 EU5 46.73 SURE S2. AS GO. 10-0.25 \ ~ NOR ZB. B~ Fig. 4. Maps of sulfate wet deposition rate, (g/m2/yr) and precipitation- weighted average concentration (peq/yr) over North America (Husar and Holloway, 1983~. so-7s ins

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34 measured sulfur wet deposition rate in that region is 0.1 g S/m2/yr, compared to the North American average of 0.34 g S/m2/yr. In estimating the natural global sulfur cycle from rain chemistry data, Granat et al. (1976) have chosen 0.15 g S/m2/yr as the representative background deposition over nondesert land areas. Considering the severe uncertainties of the deposition estimates, 0.07 g S/m2/yr would also seem reasonable as a North American average background. Scaled to the nondesert land area of the world (120 x 1012 m2), the global S wet deposition from the above "background" sources would be 9-18 Tg S/yr, which is below the estimate of 18 Tg S/yr by Granat et al. (1976~. An increasing data base of precipitation sulfur values obtained in the southern hemisphere, e.g., Stallard and Edmond (1981), from the Amazon region, and data from the polar ice caps dictates that continental average background deposition outside the 0.07 to 0.15 g S/m2/yr range is unlikely. A documentation of this statement is best available in Granat et al. (1976), but will require continuous reevaluation as the global data base expands. The lower (0.07 g S/m2/yr; 1.3 Tg/yr) and the upper bound (0.15 g S/m2/yr; 2.5 Tg/yr) of the estimated "natural background" sulfur deposition constitutes 20 to 40% of the measured total sulfur deposition (0.34 g S/m2/yr; 6.2 Tg/yr) over North America. For eastern North America the average excess sulfur deposition rate is about 0.63 g S/m2/yr (3.7 Tg/yr) and the "background" (0.5-1.0 Tg S/yr) deposition would account for 12-25% of the total ENA wet deposition. This is a substantial upward revision of previous estimates of "natural contribution" for North America (Galloway and Whelpdale 1980~. It is also instructive to compare the measured wet deposition pattern and rates to the emission field of man-made sulfur over North America (Fig. 1~. The average emission density over eastern North America is 1.9 g S/m2/yr while the average wet deposition over the same region is 0.63 g S/m2/yr, i.e., about 30% of the known man-made emissions. If we further assume that the natural sources contribute on the average 0.07 g/m2/yr, the measured wet deposition of sulfur amounts to only 25-30% of the man-made sulfur. The remaining 70-75% of the man-made sulfur is then either dry deposited as S02 or S04, or exported to the Atlantic by the prevailing winds. The sulfate concentration in precipitation over Europe (Fig. 5) shows the highest yearly average values over eastern Europe (GDR, Poland, Hungary, Romania). In those areas it exceeds 100 peq/1 which is more than double the highest concentrations over eastern North America. Nitrate The average deposition and concentration pattern of nitrate over NAM is given in Fig. 6. The average NAM wet deposition rate is 0.13 g N/m2/yr corresponding to 2.4 Tg/yr. The highest deposition rates (0.4g N/m2/yr) are observed in the area surrounding the eastern Great Lakes. The average over ENA is 0.23 g Nlm2/yr. Using again the deposition data for remote western Canada (0.03-0.06 g N/m /yr), approximately an order of magnitude increase may be observed from remote to high emission regions. Similarly as for the sulfate, the nitrate concentration in rain has only a fivefold increase from remote (3-6 /leq/l) to industrialized regions (25 peq/1~. If we take the "background" nitrate wet deposition rate to range between 0.03-0.06 g N/m2/yr over the entire continent, the "background" wet removal will contribute 0.5-1.0 Tg N/yr. As for sulfate wet deposition, the "background" nitrate deposition would account for 20-40% of the measured nitrate deposition in precipitation. Over eastern North America, the corresponding background contribution would be 10-25% of the total measured wet deposition. Taking 0.03-0.06 g N/m2/yr as representative over the nondesert land areas ( 1.2 x 1014 m2) of the world, the global nitrate wet deposition would range between 3 and 7 Tg N/yr. This is an order of magnitude less than the total

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35 S04 in PRECIPITATION El''EP Network nuerage I, i ~_~ NOS in PRECIPITATION El'1EP Network Overage K:N _s _ 100 ~ ~ Fig. 5. Sulfate and nitrate concentration, (,ueq/l) in Europe (EMEP network average).

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36 (dry and wet removed, natural and man-made) nitrate deposition (32-83 Tg N/yr) estimated by Soderland and Svennson (1976~. Using the average measured NAM nitrate wet deposition of 0.13 g N/m2/yr and applying it to the global nondesert land area, yields 16 Tg N/yr which is still a factor of two to five less than the nitrate deposition (dry and wet) estimated by Soderland and Svennson (1976~. An emission map (Fig. lb) for man-made NOX over eastern North America shows that the highest emission density is in excess of 3 g N/m2/yr in the Great Lakes region and it roughly coincides with the location of the highest deposition density (Fig. 6~. The comparison with the deposition data also reveals that wet deposition accounts for only 20% of the known man-made NOX emissions over eastern North America. The remaining 80% is thus either dry deposited or exported from the continent. The nitrate concentration in precipitation over Europe (Fig. 5) shows the highest values over western Europe including northern Italy and France. In those areas it exceeds 80 peq/1 which is more than double the highest nitrate concentrations over eastern North America. Ammonia The yearly average ammonium ion deposition field is shown in Fig. 7. The North American average deposition is 0.11 g N/m2/yr (2.0 Tg N/yr) and for eastern North America 0.16 g N/m2/yr ( 1.0 Tg N/yr). The highest deposition rates, ranging 0.25-0.50 g N/m2/yr, occur from the Great Lakes to the Rocky Mountains, an area generally known as the corn belt region. West from the Rockies and in northern Canada the deposition rate is below 0.1 g N/m2/yr. Conspicuously, the industrialized northeastern part of the U.S. does not show high ammonia deposition values. Here again, it is useful to compare the measured NAM ammonium wet deposition rates (2 Tg N/yr), scaled to the globe (13 Tg N/yr), to the global dry and wet ammonium/ammonia deposition rates (91-186 Tg N/yr), estimated by Soderland and Svennson (1976~. Possible sources of ammonia. Unlike SOX and NOX, an emission inventory of man-made or "known" ammonia sources over North America currently does not exist. As an aid to interpret the measured ammonia wet deposition field, we have constructed a tentative inventory for the U.S. The emission factors for domestic animals were taken from Bottger et al. ( 1978~. For nitrogen fertilizer it was assumed that 10% N is volatilized to the atmosphere. Minor contributions from known industrial sources were also included. The resulting ammonia emission density map is shown in Fig. 8. The total U.S. ammonia emission from the above "known" sources is estimated at 3.4 Tg N/yr with the highest emission density in the corn belt region exceeding 1 g N/m2/yr. There is a rough coincidence of the area of high measured deposition rate and estimated emission density, both extending through the corn belt region. The estimated U.S. ammonia emission rate from "known" U.S. sources is 3.4 Tg N/yr, which is comparable to the wet deposition integral of 2 Tg N/m2/yr for NAM. However, considering the severe uncertainties of the "known" source estimates, little significance is attached to the emission values beyond suggesting that the inventory is within the right order of magnitude. The ammonia concentration in precipitation over Europe (Fig. 9) shows the highest yearly average values over central and eastern Europe stretching from Great Britain through Poland and Romania. In many areas the concentration exceeds 100 peq/1 which is more than three times the highest concentrations over eastern North America. In spite of the uncertainties associated with the spatial-temporal coverage, sampling, and analytical procedures, and the interpretation of the wet deposition data, it is most gratifying that such continental-scale data bases currently exists for North

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37 NO3 -N DEPOS I T I ON >~ JON77-DEC80 ~ :~0R~ ,~ 1 1 - c ~ .\ , ~ U ..N a, ~ . . . . . . . . . . . . . +. ~ ~ ~ G. / M , T ~ ~ ~NRM J-13 - ~ 30. 25—0.50 / END O.22 ~ ~ >0.50 EN 0.27 . SURE 0; 30 ~ ~'~'~?\ ~ NO3 -N CONCENTRRT I ON ,,~ JON 7 7 -OK C so ............ __ 'if - < / END l7.l. EU5 20.16 SURE 22. 24 uEO/L 0 5-10 E} 10-25 25-50 >50 Fig. 6. Maps of nitrate wet deposition rate (g/m2/yr) and precipitation- weighted average concentration (peq/l) (Husar and Holloway, 1983~.

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38 NH~ -N DEPOS I T I ON If. Oo.os-o. 10 Elo. 10-0.2s ~0.2s-o.so (~ ~ ENa o le ~ · >O.50 ~ ~ SU" 0.21 NH~ -N CONCENTRRT I ON \\ ~ ~ ~ - ·~] 4-~- :~ <~ V-~ uEO/L 51 ~ ~ ~ o s-~o - 25-50 £ - 12 ~ ~ ~ ,50 SV" 16-0. Fig. 7. Maps of ammonia wet deposition rate, (g/m2/yr) and precipitation- weighted average concentration, (peq/l) (Husar and Holloway, 1983). NH3 -N EM I SS I ON ,\N ~_ CU, o. ~S '.\ ~ stmc o. S6 . Oo. 10-0.25 E~ o. 25-0. so E3 0. SO- 1. 00 ,=', · ' 1 . 00 Fig. 8. Estimated emission density of ammonia emissions for the U.S. (Husar and Holloway, 1983~.

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39 HYDROGEN ION in PRECIPITATION EMEP Network average . 100 peq NH4 in PRECIPITATION EMEP Network peerage it, i ~W Fig. 9. Ammonia and hydrogen ion concentration, (peq/l) in Europe (EMEP network average).

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40 America and Europe. Prudent use of such data bases will undoubtedly provide us with a much improved perspective on both scientific and other aspects of the "acid rain" problem. AIRBORNE OZONE CONCENTRATION It is well established that high airborne ozone concentrations can damage forests. Regrettably, national climatic maps for ozone are not available. One of the most difficult problems pertains to the calculation of "representative" concentrations. The concentration at a given location has a diurnal, synoptic scale (3 to 5 days), seasonal and long-term trends that result from the interacting NOX, hydrocarbon sources physical and chemical removal processes, and atmospheric transport. Nevertheless, attempts have been made to compile the ozone distribution pattern. Vukovich et al. ( 1985) have explored the daytime ozone concentrations for nonurban locations over the eastern U.S. for August 1978- 1981 as displayed in Fig. 10. The daytime ozone concentrations exceeding 60 ppb cover the area of the Ohio River valley and the mid-Atlantic states, New Jersey-North Carolina. It is worth noting, however, that the concentrations are about 40 ppb in the upper Midwest. Hence, the regional variability of the summer daytime ozone is only about a factor of two. It is likely that the spatial gradients in the winter time are more pronounced. Singh et al. (1978) have proposed a semi-quantitative picture of the seasonal variation of the ozone in the lower layers of the atmosphere (Fig. 1 1~. The natural ozone pattern is indicated by the shaded area marked A. Superimposed on this natural background is a man-induced perturbation marked with B and C. At remote sites the natural ozone concentration reaches the maximum in the early spring. In areas influenced by man-made sources, a summer peak may arise. Here's again, we emphasize that on the average the man-made ozone is a mere perturbation over a substantial natural background. VISIBILITY Atmospheric optical data are much more abundant on a continental scale than chemical composition data, with the exception of the water vapor content. Hence, if a reasonably well defined relationship can be established between, for example, the visual range or turbidity and the fine particle content of the atmosphere, then the extensive meteorological observations by human observers inherent in the interpretation of the visibility data obtained by routine meteorological observation networks. These include the subjectivity of the human observer, the lack of suitable visual targets, and the numerous natural phenomena that perturb the visual environment (rain, fog, blowing dust, natural haze, etc.~. The spatial and trend analysis of the visibility data, therefore, needs to be conducted with utmost caution. The data presented below arise from the analysis of 147 U.S. and 177 Canadian stations from 1948 to 1980. human observer, the lack The quarterly average extinction coefficient (3.9/visual range, km) from noon observations in the absence of precipitation and fog is shown in Fig. 12. At midlatitudes (40-60° N) there are three hazy regions: surrounding the Great Lakes, the Mississippi Delta and southern California including the San Joaquin Valley. A conspicuously hazy region also exists north of the arctic circle, the cause of which is unknown and will be ignored here. The lowest mean extinction coefficient (<0.1 ~ /km) occurs in the U.S. Rocky Mountain Region. The long-term availability of visibility data also permits the examination of secular trends of continental haze from about 1950 to 1980. The five-year average map for 1950- 1954 shows substantially lower haziness than the 1976- 1980 period. The important changes in the three decades occurred in the southeastern U.S. (mostly in the summer months) as well as over the northwestern U.S. and adjacent southwestern Canada (mostly in the winter months).

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41 ~~- e: Fig. 10. Mean diurnal maximum ozone concentration isopleths for August 1978- 1981 (Vukovich et al., 1985~. Ion . . too _ 80 20 I T I I -I --it DIRECT 03 TRANSPORT f ROM URBAN CENTERS LOCAL OZONE \ SYNTHESIS ~ NO_ I NTRUSIC)N, ~':',',',W'' · ~ YOU JAN '4AR WAY JUL SEPT NOW Fig. 11. Idealized ozone variations at remote locations (Singh et al., 1978).

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to ~ - - ~ - 1 a, - 42 Levi ' :11:~ 'a , Cot c, a C) 1 C) 0 CL~ V] . U. lo_ ~ 0 - - } 4 00 ED O ° I.4 I O ~ O Q ~ ~ Do C CO C) ~ ~ Ct-—X Cot _ ~ C~ ~ ~ ~ O Ce _ C: en. - o _ 4 - c: C:.—0 4_ Cd ~ ~ Q Ce (1) ·— Ct ~ ~ ~u Q Ed ct o Go

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43 A more detailed trend pattern for eastern U.S. haze is shown in Fig. 13. It is apparent that in some areas, such as in New England, the haze decreased for the first quarter of the year. Other regions, such as surrounding the Smoky Mountains, exhibit a strong increase, particularly during the third quarter (July, August, September). In the New England and New York area the SOX emissions have declined since the 1 950s primarily because of the shift from coal to oil. The Smoky Mountain region on the other hand, experienced a substantial increase of SOX emission nrimarilv rl~,`` tn in~r-~cin~ electric utility coal consumption (see Emission section above). - _ ~_^ ~e, Beyond demonstrating that substantial changes have occurred in the optical environment, the visibility data demonstrate that within the eastern U.S. some subregions may exhibit a decline, others an increase of haziness. This implies that in spite of Long range transport" subregions covering several U.S. states will exhibit trends in fine particle concentration consistent with their own emission trends. In other words, the long-term haze data base confirms the most revealing conclusion of the European OECD project, namely that-every source region impacts on itself more than on any other region. _ ACKNOWLEDGMENTS This research was partially supported by the U.S. Environmental Protection Agency cooperative agreement #CR 810351-02, the National Academy of Sciences, and Washington University. Special thanks to Dr. Ellis Cowling for his encouragement to pursue and refine our concepts of Chemical Climatology.

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44 a Thu. u''' J~\ b NEW ENGLAND ~~-~ my, 5F-— ~Y~R7iR ~ -me 5 rout OR ~ __, ~ ~~.: _ _~ NEWEST =~ C EASTERN SUNBELT 44 4E ., ~,' NO I3SO 1960 i970~ 1 ~ [i5~50 ''to i 'co Alto ',—Am. ~ ~ ~J ,~9II! 'It 2j ~ ~~ .2'~ ~ 3 'L l 19~0 t950 i960—l',o i980 into 1~ - d SMOKY MOuNtR I NS ' E '32 31- .2 .1: ::: ~ it, 1 19S'O 1~0 isio it Also ousa tIR ~ vats , ] ' t '9q~ Isso 1960 1970 ~ ~ '950 ~i60 '9~0 19~0 '990 19~o ;9SO t980—197 ·SE~!=,.CR ~._, ~ _ ,~arIs 11 ~ ouasT ~ \ _ e 2L ~ 2F ~ ~ 2 ~ ~ 2t ~ 19~0 t9S0 1950 1970 19~0 1990 1940 1950 1960 1870 1~0 15 - 't ~ 3 '; 19q0 t9SO'` 1960''l~19 - 1~9aO 1990 Fig. 13 Location of 70 eastern U.S. sites where detailed trend analysis was performed. The trend lines indicate mean and arithmetic standard deviation among the stations within each region, b.-d. Trends of extinction coefficient by yearly quarters for New England southeastern sun belt, and the Smoky Mountain region (Husar et al.,

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45 REFERENCES Bottger, A., Ehhalt, D.H., and Gravenhorst, G. 1978. Atmospherische Kreislaufe von Stickstoffen und Ammoniakum. Berichte der Kernforschunsanlage Julich, 1558. Galloway, J.N. and Whelpdale, D.M. 1980. An atmospheric sulfur budget for eastern North America. Atmospheric Environ. 14, 409. - Granat, L., Rodhe, H., and Hallberg, R.O. 1976. The global sulfur cycle. In Nitrogen Phosphorus, and Sulfur-Global Cycles. Svensson, B.H. and Soderlund, R., eds., SCOPE, Report 7. Ecol. Bull. (Stockholm) 22, 23. Husar, R.B. and Patterson, D.E. 1987. Project Summary: Haze Climate of the United States. U.S. Environmental Protection Agency, EPA/600/SB-86/071. Husar, R.B., Patterson, D.E., Holloway, J.M., Wilson, Jr., W.E., and Ellerstad, T.G. 1979. Trends of Eastern U.S. Haziness Since 1948. Proceedings of the Fourth Symposium on Turbulence, Diffusion, and Air Pollution, Jan. 15-1S, 1979. Husar, R.B. and Holloway, J.M. 1983. Sulfur and nitrogen over North America. In Ecological Effects of Acid Deposition, National Swedish Environment Protection Board-Report PM 1636, Stockholm, Sweden. Husar, R.B. 1985. Manmade SOX and NOX emission and trends of Eastern North America. Background paper for the National Academy of Sciences Committee on Monitoring and Assessment of Trends in Acid Deposition. Singh, H.B., Ludwig, F.L., and Johnson, W.B. 1978. Tropospheric ozone: concentrations and variations in clean remote atmospheres. Atmospheric Environ., 12, 2185-2196. Soderland, R., Svensson, B.H. 1976. The global nitrogen cycle. In Sevensson, (B.H. and Soderland, R., eds., Nitrogen, Phosphorous, and Sulphur - global cycles. SCOPE, Report 7. Ecol. Bull. (Stockholm) 22:23-73. Stallard, R.F. and Edmond, J.M. 1981. Geochemistry of the Amazon. 1. Precipitation chemistry and the marine contribution to the dissolved load at the time of peak discharge. J. Geophys. Res. 86,9844. Vukovich, F.M., Fishman ]. and Browell E.V. (1985~. The reservoir of ozone in the boundary layer of the Eastern United States and its potential impact on the global tropospheric ozone budget. J. Geophysical Res., 90-03, 5687-5698.

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46 LIST OF FIGURES Fig. 2. Fig. 3. Fig. 4. Fig. 5. Fig. 6. Fig. 7. Fig. 8. Fig. 9. Fig. 10 Fig. 11 Fig. 12 Fig. 13 Emission densities, g/m2/y for the year 1977-78 of manmade SO and NO over North America (Husar and Holloway, 1983~. x x Sulfur emission density, g/m2/yr for regions north and south of the Ohio River (Husar, 1985~. NO emission trend for regions north and south of the Ohio River (Husar, 1983~. Maps of sulfate wet deposition rate, (g/m2/yr) and precipitation-weighted average concentration (,ueq/yr) over North America (Husar and Holloway, 1983~. Sulfate and nitrate concentration, (,ueq/l) in Europe (EMEP network average). Maps of nitrate wet deposition rate, (g/m2/yr) and precipitation-weighted average concentration, (peq/~) (Husar and Holloway, 1983~. Maps of ammonia wet deposition rate, (g/m2/yr) and precipitation-weighted average concentration, (,ueq/l) (Husar and Holloway, 1983~. Estimated emission density of ammonia emissions for the U.S. (Husar and Holloway, 1983~. Ammonia and hydrogen ion concentration, (peq/13 in Europe (EMEP network average). Mean diurnal maximum ozone concentration isopleths for August 1978- 1981 (Vukovich et al., 1985~. Idealized ozone variations at remote locations (Singh et al., 1978~. The quarterly (Q1 Jan-March, Q2 April-]une, Q3 July-Sept, Q4 Oct-Dec) average extinction coefficient from noon observations in the absence of precipitation and fog (Husar and Patterson, 1987~. Location of 70 eastern U.S. sites where detailed trend analysis was performed. The trend lines indicate mean and arithmetic standard deviation among the stations within each region; b.-d. Trends of extinction coefficient by yearly quarters for New England, southeastern sun belt, and the Smoky Mountain region (Husar et al., 1979~.