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4 Empirical Observations and Sou rce- Receptor Re~ati onsh i ps The analysis of data taken in the field complements the development of theoretical models as a means for understanding both the phenomenon of acid deposition and the responses of the atmospheric system to alternative emission-control strategies. Field measurements not only reveal insights into the nature of the atmospheric processes involved in the deposition of acid-forming materials but also may hold the greater promise of providing direct, unequivocal evidence from which responses to mitigating strategies might be assessed. Data from which the spatial and temporal distributions of SO2 and NOx over large regions of North America might be derived have not been available until recently. Similarly, reliable sampling of the chemistry of precipi- tation in North America is a recent development. Even so, sampling of ambient pollutants in the atmosphere and sampling of those in precipitation generally have not been simultaneous. There are no direct measurements of regional dry deposition for gases or particles. [For a description of the monitoring systems in North America and data interpretation, see U.S./Canada work Group #2 (1982).] Although data are relatively sparse in North America, those that are available tell us much about the phenome- non. A more extensive data base exists for northern Europe, where atmospheric SO2 and sulfate in precipitation have been monitored systematically for several decades. Much of our understanding of acid deposition has come from analysis of the European experience and the transfer to and replication of those data on this continent. That being so, differences in patterns of emissions, climatic factors, ground cover, and influences of marine air between Europe and North 87

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88 America require that our understanding be tested against North American data. In this chapter we review the existing data for North America and use the data to assess the extent to which the relationships between emissions and deposition can be inferred from observations. The limitations of available data, based on conventional sampling and measurement methods, are discussed first. Results from some pertinent field programs are surveyed to illustrate the importance of meteorological processes for the variability of ambient air pollutants and acid-forming components in precipita- tion. Results of statistical analyses of data on ambient concentrations of pollutants are reviewed, and data on both emissions and deposition are analyzed to infer the influence of sources on deposition. Our survey is not a comprehensive one; it focuses on the results that bear most directly on atmospheric transport and transformation. In this context, the issues addressed are whether data are sufficient to infer (1) the extent to which a given region of sources affects receptor sites in remote locations and (2) the importance of nonlinear processes in determining the relationships between the magnitudes of emissions in source regions and the quantities of acid-forming substances deposited in . ~ receiving regions. One of the greatest difficulties in establishing relationships between sources of pollution and conditions at receptors is accounting for the influences of atmo- spheric processes on the behavior of pollutants. The atmospheric processes involved include airflow, mixing, and chemical transformations. These processes are responsible, directly or indirectly, for the distribution and rate of deposition. Attempts to discern the influences of atmospheric processes on source-receptor relationships have taken different routes, including (1) descriptive accounts of observations, (2) analysis of data segregated by airflow from source areas (trajec- tories), (3) statistical analyses of data, and (4) inference from source tracers. In this chapter we describe the approaches used in these types of analyses and the results obtained. We also analyze existing data on emissions and deposition to discern trends and the relative behavior of sulfur and nitrogen emissions in the atmosphere.

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89 AEROMETRIC DATA AND THEIR LIMITATIONS Most of the historical data on ambient air concentrations describe urban conditions. A large body of monitoring data exists in the National Aerometric Data Base (NADB), but few of these observations have been analyzed or interpreted. One of the more reliable and complete data sets available that describes regional air quality in the eastern United States was taken during a single year, 1977-1978: the Sulfate Regional Experiment (SURE) (Mueller and Hidy 1983). Unfortunately, few precipita- tion chemistry data were collected during the period of the study. In contrast to other data sets, those obtained from the SURE experiment have been analyzed in detail. Complementary data are or will be available from the western United States for 1980-1982 as a result of several studies (Allard et al. 1981, Pitchford et al. 1981). Observations of precipitation chemistry have bee n made periodically in the mid-1950s, from 1959 to 1966, and from 1972 to date in the programs described, for example, by Wisniewski and Kinsman (1982). Air-quality and precipitation data have been collected in Canada since 1974 in the Canadian (CANSAP) monitoring program. Precipitation data provide a direct measure of we t deposition. Since there are no direct measurements of dry deposition, regional patterns are estimated from ambient concentrations and deposition velocities (Appendix C). Since deposition velocities depend on the airflow near the surface, surface properties, and cover, these calculations are believed to be uncertain for quantitative evaluation. The quality of the data describing regional-scale processes is variable. The precision and accuracy of the measurements are generally not well defined in work reported prior to the late 1970s. Recent programs, however, have made considerable progress in reporting data with supplemental information on the calibration o f instruments, as well A: l~h" "rr~rc: ^^n. :~;r`=A ;r' she observations . A definitive discussion of aerometric data is included in Mueller and Hidy (1983). The quality of recent data on wet deposition is less formally documented, but extensive information on the comparability of data is available from the Illinois Water Survey Laboratories, the Department of Energy's Environmental Measurement Laboratories in New York, and the Environmental Protec- tion Agency's Environmental Monitoring Systems Laboratory. A statistical analysis of data from two independent

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go precipitation sampling networks (SURE and MAP3S) covering a similar region in the eastern United States indicated that there is good agreement between data on the concentrations of the major ions, H+, sulfate, nitrate, and ammonium, between the two networks (Pack 1980). Documentation of the quality of data is essential to ensure that comparisons will be possible with data from future studies. Experiments and monitoring programs should incorporate such efforts to avoid or at least minimize controversies about data interpretation. The integrity of the sampling and analytical methods employed is another important consideration in measurement quality. Despite years of effort in the development of methods, the instrumentation used in past and current programs is subject to serious interferences. Uncertainties in results can be large. Interferences or ambiguities in the methods are particularly serious under rural or remote conditions, in which concentrations of pollutants or deposition levels are low. Listed in Table 4.1 are examples of sampling or mea- surement uncertainties that exist in currently available methods of sampling and analysis. From the table it is evident that few of the important chemically related measurements can be made without ambiguity because of uncertainties in the methods. RELATIONSHIPS AMONG AEROMETRIC PARAMETERS Atmospheric measurements provide a basic description of spatial and temporal distributions of pollutants. On a regional scale, the distribution of SO2 shows strong gradients near sources, but airborne sulfate shows relatively weak gradients. Concentrations of both SO2 and SOi are elevated over the industrialized or urbanized parts of the eastern United States (Figures 4.1 and 4.2). High concentrations of sulfur oxides are found through the Ohio River Valley into Pennsylvania and New York to the Atlantic Coast. From this region of high concentration, there is a strong gradient in ambient SO2 concentration northeastward toward the Adirondacks, Vermont, and New Hampshire. The distribution of nitrogen oxide concentrations is generally similar to that of sulfur oxides, but it may show more localized maxima near urban areas (e.g., Mueller and Hidy 1983). Paralleling the distributions of ambient concentrations are the distributions of sulfate and nitrate in precipi

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91 c~ o - - ;^ c c 04 ~: - en c~ - ct o . - ~: :> c ~: .- c~ - c ct c) c :~ - m ~: E~ _ E U I, ~e y ~ ' ~ i ~ I 3 1 ' ~o ~C ,= ~ o C ,_ f~, Y `,: ~ y e ~ 0 's ~C 0 O ~O ~- ~ ~e ~0 3 ~C C o .o ~3 ~ ~ ~ ~ ~ c 3 ~ 3 ~ ~ P~ ~ ~ = ~ E c o`> to o ' E ~ C E o c 3, ~Z~= ._ c o E o ~ o _ Ut o eD c C o~ C Co _._ ~_ o~ _~C ~4.) X3 C)~, ~o _._ e~_ 3~ _ o _~ - C)~ e~ ~3 c D~ ;>, ~ O - C~ C ~.- o os ~ - C _ ~ ~ o ~ ~ en - E ~ D D ~ r ~ ~ Z _ 0,~ O C . _ Ct ._ _ ._ ~ .- ~ ~_ - 4,) <: 3 o ~ . _ _ ~c, ~ 3 E ~ ~ .- ~e ~e E v D ~E es 4' ~ * U, -c ~o ', ~E E ~ y U ~ e ~ y ~ ~ ~ y c~ - C`3 ._ ca et - ._ ,~ ~o ~ ~0 O O - D ~-=v E E E 3 _ ~ c ~ E! _ _ ~ ct 3 3 c c ._ ._ ~ CJ u' Z 3 c ; - 6 4D en c~ c, - c.> ~ 04 c~ 0 0 0 0 ._ ._ _ _ u~ C~3 c, u, c o - -

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92 ~ ~_ J2,~6,\) I\ . ^? 2(6) ' 7! /~( 20 (601~' 1 ~ ~ , ) ~ ~~~4 ) Off \ ~ _ _ fin 1 _J ~10 ('30) , - _ lo/ 10 (30) ~ ~\ N ~ _! j \ 'N 1 \ 1 ~l 4_ ~ _ _-~ 1-Hour SO2 (ppb) FIGURE 4.1 Composite spatial distribution of 1-hour average concentrations (ppb) of SO2 from one representative month in each season between 1977 and 1978. This average is approximately equivalent to an annual average. Dashed isopleth is based on limited quantities of SURE data for 1977-1978 and on data taken at Whiteface Mountain, New York, after 1978. Numbers in parentheses are calculated values of dry deposition rates in kilograms of sulfur per hectare per year assuming a uniform deposi- tion velocity of 0.8 cm/s. SOURCE: Data on concentrations from Mueller et al. (1980) and, for Whiteface Mountain, from V. Mohnen, State University of New York, Albany, personal communication (1983~. tation, as indicated in Figures 4.3 and 4.4. These distributions may be compared with that of the hydrogen ion concentration in precipitation in Figure 4.5. From these figures, the pattern of deposition of hydrogen ion in precipitation appears to correspond to regions of elevated sulfate and nitrate concentrations. This finding does not necessarily follow without accounting for all the cations (such as NH4 and Ca++) and anions that

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93 7 'it ~_~l \~z 8 ( '\~ J ~ 24-Hour c~-~- _;- ~SO4 ("g/m3) ran ~ a./ .~ FIGURE 4.2 Composite spatial distribution of 24-hour average concentrations of sulfate (,ug/m3 ~ from one representative month in each season between 1977 and 1978. This average is approximately equivalent to an annual average. Numbers in parentheses are calculated values of dry-deposition rates in kilograms of sulfur per hectare per year assuming a uniform deposition velocity of 0.2 cm/s. SOURCE: Data on concentrations from Mueller et al. (1980~. may be important factors affecting the acidity of precipitation. A qualitative comparison between dry- and wet-deposition rates can be made from observation. Wet-deposition rates for sulfate are shown in Figure 4.3 for data taken in 1980. Estimated dry-deposition rates for SO: and particulates are shown in parentheses in Figure 4.1 and 4.2 (estimated values in kilograms of sulfur per hectare per year). The rates have been calculated using uniform deposition velocities considered typical of values reported in the literature for SO2

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94 ~4 3 4, ~ l l 5.1 i " 10 " 16 I - t, v ' \' - r t - ~ /~6 3 5 2-' t . , 1 t__ j .~ L-_ 49 ~48 \ \` 53" an, I ? ~37 t~'-~-~w 1 m mole/m2 ~ 0 961 kglha l , ._ _ ~ l _ ~1 4 I 45 ~ t- _ 1 ~ - or'_ ~ ~ 11 ~ ~] I I 2 :~,~!, -~I' In' ..~-.;..:c..',fyIt. ~ - - ~ \ , ~ , '' 20 FIGURE 4.3 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 (1982~. gas and for submicrometer particles (sulfate). Note that in much of the region of high ambient concentrations, dry deposition apparently dominates total deposition. This result suggests that dry deposition of SO2 exceeds wet deposition in parts of the Ohio River Valley and the Ohio-Pennyslvania-western New York area but becomes progressively less important farther from the region of major emissions, to the northeast in New England and Canada. At large distances from sources, ambient concentrations of sulfur oxides are low; wet deposition will dominate dry deposition far from sources if precipitation is significant.

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95 ~=O.5 _ ~ 0. 9, ~ I ~ I 8.2 e/ ~ 3.5 / ~- 5.5 / 4 ~'\ _q ~1 ~ A_ . t_ _ 11~ Of) ~0 Cl i_ _. ~< ;~3~\ '"' 8~.9 ''it ~ _ _ l ' - I =~ ~ <-'9 ;W -' to _ ~ ~6.0 ~ $43~~\ 1 m mole/m2 ~ 0.62 kg/ha / FIGURE 4.4 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~. THE INFLUENCE OF METEOROLOGICAL CONDITIONS The spatial distributions of deposition for acid-forming materials are similar, and elevated concentrations appear to be associated with areas in which emissions are high. The temporal behavior of sulfur oxides and nitrogen oxides is dominated by meteorological variability. For example, analysis of the SURE data indicates that the variability in ambient concentrations of SO2 was as much as an order of magnitude greater than the vari- ability in SO2 emissions over the eastern United States (Mueller and Hidy 1983). This finding suggests that advanced and sophisticated analyses are needed to separate the influences of emissions from those of aerometric parameters in such data. These analyses have

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96 '` .1.0 ~ \ ~ aim_ ;ir;; TV j i ret I\. 1~. 5.7 ~ ~ i,> . ~, ~ 7-- ~ use\ tj 0. ~\ ~ . - ~-0 6-' I 3. _ _ _ + _ _ ~ 0 .4 I ~,) ,' t~o.5 ; $~ i\ 1 m mole/m2 ~ 0.01 kg/ha = 1- FIGURE 4.5 Spatial distribution of mean annual wet deposition of hydrogen ion weighted by the amount of precipitation in North America in 1980 (mmoles/m2 ). SOURCE: U.S./Canada Work Group #2 (1982~. followed two directions in the recent literature. The first is descriptive, taking into account the behavior of sulfur oxides in the atmosphere as a function of climato- logical or meteorological features. The second stems from the first by adapting statistical techniques to separate influences of different processes. In this section we review results of analyses of data according to meteorological conditions and studies of air-mass trajectories. In the next section we present the results of statistical methods of analysis. Classification of Meteorological Conditions One of the primary conclusions of many studies of field observations is the absence of a definitive relationship

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97 between SO2 emissions and sulfate concentrations in dry air or in precipitation patterns. Sulfur emission rates are relatively constant, whereas the concentrations of sulfate aerosol and SC: in the air are highly variable and dominated by meteorological conditions (Electrical Power Research Institute 1981). The concentrations of sulfate particles tend to be high in summer, presumably because of more rapid photochemical oxidation of SO2, high in the central and western region of high-pressure systems that move slowly toward the Atlantic, and high in maritime tropical air emanating from the coastal region of the Gulf of Mexico. Concentrations of SC2 also tend to be high under stagnant air conditions of slow-moving, high-pressure, anticyclonic systems but tend to be low in the maritime tropical air. Trajectory analyses applied to specific transient anticyclonic systems have further demonstrated the tendency of such systems to favor air- stagnation situations in which high levels of pollutants accumulate (King and Vukovich 1982). Evidence based on chemical and meteorological analyses as well as satellite photos suggests that, on occasion, pollutants originating in the Midwest and Northeast can be entrained in the clockwise flow around a transient high-pressure system and transported to the Gulf Coast region. The pollutants are believed then to be adverted back through the Midwest and Northeast in the south- westerly flow of maritime tropical air (Wolff et al. 1981, 1982). The slow-moving flow of southwesterly maritime air from the southern states entrains pollutants from sources in its path and eventually moves over the northeastern United States and eastern Canada. Summer- time convective storms that occur in this air tend to be quite acidic compared with precipitation during the cooler months (MAP3S/RAINE Research Community 1982, Raynor and Hayes 1982a). Precipitation during autumn, winter, and spring in eastern North America also tends to occur in moist southwesterly air that frequently emanates from the southern states. During these seasons, most of the precipitation occurs in the vicinity of warm fronts associated with cyclonic low-pressure storm systems. Precipitation develops because the southwesterly air travels faster than the warm front, which represents the boundary of colder and heavier air lying to the north of it. The southerly air ascends over the cold air and is cooled, producing large areas of precipitation. The situation is illustrated in Figure 4.6. The center of -

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137 TABLE 4.9 Molar Ratios of SOx to NOX in the Region of the SURE Experiment in 1977-1978a Emissions Ambient Airb Precipitation 1.2 (all sources) Gas and particles 0.66 0.73 to 1.3 2.2 (utilities)C Gas only 0.42 Particles only 24.0 SOURCE: Adapted from Mueller and Hidy (1983). aRatio taken as SO2/NO2. Sated at ground level. CWeighted heavily toward emissions injected at altitudes at or above 300 m. concentration data that are averaged over all meteoro- logical conditions with precipitation data that reflect only those conditions specifically favoring precipitation. As indicated earlier, the ratio of sulfate to nitrate in precipitation varies regularly throughout the year, being high in summer and low in winter (see Table 4.8). It is unclear how the specific nrn~=cmc -~1 1 ;~ _ = ~= ~_ _ ~ ~ ~ ~ ~ ~ ^ ~ l y Renege variations average out annually +^ h" =^ 1ln;.F~-m spatially and so similar to the emission ratios over much of the area when ambient atmospheric conditions apparently are so variable. Conceivably the uniformity of the annual SO ANON ratio in precipitation in the northeastern United States could reflect some as yet unidentified stoichiometric linkage between the atmospheric chemistry of SO2 and NO2. For example, the reaction of HOSO2 radicals formed in reactions of HO with SO2 could lead to HOSO2O2 and HOSO2O radicals, which could ultimately react preferentially with NC2 to form a one-to-one, S to N compound such as HOST ONO2 (see Chapter 3 and Appendix A). . This compound wood an principle hydrolyze in cloud water to form equal amounts of H2SO4 and HNO3 acids. However, this reaction mechanism would have to be the dominant ~, rim of on ~ _ HA x~ ~ ~ _ . . a..- ,~3 ~ ~nca~n one ratio or moles of SO4 to moles of NC] near unity, as observe in nr"~;~;' =~;~- : eastern United States. rat A- -- ^~ ~= There is no laboratory or field evidence of the significant participation of nitryl sulfuric acid (HOSC2ONC2) or other similar compounds in acid development; in fact, the existing limited evidence appears to discount the possible importance of such a product as an intermediate in the HO-SC:-NOk reaction system (see Appendix A). From theoretical considerations (e.g., Chapter 2 and Appendix A), we expect that ambient concentrations of oxidants (such as H2O2, C3, and HO) will have

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138 maximum values in summer and minimum values in winter. It is expected, therefore, that in winter in areas at higher latitudes, aqueous oxidation is slow because of low concentrations of oxidants in cloud water. Hence, if a limitation of oxidant were to lead to nonlinearity in the relationship between emissions and deposition, that effect would be most likely to be observed in winter. The evidence that the relationship between average annual emissions and deposition is not strongly nonlinear may result from the fact that most of the total annual deposition of acid occurs during the warm months. Comparison of emission ratios and deposition ratios constitutes indirect evidence for the absence of a serious nonlinearity even though we lack an adequate understanding of the intermediate chemical and meteoro- logical processes to predict, either qualitatively or quantitatively, the deposition ratios from the emission ratios. It is our opinion that the necessary data and theoretical understanding to model the effects of emissions from distant sources on the composition of precipitation reliably and accurately will not be available in the near future. The evidence based on the empirical observations appears incompatible with a seriously nonlinear system in terms of our current knowledge. Unquestionably, however, it would be preferable to reinforce such conclusions with a more complete understanding of atmospheric chemistry and meteorology under conditions prevalent in eastern North America. Additional uncertainties are introduced by the inherent variabilities of natural processes (e.g., annual rates of precipitation), imprecision of measurements, and errors in determining emission rates. For example, the ratios of emissions and wet depositions in the Northeast are uncertain by a factor of about 30 percent. The concen- trations of sulfate and nitrate in the MAP3S study are estimated to be uncertain by a factor of 15 to 20 percent. A significant reduction in the uncertainties related to natural variability, measurement imprecision, or establishment of emission rates is unlikely in the near future. FINDINGS AND CONCLUSIONS Although data from which to assess the relationships between emissions and deposition are relatively sparse in

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139 North America, analysis of the available empirical data provides insight into the nature of the atmospheric processes involved in acid deposition. Nonlinearity Data indicate that the variability in ambient concentra- tions of SO2 vapor and SO4 particles do not necessarily correlate with the variability in SO2 emissions but are predominantly controlled by meteoro- logical conditions. Variations in sulfate particle concentrations tend to correlate with variations in SO2 vapor concentrations in rural areas of the Northeast. Regression on principal components and empirical orthogonal-function analysis suggest that if other factors were constant, ambient concentrations of SO2 and soi would be determined largely by patterns of SC2 emissions. Direct evidence of a strongly nonlinear relationship between wet deposition of sulfate and SO2 emissions is limited to extensive historical data in Europe. The continuous historical record of reasonably reliable data in North America, at Hubbard Brook, indicates no evidence for a strongly nonlinear relationship between annual depositions and annual emissions in the Northeast. Indirect evidence based on patterns of the ratio of SO4 to Ned in annual deposition do not support the hypothesis of a strongly nonlinear relationship between SO2 emissions and sulfate deposition in eastern North America. Differences in the relationship between emis- sions and deposition in Europe and eastern North America may be the consequence of differences in meteorology, latitude, or other factors, such as the spatial dis- tribution of sources, between the two regions. On the basis of currently available empirical data and within the limits of uncertainty associated with the data and with estimating emissions, we therefore conclude that there is no evidence for a strong nonlinearity in the relationships between long-term average emissions and depositions in eastern North America. The conclusion that nonlinearity is probably not sig- nificant for annual average deposition in eastern North America is clouded by three types of uncertainties. First, direct evidence based on long-term time-series data is severely limited in North America to only ten stations, all of which have collected bulk deposition

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140 data. The data from only one site, Hubbard Brook, are considered to be reasonably reliable. Therefore we have relied on the historical record at only one station combined with the indirect evidence provided by data on sulfate and nitrate deposition compared with SO2 and NOX emissions. Second, there remain uncertainties in our detailed understanding of the meteorological, physi cal, and chemical processes that relate emissions to deposition. Third, the unknown influences natural vari- ability in the composition and occurrence of precipi- tation, imprecision in sampling and analysis, and uncertainties in estimation of emissions further limit confidence in this conclusion. Influence of Local and Distant Sources Both observational and theoretical evidence exists for the long-range transport of pollutants leading to acid deposition. It is apparent that any receptor site will be influenced to one degree or another by both local and distant sources. The issue of concern is the extent of this zone of influence for sensitive ecological areas, including the relative contributions to deposition of nearby and distant sources. In the case of the Hubbard Brook data, the trends in concentrations of sulfate and nitrate appear to reflect general trends in emissions. Analyses of the trajectories of precipitating systems delivering acidic precipitation to Whiteface Mountain and Ithaca in New York and south central Ontario in Canada indicate that most of the acidity in precipitation at these sites--as well as most of the precipitation--is associated with air masses that have passed over source regions to the south and southwest. The spatial distribution of the annual average molar ratios of pollutants in emissions and deposition suggest that atmospheric processes in eastern North America lead to a thorough mixing of pollutants over a wide geographic area, making it difficult to distinguish between the effects of distant and local sources. On the basis of currently available empirical data, we cannot in general determine the relative importance for the net deposition of acids in specific locations of long-range transport from distant sources or more direct influences of local sources. We regard the problem of relating emissions from a given region to depositions in

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141 a given receptor region to be of primary importance and recommend that high priority be given to research relevant to its solution. The SURE data have indicated a relatively small zone of influence (of the order of 300 to 600 km) on ambient sulfate concentrations in general, with occasional long-range influence during ducting situations involving southwesterly flows of air. Similarly, warm frontal precipitation may involve cloud formation in air parcels adverted for hundreds of kilometers from a southwesterly direction. The relative contributions of such long-range effects and of more local regional effects are currently unknown and cannot be reliably estimated using currently available models. REFERENCES Allard, D.W., I.H. Tombach, H. Mayrsohn, and C.V. Mathai. 1982. Aerosol measurements: western regional air quality studies. Presented at the 75th Annual Meeting of the Air Pollution Control Association, New Orleans. Alpert, D.J., and P.K. Hopke. 1981. A determination of the sources of airborne particles collected during the regional air pollution study. Atmos. Environ. 15:675-688. Altshuller, A.P. 1980. Seasonal and episodic trends in sulfate concentrations (1963-1978) in the eastern United States. Environ. Sci. Technol. 14:1337-1348. Altshuller, A.P., and R.A. Linthurst, ed. 1982. Critical Assessment Document: The Acidic Deposition Phenomenon and Its Effects. Draft. Prepared for the U.S. Environmental Protection Agency. Raleigh, N.C.: The North Carolina State University Acid Precipitation Program. Atkinson, R., A.C. Lloyd, and L. Winges. 1982. An updated chemical mechanism for hydrocarbon/NO~/SO2 photooxidations suitable for inclusion in atmospheric simulation models. Atmos. Environ. 16:1341-1356. Beleley, D.A., E. Kuh, and R.E. Welsch. 1980. Regression Diagnostics: Identifying Influential Data and Sources of Collinearity. Pp. 85-191. New York: John Wiley and Sons. Chamberlain, J., H. Foley, D. Hammer, G. MacDonald, D. Rothaus, and M. Ruderman. 1981. The Physics and Chemistry of Acid Precipitation. Technical Report JSR-81-25. Arlington, Va.: SRI International.

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142 - Core, J.E., P.L. Hanrahan, and J.A. Cooper. 1981. Air particulate control strategy development: a new approach using chemical mass balance methods. In Atmospheric Aerosol: Source/Air Quality Relationships, E.S. Macias and P.K. Hopke, eds. ACS Symposium Series No. 167. Washington, D.C.: American Chemical Society. Cunningham, W.C., and W.H. Zoller. 1981. The chemical composition of remote area aerosols. J. Aerosol Sci. 12:367-384. Department of Environmental Conservation. 1976. New York State Air Quality Report. DAR-77-1. Albany, N.Y.: New York State Department of Environmental Conservation. Department of Environmental Conservation. 1978. New York State Air Quality Report. DAR-79-1. Albany, N.Y.: New York State Department of Environmental Conservation. Department of Environmental Conservation. 1981. New York State Air Quality Report. DAR-82-1. Albany, N.Y.: New York State Department of Environmental Conservation. Dzubay, T.G. 1980. Chemical element balance method applied to dichotomous sampler data. In Aerosols: Anthropogenic and Natural, Sources and Transport, T.J. Kneip and P.J. Lioy, eds. Annal 338. New York: New York Academy of Sciences. Eichenlaub, V.L. 1979. Weather and Climate in the Great Lakes Region. South Bend, Ind.: The University of Notre Dame Press. Electric Power Research Institute. 1981. EPRI Sulfate Regional Experiment: Results and Implications. EPRI EA-2165-SY-LD. Palo Alto, Calif. Fisher, B.E.A. 1982. The transport and removal of sulphur dioxide in a rain system. Atmos. Environ. 16:775-784. Friedlander, S.K. 1973. Chemical element balances and identification of air pollution sources. Environ. Sci. Technol. 7: 235-24 0 . Galloway, J.N., and G.E. Likens. 1981. Acid precipitation: the importance of nitric acid. Atmos. Environ. 15:1081-1086. Gartrell, G. Jr., and S.K. Friedlander. 1975. Relating particulate pollution to sources: the 1972 California aerosol characterization study. Atmos. Environ. 9:279-300. Golomb, D. 1983. Acid deposition-precursor emission relationship in the northeastern U.S.A.: the effectiveness of regional emission reduction. Atmos. Environ. In press.

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