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5 Precipitation Chemistry Gary J. Stensland, Douglas M. Whelpdale, and Gary Oehlert INTRODUCTION The Role of Precipitation Composition in This Study The chemical composition of wet deposition has been central to the question of the effects of acid deposition and the long-range transport of air pollution since the issue of "acid rain" rose to prominence in Europe two decades ago. Early evidence linked acidic substances in precipitation to harmful effects on surface waters and fisheries in Scandinavia, and as a consequence interest has since focused on precipitation acidity and acid deposition rather than on the more general case of depo- sition of air pollutants. This focus can be attributed to several factors: Changes in precipitation composition could actually be detected, and techniques were available to make measurements; this was not true for dry deposi- tion. In addition, precipitation composition was starting to be used as an indicator of atmospheric quality, so researchers were aware of the value of such measurements. Finally, the correlation between adverse effects and wet deposition could be made more directly than that between effects and emissions. The chemistry of precipitation has been studied both in the expectation that it will provide a measure of the consequences of emissions in time and space and also as an indicator of effects on the environment. Unfortu- nately, problems exist both with the quality of past data and with the interpretation of the data. These difficul- ties have stimulated the examination of other historical data in efforts to improve our understanding of the phenomenon of acid deposition and to corroborate the available precipitation-chemistry records. 128

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129 Wet and Dry Deposition Atmospheric constituents, and thus pollutant emissions, reach the surface of the Earth by a variety of processes: those in which precipitation is involved contribute to wet deposition; those that (primarily during dry periods) involve sedimentation, turbulent and molecular diffusion, impaction, and interception contribute to dry deposition; and those that include cloud and fog impaction, dew, and frost are sometimes termed occult deposition. Wet deposi- tion and bulk deposition (i.e., the sum of wet, dry, and occult, obtained with the collector always open) have been measured with varying degrees of reliability for decades; dry deposition has been measured sporadically in a research mode but not routinely; and measurements of the third type of deposition are just beginning. Even his- torical air-concentration data, from which dry deposition might have been estimated, are sparse and incomplete. Out of necessity, therefore, we focus on wet deposi- tion data. From the point of view of determining his- torical trends, this is not necessarily a serious limitation on the regional-to-continental scale, where the wet deposition is expected to reflect total deposition to a reasonable extent. location and at a specific time, At a specific however, wet deposition may not be representative of total deposition. In addition, omitting dry and occult deposition is certainly a serious constraint on determining mass balance and interpreting effects. Over eastern North America the total wet and total dry deposition are thought to be of approximately equal magnitude. Dry deposition is, in general, greater than wet near emission Arms ration where ambient concentrations of pollutants are higher. An additional complexity for dry deposition is that the chemical species behave differently: in general, gaseous nitric acid, sulfur dioxide, and ammonia are removed more efficiently than their particulate forms, nitrate, sulfate, and ammonium. In high-elevation ecosystems the direct input from cloud and fog water is likely to exceed that from either of the other pathways. Interpreting observations of surface ecosystems must allow for these other inputs, even though the majority of past deposition data is for wet or bulk deposition. ~ ~ ~4, _,

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130 Constituents of Interest We limit this discussion of wet deposition to the major soluble species in precipitation that account for most of the measured conductance of the samples. The species include the following ions: hydrogen (H+), bicarbonate (HCO3), calcium (Ca ), magnesium (Mg ), sodium (Na+), potassium (K+), sulfate (SO4 ), nitrate (NO3), chloride (C1-), and ammonium (Nut). Experience has shown that a pH value can be calculated from measurements of the last eight ions in the list, and the calculated value is usually in good agreement with the measured pH value. The fact that we can often successfully calculate the pH of precipitation samples indicates that the small list of measured ions is probably sufficient for studies of wet deposition that emphasize the acid precipitation phenomenon. However, there are exceptions. For example, samples from remote locations can be strongly affected by organic acids (Galloway et al. 1982). The data in Table 5.1 demonstrate the relative importance of these ions at three monitoring sites of the National Acid Deposition Program (NADP) in the United TABLE 5.1 Median Ion Concentrations for 1979 for Three NADP Sites (~eq/L) GA MN NY Ion (42 Samples)a (37 Samples)b (49 Samples)C SO4- 38.9 45.8 44.8 NO3 11.6 24.2 25.0 Cl- 8.2 4.2 4.2 HCO3 (calculated) 0.3 10.3 0.1 Anions 59.0 84.5 74.1 NH4+ 5 5 37 7 8.3 Ca2+ 5.0 28.9 6.5 Mg2+ 2.4 6.1 1.9 K+ 0.7 2.0 0.4 Na+ 17.6 13.7 4.9 H+ 17.8 0.5 45.7 Cations 49.0 88.9 67.7 Median pH 4.75 6.31 4.34 NOTE: See Georgia, Minnesota, and New York in Figure 5.23 for location of these sites. aThe Georgia Station site in west central Georgia. bThe Lamberton site in southwestern Minnesota. C The Huntington Wildlife site in northeastern New York.

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131 States. Concentrations are expressed in microequivalents per liter (peq/L) to permit a direct evaluation of each ion's contribution to the anion or cation sum. If all ions were measured and there were no analytical uncer- tainty, then the anion sum would equal the cation sum. The values for hydrogen ion concentration were calculated from the measured median pH value, and the values for bicarbonate were calculated by assuming that the sample was in equilibrium with atmospheric carbon dioxide. Although the sulfate and nitrate levels shown are similar at the Minnesota and New York sites, the pH differs greatly owing to the higher levels of ammonium, calcium, magnesium, sodium, and potassium ions at the Minnesota site. These ions are frequently associated with basic compounds. The data in Table 5.1 suggest, therefore, that the concentrations of all the major ions must be considered if we are to understand the time and space patterns of pH. The data also indicate that sites in different regions of the United States exhibit large differences in ionic concentrations. Currently, sites in states such as Ohio, New York, Pennsylvania, and West Virginia have the feature shown for the New York site in Table 5.1, for which hydrogen, sulfate, and nitrate are the dominant ions. For the New York site, 98 percent of the acidity coula be accounted for if all the sulfate were sulfuric acid, whereas nitrate, as nitric acid, could have accounted for about 55 percent of the acidity. Bower sax and de Pena (1980) have concluded, by applying multiple linear-regression analysis for a central Pennsylvania site, that on the average, sulfuric acid is the principal contributor to hydrogen-ion concentration in rain, but the acidity in snow is principally from nitric acid. Variability in Acid Deposition: The Role of Meteorology Any discussion of short- and long-term trends in acid deposition cannot ignore the role of meteorological and climatological variability. Even if pollutant emissions were absolutely constant every hour of the day and every day of the year, variations in atmospheric conditions would bring about uneven deposition patterns in both time and space. For example, both variability throughout the course of a year and long-term trends in precipitation records reflect changes in storm tracks and in the fre-

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132 quency of precipitation-producing circulation patterns that affect a given region each year. Differences in precipitation amount from year to year affect total wet deposition, but even if the precipitation amount were constant every year, wet deposition of sulfate, nitrate, and other common ions would not necessarily be, because of the dependence of the concentrations of these ions on the size of the precipitation events and the time interval between the events. Atmospheric processes vary continuously across a wide temporal and spatial range; at whatever scale one selects, atmospheric variability plays a fundamental role in the acid deposition process. Unless we understand this role thoroughly, the meaning of trends in acid deposition will be difficult to resolve unequivocally. (See Chapter 3 for a more detailed discussion.) DATA EVALUATION Given that the object of our evaluation is to deter- mine trends in concentrations (or deposition) of chemicals in precipitation, our first task is to identify approxi- mate data sets and determine their quality. Since adequate methods to determine dry deposition rates for most chemical species do not exist, wet-only deposition data collected with automatic samplers are generally the most reliable and useful. However, owing to the sparsity of such data we must also consider data bases in which less desirable sampling methods were used. Such methods include bulk sampling and manual wet-only sampling in which dust leakage into the sampler and water evaporation and snow sublimation from the collection vessels have apparently introduced serious bias into the data sets. The same biases may occur even in an automatic wet-only sampling device when mechanical failure causes exposure of the sample to the open atmosphere. In this chapter we emphasize trends in concentration rather than in depo- sition to avoid the introduction of spatial variability caused by variability in the amount of precipitation. A primary tool in evaluating possible biases in monitoring techniques is comparison of different data sets, such as bulk data with concurrent wet-only data. This approach has not always been used in previous reports but may be one of the most important evaluation techniques. However, since local sources can affect bulk data through high levels of dry deposition, the only

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133 satisfactory comparison must involve concurrence in space and time. Unfortunately, only in a few cases are the two sampling methodologies concurrent spatially and temporally, so that such comparisons are usually far from ideal. Several different evaluations are described in this chapter, with emphasis on evaluating those United States data sets quoted most frequently in trend studies. Comparing Three-Month Regional Bulk Data Concurrently with Wet-Only Data Most of the published comparisons between bulk and wet-only ion concentrations are for eastern sites. The difference between these two sampling methods would be expected to be greater in locations where local sources may be influential, for example, where sampling occurs near tilled fields with an abundance of windblown dust. Peters amd Bonelli (1982) have reported the results of a bulk sampling program for the period from about December 1, 1980, to March 1, 1981. They indicated that consider- able effort in siting was made to avoid the occurrence of local contamination of regional deposition. States from Minnesota and Illinois eastward to Maryland and Maine were included in the study. The NADP network had a large number of stations operating in the same area for the same time period. Therefore we are able to present a simple comparison of the results for the wet-only NADP concentration data versus the bulk concentration data and examine geographic variations in the differences between the two methods. The wet-only and bulk concentration data were plotted and examined visually to select five contiguous geo- graphical areas in which there was some degree of uniformity in values. The areas selected are shown in Figure 5.1. The bulk sample at each site was a single time-integrated sample collected with a continuously open cylindrically shaped sampler. The concentration ratios of the, ions (mean of bulk values divided by mean of wet-only values) are shown in Table 5.2 for each area. One may observe that in all cases the value of the ratio is greater than or equal to 1.0 and that there is considerable variation in the ratio by area and by chemical parameter. The ratios for calcium, magnesium, and potassium are greater than 2 (and as high as 30) for all areas, and bulk sulfate concentrations exceeded

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134 :___ 1 15 V · 1 }41 ~ 1~ Jo-'' 70 \ 23 ~ . ~ 3: , 1 -_ _` r~ ~ ~~ my FIGURE 5.1 Map of five areas (indicated by roman numerals) used for comparing concentrations of various ions in wet-only deposition with those in bulk samples. The arabic numbers indicate the number of bulk sampling sites within each area. The NADP sites are indicated by circles. Bulk-measuring sites are distributed uniformly about the areas but are not co-located with NADP sites. See also Table 5.2. wet-only concentrations by factors of 2.0 and 8.1 in areas III and V, respectively. All ion concentrations were elevated by more than 90 percent in areas III and V, with a median ratio value of 3.7 for these two areas. Peters and Bonelli (1982) reported both sample volume and an estimated sample volume derived from the data from the nearest National Weather Service precipitation gauge site. Using these data, we calculated the medians of the ratios of the measured divided by estimated volumes to be 0.88, 0.79, 0.81, 0.69, and 0.45 for areas I, II, III, IV, and V, respectively. These ratios suggest that bulk samples may be concentrated up to a factor of 2 by evaporation. Thus, the many ratios greater than 2.0 in Table 5.2 suggest that differences between concentrations of ions in bulk samples and in wet-only samples cannot by fully explained by evaporation. Even when an average bulk to wet-only ion concentration ratio for an area in Table 5.2 has a value close to 1, an individual bulk site in that area may still differ widely in concentration from the wet-only data. Each site needs

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135 TABLE 5.2 Ion-Concentration Ratios (Mean of Bulk Values Divided by Mean of Wet-Only Values) for All Sites in Areas I to V for the Period December 1980- February 1981 Ion Area Ca2+ Mg2+ K+ Na+ Cl- NO3 NH4 SO2- NBa NWb I 2.5 5.6 2.7 1.2 1.0 1.1 1.1 1.0 23 3 II 3.7 4.8 5.9 2.9 2.0 1.5 2.0 1.3 70 11 III 7.2 3.5 11.2 2.7 2.1 1.9 3.5 2.0 45 4 IV 2.7 2.3 3.1 1.9 1.6 1.1 1.3 1.3 26 3 vc 30.5 19.2 6.0 2.8 6.6 3.9 2.1 8.1 15 - NOTE: Areas are depicted in Figure 5.1. a Number of bulk sampling sites used to determine the means. b Number of wet-only sampling sites used to determine the means. C Since there were no NADP sites in area V, computer-generated contour maps (for weighted-average concentration) for all data available for the period 1978 - mid-1983 were used to estimate average NADP ion concentrations for this area. SOURCES: NADP data were provided by Illinois State Water Survey. Bulk data were obtained from Peters and Bonelli (1982~. to be evaluated separately for each chemical parameter to determine if bulk concentrations are likely to be similar to wet-only concentrations. Comparing Bulk and Wet-Only Data at Hubbard Brook, New Hampshire A comparison of bulk and wet-only data from the Hubbard Brook site shows good agreement for most major ions, in contrast to other bulk versus wet-only com- parisons (cf. Tables 5.3 and 5.4 (below)). Weighted- average concentrations of the colocated Hubbard Brook bulk samples and NADP wet-only samples were determined for the period from June 1979 to May 1982. Table 5.3 presents the results as 3-year averages except for pH, for which the averages of three 12-month periods are given. Bulk concentrations of sulfate plus nitrate were 4.8 peq/L (7 percent) higher than for the wet-only values. The sum of calcium plus magnesium, an indicator of dust input, was actually lower for the bulk than for the wet-only data. Apparently, dry deposition of sulfate and nitrate to the continuously exposed bulk collector, either through gaseous precursors or in association with dust particles, was not an important factor. Another

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136 TABLE 5.3 Comparison of Bulk and Wet-Only Weighted-Average Concentrations (~eq/L) at Hubbard Brook, New Hampshire Hubbard Brook NADP Ion (bulk)a (wet only)b SO2- 49.9 48.5 NOT 27.3 23.9 C1- 6.6 62 NH4 11.0 9.4 Ca2+ 5.6 5.8 Mg2+ 2.2 3.0 K+ 1.0 0.4 Na+ 5.3 8.4 H+ 60.3 47.9 pH 4.24, 4.19, 4.22 4.39, 4.27, 4.32 Weighted-average concentrations for the period June 1979-May 1982. Data provided by John Eaton, Cornell University, personal communication 1985. Weighted-average concentration for wet-only samples from the NADP collector for the period June 1979-May 1982. typical problem with bulk data, concentration effects owing to sample evaporation, was probably not a major factor given the close agreement of bulk ion concen- trations with the wet-only concentrations. This finding suggests that the funnel and bottle design of the Hubbard Brook bulk sampler effectively prevented this problem for long-term averages at this site. The three (12-month) weighted averages of pH for the bulk samples are lower than the corresponding values for the wet-only samples. The 3-year weighted-average concentration of hydrogen ions in bulk samples is 26 percent larger than in wet- only samples. Such a difference is likely to be significant for these data. In addition to different sampling devices and dif- ferent laboratories for sample analyses of the Hubbard Brook data, different screening and sampling procedures were used for bulk data and the wet-only data. More bulk data than wet-only data were deleted from the data base for the 3-year period. Since the two sampling devices were generally serviced on different days of the week, it was not possible to calculate the data as exactly matched pairs. It would be useful to undertake a more detailed study involving a week-by-week comparison--both to learn more about the long-term record at this important site and to help to relate trends in bulk measurements to those in wet-only measurements.

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137 Comparing U.S. Geological Survey Bulk Data with National Acid Deposition Program Wet-Only Data Nine U.S. Geological Survey (USGS) bulk collection sites have operated in New York state and Pennsylvania since 1965 (Barnes et al. 1982), and several investigators have summarized and analyzed these data. (See later discussion.) Here, we compare these data with NADP wet-only data (Table 5.4). The first column of the table gives the mean and the standard deviation of all New York NADP data available through the end of 1982; the seven NADP sites began operation at different times between November 1978 and TABLE 5.4 Ion Concentrations (~eq/L) and pH for All New York NADP Sites and for Two New York USGS Bulk Sites Time-Adjusted Weighted Average Weighted Average Mean and Standard Deviation for Mays Mays Ion Seven NADP Sitesa Hinckleyb Points Hinckley Point Ne 23 24 23 24 Ca2+ 8.4 + 2.0 30.2 74.2 26.3 71.6 Mg2+ 3.5 + 0.8 7.7 17.1 6.7 16.5 K+ 0.8 + 0.6 2.1 2.9 1.8 2.8 Na+ 6.2 + 3.5 5.4 10.4 4.7 10.0 NH4+ 16.5 + 5.5 31.2 22.7 37.3 27.5 Sumf 35.4 76.6 127.3 76.8 128.4 NO3 30.2 + 5.4 41.7 46.5 62.1 54.7 SO2- 64.8 + 13.4 72.5 82.2 65.9 76.4 C1- 5.8 + 3.3 8.9 15.2 8.9 15.2 pH 4.22 (4.15-4.33)g 3.99 3.99 3.90 3.86 H+ 60.4 + 10.3 102.3h 102.3h 126.4h 138.3h aVolume-weighted concentrations for 1978-1982. bHinckley USGS bulk data for 8/31/75 to 9/15/77. CMays Point USGS bulk data for 8/31/75 to 8/31177. Time adjustments made using time-trend results for 1965-1978 period as determined by Kendall's test (Barnes et al. 1982). An adjustment factor for a 4.5-year time shift was multiplied by the USGS bulk concentrations: Ca2+, Mg2+, K+, and Na+ concentrations were multiplied by 0.87 and 0.965 for Hinckley and Mays Point, respectively; SO2- concentration by 0.909 and 0.93; NO3 concentration by 1.489 and 1.176; NH4+ concentration by 1.194 and 1.211; and H+ concentration by 1.236 and 1.352. eN is the number of samples at USGS sites. f"Sum" gives the total microequivalents per liter potentially available for neutralizing acids in precipitation. g Range. h USGS weighted-average H+ concentrations are likely influenced by a few outliers. Median concentrations are ~50% less.

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138 June 1980. The sample volume weighted-average concentra- tions were calculated for each NADP site, and the means of these values are shown in Table 5.4. The table also presents weighted-average concentrations of data from Hinckley and May s Point, two of the USGS bulk sampling sites in New York; later in this chapter these two sites are listed with three others for con- sideration of regional time-trend analysis. The Hinckley and May s Point sites were chosen for inclusion in Table 5.4 because these sites tend to represent the extremes in concentrations found at the USGS stations, with the Hinckley site tending to have the lower concentrations and the May s Point site the higher concentrations (Barnes et al. 1982). Because the two data sets do not overlap in time (1978-1982 for NADP and 1975-1977 for USGS) the USGS data are presented in two ways. The two middle columns in Table 5.4 present the data from 1975-1977. The two right hand columns present the USGS data adjusted to account for the difference in time periods. Temporal trends of the concentrations of the ions listed in the table were determined by Barnes et al. (1982) for the period from 1965 to 1978. We estimated the data for the 1978-1982 period by multiplying the USGS data (1975-1977) by a factor derived from the calculated trend from 1965-1978 (see footnote d in Table 5.4 for details) . Regardless of whether the actual or adjusted USGS values are used, large differences exist between the NADP and USGS data sets. Calcium concentrations are three to nine times higher in the bulk data. Magnesium, potassium, ammonium, and nitrate concentrations are also higher in the bulk data (factors of 1.4 to 4.9). Sodium concentra- tions are about the same, and the hydrogen-ion concentra- tion is much higher in the bulk data, although one might anticipate the reverse. The bulk sulfate values are somewhat higher than the wet-only values at May s Point but are about the same as Hinckley values. The overall conclusion is that the USGS bulk concen- tration data are significantly different from NADP wet-only data for many ions. The sum of the basic cations is larger at the bulk sites than at the wet-only sites by more than twofold. -

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189 120 100 - cn 0 80 LLl 60 by o C' Z 40 _: \ so2- +1 \NO3 + Cl -/\ \ so2 _^ _ \\ H ~ 1 ~ _~_ ~ NO3 20 —NH4 1 1 1 1 o NY 20 Sow + on ~ ~ SHO 2- _N, {~' NO3 ~ _ / NH4 80 82 NC 41 ~so2~+ _ NO3 + Cl - ~ iso2~ _ _ ,~ Hit 1 / ~ NO3 — N H 4 1 1 1 1 80 82 80 82 CALENDAR YEAR ~ Ca2 + | M92+ + 1 Na + K + NH4 FIGURE 5.24 Calendar year median ion concentrations for three NADP sites for 1979-1982. NADP Data Because the data have only recently become available, no studies have been published that examine the time pattern for NADP stations (in particular for the 1979-1982 time period). Here an initial examination is presented. When the NADP began in 1978 most of the stations were located in the northeastern quadrant of the United States, although there were fewer stations in other areas. These stations have compiled relatively complete records for the 1979-1982 period. Samples were collected for each week that had precipitation and problems of contamination or equipment malfunction were infrequent. Results are presented for 12 sites whose locations are shown in Figure 5.23. The median annual concentrations for major ions and combinations of ions are shown in Figure 5.24 for three of the sites (ILll, NY20, and NC41), and volume-weighted

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190 ~ N ~ I ~ ~ ~ I ~ ~4 '' ~ ~ ~ ~ ~ N; I ~ I I ° ° — 1 1 1 1 1 + ~ N ~ I ~ ~ _ 1 1 1 1 1 1 1 1 O O O O C-l O 00 ~ N ~ , O ~ O I v, I _ z Z ~1 1 1 1 1 ~ ¢ 141 - o l , ~ ~ ~ I _ Oz I C~ / _ g ~ _ O O d. (l/bad) SNOIlVUlN3~:)NOO G31H!313M-3WnlOA c: z LL c, z LL J ~: C~ o N N , + ~ ~ ' ~ Y Z ~ ~ Z ~ Y ~ ~ ~r U) (D N O + O I — Z Z t~ N tD 1 1 1 1 14 1 1~ ° N ~ O I i/ ~ ~: LL t~d (D t(~0 Z J :: 1 - N ~ ~ ~ ~ ~ _, O — _ Z Z ~, ,,, C`' O O O O O O O C~l O 00 C.C) ~ ~ r~ _ (l/bad) SNOIl~lUlN3ONOD 031HE)13M-3Wnl OA

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191 + + - OCR for page 128
192 average concentrations (to be referred to as weighted concentrations) are shown for all twelve sites in Figure 5.25. The results shown in Figures 5.24 and 5.25(a) can be compared to evaluate differences in the two data presen- tations. For site ILll the anions and hydrogen ion have similar median and weighted concentration patterns, although hydrogen ion is lowest in 1979 for the weighted concentration and in 1982 for the median concentration. For ammonium and the base cation sum (the sum of calcium, magnesium, potassium, sodium, and ammonium) the year of maximum concentration differs for the median and weighted concentrations. There are also differences in median and weighted concentrations at sites NY20 and NC41. For site NY20 the concentration patterns of ammonium ion and the base-cation sum differ between the two figures, while for site NC41 the sulfate and hydrogen ion concentration patterns differ in the two presentations. To compare the 12 sites, volume-weighted average concentrations are shown in Figure 5.25. From Figure 5.25 (b) for the three sites in Ohio, we see that almost every curve has some site-to-site differences even though the sites are spatially close together. In particular, sulfate has a different pattern for each site. From Figure 5.25(c) we see that the various curves of the first two sites, GA41 and NC25, are quite similar in shape but different from NC03. However, the patterns for site NC03 are quite similar to those for site NC41 in Figure 5.25(a). From Figure 5.23 we see that GA41 and NC25 are adjacent, as are NC03 and NC41, suggesting a spatial consistency for the annual concentration patterns. In summary, the most persistent pattern for all ions and all sites is higher concentrations in 1980 and 1981 and lower concentrations in 1979 and 1982. However, not all sites or all ions follow the general pattern. Even multiple sites within a single state have different time patterns (for example, in North Carolina and Ohio). It is not uncommon to have most of the major conservative ions (sulfate, nitrate, ammonium, and calcium) following the same time pattern (for example, see ILll). This suggests that for the 1979-1982 period meteorology, as opposed to the temporal changes in source emissions, was important in determining time trends of ion concentrations.

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193 CONCLUSIONS AND RECOMMENDATIONS Conclusions · The eastern half of the United States and south- eastern Canada south of James Bay experience concentra- tions of sulfate and nitrate in precipitation that are, in general, greater by at least a factor of 5 than those in remote areas of the world, indicating that levels have increased by this amount in northeastern North America since sometime before the 1950s. · The northeastern quadrant of the United States, consisting of Region B. the northernmost states of Region C, and most of Region D, is the area most heavily affected by acidic species (the ions of hydrogen, sulfate, and nitrate) in precipitation. · Spatial distributions of annual (precipitation- weighted-mean) concentrations and wet deposition for the major ionic species in precipitation are adequately known on the regional to continental scale in North America to afford comparison with spatial patterns of emissions and other possible indicators of acid deposition, such as visibility and the quality of surface waters. The 1980 concentration and deposition fields are acceptably representative of the broad-scale patterns present in the 7-year period 1977-1983. · Data on the chemistry of precipitation before 1955 should not be used for trend analysis. . Precipitation is currently more acidic in parts of the eastern United States than it was in the mid-19SOs or mid-1960s; however, the amount of change and its mech- anism are in dispute. Changes in natural dust sources, anthropogenic acid sources, and sampling methods are all major factors that need to be considered in interpreting the changes in precipitation. . Precipitation sulfate concentrations and possibly acidity leave increased in the southeastern United States (Region C) since the mid-1950s. · For Hubbard Brook in New England, general agree- ment exists among several reports on the magnitude and direction of trends since 1964 in some species: hydrogen ion shows no overall significant trend; sulfate has decreased at approximately 2 percent/yr; sodium, chloride, calcium, magnesium, and potassium have shown strong decreases with time; nitrate appears to have increased until about 1970-1971 and subsequently leveled off; the analyses for the ammonium record are contradictory.

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194 · On the basis of past analyses of the USGS bulk data from New York and Pennsylvania since the mid-1960s a consistent network-wide trend does not emerge: results differ from station to station (with a few exceptions). Thus the USGS data base should be screened carefully when general conclusions about regional-scale trends are drawn. The USGS stations at Mays Point and Hinckley, New York, show sulfate trends based on time-series analysis to be consistent with those at Hubbard Brook, making it more likely that a regional trend exists in this parameter. . On the basis of preliminary analyses of CANSAP- APN and NADP data for the eastern half of the continent, changes in anthropogenic emissions during the period 1979-1982 are not readily apparent; concentration changes from year to year occur concurrently in both anthropogenic and natural constituents and are thought to be more strongly affected by meteorological rather than emissions changes during this period. . Because data from the WMO BAPMoN network have not yet been rigorously quality assured, trend analyses have not yet been carried out. . The uncertainties associated with available time- trend estimates of precipitation chemistry make it unwise to draw quantitative conclusions on the basis of direct comparisons between changes in concentrations or deposi- tion and changes in emissions. In cases in which artifacts may be introduced in a precipitation-chemistry data set, for example, by changes in analytical methods over time, even qualitative conclusions related to emission changes may not be possible. . In general, individual sites or groups of a few neighboring sites cannot be assumed a priori to provide regionally representative information; regional repre- sentativeness must be demonstrated on a site-by-site basis. · General conclusions drawn from merging bulk precipitation data sets with wet-only precipitation data sets are not valid unless detailed site-by-site analyses justify the approach. Similar evaluations should be made when different wet-only precipitation data sets are merged. · Conclusions regarding time trends depend to some extent on the statistical analysis technique chosen (see Table 5.12). They also depend on the period of the record used for analysis, and this point is frequently ignored.

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195 · Trends for particular ions in precipitation (including hydrogen) should be interpreted only in relation to trends of all the major ions present. Recommendations · Expand the scope of deposition monitoring to include dry and occult deposition. · Strive for improved understanding of the char- acteristics of all forms of deposition, particularly as a function of altitude and species. . Continue long-term, wet-only national monitoring, preferably under the auspices of a single jurisdiction. · Use measures other than pH or hydrogen-ion concentration as a measure or an indicator of the acid deposition problem. . Investigate the role of organic acids in precipitation acidity. . sites. Discourage bulk sampling except in specific circumstances in which it can be demonstrated that bulk and wet-only samples are providing comparable data. · Use other major ionic species (in addition to hydrogen and sulfate) in future trend analyses; also use ancillary information such as meteorological/ climatological data. · Investigate the role of meteorological and climatological variability in temporal and spatial variations of acid deposition. · Monitor air concentrations in closely located · Take great care in all aspects of selecting and maintaining a station, particularly its siting and operation, if it is to be used in the future for trend analysis. . Devote more effort to the analysis of available data. · Emphasize network design to ensure that networks are capable of detecting changes, particularly those changes that may result from strategic emissions reductions. · Establish a mechanism and a procedure to compile data, to perform quality control and assurance, to analyze routinely, to synthesize results, and to publish multinetwork data.

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