## Trends in Precipitation Chemistry in the United States, 1983-94:       An Analysis of the Effects in 1995 of Phase I of the            Clean Air Act Amendments of 1990, Title IV

Principal Investigators

James A. Lynch, Ph.D.                               Pennsylvania State University

Van C. Bowersox                                       Illinois State Water Survey

Jeffrey W. Grimm                                       Pennsylvania State University

This research has been funded by the U.S. Geological Survey as a contributionto the National Acid Precipitation Assessment Program.



---

**TABLE OF CONTENTS**

LIST OFTABLESAPPENDIX ILIST OF FIGURESAPPENDIX IIINTRODUCTIONMETHODS AND RESULTS







## LIST OF TABLES

















## APPENDIX I

























## LIST OF FIGURES















































## APPENDIX II

























---

INTRODUCTIONTitle IV of the Clean Air Act Amendments of 1990 (CAAA-90) (PublicLaw 101-549) seeks to reduce acidic deposition in the United Statesthrough phased reductions in sulfur dioxide and nitrogen oxideemissions.  One of the first steps in assessing the effectivenessof emissions reductions is to evaluate spatial and temporal trendsin sulfate (SO42- ), nitrate (NO3-), and hydrogen (H+) ionconcentrations in precipitation.  Lynch et al. (1995a) reported themost recent comprehensive summary of temporal trends inprecipitation chemistry in the United States using data from 58National Atmospheric Deposition Program/National Trends Network(NADP/NTN) sites from 1980 through 1992.  Results showed widespreaddeclines in SO42-  concentrations accompanied by significantdecreases in all of the base cations, most noticeably calcium(Ca2+) and magnesium (Mg2+).  As a result, only 17 of the 42 siteswith significant (p<0.05) decreasing SO42-  trends had concurrentsignificant decreasing H+ trends.  Lynch et al. (1995a) cautionedthat they found considerable inter- and intra-regional variabilityin pH trends even among multiple sites within the same state.  Theyspeculated that significant site-specific changes in NO3- andammonium (NH4+) concentrations, as well as varying magnitudes ofSO42-  and Ca2+ changes could help explain much of this variability.

In the National Acid Precipitation Assessment Program (NAPAP)report on deposition monitoring, Sisterson et al. (1990) applied aKendall seasonal tau test for trend detection in the presence ofseasonal cycles (see Hirsch et al., 1982).  Seasonal observationswere defined as monthly mean precipitation-weighted concentrationsor monthly depositions for each site.  Data from six North Americannetworks, including NADP/NTN, were used in the analysis.  Sen'smedian slopes (see Hirsch et al., 1982) were calculated to estimatethe magnitude of changes in concentrations and depositions forsites meeting predetermined data completeness criteria (Olsen etal., 1990; Sisterson et al., 1990).  The NAPAP trends analysisaddressed two time periods: a 39-site (24 NADP/NTN sites), 9-yeardata set (1979-1987) and a 148-site (76 NADP/NTN sites), 6-yeardata set (1982-1987).  Statistically significant (p<0.05) trendswere found at a higher proportion of sites for all ions, exceptNO3-, in the 9-year data set than during the 6-year period,suggesting that the largest changes occurred early in the datarecord from 1979-1982.  Except for NO3- and potassium (K+), theabsolute magnitudes of the median percent changes were larger forthe 9-year period, as well.  For the ions that most affected pH(SO42- , NO3-, NH4+, and Ca2+), the strongest evidence for changewas in the base cation, Ca2+.  Calcium decreased at 38 of the 39sites over the 1979-1987 period; at 13 sites these changes, alldown, were significant (p<0.05).  Sulfate decreased at 35 of the 39sites; at seven sites these changes, all down, were significant(p<0.05).  Nitrate, NH4+, and H+ ions exhibited a more even splitof increasing and decreasing changes (Sisterson et al., 1990, pages6-154 through 6-163).  These results led the authors to concludethat the acidity of precipitation did not decrease because of theconcurrent decrease in cation concentrations.

Lynch et al. (1995b) also evaluated trends in precipitationchemistry at NADP/NTN sites for multiple summary periods.  Thisanalysis utilized a general, linear least-squares model to evaluatetrends for three summary periods: 1980-1993, 1983-1993, and 1985-1993.Major growth in the NADP/NTN occurred between 1980 and 1985effectively tripling the number of network sites.  Choosingsuccessively later start dates allowed comparison of results amonglarger numbers of sites having greater spatial coverage.  It alsoallowed the assessment of the effect of summary period length ontrends.  Regardless of the summary period length, the vast majorityof NADP/NTN sites exhibited decreasing SO42-  concentrations.Consistent with the findings of Sisterson et al. (1990), thisanalysis also indicated that SO42-  concentrations decreased morerapidly during the early 1980s and less rapidly thereafter.Nitrate and NH4+ concentrations exhibited considerable variabilitywith only a few sites showing statistically significant trends,some positive and some negative.  The larger SO42-  decreases in theearly 1980s were similar to sulfur dioxide emissions changes, whichdecreased more rapidly between 1980 and 1983, then vacillated abouta nearly constant rate in many states (Lins, 1987).

Hedin et al. (1994) reported steep declines in base cations inprecipitation from Sweden, the Netherlands, and the United States.In the United States, annual volume-weighted mean concentrations ofSO42-  and base cations (defined as the sum of non-sea-salt Ca2+,Mg2+, Na+, and K+) were calculated using data from 32 NADP/NTNsites (1979 or 1980 to 1990), nine Multistate Atmospheric PowerProduction Pollution Study (MAP3S) sites (1978-1988), and theHubbard Brook Experimental Forest (HBEF) (1965-1989).  A regressionof these annual means against time in years yielded trend estimatesfor SO42-  and cations (NH4+, Ca2+, Mg2+, Na+, K+).  At the HBEF andat NADP/NTN sites in the Northeast, Southeast, and Midwest, bothSO42-  and base cations decreased and the trends were significant(p<0.05 or p<0.001).  At the nine MAP3S sites, SO42-  decreases werealso statistically significant; however, base cations decreased atonly five sites, none of which were statistically significant.Hedin et al. (1994) concluded that recent declines in both basecation and SO42-  concentrations had offset one another in varyingproportions in many regions in the Northern Hemisphere.

Other authors have also focused their attention on describingtrends in precipitation concentrations or depositions (for easternNorth America, see Sirois, 1993 or Oehlert, 1993; for NADP/NTNdata, see Baier and Cohn, 1993; for MAP3S data, see Dana andEaster, 1987; for data from the Netherlands, see Ruijgrok andRomer, 1993 or Buishand et al., 1988; for data from Texas, seeKessler et al., 1992; and for data from New York, see Hirsch andPeters, 1988).  All have shown general decreasing trends in SO42- and base cation concentrations, with small and generallystatistically insignificant changes in free acidity.  In thesestudies, both parametric (e.g., Buishand et al., 1988) andnonparametric (e.g., Baier and Cohn, 1993) approaches have beenused.

Measuring and reliably quantifying trends in precipitationchemistry are essential tools for assessing the effectiveness ofsulfur and nitrogen emissions reductions programs designed toprotect the environment.  Trend estimation techniques were comparedat a recent workshop (Holland et al., 1995).  Four approaches wereexamined, including the linear least-squares model used in thisanalysis and several modifications of the seasonal Kendall test(Hirsch et al., 1982).  In one case, a modification of Sen's slopeestimator (Hirsch et al., 1982) was used to estimate the annualpercentage change in precipitation-weighted mean concentrations.This comparison indicated that all four approaches were valid andgenerally yielded similar long-term trend results, althoughstrengths and weaknesses were noted for each approach.  The linearleast-squares approach provided the added advantage of quantifyingseasonal changes in concentration over time.

Phase I of Title IV of the CAAA-90 requires specific reductions insulfur dioxide emissions on or before 1 January 1995 at selectedelectric utility plants, the majority of which are located instates east of the Mississippi River.  As a result of thislegislation, large reductions in sulfur dioxide emissions arelikely to have occurred in 1995 which should have affected SO42- and H+ concentrations, and to a lesser extent NO3- concentrations,in precipitation in this region.  The purpose of this study was toevaluate this effect, if any, at NADP/NTN sites in the region in1995.  This assessment was based on a comparison of observed 1995SO42- , H+, and NO3- concentrations at these NADP/NTN sites withestimates obtained from linear least-squares trend models ofprecipitation chemistry data collected from 1983 through 1994.  The1983 through 1994 summary period was selected for the followingreasons: (1) the 1980-1994 record limits the number of sites to 57,too few for meaningful inter- and intra-regional comparisons; (2)the 1985-1994 summary period increases the number of sites to 168,but reduces the strength of the trend models estimates because ofthe shortness (10 years) of record; (3) the 1983-1994 summaryperiod represents the best compromise between both the spatial andtemporal strengths of the analysis; (4) the 1983-1994 summaryperiod avoids potential start-up problems associated with fieldsampling and laboratory protocols; (5) the 1983-1994 period hasstable emissions relative to the early 1980s (Lins, 1987); and (6)the National Trends Network (NTN) became a formal part of NADP in1983, so many of the sites added in the 1983-1994 summary periodover the 1980-1994 period were NTN sites.

## METHODS AND RESULTS

Since its inception in 1978, NADP/NTN sampling protocols requiredsite operators to send precipitation samples to the CentralAnalytical Lab (CAL) at the Illinois State Water Survey in the~14-liter HDPE (high density polyethylene) buckets used for collection.The bucket lid contained a rubber o-ring, which sealed the lid andbucket, preventing leaks during shipment.  This o-ring was found tobe a source of many of the same cations and anions measured inprecipitation.  For most samples, the concentration biases due too-ring contamination were small and unimportant.  Most affectedwere free H+ concentrations (pH) at sites in the western states,where pH values are typically above 4.8.  Samples lost free aciditydue to the o-ring so sample pHs were biased high.  Efforts toimprove lid cleaning procedures reduced, but could not eliminate,o-ring contamination.

Although o-ring contamination was persistent, occasional changes bythe lid manufacturer added to the uncertainty in the size of thebias over time.  This had the potential for interfering with long-termtrend analyses.  Studies had shown that samples sent to theCAL in bottles had much smaller losses of acidity than samples sentin the buckets with the lid o-ring.  As a result, the NADP/NTNelected to change its procedures effective 11 January 1994, endingthe use of the buckets for sample shipment and eliminating anycontact of the sample with the o-ring.  After 11 January 1994, asnap-on lid with no o-ring was used to cover the collection bucketduring transport from the site to the field laboratory and a one-liter,wide-mouthed, HDPE bottle was used for shipment to the CAL.

A special study was conducted at 11 sites scattered across thenetwork to assess the size and direction of the concentrationchange of each analyte due to lid o-ring contamination.  Theresults of this study were analyzed with the intent of identifyinga set of algorithms that could be used to adjust the pre-1994 datafor the effects of o-ring contamination.  Side-by-side samples werecollected at these sites, one using the old procedure with thebucket and lid with the o-ring and the other using the newprocedure with the bottle.  Paired concentration differences werecalculated from the two samples at each site.  Nonlinear, leastsquares regressions were performed to evaluate these differencesfor dependence on sample volume or on pH or on a combination ofthese factors.  CAL experimental results suggested that bothfactors could be important in explaining the differences betweenthe bucket and bottle samples.  Specifically, these resultsindicated that as the volume increases the o-ring contaminationincreases, approaching a maximum value.  For a fixed volume, the o-ringcontamination is large at low and high pH values and small atmid-level pHs.  These volume and pH relationships dictated the formof the equations used in the analysis.

Six different models (algorithms) per analyte were evaluated foreach of the conservative ions.  These included:  (1) a constantmass model, (2) a simple linear regression model, (3) a massdifference model as a function of sample volume, (4) a massdifference model as a function of sample volume and pH, (5) a powerlaw model, and (6) hybrids of 2 and 3 or 2 and 4.  To select thebest algorithms, the root mean square (RMS) bucket/bottleconcentration differences and the RMS corrected bucket/bottleconcentration differences from the six models were calculated.  Asensitivity test of the RMS calculations was performed using a"bootstrap" experiment with 100 repetitions.  Following theseexperiments, the most robust algorithms with the lowest RMS of thecorrected bucket/bottle concentration differences were identified.Using the overall "best fit" models for the conservative ions, thepH (H+) correction was evaluated.  This evaluation revealed thatthe best model was one based on adding the effects of all of thecorrections plus an unmeasured cation (probably zinc, which can beleached from the lid seal), then calculating a corrected pH.  Theequations that best fit the special study results for allmeasurements are presented in the Appendix (Table A.1).  Four ofthe correction equations (K+, NO3-, Na+, and SO42- ) incorporateboth sample volume and sample pH as predictor variables.  For fourothers (Ca2+, Mg2+, NH4+, and Cl-), sample volume alone was thebest predictor.

A comparison of volume-weighted mean annual concentrations inprecipitation for corrected and uncorrected NADP/NTN data from1983-1993 (Appendix, Table A.2) illustrates the effects thecorrections (Appendix, Table A.1) had on NADP/NTN precipitationchemistry data.  At all sites, the corrections resulted in lowermean annual concentrations for all ions, except H+.  Hydrogen ionconcentrations increased as reflected in the lower pH values.  Witha few exceptions, the reductions in pH were less than 0.04 unit inthe eastern states; reductions in pH were higher in the West, withsome sites experiencing reductions in mean annual pH of more than0.3 unit.  Sulfate concentrations generally decreased from 0.03mg/L to 0.04 mg/L in the western region of the country; at Easternsites, SO42-  concentration decreases were about 0.01 mg/L withconcentration changes at some sites exceeding 0.1 mg/L.  Decreasesin NO3- concentrations were fairly uniform across the country andaveraged around 0.03 mg/L.  Chloride concentrations decreasedapproximately 0.005 mg/L and did not exhibit significant regionaldifferences.  Ammonium concentrations decreased approximately 0.01mg/L and were considerably more variable in the western than theeastern states.  Calcium and Mg2+ concentrations decreased between0.003 and 0.004 mg/L at most sites and exhibited very littlespatial variation across the country.  A similar pattern andmagnitude of change was also evident for K+ and Na+ concentrations.Overall, the correction equations did not result in any majoraberrations in the data set and are consistent with results frombucket versus bottle comparison studies.

**Temporal Trend Analyses**

Temporal trend analyses covered the entire NADP/NTN network,included all analytes except orthophosphate, and was based oncorrected data through 1993 and 1994 data.  Trends in analyteconcentrations were examined for the 1983-1994 summary period.Results of this analysis provided a baseline against which 1995concentrations were contrasted to assess the effects of the CAAA-90,Phase I emissions reductions on precipitation chemistry.

In the trend analysis of the summary period (1983-1994), weeklyprecipitation volume and corrected chemical observations wereaccumulated into bi-monthly precipitation totals and volume-weightedmean concentrations of H+ (from pH), SO42- , NO3-, Cl-,NH4+, Ca2+, Mg2+, K+, and Na+ ions.  Orthophosphate was omittedbecause a large percentage (>80%) of weekly samples haveconcentrations below the analytical detection limit (0.003 mg/L).Only valid weekly samples with a complete set of analyses were usedto calculate bi-monthly volume-weighted mean concentrations.  Sitesand bi-monthly records were selected for the trend analysisaccording to the following completeness criteria:



1. Only those monitoring sites having weekly precipitationchemistry records from   January 1983 through December 1994, wereconsidered.  At least 75 percent of the      precipitation recordedduring this summary period had to have valid chemical  analyses inorder for the site's data to be accepted for trend analyses.
2. For a bi-monthly record to be accepted, a valid analysis foreach ion had to be  available for at least 75 percent of the bi-monthlyprecipitation.
3. During each bi-monthly period, at least 50 percent of theweekly samples having         sufficient volume for analysis (>35mL) had to have a valid analysis for each ion.



Log-transformed concentrations were used because the modelresiduals have a more nearly normal distribution (Lynch et al.,1995a).  After initially fitting the model to a site'sconcentration data (expressed as μeq/L) for a given ion,studentized residuals were calculated.  Bi-monthly observationshaving a studentized residual >3.5 in absolute value wereeliminated from the data set and a second calculation of modelcoefficients was performed using the remaining observations.  Theselected cut-off value applied to the studentized residuals wouldbe exceeded by chance at a rate less than 0.001 under theassumption of normally distributed residuals of constant variance.

A tabular summary of the trend results is presented in Table A.3(Appendix).  This summary table contains information on thedirection (byear) and statistical significance (p) of the trend ineach analyte for each site that met the above completenesscriteria.  Sites are identified by their NADP/NTN CAL code.  TheMississippi River was used to segregate those sites located in theeastern and western sections of the country.  The number of sitesexhibiting increasing trends and the number exhibiting decreasingtrends are presented in Table 1.  These sites are furthersubdivided by statistical significance (p<0.05).

The direction and statistical significance (p<0.05) of trends foreach analyte are also presented graphically in Figures 1-9.  Upwardor downward pointing triangles indicate the direction of the trend.A solid triangle indicates a statistically significant (p<0.05)trend; an open triangle indicates that the trend is not significant(p>0.05).   NADP/NTN sites located in Alaska and Hawaii are notshown on the maps but were included in the analyses if they met thecompleteness criteria.

Changes in ionic composition of precipitation at each site from thebeginning (1983) to the end (1994) of the summary period werecalculated as the difference between the average of six bi-monthlymean concentrations (μeq/L) estimated from the models for 1983 and1994.  Cation and anion concentration (μeq/L) changes and percentchanges from 1983 to 1994 are presented in Table 4.A (Appendix).Table 2 lists the mean concentration (μeq/L) changes and percentchanges for all sites and for only sites with statisticallysignificant (p<0.05) trends.  The coincidence of significant(p<0.05) decreasing SO42-  and H+ concentrations is summarized inTable 3.  Sites are identified by the NADP/NTN CAL code.

**Evaluating the Effectiveness of Phase I of the Clean Air ActAmendments of 1990**

Phase I of the CAAA-90, Title IV was implemented on 1 January 1995.One-hundred and ten (110) electric utility plants were affected in21 states, 17 of which are located east of the Mississippi River.Sixty-three (63) of these targeted plants are located in states inthe Ohio River Valley.  Notwithstanding the effects of trades ofemissions allowances, large reductions in sulfur dioxide emissionsare likely to have occurred in 1995, particularly in the easternstates.  These reductions should have affected precipitationchemistry, particularly SO42-  and H+ concentrations, and to alesser extent NO3- concentrations.  In order to evaluate thiseffect, if any, at NADP/NTN sites in the eastern states in 1995,the linear least-squares models discussed above were used toestimate 1995 bi-monthly and annual mean concentrations for eachanalyte.  The model estimates were compared to actual bi-monthlyvolume-weighted mean concentrations for each NADP/NTN site that metthe above completeness criteria.  Because the bi-monthly means weresummed to obtain annual means, only 109 sites with six valid 1995bi-monthly means were included in this analysis.  For comparativepurposes, separate, identical analyses were conducted for siteslocated east and west of the Mississippi River.  The impact ofPhase I emissions reductions in the western states should have beenminimal given that only 16 of the affected plants are located inthis region (Missouri-8, Iowa-6, Kansas-1, Minnesota-1).  As aresult, a comparison of predicted with observed concentration meansin both regions provides a means of evaluating model performanceand the effect of CAAA-90 emissions reductions on precipitationchemistry.

The results of the comparisons of 1995 measured and estimatedbi-monthly and annual mean concentrations for individual siteslocated in eastern and western regions of the country are presentedin Tables 4, 5, and 6 for SO42- , H+, and NO3- concentrations,respectively.  Similar comparisons for the remaining cations andanions are presented in the Appendix (Tables A.5-A.10).  Easternresults are further stratified into Northeast (NE) and Southeast(SE) regions.  Table 7 list the frequency of occurrence of observed1995 cation and anion concentrations that were less than predictedconcentrations.  Regional mean and percent departures from the trendmodel estimates are presented in Table 8 for annual and bi-monthlycation and anion concentrations.

One way to assess the statistical significance of the effects ofPhase I emissions reductions on precipitation chemistry is todetermine the significance of the deviations of the 1995 bi-monthlyobservations from the trend model estimates derived from the1983-1994 data.  Because only one year (1995) of data is availablesubsequent to Phase I implementation, quantifying a slope change inthe long-term trend is inappropriate.  However, the data do permitestimation of changes in the intercept of the trend line coincidingwith the implementation of emission reductions.  Estimating thechange in the intercept is appropriate because it directlyquantifies the "step-function" that would occur in precipitationchemistry if emissions were reduced suddenly, i.e., large reductionsover a short period of time.  The change in intercept for the 1995data was estimated by adding a binary indicator variable to theseasonal trend models.  This indicator variable was given a value of1 for 1995 observations and a value of 0, otherwise.  Thisvariable's estimated coefficients and its significance are presentedin Table A.11 (Appendix) which indicates in what direction and atwhich sites the 1995 data departed from the historical trend.Because there are only six bi-monthly observations in 1995, thepower of this statistical test for change is rather low.Nevertheless, the results of this analysis reaffirm the results fromthe linear least-squares models and illustrate that the 1995precipitation chemistry data were different from the historicaltrends at NADP/NTN sites located in the Eastern United States.Sulfate and H+ concentrations in 1995 at 12 of the 62 sites locatedeast of the Mississippi River were significantly (p<0.05) differentwhen compared to the historical trend models at these sites; forNO3- concentrations, only two sites were significantly different,both positive indicating higher concentrations in 1995 than themodel estimates.In addition to the above analyses, 1983-1994 trends for selectedsites are presented graphically by plotting observed bi-monthly meanconcentrations along with two corresponding estimates ofconcentration against time.  One set of estimates was from theleast-squares general linear models described above, the other setwas from a LOWESS regression of the observed bi-monthly meanconcentrations.  The LOWESS smoothing method, described by Cleveland(1979 and 1985), was added because it does not assume a functionalrelationship between concentration and time and can depictnonlinearities in trends.  The LOWESS method was not used tostatistically assess concentration trends because assessment ofchanges in ionic concentration from one time to another, bydefinition, involves linear hypotheses and because LOWESSregressions do not provide an overall test of trend or model fit forthe data set as a whole.  The "moving window" of data points was setat 2 years (12 bi-monthly points) before and after the date to beestimated.  The distance weighting function for the LOWESSregressions was,





The robustness weights for the second stage of the LOWESSestimation procedure were calculated as,





For comparison purposes, a disjunct LOWESS regression line wasplotted for the six bi-monthly mean concentrations in 1995.  ThisLOWESS regression was based on 1993-1995 bi-monthly meanobservations.  It was separated physically from the 1983 through1994 LOWESS line to emphasize the step-function change in 1995 fromthe preceding 12-year summary.  Six examples of H+, SO42- , and NO3-concentration trends showing the linear model (solid line) andLOWESS regression (dashed line) and the observed bi-monthly meanconcentrations (solid circles) are presented in Figures 10-15 forthe 1983-94 and 1995 summary periods.  Figures 10 and 11 (KY03 andIL63, respectively) illustrate a dramatic change in 1995 bi-monthlymean concentrations relative to the historical trend, while Figures12 (NC36) and 13 (VT01) illustrate a moderate decrease and Figures14 (IN34) and 15 (MA08) illustrate no change.  The remaining cationand anion comparisons for these six sites appear in the Appendix(Figures A.1-A.12).

Color-scaled raster maps based on surface estimation algorithms arefrequently used to display precipitation chemistry data over aregion.  This approach was used to illustrate the departures of 1995observed concentrations (μeq/L and percent) from the trend modelestimates for 1995.  Maps were prepared for the eastern half of thecountry, where the greatest impact of Phase I emissions reductionswas most likely to occur.  The differences were plotted using theMultiquadic Equations (MQE) surfacing function described by Hardy(1971) and evaluated as a tool for depicting regional wet depositionby Grimm and Lynch (1991).  Also included is a color-scaled rastermap showing deviations in 1995 precipitation volumes from the 1983through 1994 annual average volumes.  All four maps are based ondata from the 109 sites included in the analysis of theeffectiveness of the CAAA-90 discussed above.  Results are presentedin Figures 16, 18, 20, and 22 for SO42- , H+, NO3-, and precipitationvolume, respectively; percent differences are presented in Figures17, 19, 21, and 23, respectively.  The location of NADP/NTN sitesare indicated by a plus (+) sign.

## DISCUSSION

Sulfate concentrations at 92% of the NADP/NTN monitoring sites in theUnited States have decreased since 1983; the trends are statisticallysignificant (p<0.05) at 38% of the sites (Table 1, Figure 1).  Nomajor regional (east vs west) differences in the number and percentageof sites exhibiting decreasing SO42-  trends areevident.  Sites with increasing SO42-  trends(Table 1) are also uniformly distributed across the country; however,only one of the increasing SO42-  concentrationtrends is significant (p<0.05).  The mean change inSO42-  concentrations across the United Statesfrom 1983 to 1994 was -4.80microequivalents per liter (16.4%) compared to -7.69μeq/L (25.9%) atsites with statistically significant trends (Table 2).  Sulfateconcentrations have decreased more rapidly in the eastern states (5.94μeq/L vs 3.71 μeq/L), although the mean percentage change hasbeen greater in the western states (18.9% vs 13.8%).  This pattern isalso evident at sites with significant (p<0.05) trends; however, theconcentration and percentage decreases are much larger (Table 2).

Nitrate concentration trends do not exhibit a consistent spatialpattern (Figure 2).  The number of sites exhibiting increasingtrends is nearly equal to the number exhibiting decreasing trends(Table 1).  Perhaps of greater importance, a larger percentage ofsites with increasing trends (14%) are significant (p<0.05), whileonly two of the 153 sites included in this analysis have significantdecreasing trends.  A larger percentage of sites with increasingtrends are located in the western states.  As expected from theseresults, the network mean change in NO3- concentrations was verysmall (<0.5 micro equivalents per liter) and positive (Table 2).  However, the mean NO3-concentration at the 23 sites with statistically significant trends(2 decreasing, 21 increasing) increased 51.6% (3.34 μeq/L).  Thelargest increases in NO3- concentrations occurred in the westernstates.

Ammonium concentrations increases were larger and more widespreadthan NO3- increases (Table 2, Figure 3).  Eighty percent (80%) ofthe sites in the NADP/NTN exhibited increasing NH4+ concentrationssince 1983 (Table 1); at 22% of the sites the trends are significant(p<0.05).  Only one of the sites has a statistically significantdecreasing trend.  Although regional patterns are similar, thelargest number of sites with significantly increasing trends arelocated in the western states.  The average increase in NH4+concentrations at these sites is 6.40 μeq/L, an increase of 86.6%over 1983 levels.  The network-wide increase in NH4+ concentrationssince 1983 is 2.33 μeq/L (28.2%).  Clearly, nitrogen concentrations(both NO3- and NH4+) in precipitation have increased in the UnitedStates since 1983.

Calcium (Figure 4), Mg2+ (Figure 5), and K+ (Figure 6)concentrations decreased markedly in the United States since 1983.Very few sites exhibited increasing concentrations (Table 1):  Ca2+(9 sites), Mg2+ (2 sites), and K+ (21 sites); only two of thesetrends (both K+) are significant (p<0.05).  Decreasing trends aresignificant at 52% of the sites for Ca2+, 77% for Mg2+, and 38% for K+.  A higher percentage of these sites is located in the eastern states.  Mean percentage decreases in these cations are fairlyconsistent across the United States, 23.5% for K+, 27.9% for Ca2+and 39.7% for Mg2+ (Table 2).  Despite regional similarities inpercent decreases, the concentration (μeq/L) changes have beenconsistently larger in the western portion of the country for allthree cations.  These same patterns are evident when only sites withsignificant trends are compared, although the percent and μeq/Lchanges are larger.  Clearly, base cation concentrations havedecreased over the past 12 years.  Similar results have beenreported by Lynch et al., (1995a, 1995b); Hedin et al., (1994); andSisterson et al., (1990).

Sodium and Cl- concentrations generally decreased across the country(Figures 7 and 8); trends are significant (p<0.05) at 20% of thesites for Na+ and 37% for Cl- (Table 1).  Regional differences inboth Na+ and Cl- trends are evident, with slightly more significantCl- and Na+ trends in the western than eastern region of thecountry.  Across the United States, Na+ and Cl- concentrationsdecreased approximately 20% on average since 1983 (Table 2).  Atsites with significant trends, Na+ and Cl- concentrations are 36.8%to 32.3% lower.  On a concentration basis, both Na+ and Cl-decreased 1.5 μeq/L since 1983.

Like SO42-  concentrations, the majority of sites (81%) exhibiteddecreases in H+ concentrations from 1983 through 1994 (Table 1,Figure 9).  However, only 39 sites (25%) have statisticallysignificant H+ decreases, 20 in the western states and 19 in theeastern states.  Of the 29 sites exhibiting H+ increases, only oneis significant.  Free acidity in precipitation across the UnitedStates decreased by 2.68 μeq/L (13.4%) on average since 1983, withthe largest concentration (smallest percentage) changes occurring inthe East (Table 2).  This same pattern is evident when only siteswith significant trends are compared, although both the percentageand concentration decreases are more than twice as large.  The lackof consistency between sites with both significant decreasing SO42- and H+ concentrations (Table 3) suggests that concurrent changes inother ions have offset varying amounts of the SO42-  decrease.  As aresult, commensurate reductions in H+ did not occur.  A similarhypothesis has been suggested by Lynch et al., (1995a); Hedin etal., (1994); and Sisterson et al., (1990).  The ions most frequentlyoffsetting the SO42-  reductions are the acid neutralizing cations,Ca2+ and Mg2+.  However, at some sites, increasing NO3-concentrations have offset SO42-  reductions leaving small changes infree acidity (Lynch et al., 1995a).

**Comparison of Predicted and Observed 1995 Concentrations**

Sulfate and H+ concentrations in the eastern states in 1995 wereconsiderably lower than predicted from the trend models for the1983-1994 reference summary period (Tables 4 and 5); NO3-concentrations remained relatively unchanged (Table 6).  Twoexamples each of relatively large decreases (KY03 and IL63),moderately large decreases (NC36 and VT01), and little to no change(IN34 and MA08) in 1995 bi-monthly mean SO42-  and H+ concentrations,compared to the 1983-1994 trend models, are shown in Figures 10-15,respectively.  Site selection was based on change in SO42- concentrations in 1995 relative to the historical trend at eachsite.  A LOWESS regression line (dashed line) is included toillustrate the step-function change in SO42-  and H+ concentrationsthat occurred in 1995.  Corresponding base cation and Cl-concentrations at these sites are shown in Figures A.1-A.12 in theAppendix.

Mean annual H+ and SO42-  concentrations in 1995 were below predictedvalues at 88.7% and 79.0%, respectively, of the 109 sites that metthe data completeness criteria (Table 7).  By comparison, only 31.9%and 36.2%, respectively, of the Western sites were below modeledestimates.  On average, 1995 measured SO42-  concentrations in theeastern states were 4.01 μeq/L (9.8%) below modeled estimates (Table8).  The SO42-  decreases were substantially larger than 4 μeq/L inmuch of the Northeast.  In contrast, at sites west of theMississippi River 1995 SO42-  concentrations averaged 1.37 μeq/L(7.5%) higher than the modeled estimates (Table 8).  The spatialpattern of H+ decreases in the East was virtually the same as thepattern of SO4 decreases, although the magnitude of theconcentration and percentage decreases were even larger than SO42- decreases (Table 8).

Unlike SO42-  and H+ concentrations, NO3- concentrations in 1995 wereabove predicted concentrations at the majority of sites located inboth regions of the country (Table 7).  Approximately 61% of thesites in the East recorded higher NO3- concentrations in 1995 thanpredicted from the 1983-1994 models; 57% of the western sites wereabove model predictions (Table 7).  Nitrate concentrations in 1995were 0.97 μeq/L (4.6%) and 0.76 μeq/L (3.8%) above modeled estimatesin the eastern and western states, respectively (Table 8).  Inaddition, there was no evidence of sharp drops in the Northeast asthere was for SO42- .  These results suggest that nitrogen oxidesemissions were not significantly affected by Phase I of the CAAA-90,Title IV in 1995, at least not on a broad regional basis.

Color-scaled raster maps of the changes in SO42- , H+, and NO3-concentrations are shown in Figures 16, 18, and 20, respectively;percent changes in these ions are presented in Figures 17, 19, and21, respectively.  The largest reductions in SO42-  and H+concentrations occurred along the Ohio River Valley and in stateslocated immediately downwind of this region.  Although no emissionsdata are presented in this analysis, this region was clearlytargeted for major reductions in sulfur dioxide emissions by Phase Iof the CAAA-90, Title IV.  In fact, of the 110 plants affected byTitle IV, 63 are located in this region.  Downwind of the Ohio RiverValley, e.g., New England or the southeastern states., decreases inboth SO42-  and H+ concentrations in 1995 were smaller.  There areonly six affected sources in New England and 18 in the Southeastthat were targeted for reductions in emissions in Phase I of theCAAA-90, Title IV.  Another important feature of these maps is thatthe SO42-  reductions in the eastern states are roughly matched inmagnitude and location by H+ reductions.  The largest decreasesoccur in the Ohio River Valley and the northern portion of theMid-Atlantic region.

To examine whether the sharp drop in SO42-  and H+ concentrations in1995 were due to precipitation anomalies, a comparison of bi-monthlyand annual precipitation volumes at each site with their 1983-1994means was undertaken.  Results of this analysis are listed for eachsite in Table A.12 (Appendix).  A summary of these results ispresented in Figures 21 and 22, which depict departures of 1995annual precipitation from the mean annual volume during 1983 through1994.  A comparison of the precipitation volume color-scaled map(Figure 21) with the SO42-  (Figure 16) and H+ (Figure 18)concentration maps reveals that where 1995 SO42-  and H+concentrations were higher than trend model estimates (e.g., thesouthwestern portion of the Eastern United States and a region southand east of Lake Michigan), 1995 volumes were below the 1983-94averages.  Ionic concentrations in weekly NADP/NTN samples depend onprecipitation volume (Baier and Cohn, 1993).  Low precipitationvolumes are associated with high concentrations and vice versa.When these factors are taken into consideration, the actualreductions in SO42-  and H+ concentrations across much ofPennsylvania, Western New York, and the southern portion of NewEngland in 1995 would have been greater had a more "average" amountof precipitation occurred in these regions.

A comparison of the differences in 1995 annual and bi-monthly meanconcentrations for those cations (NH4+, Ca2+, Mg2+, K+, and Na+) andCl- that would not have been substantially affected by Phase Isulfur dioxide reductions (Tables A.5-A.10, Appendix), supports theargument that the changes in SO42-  and H+ concentrations in 1995were not the result of lower precipitation volume.  Precipitationvolumes would not selectively affect H+ and SO42-  concentrations;all ionic concentrations would be affected.  A review of theindividual site data in Tables A.5-A.10 (Appendix) and observeddifferences in NO3- concentrations (Figure 20) and percent changes(Figure 21) support this statement.  It is not apparent that NO3-concentrations were affected over the eastern states by Phase Iemissions reductions, only that NO3- was affected by the belowaverage precipitation volumes.  For example, regions with higherthan predicted 1995 NO3- concentrations had below averageprecipitation and regions with lower concentrations in 1995generally had above average precipitation.  Although nitrogenemissions may have been affected by the CAAA-90, Title IV, thereductions would have been much smaller than the sulfur dioxidereductions.  In addition, since Phase I, Title IV of the CAAA-90targeted only stationary (utilities) sources and since these sourcescontribute only one-third of the total United States nitrogen oxideemissions (Placet, 1990), it is highly unlikely that large regionsof the East would experience major reductions in nitrateconcentrations in 1995.

## CONCLUSIONS

Acknowledgment---This work was supported in part by the NationalAtmospheric Deposition Program/National Trends Network (NADP/NTN)through funds provided for its Central Analytical Laboratory at theIllinois State Water Survey.  The NADP/NTN is a NRSP-3 Project ofthe State Agricultural Experiment Stations (SAES).  Funding isprovided by federal and state agencies, universities, publicutilities and industries, as well as the SAES.

## REFERENCES

Buishand, T.A., Kempen, G.T., Frantzen, A.J., Reijnders, H.F.R. andvan den Eshof, A.J. 1988. Trend and seasonal variation ofprecipitation chemistry data in the Netherlands.  *AtmosphericEnvironment*, 22:339-348.

Cleveland, W.S.  1979.  Robust locally weighted regression andsmoothing scatterplots.  *Journ. of the American Statistical Assoc.*74:829-836.

Cleveland, W.S.  1985.  *The Elements of Graphing Data.*  Wadsworth,Monterey, CA, USA.

Dana, M.T. and Easter, R.C. 1987.  Statistical summary and analysesof event precipitation chemistry from the MAP3S Network, 1976-1983. *Atmospheric Environment,* 21:123-128.

Grimm, J.W. and J.A. Lynch.  1991. Statistical analysis of errorsin estimating wet deposition using five surface estimationalgorithms.  *Atmospheric Environment,* 25A(2):317-327.

Hardy, R.L. 1971.  Multiquadic equations of topography and otherirregular surfaces. *Journ. Geophys. Res.* 76:1905-1915.

Hedin, L.O., Granat, L., Likens, G.E., Buishand, T.A., Galloway,J.N., Butler, T.J. and Rodhe, H. 1994.  Steep declines inatmospheric base cations in regions of Europe and North America.*Nature*, 367:351-354.

Hirsch, R.M. and Peters, N.E. 1988.  Short-term Trends in sulfatedeposition at selected bulk precipitation stations in New York.*Atmospheric Environment*, 22:1175-1178.

Hirsch, R.M., Slack, J.R. and Smith, R.A. 1982.  Techniques ofTrend analysis for monthly water quality data.  *Water ResourcesResearch*, 18:107-121.

Holland, D., C. Simmons, L. Smith, T. Cohn, G. Baier, J. Lynch, J.Grimm, G. Oehlert, S. Lingberg. 1995. Long-term Trends in NADP/NTNprecipitation chemistry data: Results of different statisticalanalyses.  *Water, Air, and Soil Pollution*, 85(2):595-601.

Kessler, C.J., Porter, T.H., Firth, D., Sager, T.W. and Hemphill,M.W. 1992.  Factor analysis of trends in Texas acidic deposition.*Atmospheric Environment*, 26A:1137-1146.

Lins, H.F. 1987.  Trend analysis of monthly sulfur dioxideemissions in the conterminous United States.  *AtmosphericEnvironment*, 21:2297-2309.

Lynch, J.A., V.C. Bowersox, and C. Simmons.  1995a.  Precipitationchemistry trends in the United States: 1980-1993, Summary Report.National Atmospheric Deposition Program. Natural Resources EcologyLab, Fort Collins CO. 103 pp.

Lynch, J.A., J.W. Grimm, V.C. Bowersox.  1995b.  Trends inprecipitation chemistry in the Untied States: A nationalPerspective, 1980-1992.  *Atmospheric Environment*, 29:1231-1246.

Oehlert, G.W. 1993.  Regional trends in sulfate wet deposition.*Journ. Am. Stat. Assoc.*, 88:390-399.

Olsen, A.R., Voldner, E.C., Bigelow, D.S., Chen, W.H., Clark, T.L.,Lusis, M.A., Misra, P.K. and Vet, R.J. 1990.  Unified wetdeposition data summaries for North America:  Data summaryprocedures and results for 1980-1986.  *Atmospheric Environment*,24A:661-672.

Placet, M. 1990.  Emissions reductions in acidic depositionprocesses.  National Acid Precipitation Assessment Program, Stateof Science and Technology Report 1, p. 1-127.  Government PrintingOffice, Washington, DC.

Ruijgrok, W. and Romer, F.G.  1993.  Aspects of wet acidifyingdeposition in Arnhem: Source regions, correlations and trends(1984-1991).  *Atmospheric Environment*, 27A:637-653.

SAS Institute, Inc.  1988.  SAS/STAT User's Guide, Release 6.03Edition.  Cary, NC.  SAS Institute Inc., 1988.  1028 pp.

Sirois, A.  1993.  Temporal variation of sulphate and nitrateconcentrations in precipitation in Eastern North America:1979-1990.  *Atmospheric Environment*, 27A:945-963.

Sisterson, D.L., Bowersox, V.C. and Olsen, A.R.  1990.  WetDeposition of Atmospheric Pollutants. In Deposition Monitoring:Methods and Results, pp. 6-104 - 6-105; 6-145 - 6-174 and 6-220.National Acid Precipitation Assessment Program, State of Scienceand Technology Report 6.  Government Printing Office, Washington,DC.

Venkatram, A., McNaughton, D. and Karamchandani, P.K.  1990.Factors Influencing Source-Receptor Relationships.  In:Relationships Between Atmospheric Emissions and Deposition/AirQuality, pp. 8-23 - 8-37.  National Acid Precipitation AssessmentProgram, State of Science and Technology Report 8.  GovernmentPrinting Office, Washington, DC.