Environ. Sci. Technol. 1989, 23, 881-807
An Interpretation of Differences between Field and Laboratory pH Values Reported by the National Atmospheric Deposition Program/National Trends Network Monitoring Program David S. Bigelow’
Natural Resource Ecology Laboratory, Colorado State University, Ft. Collins, Colorado 80523 Douglas L. Sisterson
Center For Environmental Research, Argonne National Laboratory, Argonne, Illinois 60439 LeRoy J. Schroder U.S. Geological Survey, Box 25046, MS 401, Denver, Colorado 80225
Differences between field and laboratory pH values reported by the National Atmospheric Deposition Program/National Trends Network (NADP/NTN) monitoring program from 1984 through 1986 are investigated. Median differences in hydrogen ion concentration between laboratory and field pH determinations at sites averaged -4.6 pequiv/L in natural precipitation samples on an annual basis. The median difference found in external quality assurance samples analyzed during the same time period was -11 pequiv/L. The results suggest a systematic bias in pH values reported by the NADP/NTN network. The bias appears to have a fixed component of approximately -7 pequiv/L, which can be attributed to the sampling bucket and lid, and a seasonal and regional component that ranges from +4 to -22 pequiv/L at the 10th and 90th percentiles. Differences were found to be independent of sample pH and sample volume. The magnitude of the bias has implications for the interpretation of previously published pH and hydrogen ion concentration and deposition values in the western United States.
Introduction Significant differences between pH measurements made soon after sample collection (field measurements) and those made in the laboratory are well documented in numerous studies designed to evaluate collectors, sampling frequencies, and sample preservation and storage protocols (1-5). These studies and others attribute the differences to a variety of both biological and chemical phenomena that result in a loss of sample integrity (6-B), such as microbial activity, degradation of organic acids, and the dissolution of particulate matter. Breaches in sampling protocols such as bucket contamination, improper sample handing and transportation, and laboratory bias also are recognized. Because the various explanations for the causes of differences in field versus laboratory measurements often acknowledge both geographic and seasonal phenomena (9, I O ) , and because each explanation may favor a different recommendation for minimizing or eliminating the differences (changing the sampling frequency, sample preservation technique, or sampling device), most major precipitation monitoring networks in North America have maintained some type of on-site measurements to provide documentation for the time when the reasons for the differences in field versus laboratory measurements are finally resolved. The examination of these “paired” network data sets provides an opportunity to further investigate the causes of the differences and to document their dependencies. In this presentation, routine weekly field and laboratory pH measurements from the National Atmospheric De-
position Program/National Trends Network (NADP/ NTN) monitoring network are examined along with their corresponding external and internal quality assurance documentation to determine the reasons for the differences between the two measurements. The length of the data record and the large number of widely distributed monitoring sites permit both spatial and temporal examination of the differences, independent of specific field experiments. Background. Both field and laboratory pH and specific conductance values are routinely obtained in the NADP/NTN monitoring network. Specific procedures used to collect and analyze the samples are documented elsewhere (11-13). Differences between the measurements made on these natural precipitation samples document changes that take place between the time the operator collects the sample and the time that the sample is analyzed at the NADP/NTN Central Analytical Laboratory (CAL). These changes might be caused by sampling bucket contamination, sample handling and transportation, microbial activity, degradation of organic acids, or dissolution of particulate matter, each of which may be exacerbated by long sample accumulation or storage times prior to chemical analysis (1-10). In addition to the routine field and laboratory pH measurements, the NADP/NTN monitoring program makes available three types of quality assurance information that may be used concurrently with concentration and precipitation data (Table I). This information includes a weekly field pH quality control sample result, known as the “pH check sample” (I2),from each monitoring site making a field pH measurement, field and laboratory pH results and laboratory reference pH values from twice-weekly quality assurance audit samples submitted as blind samples through the field sites, to the CAL [Blind-Audit Program (14)],and results of quarterly (now semiannual) quality assurance audit samples sent as unknowns directly to the sites to audit field pH and specific conductance measurements [Intersite Comparison Program (14)]. In 1988, the CAL reported on a series of experiments that were undertaken in 1985 to address the effectiveness of sampling bucket cleaning operations (15). During the course of this study, a one-time assessment was made of contamination contributed by sampling buckets and lids. The integration of all of the natural precipitation sample and external quality assurance information with the results of the one-time CAL assessment provides an opportunity to address many aspects of the sample integrity question.
Experimental Section Sample Collection Methods. The NADP/NTN monitoring program collects weekly precipitation samples
OQ13-936X109/Q923-Q08 1$Q1.5O/Q 0 1989 American Chemical Society
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NO. 7, 1989 881
Table I. NADP/NTN Monitoring Program Information Used To Investigate Field versus Laboratory pH Values item
frequency
source
natural precipitation samples field and laboratory pH values
weekly at each site
routine measurements made at each NADP/NTN field site and at the CAL; from NADP/NTN data reports and data tapes
pH check samples dilute HN03 solution having a known pH of 4.30
weekly at each site
routine measurements made at each NADP/NTN field site and at the CAL; from NADP/NTN data reports and data tapes
Blind-Audit Program field and laboratory pH values from known synthetic quality twice per week to the CAL; sent USGS External Quality Assurance assurance audit solutions that are processed following the from two different field sites Program; reported in USGS water same protocols used for natural precipitation samples; an resources investigative reports additional sample sent directly to the laboratory for analysis provides a laboratory reference pH value Intersite Comparison Program field pH results of quality assurance audits of field measurements made at each site
quarterly/semiannual
in a 16-L, linear polyethylene (LPE) bucket. The bucket is mounted in an Aerochem Metrics Wet/Dry Collector (16) that exposes the bucket during periods of precipitation and protects the bucket from other forms of deposition during times of nonprecipitation. (Use of trade names is used for identification purposes only and does not constitute an endorsement by the U.S. Geological Survey.) At the end of each weekly collection period, the site operator is instructed to remove a 20-mL aliquot from the bucket and to electrometrically determine pH and specific conductance (12). A CAL provided, low ionic strength HNO, solution of known pH (pH check sample) is also measured at this time as a quality control check. The bucket is then sealed with an LPE, butadiene-gasketed lid and shipped to the CAL. A t the CAL, pH and specific conductance measurements are repeated, and additional determinations of Ca2+,Mg2+,Na+, K+,NH4+,SO-,: NO3-, C1-, and are made. The CAL analytical methods and internal quality assurance results are described by Peden et al. (13), Peden (15), and Lockard (17). Buckets are cleaned and returned to the sites by the CAL. New lids are cleaned, but are used only once. The CAL also supplies field sites with 20-mL syringes for aliquot removal, and with pH and specific conductance calibration and quality assurance solutions. Field pH determinations and their corresponding pH check sample values are reported as a standard part of network data reports along with the pH values determined by the CAL (18-25). Supplementing the weekly determinations of field and laboratory pH is an external, double-blind audit program sponsored by the U.S. Geological Survey (USGS) (14). In this program approximately 100 sites per year selected randomly from four quadrants of the continental United States are instructed to submit one additional sample to the CAL. The extra submission schedule is arranged so that the rate of double-blind audit solutions submitted to the CAL approximates two per week. To produce the audit sample, each site operator is asked to pour approximately two-thirds of a USGS-provided, 250-mL audit solution into a clean bucket and treat it as if it were the natural precipitation sample that had been collected during the previous week. On-site measurements are performed, recorded, and submitted to the CAL along with the sample in the usual manner. The natural precipitation sample for the previous week is recoded at the site as an audit sample 882
Environ. Sci. Technol., Vol. 23, No. 7, 1989
USGS External Quality Assurance Program; routinely reported in NADP/NTN data reports and in USGS water resources investigative reports
and is also submitted to the CAL in the usual manner. The remaining one-third of the double-blind solution is then submitted directly to the CAL in its original bottle to provide the known analyte concentrations for each double-blind audit solution. In this paper we refer to this directly submitted portion of the blind-audit solution as the laboratory reference measurement. The laboratory reference measurement is used in lieu of a calculated or theoretical value to isolate sample integrity problems that arise during the handling of the sample by the site operator, through the time the chemical analyses are performed a t the central laboratory. Sample accumulation and storage effects are specifically excluded since these are best assessed by more direct methods. Since the blind-audit submission protocol specifies that a portion of the audit solution be poured into the sampling bucket and then removed, the effects of the sample bucket contamination contribution can be assessed by comparing the laboratory reference measurement to the expected value. Prior to late 1984 the procedure was only slightly different. Sites were selected randomly but not by region, and sites submitted double-blind samples in lieu of a natural precipitation sample during weeks when no precipitation occurred instead of in addition to a natural precipitation sample on a strict schedule. Preparation of the double-blind samples is described by Brooks et al. (14). The quality of field site measurements can be inferred from the previously described pH check sample measurements (12). Semiannual or quarterly audit solutions supplied by the USGS to the sites can also be used. In the semiannual and quarterly audits, known as “intersite comparisons” (141, the composition of the audit sample is known only by the USGS. Site operators are instructed to remove an aliquot of the intersite comparison solution and to determine its pH and specific conductance. The measured values and unused portion of this solution are returned to the USGS within 45 days. Results of this program are published in NADP/NTN program reports (26-28). Preparation of the audit solutions is described in USGS program reports (29, 30). Evaluation and Screening of Natural Samples. Unlike laboratory pH values, whose quality is assessed as a part of the standard NADP/NTN quality assurance program (15, 17), field pH values are reported in the NADP/NTN monitoring program without the benefit of
Table 11. Selection Criteria for Identifying Well-Documented, Quality Field pH Measurements selection criterion data intervals (12/28/83-01/04/87) sites with at least seven intersite measurements sites meeting field pH measurement criterion valid and complete pH data pH check sample values 4.30 f 0.1
available data records sites 203 152
28 069 22 681
87 87 87
13017 7 614 6 623
a quality assurance assessment. To provide the best uncertainty as to the origin of the differences detected in the examination of natural precipitation samples, a field pH data screening method was developed (See Table 11). As a first step, data analysis was limited to the three most current annual data sets (1984-1986). These data contained the greatest amount of quality assurance information and had the widest possible geographic distribution compared to other data years. Nine USGS intersite comparison audits were conducted during this time period, and weekly NADP/NTN pH check sample quality control values were potentially available for each field pH measurement. A total of 203 sites operated during this time period. After the selection of the 1984-1986 time period, initial consideration in the selection of quality natural precipitation sample results was directed toward a site’s participation and performance in the USGS Intersite Comparison Program. Participation in seven of the nine intersite comparisons with acceptable performance in 75% of the comparisons was considered a minimum requirement for demonstrating a site’s ability to perform quality pH measurements. A lack of participation or poor performance resulted in the elimination of all of that site’s data from further analysis. Acceptable performance was taken from guidelines in the NADP quality assurance manual (31). These guidelines establish acceptable field pH values as being within f O . l pH unit of the expected value. The derivations of the expected values have been described by Schroder et al. (14,29, 30). The strictness of the participation requirement has the unfortunate effect of removing sites from analysis that had not been a part of the network for the entire time period being studied, even though they may have achieved the desired performance level. The strict criteria were deemed necessary to ensure that sites had demonstrated their ability to perform quality field measurements and that the differences examined were not biased toward sites having a poor performance record. Weekly data from sites meeting the above participation and performance requirements that were determined to be free from sampling protocol deficiencies (32) and had field, laboratory, and pH check sample values were identified and further screened to eliminate data records where the reported pH check sample value was >f0.1 pH unit from the theoretical value of 4.30. Table I1 summarizes the number of sites and records available after applying each criterion, including the initial limitation of the data analysis to the 3 most recent years. The table indicates that most of the data are lost by requiring sites to have achieved acceptable results (participation and performance) in seven of the nine intersite comparisons. These two criteria account for a 57% reduction in the number of available sites (less 116) and for a 54% reduction in available data (less 15052 records). They also affect somewhat the geographic representativeness of the data set by eliminating a larger proportion of western and midwestern sites. More western and
Flgure 1. Number of screened natural samples available at each selected site: 1984-1986.
midwestern sites are removed because a disproportionate number of them were established toward the latter portion of the time period studied. The fourth criterion in Table 11,valid and complete data (eliminating samples where the lab, field, or check sample data are missing), may also bias the subsequent analyses toward eastern sites because the higher incidence of low precipitation samples a t western sites commonly results in insufficient sample volumes for field analyses. This criterion further reduces the data set by 19%. The final numbers of samples available a t each site are displayed in Figure 1. Data Analysis. For natural precipitation samples, four basic data sets organized into either by-site or by-network groupings were used for data analysis. Data distributions of the differences between field and laboratory pH values were tabulated and graphed, both as pH and as hydrogen ion concentration [H+],for the various groupings and data sets. When appropriate, spatial maps were produced to determine any regional or seasonal attributes of the data sets. The four data sets were as follows: (1)network, all screened data for the 1984-1986 period; (2) annual, screened data covering each annual time period; (3) cool season, data for each of the 3 years, but limited to October through March; (4) warm season, data for each of the 3 years, but limited to April through September. When differences in stratified data were suspected, further tests for significance were conducted on data from each site using a paired t test (33).
Results and Discussion For natural precipitation samples, the median difference, as [H+], between laboratory and field pH values in network-wide data was -4.3 pequiv/L (as laboratory minus field). On a by-site basis the differences were significant at the level in all but 5 of the 87 sites used in the analysis. Median site differences averaged -5.3 pequiv/L in 1984, -3.8 pequiv/L in 1985, and -4.7 pequiv/L in 1986. Though significant, the differences are generally within the guidelines of fO.l pH unit established by NADP/NTN and used in this analysis to screen natural precipitation samples. At pH 4.6, for instance, which is the overall network median pH value, acceptable [H’] values would differences be 20.0-31.6 pequiv/L. The range of [H+] between the loth and 90th percentile in the 3-year network data set was +3.1 to -18.5 pequiv/L, with more than 70% of the differences falling within the acceptable range at the median value. Differences between laboratory and field specific conductance values support the findings for [H+],exhibiting a range of -4 to +3 pS/cm between the 10th and 90th percentiles. Results for annual data sets produced similar findings. Environ. Sci. Technol., Vol. 23, No. 7, 1989
883
- Network pH
Table 111. Difference in [H+] (in p3quiv/L)"
ti
Warm Season H'
_ _ _ _ Cool S e a m
H+
percentiles 0 8-
10 30 50 70 90
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4
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Flgure 2. Median pH and [H'] differences (laboratory minus fleM measurements) of natural precipitation samples at 0.1 pH intervals. Figure Includes a percentile frequency distrlbution at each pH Interval (left-hand axls), a warm season and cool season median A[H+] at each Interval (right-hand axis), and a median ApH for each interval (extreme right-hand axls).
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Figwe 3. SABL decomposition of median weekly [H+] differences between laboratory and flekl natural preclpltatbn sample pH values. Original data (DATA), along wlth a trend (TREND) and seasonal (SEASONAL) apportionment of the data,are displayed. The gaph labeled IRREGULAR displays that portion of the data that can not be explained by either trend or seasonality. Scaling bars along the extreme right axis show the relative difference In scales used in each graph.
pH and Seasonal Dependencies. Figure 2 shows the sample frequencies and median differences between laboratory and field [H+] and pH values a t intervals of 0.1 pH unit. Differences in [H+]values remain nearly constant between pH 4.0 and 6.5 a t approximately -4 to -5 pequiv/L. Median cool season differences are only slightly greater than warm season differences and average only 0.35 pequiv/L more than the warm season values on a per-site basis. Plots from a SABL time series decomposition (34,35) of weekly median laboratory minus field [H+]differences (Figure 3) also indicate that the natural precipitation sample differences exhibit some seasonality. The seasonality, however, is masked by differences in [H+]variability ("trend") in each of the three annual data sets. The 1984 and 1986 differences exhibit biannual trend cycles, while the 1985 differences show an almost continuous differences throughout the year. When decrease in [H+] this trend component is subtracted from the overall weekly median [H+] differences, the seasonal component can be seen to change from approximately -3 pequiv/L in winter months to a more constant 0 to +2 ctequiv/L in the remaining months in each year. 884
Environ. Sci. Technol., Vol. 23, No. 7 . 1989
(1768) +3.5 -1.8 -5.2 -10.0 -21.8
yearb 1985 nat blind
nat
1986 blind
(30) (2252) (75) (2603) (90) -1.3 +3.5 -1.8 +2.2 -1.7 -4.4 -0.07 -6.4 -1.7 -4.5 -12.0 -3.5 -10.6 -4.3 -8.5 -19.8 -7.5 -18.4 -8.0 -14.1 -43.5 -17.6 -34.7 -17.0 -19.6
OCalculated by subtracting field pH as [H+] from laboratory pH as [H+]for natural samples, and nonbucket direct pH as [H+]from laboratory bucket pH as [H'] for external quality assurance samples. *Nat, natural samples; blind, external quality assurance blind-audit samples; numbers in parentheses, number of samples.
0 2-
3
1984 nat blind
The trend differences (Table 111) are further supported by the external quality assurance samples. Distributions of laboratory minus field [H+] differences in quality assurance samples, like those of the natural precipitation samples, indicate that 1984 data are more variable than either 1985 or 1986 data. Because of this, 1984 data tend to dominate the analysis of natural precipitation sample data. The 1985 [H+]differences, on the other hand, are more uniform than either 1984 or 1986 differences and exhibit an almost steady decrease in [H+]differences from January through December. The reasons for the anomalies in the 1985 data are unknown. We suspect that the changes in [H+] neutralization indicated by the trend component of the SABL decomposition are caused by changes in bucket and lid suppliers and their manufacturing processes, laboratory bucket washing procedures, and other colledor and laboratory maintenance procedures that have some degree of periodicity. The seasonal differences, though slight, support the findings of others (6, 8, 9, 36) who reported significant increases in H+ neutralization during summer months. The direction and magnitude of the seasonal trend in the [H+]differences will depend upon which proceas dominates in any given season. We assume this to be somewhat site dependent. Neutralization of [H+] by soil-derived Ca2+, Mg2+,and NH4+ is thought to be associated with areas having or receiving particulate matter derived from alkaline soils (8,9).Windy conditions, extended dry periods, and certain types of storms and storm tracks are all thought tr exacerbate the effects. Changes in ground cover, whi :h occur during snowmelt, has also been suggested ( 9 ) Keene and Galloway (36) have noted that organic a ids coinciding with growing season organic matter production appear to contribute to the larger increases in [H+] differences during the warm seasons, and that there may also be differences that coincide with specific storm types, storm tracks, or the volume of precipitation (36). These increased differences have been shown to be the result of the large losses of organic acids that occur in unpreserved samples (6,37). Microbial activity, resulting in the production or consumption of NH4+, other nitrogen species, and organic acids has also been identified as a contributor to seasonal differences (6). Warmer temperatures found inside the sealed buckets while the samples are in the field are also conducive to most of the above processes. In the NADP/NTN natural precipitation samples, seasonal differences account for only a small portion of the difference between laboratory and field pH values. These seasonal processes, however, may well change the composition of NADP/NTN natural precipitation sample free acidity prior to the first pH measurement made by the network.
Bucket Effects. The lack of a strong seasonal component and the near-constant reduction in [H+] between laboratory and field measurements across most pH intervals suggest a bias in NADP/NTN reported values that is due to the sampling and analysis protocols. Previous independent studies conducted at the Department of Energy’s Environmental Measurements Laboratory (38) had noted a slight H+ neutralization in solutions that had contacted the butadiene O-ring used to provide a watertight seal between the sampling bucket and the lid. More recently, Peden (15)reported a reduction of approximately 12 pequiv/L in [H+] values for laboratory-washed buckets and lids containing fixed volumes of dilute nitric acid. Peden’s report (15)describes 1-day and 1-week experiments, performed a t the CAL in 1985, to test bucket cleanliness. Two different solutions were used: (a) pH 4.3 or 50 pequiv/L [H+] and (b) pH 4.60 or 25 pequiv/L [H+]. Both concentrations were used for the 1-day equilibrium tests,but only the pH 4.60 solution was used for the 1-week equilibrium test. The buckets were tested by adding various volumes of the two nitric acid solutions and leaving the buckets in an upright position. The median pH change was 0.05 pH unit for the 4.30 pH solution and 0.19 pH unit for the 4.60 pH solution. The median [H+]decrease in the two solutions was -7 pequiv/L in upright buckets. The bucket-and-lid combination was similarly tested by adding various volumes of the two nitric acid solutions to the bucket, sealing the bucket with the lid, and inverting the bucket. The inverted-bucket experiments resulted in a median [H+] decrease of 12 pequiv/L for both solutions. The results of the upright-bucket 1-week equilibration were similar to those of the 1-day equilibration. These experiments suggest that a limited capacity for H+ neutralization exists in the buckets and lids, and that the neutralization occurs during the first day of the solution contact with the bucket. These findings led to a comparison of the field and laboratory pH values reported in the external quality assurance blind-audit samples. External Quality Assurance Samples. Table I11 compares the [H+] differences in natural precipitation samples to [H+] differences measured in the external quality assurance blind-audit samples. The table indicates that reductions in [H+] occurred in both natural precipitation and quality assurance samples in all years. Blindaudit samples, however, exhibited a median [H+] decrease of 11pequiv/L, as compared to a decrease of 4 pequiv/L in the natural precipitation samples. The discrepancy between [H+]differences in natural precipitation samples and external quality assurance blind-audit samples occurs because the calculated differences in the blind-audit samples compare a routine laboratory pH measurement to a directly shipped, nonbucket laboratory reference pH measurement, rather than comparing two measurements made on a sample from the same bucket. The reduction in [H+] that is seen in the blind-audit samples can be attributed to field handling, shipment, and bucket-and-lid effects. Differences in [H+] calculated from the natural precipitation samples can also be attributed to these same effects with one important exception. Changes that might occur when the sample contacts the bucket have already taken place by the time the first field :iH measurement is made by the site operator. Evidence that reductions in [H+] occur prior to the first field pH measurements made by the site operator comes from a comparison of blind-audit sample field pH measurements made at the site, by the site operator, with those routine laboratory measurements made at the CAL. These median differences, between laboratory and field mea-
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20
40
PERCENTILES OF [H’]
60
80
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Flgure 4. Percentile distribution of [H+] differences In natural precipltatlon and external quality assurance samples: 1984-1986.
surements of the blind-audit samples, are identical with the [H+] differences of 4 pequiv/L found in the natural precipitation samples. Since Peden’s data (15)also indicate that the reduction in [H+] of nitric acid solution occurs after only 1 day of contact with the sampling bucket. It follows, then, that the wet-deposition [H+] probably decreases during the 1-week sample collection period. This conclusion was also reached by Sisterson et al. (8), although they attributed the changes to neutralization of H+ by soil particles, and by Keene and Galloway (6),who attributed the differences to the loss of organic acids. Our analysis does not necessarily preclude these finding, but rather apportions a constant part of this neutralization to bucket effects. We suggest that the collection process for natural precipitation samples causes the sample to come in contact with most of the inner surfaces of the sampling bucket, and that it is this contact that most likely results in a [H+] decrease of -6 pequiv/L. Further, because this amount is very similar to the decrease of 7 pequiv/L reported by Peden (15),we believe that the probable median difference between the actual wet-deposition [H+] after rainfall and the laboratory measured [H+] is about 11 pequiv/L-7 pequiv/L from the bucket and 4 pequiv/L from the lid. Although the decreases in median [H’] values obtained from the external quality assurance blind-audit samples (laboratory measurement minus laboratory reference of -11 pequiv/L) are very similar to the median CAL-reported inverted-bucket median results (12 pequiv/L), we cannot ignore the fact that the [H+] changes exceed 20 pequiv/L in 20% of the external quality assurance samples and approximately 10% of the natural precipitation samples (Figure 4). In an attempt to explain these larger [H+] differences, sample volume and geographic dependencies were investigated.
Sample Volume and Geographic Dependencies. Because both the external quality assurance blind-audit data and the 1985 CAL study used a very narrow range of sample volumes (50-500 mL), only natural precipitation samples can be used to investigate sample volume dependencies. Unfortunately, the small differences in [H+] in natural precipitation samples (-4 pequiv/L) and the large differences in sample volumes (0.03-14 L) caused results from regre’ sion tests to be inconclusive. Noteworthy, however, i the fact that the sample volume dependencies were nc b obvious, even when the analysis was limited to the uppz c 20% of the sample volume distribution. Geographic differences in natural precipitation samples are displayed in Figure 5. Though the differences are, for >
Environ. Scl. Technol., Vol. 23, No. 7, 1989 885
Figure 5. Spatial distribution of median laboratory minus field [H'] differencesin NADPlNTN monitoring network data: 1984-1986. Shaded area. A[H+] 2 -4 pequiv/L.
the most part, within the error limits referenced in the NADP/NTN monitoring program, regional differencesare noticeable. We attribute these regional differences to the same processes that control seasonal dependencies. For instance, the small differences noted in the central plains states are most likely the result of the increased soil-derived CaH, M$+, and NH4+which characterizesthisregion of the country (39). Neutralization of H+ probably takes place prior to the first field measurement made by the network, thereby minimizing the differences detected between the field and laboratory values. In eastern regions, where windborne alkaline analytes are less prevalent and where both natural and anthropogenic emissions sources contribute to higher [H+], differences between laboratory and field measurements are more pronounced (6). Near year-round microbial activity in the deep south and Florida may be the reason why differences are not as pronounced in this region. We cannot exdain the smaller differences noted along the Ohio River vhley, especially the central West VirginG site that exhibits a positive laboratory minus field [H+l difference. Only one other site, in the westem suburbs of Boston, had positive [H+] differences. We speculate that a more in-depth study of these and other stations exhibiting extreme differences (Pennsylvania, Maine, Oregon) will reveal local conditions that will explain the extreme values. In the northeastern United States where [H+l values are generally 3-4 times higher than the 11 pequiv/L [H+l bias, the choice of NADPjNTN field or laboratory pH measurements to estimate [H+] is perhaps moot. For selected areas in the East and certainly most areas of the West, the choice of measurements and the exact interpretation of the bias become more critical. For some locations in the West, the use of the bias in calculating [H+] could change H+ deposition values by a factor of 2.
Conclusions Annual median differences in [H+] between laboratory and field natural precipitation sample pH measurements were approximately 4 pequiv/L. External quality assurance results support these findings but identify the bias at the 50th percentile to be approximately 11 pequivll. The difference in the two estimates is explained by using internal laboratory quality control data, which apportion 7 pequiv/L to bucket effects and an additional 5 pequiv/L to lid effects. The 7 pequiv/L difference (laboratory measurement minus laboratory reference) found in the external quality assurance blind-audit samples is most likely attributed to a neutralization of H* in the sampling 886
Environ. Sci. Technol., Vol. 23. NO. 7. 1989
bucket that takes place before field measurements are made by the site operator. While median values estimate biases in measured laboratory pH values a t approximately 11 pequiv/L, the percentile distributions of both natural precipitation and quality assurance samples indicate that more than 20% of the samples may have biases in excess of 20 pequiv/L. This implies that researchers should be cautious when applying any uniform correction to NADPjNTN monitoring data. Regional and seasonal differences may exist in these data, but, they are not due to effects associated with the handling and transportation of samples to the CAL. Changes approximating 11 pequiv/L are associated with sample contact with the bucket and lid. Departures from the median values of [H+] differences are most likely the result of site- or season-specific phenomena. Registry No. H+,12408-02-5; water, 7732-18-5. Literature Cited (1) Galloway,J. N.; Likens, G. E. Water, Air, SoilPollut. 1976, 6,241-258. (2) Galloway, J. N.; Likens, G. E. Tellus 1978, 30, 71-82. (3) Peden, M. A,; Skrowon,L. M. Atmos. Enuiron. 1978,12, 2343-2349. (41 Madsen. B. C. Atmos. Enuiron. 1982. 16. 2515-2519. (5) de Pena; R. G.; Walker, K. C.; LeboAtz,'L.; Micka, J. G. Atmos. Enuiron. 1985, 19, 151-156. (6) Keene, W. C.; Galloway, J. N. Atmos. Enuiron. 1984, 18, 2491-2497. (7) Mahendrappa, M. K. Atmos. Enuiron. 1985,19,1681-1684. (8) Sisterson, D. L.; Wurfel, B. E.; Lebt, B. M. Atmar. Enuiron. 1985,19,1453-1469. (9) Chan, W. H.; Tang, A. J. S.; Chung, D. H. S.; Reid, N. W. Enuiron. Sci. Technol. 1987,21, 1219-1224. (10) Tang, A. J. S.; Chan, D. B.; Orr, D. B.; Bardswick, W. S.; Lusis, M. A. Water, Air, Soil Pollut. 1987, 36, 91-102. (11) Bigelow, D. S. NADP Instruction Manual: Site Operation.
National Atmospheric Deposition Program, Natural Resource Ecology Laboratory, Colorado State University, Ft. Collins, CO, 1982. (12) Bigelow, D. S.; Dossett, S. R. NADP/NTN Instruction Manual: Site O p t i o n s . National Atmospheric Depition Program, Natural Resource Ecology Laboratory, Colorado State University, Ft. Collins, CO, 1988. (13) Peden, M. E.; Bachman, S. R.; Brennan, C. J.; Demir, B.; James, K. 0.; Kaiser, B. W.; Lockard, J. M.; Rothert, J. E.; Saur, J.; Skowron, L. M.; Slater, M. J. Development Of Standard Methods For The Collection And Analysis Of Precipitation. Illinois State Water Survey, Champaign, IL, 1986; ISWS Contract Report 381. (14) Brooks, M. H.; Schroder, L. J.; Willoughby, T. C. Results Of External Quality-AssuranceProgram For The National Atmospheric Deposition Program And National Trends Network During 1985. US. Geological Survey, Denver, CO, 1988; Water-Resources InvestigationsReport 87-4219. (15) Peden, J. L. Quality Assurance Report: NADP/NTN Deposition Monitoring, Laboratory Operations, Central Analytical Laboratory, 1984 through 1985. National Atmospheric Depasition Program, Natural Resource Ecology Laboratory, Colorado State University, Ft. Collins, CO, 1988. (16) Aerocbem Metrics, Inc., Route 2, Box 112, Bushell, FL, 33513. (17) Lockard, J. M. Quality Assurance Report: NADP/NTN
Deposition Monitoring, Laboratory Operations, Central Analytical Laboratory, 1978 through 1983. National Atmospheric Deposition Program, Natural Resource Ecology Laboratory, Colorado State University, Ft. Collins, CO, 1987. (18) National Atmospheric Deposition Program. NADP/" Data Report: Precipitation Chemistry, First Quarter 1984.
Natural Resource Ecology Laboratory, Colorado State
University, Fort Collins, CO, 1985. (19) National Atmospheric Deposition Program. NADP/NTN Data Report: Precipitation Chemistry, Second Quarter 1984. Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO, 1985. (20) National Atmospheric Deposition Program. NADP/NTN Data Report: Precipitation Chemistry, Third Quarter 1984. Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO, 1986. (21) National Atmospheric Deposition Program. NADP/NTN Data Report: Precipitation Chemistry, Fourth Quarter 1984. Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO, 1986. (22) National Atmospheric Deposition Program. NADP/NTN Data Report: Precipitation Chemistry, 1January-30 June 1985. Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO, 1987. (23) National Atmospheric Deposition Program. NADP/NTN Data Report: Precipitation Chemistry, 1July-31 December 1985. Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO, 1987. (24) National Atmospheric Deposition Program. NADP/NTN Data Report: Precipitation Chemistry, 1January-30 June 1986. Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO, 1987. (25) National Atmospheric Deposition Program. NADP/NTN Data Report: Precipitation Chemistry, 1July-31 December 1986. Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO, 1987. (26) National Atmospheric Deposition Program. NADP Annual Data Summary: Precipitation Chemistry In The United States, 1984. Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO, 1986. (27) National Atmospheric Deposition Program. NADP Annual Data Summary: Precipitation Chemistry In The United States, 1985. Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO, 1987. (28) National Atmospheric Deposition Program. NADP Annual Data Summary: Precipitation Chemistry In The United States, 1986. Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO, 1987. (29) Schroder, L. J.; Brennan, J. 0. Precision Of The Measurement Of pH And Specific Conductance At National Atmospheric Deposition Program Monitoring Sites, October 1981-November 1983. U.S. Geological Survey, Lakewood, CO, 1985; Water Resources Investigations Report 84-4325. (30) Schroder, L. J.; Brooks, M. H.; Willoughby, T. C. Results Of Intercomparison Studies For The Measurement Of pH And Specific Conductance At National Atmospheric De-
(31)
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(33) (34) (35)
(36) (37) (38) (39)
position Program/National Trends Network Monitoring Sites, October 1981-0ctober 1985. U.S. Geological Survey, Lakewood, CO, 1987; Water Resources Investigations Report 86-4363. National Atmospheric Deposition Program, NADP Quality Assurance Steering Committee. The NADP Quality Assurance Plan: Deposition Monitoring. Natural Resource Ecology Laboratory, Colorado State University, Ft. Collins, CO, 1984. Bowersox, V. C. Data Validation Procedures for Wet Deposition Samples at the Central Analytical Laboratory of the National Atmospheric Deposition Program. Transactions of the APCA/ASQC Specialty Conference on Quality Assurance in Air Pollution Measurements, APCA, Pittsburgh, PA, 1985; pp 500-524. Dixon, W. J.; Massey, F. J. Introduction T o Statistical Analysis, third ed.; McGraw-Hill, New York, 1969; pp 119-121. Becker, R. A,; Chambers, J. M. S: An Interactive Environment For Data Analysis And Graphics; Wadsworth, Inc.: Belmont, CA, 1984. Cleveland, W. S.; Devlin, S. J.; Terpenning, I. J. In T i m e Series Analysis: Theory and Practice 1; North-Holland Publishing Co.: Amsterdam, The Netherlands, 1982; pp 539-564. Keene, W. C.; Galloway, J. N. J. Geophys. Res. 1986, 91(D13), 14466-14474. Bachman, S. A.; Peden, M. E. Water, Air, Soil Pollut. 1987, 33, 191-198. Bogen, D. C. U S . Dept. of Energy, Environmental Measurements Laboratory, New York, NY, personnel communication, 1986. Munger, J. W.; Eisenreich, S. A. Enuiron. Sci. Technol. 1983, 17, 32A-42A.
Received for review August 22,1988. Accepted January 24,1989. This research was supported in part by a Cooperative Agreement (CR-813910-02-0) with the US.EPA’s Environmental Monitoring Systems Laboratory, Research Triangle Park, N C . Although the research described has been funded in part by the US.Environmental Protection Agency, it has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred. Additional funding was provided by the PRocessing of Emissions by Clouds and Precipitation (PRECP)program as a part of the National Acid Precipitation Assessment Program by the Office of Health and Environmental Research of the US. Department of Energy under Contract W-31-109-ENG-38.
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