Sulfate Sampling Artifact from SO2 and Alkaline Soil - American

To test this hypothesis, we compare measurements from Teflon filters without a denuder to measurements from nylon filters following a carbonate denude...
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Environ. Sci. Technol. 1997, 31, 1320-1324

Sulfate Sampling Artifact from SO2 and Alkaline Soil ROBERT A. ELDRED* AND THOMAS A. CAHILL Crocker Nuclear Laboratory, University of California, Davis, Davis, California 95616-8569

A significant sulfate artifact may be produced by the interaction of SO2(g) with basic particles on filters when the SO2 is not first removed by a denuder. Such an artifact could invalidate most historical sulfate measurements in regions with alkaline soil. To test this hypothesis, we compare measurements from Teflon filters without a denuder to measurements from nylon filters following a carbonate denuder from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network. The data at 11 remote sites in the Desert Southwest show no evidence of a sulfate artifact. Using a normal error distribution, we show that any artifact is much less than 20 ng/m3. In addition, data from 16 collocated samplers at Meadview, AZ, show that removing the SO2 by denuders does not change the measured sulfate. The mean difference between no-denuder and denuder is -4 ( 10 ng/m3 for Teflon filters and 3 ( 17 ng/m3 for nylon filters. Brigham Young University obtained large differences between their denuder and filter pack samplers at Meadview. Our analysis shows that these differences are explained better by a bias in their denuder sampler than by an SO2 artifact. Our analysis also shows that their measurements are too imprecise to determine an artifact less than about 500 ng/m3.

Introduction The National Park Service and the IMPROVE programs have been monitoring particulate concentrations at numerous sites in the Desert Southwest of the United States since 1979 (1, 2). Sulfate is the most important single species at these sites, accounting for 30-40% of the fine mass budget and 32-46% of the reconstructed extinction budget (2). Since emission strategies depend on accurate ambient measurements, it is essential that the measured sulfate be free of any significant artifact. Because of the importance of sulfate, the IMPROVE network has measured sulfate in two independent sampler modules since 1988. One module has no denuder and collects on a Teflon filter, while the other has an annular denuder to remove SO2 and nitric acid before the particles are collected on a nylon filter. Our analysis of the IMPROVE data showed no significant difference between the sulfate measurements from the Teflon filter without denuder and from the nylon filter following a denuder at sites in the western United States (3). However, Brigham Young University (BYU) obtained large differences between the sulfate collected by a denuder sampler and that collected either by a filter pack without denuder or by a high volume sampler without denuder at several sites in the Desert Southwest (4). They proposed a sulfate artifact averaging * Corresponding author phone: (916)752-1124; fax: (916)752-4107; e-mail: [email protected].

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FIGURE 1. IMPROVE sampling sites used in this paper. Meadview (+) is the site of the November 1991 special study. The 11 Desert Southwest network sites are shown as solid circles, while the 11 Pacific Northwest network sites are shown as open circles.

between 200 and 500 ng/m3, which is large as compared to the average sulfate concentration of 1000 ng/m3 at IMPROVE sites in the Desert Southwest. An artifact of this magnitude would invalidate most historical measurements of sulfate in this region. The sulfate artifact would also have a major impact on Project MOHAVE, a regional study conducted by the Environmental Protection Agency in 1992. Therefore, a special study to focus on this issue was conducted jointly by the University of California at Davis (UCD) and BYU at Meadview, AZ, in November 1991. The location is shown in Figure 1. The specific question was whether the removal of SO2 from the airstream would give different values of sulfate on the filter. UCD had an array of 14 PM2.5 (particulate matter smaller than 2.5 µm aerodynamic diameter) samplers with Teflon filters and two PM2.5 samplers with nylon filters, with seven samplers having no denuders and nine having denuders. BYU had three samplers without denuder and six samplers with denuder. One-half of the BYU denuder samples were exposed to SO2 in the laboratory before analysis. The current paper examines the results that both laboratories obtained from this study, based on the original data reports (5, 6). The BYU results from this study are discussed in Eatough et al. (7). BYU has also discussed the artifact measured at other sites in the Desert Southwest (8, 9). The hypothesis is that the SO2 will interact with basic particles present on the filter. The primary source of basic particles in remote desert regions would be alkaline soil. At the IMPROVE sites in the Desert Southwest, the average PM2.5 soil concentration is around 0.7 µg/m3, which is 75% of the sulfate concentration. Measurements from the IMPROVE network find that approximately 80% of the PM10 (particulate matter smaller than 10 µm) soil mass is larger than 2.5 µm and is removed by the cyclone.

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TABLE 1. Configurations of Collocated Samplers and Analytical Methods Used in IMPROVE Network and in Meadview Study filter type

denuder no denuder IMPROVE denuder

collocated samplers

IMPROVE Network Teflon 1 nylon 1

no denuder IMPROVE denuder URG denuder no denuder IMPROVE denuder

Meadview Teflon Teflon Teflon nylon nylon

6 6 2 1 1

method of analysis PIXE IC PIXE PIXE PIXE IC IC

Experimental Section IMPROVE Sampler. The standard IMPROVE sampler consists of three PM2.5 modules with Teflon, nylon, and quartz filters and one PM10 module with Teflon filters (1). The airstream enters the PM2.5 module through a rain cap, which also removes large particles, and passes through a vertical stack to a cyclone. The module with a nylon filter has a vertical annular denuder in this inlet that consists of five concentric aluminum tubes giving four annular denuder sections. A low Reynolds number of 160 indicates that the flow in the denuder is laminar. All surfaces are coated with Na2CO3. The modules with Teflon and quartz filters have no denuder. The cyclone provides a 2.5 µm aerodynamic diameter cut point at a flow rate of 22.8 L/min. The filters follow the cyclone directly. The flow rate of each filter is measured before and after sampling using two gauges, which are calibrated using an orifice meter system that has been calibrated with a spirometer at UCD. The uncertainty in the flow rate and volume, based on third-party audits, is estimated to be 3%. The flow rate is controlled by a critical orifice downstream of the filter; the standard deviation of annual flow rates is less than 3%. The probability of SO2 striking a denuder wall is >99.9% at the normal flow rate of 22.8 L min-1 (10). Laboratory measurements were conducted at UCD to measure the capture efficiency of SO2 for various denuder configurations: no coating, Na2CO3 without glycerin, and Na2CO3 with 3% glycerin. The temperature was 16 °C, and the relative humidity was 25% to simulate the dry conditions of the desert environment. At 22.8 L min-1, the efficiencies for SO2 capture are 95% with no coating, 98% with Na2CO3, and 99.2% with Na2CO3 plus glycerin. Although we currently include 3% glycerin as a wetting agent, this was not done for the samples in this paper. Similar laboratory measurements for the inlet stack without a denuder show that 20% of the ambient SO2 is captured in the inlet tube, so that 80% reaches the filter. The denuders in the network are changed annually. Additional laboratory tests show that it would require several years for the denuder to lose efficiency from neutralization by SO2. IMPROVE Network Collection and Analysis. Two 24-h samples are collected each week in the IMPROVE network. The cassettes are changed weekly by local operators while all filter handling is done at UCD. In this paper, we will consider the two PM2.5 modules listed in Table 1: Teflon filter without denuder and nylon filter following a denuder. Sulfur is measured on the Teflon filter using particle induced X-ray emission (PIXE), while sulfate is measured on the nylon filter by ion chromatography (IC). The analytical precision for each method is monitored by replicate analyses: 4% for PIXE and 2% for IC. PIXE counting statistics add a small contribution to the sulfur uncertainty, and the uncertainty in the blank subtraction will add a small contribution to the sulfate uncertainty. For 2149 samples collected at the Desert

Southwest sites between September 1992 and August 1994, the sulfur uncertainties ranged from 5% to 14%, with a median value of 5.1%. Only 0.3% of the samples have a sulfur uncertainty exceeding 7%. The sulfate uncertainties ranged from 3.6% to 44%, with a median of 4.2%. Separate collocated measurements using Teflon and nylon filters have verified that these uncertainties are realistic (3). Meadview Collection and Analysis. In the Meadview study of November 1991, UCD and BYU operated separate arrays of samplers for eight periods. The first period had a duration of only 6.8 h; periods 2-6 were around 12 h; and the last two periods were around 24 h. Table 1 lists the five configurations of IMPROVE PM2.5 samplers, 14 with Teflon filters and 2 with nylon filters. Six samplers with Teflon had no denuder, six had IMPROVE denuders, and two had URG2000-30F denuders, coated with carbonate by BYU. The efficiency of a single URG denuder is 97% at flow rates at 18 L min-1 (6). This will drop to around 95% at a flow rate of 22 L min-1. Thus, the efficiency of the single URG denuder in the IMPROVE sampler is similar to the 98% efficiency of the IMPROVE denuder. There were also two samplers with nylon filters, one with no denuder and one with an IMPROVE denuder. Again, sulfur was measured on the Teflon filter using PIXE, and sulfate was measured on the nylon filter using IC. SO2 concentrations were measured by UCD using carbonate-impregnated quartz filters following the Teflon filters in the modules without denuders. The carbonate filters were analyzed for sulfate by IC. BYU measured SO2 using the denuders in the denuder samplers and carbonate-impregnated filters in the filter pack samplers. The correlation coefficients (r 2) between the IMPROVE sampler and the two BYU samplers were 0.94 and 0.96. The mean SO2 from the IMPROVE sampler was 20% lower than the mean from the BYU filter pack sampler, consistent with UCD laboratory studies that 20% is lost in the inlet tube.

Results and Discussion IMPROVE Network. If the artifact exists, it would produce higher measured sulfate concentrations on the Teflon filter than on the nylon filter, since more SO2 passes through the Teflon filter. (Eighty percent of the ambient SO2 will reach the Teflon filter, while 2% will reach the nylon filter.) We will examine the relationship between the two sulfate concentrations at the 11 Desert Southwest sites and the 11 Pacific Northwest sites shown in Figure 1. Figure 2 compares concentrations for 4000 samples collected over a 2-year period in these two regions. The error bars show the propagated precision calculated for each sample. An artifact would shift a point toward the right, so that we would find more points to the right of the 1:1 line than to the left. Since the northwest soil does not tend to be alkaline, we would not expect basic particles on the filter and thus would not expect any artifact. The northwest plot shows that there is no bias between the two measurements in an environment where no SO2 artifact is expected. The southwest plot looks similar to the northwest plot, indicating that the presence of desert soil on filters from the southwest does not result in a visible difference. More information can be obtained by examining the distribution of differences and the error distribution (the ratio of difference divided by the uncertainty in the difference). The uncertainty of the difference is the quadratic sum of the calculated uncertainties of the two measurements, estimated by propagating the collection and analytical uncertainties. (These measurement uncertainties are the error bars in Figure 2.) The left plot of Figure 3 shows the distribution of the differences of the two sulfate measurements at the southwest sites, while the right plot shows the normal error distribution. An SO2 artifact would shift the distributions in both plots to the right. The left plot shows that the actual sulfate differences are centered on zero. A few cases have differences greater

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FIGURE 2. Comparison of sulfate concentrations in ng/m3 measured by PIXE on Teflon with no denuder with that measured by IC on nylon following a denuder that removes SO2, for a 2-year period ending in August 1994. The left plot shows 2149 points from 11 sites in the Desert Southwest, while the right plot shows 1832 points from 11 sites in the Pacific Northwest. The error bars show the calculated precision, which is approximately 5% for all except the most lightly loaded. The 1:1 lines are included. The slope of the regression line minimizing the perpendicular deviations is 0.99 for the southwest and 1.01 for the northwest. The correlation coefficient (r 2) is 0.98 for both plots.

FIGURE 3. Distribution of the difference between the sulfate measured on Teflon with no denuder and the sulfate measured on nylon following a denuder for the same Desert Southwest samples as in Figure 2. The black area indicates the positive differences, which would correspond to a positive artifact. The left plot gives the distribution of differences in ng/m3, while the right plot divides each difference by the uncertainty in the difference. The curve on the right plot is the predicted Gaussian distribution for a zero mean. than 200 ng/m3, as would be expected from the analytical uncertainty. For example, with a sulfate concentration of 5 µg/m3 and both measurement uncertainties of 5%, the uncertainty in the difference would be 310 ng/m3. The error distribution plot provides more information because its shape can be predicted using a Gaussian distribution (11). The normal error plot shows that the actual and predicted distributions are similar, with no more positive differences than predicted. The only discrepancy is a small variation for negative differences. In order to check the sensitivity of the normal error distribution to an artifact, we added a constant to the measured Teflon concentrations. This constant would mimic the effect of an artifact on the Teflon filter without denuder. Figure 4 shows the effect on the normal error distribution from two scenarios: adding 200 ng/m3 to 10% of the Teflon concentrations (left plot) and adding 20 ng/m3 to every concentration (right plot). Both changes would increase the mean annual sulfur concentrations from the Teflon filters by 2%. The distributions clearly no longer follow the predicted Gaussian distribution. This indicates that we could observe any artifact that would increase the annual concentrations by 1-2%, whether it is a large artifact on a few samples or a small artifact on many samples. We could, for instance, easily observe the average artifacts of 200-500 ng/m3 suggested by BYU, which are 10-20 times these additions. We conclude that there is no statistical evidence for an SO2

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FIGURE 4. Normal error distribution (difference/uncertainty of difference) after adding a constant concentration to all or a fraction of the Teflon concentrations. The left plot adds 200 ng/m3 to 10% of the concentrations, while the right plot adds 20 ng/m3 to every concentration. The last bar in both plots includes all values greater than 4.5. The curves are the same predicted Gaussian distributions. artifact in the IMPROVE network measurements, even at a much lower level than that suggested by BYU. Similar tests with a Teflon filter operating in parallel with a nylon filter following a denuder were conducted in Australia using a sampler based on the IMPROVE sampler (12). The only difference was that all filters were analyzed by PIXE. The ratio of the mean sulfur with denuder to the mean sulfur without denuder for 99 pairs of samples was 1.00 ( 0.09. Thus, the Australian comparisons also observed no artifact. UCD Results from the 1991 Meadview Study. The advantage of the Meadview comparison over the IMPROVE network comparison is that it eliminates any potential bias associated with different filter types and analytical methods. In addition, the use of a large array improves the sensitivity to small differences as compared to a single pair. The first three columns of Table 2 give the mean concentrations and standard errors of sulfate for the various combinations of denuder with Teflon filters. Only those concentrations measured by PIXE at UCD are included. The root mean square standard deviation for all periods for these measurements is 17 ng/m3. The fractional standard deviations range from 20% for period 1 (6.8 h), to 8% for periods 2-6 (9.5-13 h), to 5% for periods 7 and 8 (24-30 h). These are similar to the calculated precision from the uncertainties in flow rate, analytical calibration, and counting statistics: 10% for period 1, 7% for periods 2-6, and 5% for periods 7 and 8. The URG and IMPROVE denuders give similar concentrations, with overall mean concentrations of 221 and 215 ng/m3, respectively. Combining both denuders does not change the

TABLE 2. Mean Sulfate and Difference (ng/m3) for Various Configurations of IMPROVE Sampler in Meadview Studya no IMPROVE URG Teflon nylon period denuder denuder denuder difference difference SO2 soil 1 2 3 4 5 6 7 8

137 ( 11 160 ( 6 191 ( 3 216 ( 9 237 ( 6 149 ( 8 282 ( 4 314 ( 3

142 ( 14 179 ( 5 203 ( 6 224 ( 4 239 ( 5 144 ( 5 269 ( 4 320 ( 10

173 ( 5 -5 ( 18 -1 ( 10 210 173 ( 17 -19 ( 8 4 ( 13 70 200 ( 5 -11 ( 7 7 ( 16 210 242 ( 2 -8 ( 10 -8 ( 16 150 219 ( 6 -2 ( 8 -1 ( 18 60 164 ( 5 5 ( 10 -4 ( 11 50 266 ( 14 13 ( 6 -3 ( 21 560 333 ( 0 -6 ( 11 30 ( 26 400

138 94 163 129 81 133 76 90

a The first three columns give the mean and standard error of the measured sulfur times 3.0 for the three configurations with Teflon filters. The fourth column is the difference between no denuder and the IMPROVE denuder; the uncertainty is from the standard error of the measurements. The fifth column gives the difference between the measured sulfate concentrations on the nylon filters; the uncertainty is from the propagated uncertainties. The last two columns give the mean SO2 and soil concentrations measured by the IMPROVE sampler. Soil is the sum of the soil elements with their oxides.

conclusions. However, since all other comparisons use the IMPROVE denuder, we will not include the measurements with the URG denuder. The fourth column gives the difference between the mean without denuders and the mean with IMPROVE denuder for each period for samples collected on the Teflon filters. The uncertainty is the quadratic sum of the two standard errors. The pair of samplers with nylon filters provides a check of the Teflon results. The fifth column gives the difference between the two samples collected on the nylon filters. The uncertainty in the difference is the quadratic sum of the two propagated precisions of approximately 5%. The last two columns give the concentrations of SO2 and soil. Soil is the sum of soil-derived elements and their normal oxides. This is an indicator of possible basic particles on the filter. For the study as a whole, the difference calculated from the UCD samplers is not significantly different from zero: -4 ( 10 ng/m3 for Teflon and 3 ( 17 ng/m3 for nylon. The difference for period 7 from the Teflon samples is significant by the Student’s t-test at the 95% significance level. Nevertheless, other data suggest that the difference may only be associated with statistics. First, period 8, with comparable soil and SO2 concentrations, has a negative sulfate difference from the Teflon filters (-6 ng/m3). There is no reason that the particles on the filter during period 7 would be significantly more basic than during period 8. Second, the modules with nylon filters give a negative difference for period 7. The conclusion is that a small sulfate difference of 10-15 ng/m3 is observed during one period, but the difference may only be statistical variation. A sulfate artifact of 10 ng/m3 on every sample would increase the annual average ambient concentration for the IMPROVE sites in the southwest by only 1%. BYU Results from the 1991 Meadview Study. The Meadview study was cited by BYU as showing the presence of the SO2-basic particle artifact (7), although differences were observed in other studies (3, 8, 9). Because this was a joint study and we have access to the original measurements for Meadview, we are able to examine these data in greater detail than we can for other BYU studies. The BYU samples were collected on Teflon filters using filter pack samplers (no denuder) and denuder samplers. PM2.5 particle sizing was done using URG cyclones. The denuder sampler had two URG2000-30F denuders in series after the cyclone, with an efficiency for removal of SO2 of >99%. The filters were analyzed for sulfate by IC at BYU. Three of the six samples collected in the denuder sampler during each period were exposed to SO2 in the laboratory before the IC analysis. The laboratory SO2 exposure matched the ambient exposure for the period. Table 3 shows the measured sulfate concentra-

TABLE 3. Measured Sulfate Concentrations (in ng/m3) from Collocated BYU Samplers and Analyzed by IC, from the BYU Data Report with One Concentration Corrected (6) measurement period 1

2

104 131 3159

177 1018 1707

52 546 1392

21 85 382

53 93 143

42 85 242

7

8

Filter Pack (No Denuder) 81 104 60 57 97 139 170 75 378 821 363 212

3

57 295 299

231 235 476

Denuder 84 49 101 90 146 183

12 84 143

17 94 191

168 257 355

Denuder + SO2 in Laboratory 35 22 43 25 131 60 97 101 143 140 190

41 230 381

228 250 260

77 129 173

4

5

6

tions in each of the three groups (in ng/m3) obtained from the 1991 BYU data report (6) with one change for an incorrect decimal place (13). The difference between the filter pack samples without denuder and the denuder samples that were not exposed to SO2 represents the ambient artifact and is equivalent to the difference with the IMPROVE samplers. The difference between the denuder samples that were exposed to SO2 in the laboratory and the denuder samples that were not exposed to SO2 represents a laboratory-induced artifact. The SO2 artifact hypothesis predicts that the ambient and laboratory-induced artifact would be equal. Table 3 shows that the BYU measurements were highly variable with high and low concentrations differing by factors of 5-30 in 12 of the 24 sets. The overall standard deviation, from the root mean square of the standard deviations for the 24 sets, is 430 ng/m3, which is 170% of the mean. This standard deviation is comparable to BYU’s average reported precision in other studies of 190 and 260 ng/m3 for the filter pack and denuder samplers, respectively (9). However it is 25 times that for UCD in the Meadview study. This large uncertainty and only three samplers in each configuration make the BYU setup insensitive to small differences. With three samplers in each configuration and a standard deviation of 200 ng/m3, the artifact must be greater than 450 ng/m3 in order to obtain a 95% confidence level. Because of the poor precision, none of the differences corresponding to either ambient of laboratory-induced artifacts in the Meadview study are significant by the Student’s t-test. Table 3 also shows that adding SO2 in the laboratory had no effect on the measured sulfate, and both configurations give much lower concentrations than the filter pack. If the first two periods are excluded because of especially large variations, the mean denuder is 131 ng/m3, the mean denuder plus SO2 is 136 ng/m3, and the mean filter pack is 231 ng/m3. The mean IMPROVE for all configurations is 233 ng/m3. The difference between the filter pack and denuder indicates either a large positive ambient artifact or a large systematic measurement bias by one of the samplers. The similarity between the two denuder means indicates that the difference is not produced by an interaction with SO2. While the BYU measurements are too imprecise to permit evaluating differences for individual periods, we can obtain adequate statistics by combining the periods. The overall averages, used above, include the variation in ambient sulfate from period to period. To remove this, we will normalize each measurement by the mean of all UCD measurements for that period. (The conclusions are the same if we normalize to the mean of all measurements in the period, including the BYU measurements.) This creates four data sets: IMPROVE (74 values), filter pack (18 values), BYU denuder (18 values), and denuder + SO2 (17 values). The means and standard errors

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of the ratios are 1.00 ( 0.01 for the IMPROVE sampler, 0.99 ( 0.20 for the filter pack, 0.55 ( 0.07 for the denuder without SO2, and 0.56 ( 0.08 for the denuder followed by SO2 in the laboratory. The Student’s t-test can be used to quantify the relationship between the three BYU configurations. There is a 95% probability that the difference between the filter pack and denuder samplers is significant, compared to a 5% probability that adding SO2 in the laboratory produces a change. Thus, there is a very high probability that there is a bias in either the denuder or filter pack sampler. The t-test can be used to determine which sampler is most likely to be biased. There is a 0.94 probability that the IMPROVE and BYU filter pack means are equal, as opposed to a 0.0005 probability that the IMPROVE and either BYU denuder means are equal. The most probable explanation for these results is that the denuder sampler underestimates the sulfate concentration. Our analysis differs from that of Eatough et al. (7) primarily in the estimate of analytical precision and the magnitude and significance of the ambient and laboratory-induced artifacts. The difference is the treatment of statistical outliers. Eatough et al. (7) deleted 13% of the valid measurements solely on the basis of statistical inconsistency. There are three major difficulties with this. The most important difficulty is that the paper fails to note that any measurements were deleted. These deletions had a major impact on the conclusions, reducing the overall precision from 430 to 80 ng/m3 and doubling both artifacts from period 7. The second difficulty is the justification for using any rejection procedure when this leads to so many rejections. An outlier in a wellbehaved system should be a rare occurrence (14), so rejection tests should not be used if they discard too many points (15). They should not be used repeatedly to remove multiple points in a given set (15) or with less than four measurements (16). The fact that the standard deviation in this study is similar to those in other BYU studies indicates that there is little justification for deleting any measurements. The third difficulty is that BYU was not consistent in using the rejection tests. If a set of criteria is used to reject any points, it must be applied for all points. BYU used what Taylor calls ‘the rule of the huge error’ rather than the more established Gibbs or Dixon criteria (13, 14). The rule deletes a point if the difference from the mean of all other points exceeds four times the standard deviation of all other points. This represents a 98% confidence level for 15 points, but a lower confidence level for fewer points. BYU removed some outliers using all nine points in a period, repeating the test on the remaining points. They also removed some outliers using only the three points in a configuration. Using this methodology for both three and nine points, we found that 15 of the 71 measurements met the criteria. However, BYU rejected only eight of these 15 points. In addition, they rejected one measurement that did not meet the criteria. The difficulties can be illustrated by two important deletions in period 7, the one period that Eatough et al. (7) used as “evidence for a filter pack sulfate sampling artifact”. BYU applied the rule for the three measured concentrations from the filter pack of 299, 295, and 57 ng/m3. The standard deviation of 140 ng/m3 for all three points is actually less than the precision of 200-250 ng/m3 that BYU normally

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obtains. Deleting the third point assumes that the standard deviation for the first two of 3 ng/m3 is a good estimate of the precision, which is clearly invalid. By deleting this measurement, the ambient artifact increases from 116 to 196 ng/m3, and the significance increases from 70% to 94%. BYU also discarded one measurement collected by the denuder sampler and one to SO2 in the laboratory, although the rejection does not meet any outlier criteria. This deletion increases the laboratory-induced artifact from 117 to 205 ng/ m3 and the significance from 63% to 84%.

Acknowledgments This work is supported by National Park Service Contract 1443CX0001-93-006.

Literature Cited (1) Eldred, R. A.; Cahill, T. A.; Wilkinson, L. K.; Feeney, P. J.; Chow, J. C.; Malm, W. C. in Visibility and Fine Particles; Mathai, C. V., Ed.; Air and Waste Management Association: Pittsburgh, 1990, pp 187-196. (2) Malm, W. C.; Sisler, J. F.; Huffman, D.; Eldred, R. A.; Cahill, T. A. J. Geophys. Res. 1994, 99, (D1), 1347-1370. (3) Eldred, R. A.; Cahill, T. A.; Feeney, P. J. Proceedings, 86th Annual A&WMA Meeting; Air and Waste Management Association: Pittsburgh, PA, 1993; Paper 93-RA-110.02. (4) Lewis, E. A.; Lewis, L. J.; Eatough, M.; Hansen, L. D.; Eatough, D. J. Proceedings, EPA/AWMA Conference on Toxic and Related Air Pollutants; Air Waste Management Assoc.: Pittsburgh, PA, 1991; pp 787-971. (5) Eldred, R. A. Meadview Sulfate Study, Project MOHAVE, November 1991; Data report submitted to the Project MOHAVE Technical Committee, February 1992. (6) Eatough, D. J. A comparison of the determination of particulate sulfate and SO(g) at Meadview using filter pack and diffusion denuder sampling systems. Data report submitted to Southern California Edison, December 19, 1991. (7) Eatough, D. J.; Lewis, L. J.; Eatough, M.; Lewis, E. A. Environ. Sci. Technol. 1995, 29, 787-971. (8) Eatough, D. J.; Eatough, M.; Lamb, J. D.; Lewis, L. L.; Lewis, E. A.; Eatough, N. L.; Missen, R. Atmos. Environ. 1996, 30, 269281. (9) Eatough, N. L.; Eatough, M.; Joseph, J. M.; Caka, F. M.; Lewis, E. A.; Lewis, L. J.; Eatough, D. J. Precision and accuracy in the determination of gas and particulate phase sulfur oxides and fluorine, and of spherical aluminosilicate fly ash particles in Project MOHAVE. Submitted to J. Air Waste Manage. Assoc. (10) Possanzini, M.; Febo, A.; Biberti, A. Atmos. Environ. 1983, 17, 2605-2610. (11) Bevington, P. R.; Robinson, D. K. Data Reduction and Error Analysis for the Physical Sciences; McGraw-Hill: New York, 1992. (12) Cohen, D. D.; Martin, J.; Bailey, G. M.; Crisp, P. T. Clean Air 1993, 27, 63-71. (13) Eatough, D. J. Personal communication, Memorandum to Project MOHAVE Technical Committee, November 10, 1995. (14) Taylor, J. K. Quality Assurance of Chemical Measurements; Lewis: Boca Raton, 1987. (15) Wernimont, G. Use of Statistics to Develop and Evaluate Analytical Methods; Association of Official Analytical Chemists: Arlington, VA, 1985. (16) Miller, J. C.; Miller, J. N. Statistics of Analytical Chemistry; Wiley: New York, 1988.

Received for review June 25, 1996. Revised manuscript received December 31, 1996. Accepted January 14, 1997.X ES9605533 X

Abstract published in Advance ACS Abstracts, March 15, 1997.