A Comparison of PAH, PCB, and Pesticide Concentrations in Air at

It is interesting to note that the χ2-test also indicates that the concentrations of these four .... We thank the rest of Team IADN and the U.S. Envi...
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Environ. Sci. Technol. 2001, 35, 2417-2422

A Comparison of PAH, PCB, and Pesticide Concentrations in Air at Two Rural Sites on Lake Superior STEPHANIE S. BUEHLER, ILORA BASU, AND RONALD A. HITES* School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405

Atmospheric concentrations of polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and pesticides were compared at Brule River and Eagle Harbor, two rural sites on Lake Superior that are part of the Integrated Atmospheric Deposition Network (IADN). Brule River lies 40 km southwest of Duluth, MN, a small industrial city, and Eagle Harbor is in Michigan’s upper peninsula, 400 km east of Brule River. Pesticide and PCB concentrations were similar at both sites. Day-by-day regression analyses of the data showed that PAH concentrations, an indication of urban contamination, were significantly higher at Brule River than at Eagle Harbor. Concentration ranges for all compounds at both sites were well within global background levels, despite the differences observed between the two sites. Clearly, pollution from Duluth is influencing PAH concentrations at Brule River more than at Eagle Harbor.

Introduction The Integrated Atmospheric Deposition Network (IADN) began in 1990 as a joint effort between the United States and Canada to monitor the atmospheric deposition of toxic pollutants to the Great Lakes. Five master stations were established for the IADN project, one near each of the Great Lakes. These stations were considered remote from local influences, and measurements at these sites were considered representative of the Great Lakes’ background atmosphere. As the IADN project progressed, satellite stations were developed to augment the master stations. One such satellite station, established in 1994, is at Brule River, WI. This station is on the south shore of Lake Superior (see Figure 1). It is approximately 400 km west of Eagle Harbor, MI, one of the original five master stations. Brule River is 40 km southeast of Duluth, MN, a city with a population of about 100 000 people and with some heavy industry. Thunder Bay, an industrial city in Ontario, Canada (see Figure 1) with a population of approximately 110 000, also sits on the shores of Lake Superior, about 140 km north of Eagle Harbor and approximately 450 km northeast of Brule River. Since its inception, IADN has made regular measurements of polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and chlorinated pesticides in the air and precipitation near the Great Lakes at each of its stations. Because PCB production was banned in the U.S. in 1977 and because many of the pesticides IADN studies were either banned or heavily restricted, the current levels measured for * Corresponding author phone: (812)855-4848 ext 0193; fax: (812)855-1076; e-mail: [email protected]. 10.1021/es001805+ CCC: $20.00 Published on Web 05/03/2001

 2001 American Chemical Society

these compounds at the Great Lakes come mainly from revolatilization from lake and terrestrial surfaces. PAHs, however, are byproducts of incomplete combustion, and thus, these compounds have many current sources, including vehicle emissions, residential heating, and industrial activity (1). Since the atmospheric concentrations of PCBs and pesticides are the result of revolatilization, concentration peaks for these compounds are seen in the warmer months. The sources of PAHs, however, maximize in the colder months (1, 2) and tend to be concentrated in urban areas, which can in turn influence the PAH concentrations in rural areas (3). All the previous IADN studies of PCBs, PAHs, and pesticides have focused on the measurements acquired at the five master stations; thus, all analyses of Lake Superior concentrations have used the site at Eagle Harbor. However, given that the Eagle Harbor and Brule River sites are relatively close to one another, we have an opportunity to compare measurements between the two sites. For example, we wondered if it was, in fact, useful to have two sites so close to each other on Lake Superior. This paper examines the question, “How close is too close?” by comparing the concentrations of PAHs, PCBs, and pesticides at Brule River to those at Eagle Harbor. We examine the possible influence of Duluth on Brule River atmospheric pollutant concentrations. In addition, we determine whether the Brule River and Eagle Harbor sites are significantly different and what implications these results might have on future studies.

Experimental Section Sampling Methodology. A brief synopsis of the sampling and analytical methodology will be presented here, but full details can be found elsewhere (4-7). Figure 1 shows the location of the two sites. The sites at both Eagle Harbor (latitude 47° 27′ 47′′; longitude 88° 08′ 59′′) and Brule River (latitude 46° 44′ 51′′; longitude 91° 36′ 30′′) are situated within 400 m of the shore of Lake Superior, and both are considered remote sampling sites, representative of the Great Lakes atmosphere. Air samples were collected using a modified Anderson high-volume sampler (General Metal Works, model GS 2310) that pulls air through a filter and an absorbent at a rate of 34 m3/h. Sampling occurred every 12 days and lasted 24 h. Particles were collected on quartz-fiber filters (Whatman QMA), and gas-phase compounds were collected on XAD-2 resin (Sigma, Amberlite 20-60 mesh). Each site is equipped with a meteorological tower that records an hourly average wind speed and wind direction at an elevation of 10 m height and air temperature, relative humidity, and solar radiation at 2 m height. Analytical Methodology. Once collected, the quartz-fiber filters and XAD-2 resin were spiked with PCB-14, PCB-65, PCB-166, dibutylchlorendate, δ-HCH, d10-phenanthrene, and d10-pyrene as recovery standards and Soxhlet extracted with a mixture of 50% acetone in hexane for 24 h. Then the extracts were concentrated and solvent-exchanged to hexane by rotary evaporation and fractionated on 3.5% (w/w) water deactivated silica gel. PCBs, hexachlorobenzene (HCB), and p,p′DDE were eluted with hexane, while the remaining pesticides and all of the PAHs were eluted using 50% dichloromethane in hexane. The extracts were then concentrated under a stream of nitrogen and spiked with PCB-30, PCB-65, PCB155, PCB-204, d10-anthracene, d12-benz[a]anthracene, and d12-perylene as quantitation standards. PCBs and pesticides were analyzed on a Hewlett-Packard 5890 gas chromatograph with a 63Ni electron capture detector VOL. 35, NO. 12, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. IADN sampling stations (2) and nearby cities (9) on Lake Superior. in splitless injection mode. A 60 m × 250 µm i.d. (film thickness 0.10 µm) DB-5 column was used for separation. PAH analyses were performed on a Hewlett-Packard 6890 gas chromatograph coupled to a 5973 mass spectrometer. Separation was achieved using a 30 m × 250 µm i.d. (film thickness 0.25 µm) DB-5MS column. The PAHs were analyzed using selected ion monitoring (SIM) and quantitated using the internal standards. All analytical procedures were monitored using strict quality control measures (8). Extraction efficiencies were monitored using matrix spikes in each batch of approximately 10 samples and surrogate standards in each sample. Average spike recoveries for PCBs in this study ranged from 75% to 105% (8). Spike recoveries for pesticides ranged from 80% to 104%, and recoveries for PAHs ranged from 63% to 94% (8). The laboratory blank values were less than 20% of actual sample values; thus, blank corrections were not necessary. Data Analysis. Gas-phase data from January 1996 through December 1998 were used for this study. Filter samples before October 1996 had been composited on a monthly basis, so all analysis involving particle-phase compounds used data from October 1996 through December 1998. Sample concentrations were determined by dividing the mass of the analyte measured by the volume of air collected. Partial pressures were calculated from the compound’s concentration using the ideal gas law, which incorporated the molecular weight of the compound and the average air temperature during the sampling. Among the PAH, acenaphthylene and acenaphthene were not reliably eluted during fractionation, and these compounds were omitted from our data analysis. For calculations and comparisons, the 16 individual PAHs that were measured were summed together to determine a total PAH concentration, which is represented in this paper as ΣPAH. The individual PAHs are fluorene, phenanthrene, anthracene, fluoranthene, pyrene, retene, benz(a)anthracene, chrysene/triphenylene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(e)pyrene, benzo(a)pyrene, indeno(1,2,3-cd)pyrene, dibenz(a,h)anthracene, benzo(ghi)perylene, and coronene. ΣPCB represents the same idea, with about 100 individual PCB congeners (4) being summed together. Sums were used instead of individual congeners and PAHs because a correlation matrix for each group of compounds and their sums indicated that individual PAHs and PCBs were significantly correlated with each other and with their totals. From these results, we concluded that sums would be good representatives of individual compounds. The exceptions for particle-phase PAHs were anthracene and retene at Eagle Harbor. Anthracene was rarely seen at Eagle Harbor (10 out of 61 samples), and both PAHs were at very low 2418

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concentrations at this site. The 17 individual gas-phase PCB congeners that did not correlate well with ΣPCB represent a small fraction of the total compared to other congeners because of their low concentrations.

Results and Discussion The average concentrations of the various analytes (with their standard errors) averaged over 1996, 1997, and 1998 are given in Table 1 for the two locations. For comparison, the averages for the same compounds for the same years at the three other United States’ IADN sites are also given in Table 1. It is clear that the concentrations at Brule River and Eagle Harbor are among the very lowest measured, and for most of these compounds, are probably representative of background atmospheric concentrations in the Great Lakes basin. Our goal is to determine if there are significant differences between the concentrations at Brule River and Eagle Harbor, presumably due to the relative proximity of Duluth to the Brule River site. Our hypothesis is that the Brule River concentrations should be systematically higher than those at Eagle Harbor because of the influence of Duluth at Brule River. Duluth is the apparent choice for possible concentration increases at Brule River for two reasons: wind patterns and lack of other prospective cities. A back trajectory analysis showed that winds generally came to Brule River from the northwest, where Duluth is located. The only other urban center in the area with the potential to influence Brule River concentrations is Thunder Bay in Ontario, Canada (see Figure 1). Thunder Bay lies northeast of Brule River, and our air trajectory analysis indicated that winds did not often come from this direction. Our statistical approach was to examine the concentrations measured on the same days at the two IADN sites. In other words, we looked at the measurements of each compound as paired data, one from each location, both taken on the same day. Because atmospheric temperature has a big effect on the atmospheric concentrations of these compounds (9-18), we first examined the paired samples to make sure that the air temperatures at the two sites were not significantly different. As expected, the average absolute temperature difference was small (0.61 ( 0.39 K). Thirty-three of the temperatures were higher at Eagle Harbor, and 25 of the temperatures were higher at Brule River; a sign test of these values showed no significance (χ2 ) 1.10, p < 0.25). Thus, we are confident that any differences in the concentrations we measured at the two sites are not due to the effects of atmospheric temperature. An example of the pairwise comparison of the data is given in Figure 2, which shows the concentrations of total

TABLE 1. Average Concentrations during 1996-1998 with Standard Errors for ΣPAHs, ΣPCBs, and Pesticides at All U.S. IADN Stations Brule River

Eagle Harbor

ΣPAH ΣHCHa Σchlordaneb dieldrin ΣDDTc

410 ( 65 1.2 ( 0.4 1.3 ( 0.2 2.1 ( 0.2 0.44 ( 0.08

160 ( 19 0.90 ( 0.19 0.81 ( 0.09 3.3 ( 0.3 0.14 ( 0.06

ΣPAH ΣHCHa Σchlordaneb dieldrin ΣDDTc HCB ΣPCBs

1500 ( 130 89 ( 7 7.8 ( 0.7 8.4 ( 1.3 2.9 ( 0.3 74 ( 3 76 ( 7

1000 ( 81 96 ( 7 8.6 ( 1.0 8.8 ( 1.4 4.4 ( 0.6 68 ( 2 63 ( 6

Sleeping Bear Dunes (pg/m3)

Sturgeon Point

Chicago

Particle Phase 320 ( 51 0.69 ( 0.13 1.5 ( 0.2 5.7 ( 0.6 0.41 ( 0.12

1200 ( 120 0.58 ( 0.11 2.7 ( 0.2 5.0 ( 0.4 0.81 ( 0.12

14 000 ( 1500 1.0 ( 0.2 21 ( 2 29 ( 3 9.2 ( 0.9

Gas Phase (pg/m3) 1500 ( 170 99 ( 11 14 ( 2 15 ( 3 11 ( 1 80 ( 4 130 ( 15

5500 ( 410 82 ( 6 38 ( 4 19 ( 3 31 ( 2 54 ( 4 260 ( 27

99 000 ( 14 000 130 ( 10 130 ( 13 130 ( 17 71 ( 7 110 ( 6 1800 ( 170

a ΣHCH represents the sum of R- and γ-HCH. b ΣChlordane represents the sum of R- and γ-chlordane and trans-nonachlor. c ΣDDT represents the sum of the p,p′-isomers of DDT and its two metabolites, DDE and DDD.

FIGURE 2. Total PAH particle-phase concentrations at Brule River and Eagle Harbor as a function of time. polycyclic aromatic hydrocarbons in the particle phase on 61 days. Notice that the Brule River concentrations are higher (in some cases, much higher) than those at Eagle Harbor in 42 out of 61 cases (χ2 ) 8.67, p < 0.005). We performed sign tests such as this for all of the compounds measured in all of the media (gas and particle phase), and Table 2 gives the χ2 values for the sign tests. χ2 values were significant at the 95% confidence level or higher for total PAHs in both the particle and gas phase, indicating that PAH concentrations at Brule River are consistently higher than those at Eagle Harbor. χ2 values for total PCBs and pesticides in the two media either lacked significance or indicated that one or the other site had higher concentrations, depending on the pesticide in question. This lack of a significant trend for the pesticides and PCBs is probably due to consistently low concentrations near blank levels at the sites. In addition, there are no current known sources for PCBs and many of the pesticides we measure. This means that volatilization from terrestrial and aquatic sources releases these compounds into the atmosphere (9, 12, 19, 20). Thus, concentrations of PCBs and pesticides at Brule River and Eagle Harbor are not related to a point source, such as the city of Duluth. PAHs, on the other hand, have a point source, and their higher concentrations at Brule River as compared to Eagle Harbor indicate that Duluth is this source.

Rather than comparing paired concentrations (see Figure 2), another approach is to regress, on a day-by-day basis, the concentrations of gas-phase and particle-phase ΣPAHs, ΣPCBs, and pesticides from Eagle Harbor against the concentrations measured on the same day from Brule River. Figure 3 shows such an example. Figure 3A is a plot of the ΣPAH particle-phase concentrations at Eagle Harbor vs Brule River using the data shown in Figure 2. Figure 3B is a similar plot for the ΣPCB gas-phase concentrations measured at the two sites. In these two cases, the regressions are significant at the 99% confidence limit. The slope for the ΣPAH plot is 0.103, which suggests that, on average, the concentrations of PAH are about 10 times lower at Eagle Harbor than at Brule River. The slope for the ΣPCB plot is 0.556, which suggests that the concentrations of PCBs are about half at Eagle Harbor compared to Brule River. The results for these analyses for all of the compounds in all of the media are given in Table 2. Twenty-five out of 26 of these correlations are statistically significant, and 21 out of 26 have significant slopes less than one. To test the strength of the regressions in Table 2 and to be sure that outliers were not driving the regressions, we performed log and rank order transformations on the concentrations at both Brule River and Eagle Harbor and then performed regression analyses on those transformations. These transVOL. 35, NO. 12, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Sign-Test χ2 Values and Slopes and r2 Values for Regressions of Paired Samples from Eagle Harbor v. Brule River χ2

p < |χ2|b slope

Gas Phase R-HCH 2.71 a γ-HCH 31.3 0.0005d γ-chlordane 1.95 a endosulfan I 0.01 a R-chlordane 4.05 0.05 trans-nonachlor 4.38 0.05d dieldrin 0.44 a endosulfan II 0.33 a p,p′-DDD 3.50 a endosulfan sulfate 0.02 a p,p′-DDT 14.5 0.0005d HCB 0.44 a p,p′-DDE 11.9 0.001d ΣPCB 1.80 a ΣPAH 5.00 0.05 R-HCH γ-HCH γ-chlordane endosulfan I R-chlordane trans-nonachlor dieldrin endosulfan II ΣDDT endosulfan sulfate ΣPAH

r2

n p < |t|c

0.796 1.114 0.913 0.668 0.766 1.008 0.787 0.792 0.484 0.681 1.002 0.432 1.003 0.556 a

0.795 0.651 0.725 0.770 0.684 0.730 0.789 0.724 0.483 0.482 0.385 0.404 0.551 0.478 a

83 83 80 81 82 81 82 82 81 81 81 81 81 80 80

0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 a

Particle Phase 3.56 a 0.347 18.8 0.0005d 0.279 10.6 0.0025 0.462 8.67 0.005d 0.537 17.3 0.0005 0.306 7.37 0.01 0.437 22.4 0.0005d 0.692 2.47 a 0.503 20.5 0.0005 0.240 0.27 a 0.826 8.67 0.005 0.103

0.225 0.077 0.353 0.388 0.332 0.228 0.222 0.623 0.253 0.654 0.122

61 63 62 61 62 63 61 61 63 63 61

0.0001 0.028 0.0001 0.0001 0.0001 0.0001 0.0002 0.0001 0.0085 0.0001 0.006

a Values were not significant at the 95% confidence level or better. Significance levels are for χ2 values. c Significance levels are for r2 values. d Eagle Harbor values were statistically significantly greater than Brule River values. b

FIGURE 3. Eagle Harbor vs Brule River concentrations for paired samples for (A) ΣPAH in the particle phase and (B) ΣPCB in the gas phase. formation analyses supported the results found by the linear regressions, indicating the robustness of our results. These analyses suggest that the concentrations of most of these compounds are higher at Brule River and are probably being influenced by Duluth. The four exceptions are the gasphase concentrations of γ-HCH, trans-nonachlor, p,p′-DDT, 2420

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and p,p′-DDE. It is interesting to note that the χ2-test also indicates that the concentrations of these four compounds are statistically significantly higher at Eagle Harbor than at Brule River. All of these compounds have either been banned or had their uses severely restricted in the U.S., and their current sources are thought to come mainly from revolatilization from the lake water (13). Thus, it is reasonable that these four compounds are more concentrated at the site that is more in the center of Lake Superior (Eagle Harbor) compared to the site that is on the far southwestern shore (see Figure 1). In general, the slope for particle-phase ΣPAH is much lower than the slopes for the pesticides and ΣPCBs. Because the sources of PAHs are largely associated with the industrial activity, space heating, and vehicular traffic localized in urban centers, the lower slope for ΣPAH in this regression analysis suggests an especially large influence of Duluth on the samples taken at Brule River. On the other hand, because PCBs and many of the pesticides are no longer produced, and because the use of most pesticides is not associated with urban centers, the slopes for most of these compounds are likely to be much closer to unity than those for PAHs. The gas-phase concentrations of semivolatile organic compounds have been shown to be strongly correlated to atmospheric temperature (9-12, 18, 21-26). The seasonal cycling (11, 12, 16, 17, 27) and terrestrial revolatilization (19, 20) of such compounds as pesticides and PCBs are strong indicators of the effect of temperature on the concentrations of these contaminants. The gas-phase behavior of pollutants is thermodynamically described using the Clausius-Clapeyron equation

ln P ) -

∆H 1 +const R T

()

(1)

where P is the partial pressure of the compound of interest in atmospheres, ∆H is the phase transition energy of the compound (kJ/mol), R is the gas constant, and T is the temperature in Kelvin (9-12, 18, 21-26). This relationship demonstrates the importance of temperature effects on gasphase concentrations. While partial pressures are the preferred representation of the pollutants, gas-phase concentrations could just as easily be used in their place. This functional relationship has been used to describe local temperature effects as well as long- and short-range transport or sources of pollutants (9, 10, 12, 21, 26). To further investigate the sources of PAHs, we investigated the effect of temperature on the concentrations at both Brule River and Eagle Harbor, and we did the same for PCBs as a comparison. The resulting graphs are shown in Figure 4. Figure 4A,B shows plots of concentration vs temperature and the natural log of concentration vs reciprocal temperature, respectively, for ΣPAH in the particle phase at both sites. Figure 4C,D shows the same plots for ΣPCB in the gas phase. Figure 4A shows a strong negative correlation (r ) -0.560) between ΣPAH and temperature for Brule River. As expected, ΣPAH concentrations increase as temperature decreases, because of increased domestic and industrial heating, increased partitioning onto the particle phase, and more numerous temperature inversions (1, 2, 28). Brule River and Eagle Harbor would both be expected to experience similar meteorological and partitioning conditions because of their close proximity to each other and the lake. The fact that a higher and statistically significant correlation for ΣPAH exists at Brule River than Eagle Harbor (0.314 vs 0.047) indicates that increased seasonal sources of PAHs have a larger impact there than at Eagle Harbor. Duluth is only 40 km from Brule River and is a city of moderate population with heavy industry. That industry relies on coal- and petroleum-based energy sources, the combustion of which

FIGURE 4. ΣPAH concentrations in the particle phase vs temperature (A), natural logarithm of the ΣPAH concentration in the particle phase vs reciprocal temperature (B), ΣPCB concentration in the gas-phase concentration vs temperature (C), and natural logarithm of the ΣPCB concentration in the gas-phase vs reciprocal temperature (D). In all cases, b refers to Brule River and O refers to Eagle Harbor. The r2 values for each regression are given near each line; values over 0.106 are significant with 99% confidence. is a major contributor to PAH release. Figure 4B represents a Clausius-Clapeyron relationship for particle-phase PAHs. The similarity of the r2 values for plots 4A for Brule River (0.314 vs 0.273) and 4B for Eagle Harbor (0.047 vs 0.028) indicates that the Clausius-Clapeyron relationship is not controlling the concentration of the PAHs. In fact, the Eagle Harbor regression in Figure 4B still shows no statistical significance. In other words, it is not temperature but sources that are ultimately controlling PAH concentrations at Brule River. The plots for ΣPCB in the gas phase (Figure 4C,D) are in clear contrast to the ΣPAH plots. The r2 values and slopes for the different sites are about the same in Figure 4C (r2 ) 0.44) and 4D (r2 ) 0.61). This similarity indicates that the source is the same at the two sites, namely revolatilization from soil and the lake. The large increase in r2 values from Figure 4C to Figure 4D for both sites indicates that the atmospheric temperature controls most of the variability at the two sites. The difference between the PCB and PAH plots as well as the differences between the Brule River and Eagle Harbor PAH regressions (see Figure 3) are indicators that nearby sources (probably Duluth) are controlling PAH concentrations at Brule River. It is likely that Duluth contributes some ΣPCB and pesticides to the Brule River site, but the urban effect on these compounds’ concentrations is much smaller than this effect on ΣPAH. Despite the apparent influence of Duluth on Brule River concentrations, PAH levels in the gas and particle phases in this study at both Brule River and Eagle Harbor are comparable to levels at other rural locations (see Table 1 (3, 16, 29-31)), and they are well below urban PAH levels (16, 30-34). Interestingly, while PAH levels at Brule River are, on average, 1-3 times greater than levels at Eagle Harbor and on some individual days 10 times greater, Brule River levels are 3 times lower than levels at Sturgeon Point, a site 20 km southwest of Buffalo, NY (3). Both the Brule River and Sturgeon Point sites are affected by nearby sources of PAHs; however, the Brule River site is affected less, presumably because it is further away (40 km) from a smaller city (Duluth) than the Sturgeon Point site, which is closer (20 km) to a larger city (Buffalo). It is interesting that a small urban center, such as Duluth, appears to be showing a real influence on a rural location

that was selected to minimize the influences from local sources. These influences could have gone unnoticed without the paired sample analysis discussed here. If such a small city can have an impact on a rural sampling site, one wonders if there is a good place to put a sampling site so that it will remain completely unaffected by local sources and be truly representative of global background levels. Are these sites too close to each other? Although the Brule River site showed PAH concentrations near background levels, the actual levels measured at this site were substantially elevated with respect to those measured at Eagle Harbor. Thus, the two sites are not providing duplicate data, and it is hard to argue that the measurements of PAH at Brule River should be discontinued. On the other hand, these two sampling sites showed similar pesticide and ΣPCB concentrations (generally within a factor of 2). Thus, the measurement of these compounds at Brule River could be discontinued. In general, the Brule River site, which might have been considered to be too close to the Eagle Harbor site, is at an important location. Samples taken there are capable of revealing details about how a small urban area can substantially increase pollutant levels at pristine locations near the Great Lakes.

Acknowledgments We thank the rest of Team IADN and the U.S. Environmental Protection Agency’s Great Lakes National Program Office for funding (GL995656).

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Received for review October 24, 2000. Revised manuscript received March 16, 2001. Accepted March 22, 2001. ES001805+