Environ. Sci. Technol. 1996, 30, 444-446
Diurnal Variations in Atmospheric Concentrations of Polychlorinated Biphenyls and Endosulfan: Implications for Sampling Protocols JEFFREY C. WALLACE AND RONALD A. HITES* School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
Polychlorinated biphenyls (PCBs) and endosulfan, both semivolatile organic compounds (SOCs), exhibited large diurnal variations in concentration in Bloomington, IN (see Figure 1). Four samples per day were collected serially over a 5-day period in September 1994. For both SOCs, air samples collected in the morning had twice the concentration of samples collected at midnight. The diurnal variations tracked atmospheric temperature, with the highest concentrations recorded at the highest temperatures. An important implication for sampling SOCs is evident from this study. If one is attempting to measure the daily concentration of an SOC, sampling should be in multiples of 24 h; sampling for more or less time could lead to inaccurate results.
Introduction The atmospheric behavior of semivolatile organic compounds (SOCs) has been well-studied for at least 20 years. Most of this knowledge has been gained by measuring SOCs in air samples taken over periods of several months (1-4), usually at 2-7-day intervals. As a result, we have a good understanding of how SOCs behave on seasonal and annual cycles. However, because of the low sampling frequency, we have much less knowledge about how these compounds behave on diurnal cycles. This is not the case with other compounds. For example, because several continuous or rapid techniques have been developed to measure volatile organic compounds in the atmosphere [including openpath FTIR (5), high-speed gas chromatography (6, 7), and cryofocusing gas chromatographic mass spectrometry (8)], we have good information on the diurnal variations of volatile compounds (9). In addition, we know that organic aerosols often show strong diurnal variations, with maximum concentrations in the daylight hours (10). In this study, we have used our new low-volume air sampling system (11) to achieve the rapid sampling frequency necessary to measure diurnal variations. Using this low-volume sampler, we have investigated the diurnal behavior of two SOCs: a family of industrial chemicals no * Corresponding author e-mail address:
[email protected].
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longer in use (the polychlorinated biphenyls, PCBs) and a pesticide still in use (endosulfan). PCBs were used from the 1930s to 1977 in the United States for many applications requiring an inert, nonflammable, electrically insulating fluid (12). PCB sales were halted in the United States in 1977 when it was realized that PCBs had become environmentally ubiquitous. Endosulfan, a contact and stomach insecticide, is still used to kill a wide variety of insects that attack cereals, coffee, cotton, fruit, oilseeds, potatoes, tea, and vegetables (13). In 1988, the worldwide production of endosulfan was about 107 kg/year (14).
Experimental Section The sampling site was near a residence located 4 km southeast of the campus of Indiana University in Bloomington, IN. The sampler was placed on a steel platform 0.5 m above a grass lawn. Samples were collected serially at 6-h intervals; a typical sample volume was 60-70 m3 [approximately one-half of the breakthrough volume measured at 27 °C (11)]. Sampling started at 7 A.M. on September 17, 1994. Soil and air temperatures were recorded continuously. Soil temperatures were recorded using a circular chart recorder equipped with a thermocouple probe. The probe was inserted into the soil to a depth of 1 cm, 1 m north of the sampler; this depth approximated the surface soil temperature. Air temperatures were measured at a height of 2 m with a bimetallic element connected to a circular chart recorder. Each day, four clean polyurethane foam (PUF) plugs were loaded into the sampler at 7 A.M., the sampling program was initiated, and the sampler was run unattended until the next day at 7 A.M. The sampled PUF plugs were removed, and the next day’s plugs were loaded. Every other day, a field blank was collected; the field blank was a clean PUF plug placed in the sampler and left there for 24 h; no air was drawn through it. The analysis procedures used in this experiment have been described previously (11); only a brief overview will be presented here. After use, the PUF plugs were returned to the laboratory, and the PCBs and endosulfan were removed from the plugs by supercritical fluid extraction (SFE). The PUF plugs were loaded into 3.47-mL extraction vessels and spiked with internal standards (PCB congeners 30 and 204 and endosulfan-d4); the vessels were sealed and placed in a heater at 60 °C. A total of 50 mL of supercritical carbon dioxide (measured as liquid in the pump) at 400 atm was passed through each vessel, and the CO2 was decompressed through a 15-cm length of 30 µm i.d. fused silica tubing into a 12-mL culture tube filled with 5 mL of methylene chloride. After the extraction was completed, the extract was solvent exchanged to hexane and loaded onto the head of a 2-g silica column. Once samples were loaded, 3 mL of hexane followed by 7 mL of 10% methylene chloride in hexane were passed through the column and collected in a 12-mL culture tube. This fraction contained the PCBs. A second fraction was eluted with 10 mL of methylene chloride. This fraction contained endosulfan. Samples were then solvent exchanged into hexane and reduced in volume under a stream of dry nitrogen to 100 µL. Recovery experiments were performed for both the supercritical fluid extraction and solid-phase clean up
0013-936X/96/0930-0444$12.00/0
1996 American Chemical Society
portions of the procedure. A recovery standard containing PCB congeners 33, 49, 114, 128, 180, 208, and 209 at approximately 100 pg/µL was used (Ultra Scientific, North Kingston, RI). This standard had one congener from each of the trichloro- through decachloro- (except octachloro-) homologue classes. For the SFE recovery experiment, the standard was spiked onto three PUF plugs, and these plugs were extracted as detailed above. The same standard was spiked onto three solid-phase cleanup cartridges, and the standard was eluted as detailed above. The resulting extracts were spiked with an internal standard (PCB congener 204) and chromatographed. The recoveries were quantitative. The SFE gave a mean recovery of 97 ( 8%, and the solid-phase cleanup gave a mean recovery of 105 ( 10%. All PCB analyses were done with a Hewlett-Packard Model 5890 gas chromatograph operated with an electron capture detector. A 30 m long DB-5 capillary column (250 µm i.d., 0.25 µm film thickness, J&W Scientific, Folsom, CA) was used with hydrogen as the carrier gas, the linear velocity of which was 40 cm/s (measured at 200 °C). All endosulfan analyses were done on a Hewlett-Packard 5985B gas chromatographic mass spectrometer system operated in the electron capture mode. A 30 m × 250 µm i.d. DB-5MS fused silica column (J&W Scientific) was used with helium as the carrier gas (at a linear velocity of 22 cm/s measured at 200 °C). The ion source temperature was maintained at 100 °C. The pressure of the reagent gas, methane, in the ion source was maintained at 0.43 Torr. PCB and endosulfan quantitations were performed identically. Relative response factors (RRFs) were generated daily. These RRFs were used to calculate the amount of the analyte of interest. All reported PCB concentrations are the sum of approximately 70 individual congeners found in a typical air sample. Blank correction was performed as follows: An average mass of each analyte was calculated for the field blanks. The average mass in the field blanks was then subtracted from each sample mass for each analyte. For a sample’s analyte concentration to be included in the total concentration, the mass of analyte in that sample must have been three times larger than in the blank. Otherwise, it was considered nondetectable. The total PCB mass in a typical field blank was approximately 20-30 ng. Endosulfan was not typically detected in the field blanks.
Results and Discussion Twenty samples were taken at 6-h intervals. The exact times and the air and soil temperatures, averaged over the sampling period, are given in the first four columns of Table 1. As expected, there is a strong periodicity to the daily temperature, with maxima in the early afternoon and minima at approximately 6 A.M. (see Figure 1). Surface soil temperatures rose faster in the midday than the air temperatures and did not drop as much in the evenings. There was no precipitation over the sampling period. It is well-known that atmospheric concentrations (or partial pressures) of semivolatile organic compounds depend strongly on ambient temperature (1, 2, 15). Since temperature fluctuates periodically during the day, it is reasonable to assume that atmospheric SOC concentrations will also fluctuate during the day. To see if this is true, atmospheric PCB concentrations (see Table 1, fifth column) were plotted versus time (see Figure 2, top), and a sine
TABLE 1
PCB and Endosulfan Concentrations Measured in Bloomington, IN, at the Dates and Times Indicateda date (1994)
time
Sep 17 Sep 17 Sep 17 Sep 18 Sep 18 Sep 18 Sep 18 Sep 19 Sep 19 Sep 19 Sep 19 Sep 20 Sep 20 Sep 20 Sep 20 Sep 21 Sep 21 Sep 21 Sep 21 Sep 22
7 A.M.-1 P.M. 1 P.M.-7 P.M. 7 P.M.-1 A.M. 1 A.M.-7 A.M. 7 A.M.-1 P.M. 1 P.M.-7 P.M. 7 P.M.-1 A.M. 1 A.M.-7 A.M. 7 A.M.-1 P.M. 1 P.M.-7 P.M. 7 P.M.-1 A.M. 1 A.M.-7 A.M. 7 A.M.-1 P.M. 1 P.M.-7 P.M. 7 P.M.-1 A.M. 1 A.M.-7 A.M. 7 A.M.-1 P.M. 1 P.M.-7 P.M. 7 P.M.-1 A.M. 1 A.M.-7 A.M.
soil air PCB endosulfan (pg/m3) temp (°C) temp (°C) (ng/m3) 22.5 25.6 19.6 16.3 19.7 25.2 17.0 14.4 19.5 25.4 17.1 14.4 20.1 26.1 17.1 14.5 18.1 26.1 17.9 15.4
20.1 23.8 17.6 13.3 18.4 24.7 16.1 12.4 19.5 24.8 15.7 12.3 18.4 24.8 16.5 13.4 19.9 24.8 17.3 15.2
3.7 4.0 2.4 1.5 5.3 5.5 1.8 b 4.8 4.4 2.8 2.7 4.8 5.6 2.8 2.1 3.9 5.0 3.1 2.1
32 23 17 12 25 23 17 b 24 24 17 14 34 30 20 19 41 39 31 35
a The air and soil temperatures averaged over the sampling period are also given. b Sample lost in preparation.
FIGURE 1. Air (solid line) and soil (dashed line) temperatures over the sampling period. Time zero is 12:01 A.M. on September 17, 1994.
wave was fitted to the data. The equation for this line is given by
[ {2π24(t + B)}] + D
conc ) A sin
(1)
where conc is the concentration at time t (in hours), A is the amplitude of the wave, B is the time offset, and D is the vertical offset. D was determined by taking the average concentration of all 19 samples (3.6 ng/m3). The values of A and B were determined by an iterative process that minimized the sum of squares between the measured and the predicted values. A was found to be 1.6 ng/m3, and B was found to be 1.5 h. The final minimized value for the sum of squares, normalized to total concentration, was 0.085. It is very clear that the daytime samples (from 7 A.M. to 1 P.M. and from 1 P.M. to 7 P.M.) had higher concentrations than the nighttime samples. On average, in the 6-h period between a midnight sample (1 A.M. to 7 A.M.) and a morning sample (7 A.M. to 1 P.M.), PCB concentrations more than doubled. We also observed that the daily average PCB concentration was about the same throughout the sampling period.
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FIGURE 2. PCB and endosulfan concentrations measured concurrently, showing diurnal variations. Each datum represents a 6-h sample. Same time scale as in Figure 1.
One possible sampling artifact that could produce the diurnal effect is relative humidity. Relative humidity has a maximum in the nighttime hours and a minimum in the daytime. It is conceivable that the increased water content of the nighttime air could coat the polyurethane foam plugs with water and prevent some fraction of PCBs from sorbing to the plugs. This could artificially decrease the apparent nighttime PCB concentrations and produce the diurnal effect. We are confident, however, that this is not the case. PCBs have been extracted efficiently from water samples (15, 16) using polyurethane plugs; if a plug saturated with water retains PCBs, a plug sampling moisture-laden air should do so as well. The endosulfan data (see Table 1, last column) were analyzed with the same approach; the data are plotted in Figure 2, bottom. For the endosulfan data, average daily concentrations increased over the sampling period. To compensate for this, D in eq 1 was replaced by a quadratic equation as follows:
[ {2π24(t + B)}] + D + D (t + B) +
conc ) A sin
0
1
D2(t + B)2 (2) where A and B are the same variables as in eq 1, and the Di terms are the coefficients for the quadratic expression. A sum of squares treatment was used to determine the values of the variables, as above. A was determined to be 8 pg m-3, B was 2.1 h, D0 was 25 pg m-3, D1 was -0.25 pg m-3 h-1, and D2 was 0.0032 pg m-3 h-2. The final minimized value for the sum of squares, normalized to total concentration, was 0.31. In comparing this value to that of the PCBs, we note that the latter data show a 3-fold better fit than the endosulfan data. These endosulfan values also showed a concentration doubling between the midnight and morning samples. It is not clear why it was necessary to add a quadratic offset to achieve a good fit for the endosulfan data. This upward trend was not a temperature effect; note from Figure 1 that the average daily temperature did not increase over
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this period. Neither was it a result of a rain eventsthere was no precipitation during the sampling period. One possibility is that the wind direction shifted at about day 3. As noted by Burgoyne and Hites (17), when the wind blows into Bloomington from the east, endosulfan concentrations increase relative to westerly winds. These authors speculated that this increase was due to endosulfan being used primarily east of the city. No conclusions can be drawn about wind direction during this experiment because it was not recorded. Cautions for SOC Sampling. This experiment demonstrates that large diurnal variations occur in the atmospheric concentrations of PCBs, endosulfan, and presumably, other SOCs. As a result, if one is attempting to measure an average daily concentration, the sampling time interval should be multiples of 24 h to obtain accurate results. For example, consider the eight September 19-21 PCB samples (see Table 1) starting at 7 A.M. The average concentration over the two days is 3.8 ng/m3. Sampling for 12, 24, and 36 h would give air concentrations of 4.6, 3.7, and 4.2 ng/ m3, respectively. The 12- and 36-h intervals give concentrations that are 10-20% higher than the true daily concentration.
Acknowledgments We thank the U.S. Environmental Protection Agency (Grant R818847) and the National Institute for Global and Environmental Change for financial support. We also thank Louis Brzuzy for useful discussions.
Literature Cited (1) Hoff, R. M.; Muir, D. C.; Grift, N. P. Environ. Sci. Technol. 1992, 26, 266-275. (2) Manchester-Neesvig, J. B.; Andren, A. W. Environ. Sci. Technol. 1989, 23, 1138-1148. (3) Halsall, C.; Burnett, V.; Davis, B.; Jones, P.; Jones, K. C. Chemosphere 1993, 26, 2185-2197. (4) Leister, D. L.; Baker, J. E. Atmos. Environ. 1994, 28, 1499-1520. (5) Marshall, T. L.; Chaffin, C. T.; Hammaker, R. M.; Fateley, W. G. Environ. Sci. Technol. 1994, 28, 224A-232A. (6) Klemp, M.; Peters, A.; Sacks, R. Environ. Sci. Technol. 1994, 28, 369A-376A. (7) Sacks, R.; Akard, M. Environ. Sci. Technol. 1994, 28, 428A-432A. (8) Davoli, E.; Cappellini, L.; Moggi, M.; Fanelli, R. J. Am. Soc. Mass Spectrom. 1994, 5, 1001-1007. (9) Singh, H. B.; Salas, L.; Viezee, W.; Sitton, B.; Ferek, R. Atmos. Environ. 1992, 26A, 2929-2946. (10) Turpin, B. J.; Huntzicker, J. J. Atmos. Environ. 1991, 25A, 207215. (11) Wallace, J. C.; Hites, R. A. Environ. Sci. Technol. 1995, 29, 20992106. (12) Erickson, M. D. Analytical Chemistry of PCBs; Lewis: Boca Raton, 1992; Chapter 2. (13) Worthing, C. A. The Pesticide Manual, 8th ed.; The British Crop Protection Council: Thornton Heath, U.K., 1987; p 335. (14) World Health Organization. Environmental Health Criteria 40sEndosulfan; WHO: Geneva, 1984. (15) Gesser, H. D.; Chow, A.; Davis, F. C.; Uthe, J. F.; Reinke, J. Anal. Lett. 1971, 4, 883-886. (16) Bedford, J. W. Bull. Environ. Contam. Toxicol. 1974, 12, 622625. (17) Burgoyne, T. W.; Hites, R. A. Environ. Sci. Technol. 1993, 27, 910-914.
Received for review February 14, 1995. Revised manuscript received September 13, 1995. Accepted November 3, 1995.X ES950084P X
Abstract published in Advance ACS Abstracts, December 1, 1995.
