Research The Atmospheric Background of Perfluorocarbon Compounds Used as Tracers THOMAS B. WATSON,* RICHARD WILKE, RUSSELL N. DIETZ, JOHN HEISER, AND PAUL KALB. Brookhaven National Laboratory, Upton, New York 11973
There are seven cyclic perfluoroalkane compounds, which can be detected in extremely low concentrations, that are used to track mass movement and transfer in a variety of research and practical applications. They are used in leak detection in underground storage and pipelines and in atmospheric transport and diffusion research on local, regional, and continental scales. They are likely to be a used globally for monitoring carbon sequestration in geological formations. The atmospheric background levels of these compounds must be accurately known, and trends in their concentrations determined for these compounds to be effective in monitoring CO2 reservoirs and because there are environmental concerns about their release. Results of measurements of perfluorocarbon background concentrations from two recent field programs are presented, and trends in these values examined using data collected over the last 25 years. The current atmospheric concentrations of these compounds are in the low parts per quadrillion levels, and their annual atmospheric growth rate is less than 1 part per quadrillion per year. The environmental effects of these compounds are examined and found to be negligible at current release rates.
Introduction The perfluorocarbon tracer compounds (PFTs) are totally fluorinated cycloalkane compounds consisting of four, five, and six atom carbon rings. Because the fluorine-carbon bond is strong, with energies in the range of 450-530 kJ mol-1 (1), these molecules are very stable. They are not susceptible to oxidation in the atmosphere. The only important sink for perfluorocarbons is photolysis in the mesosphere (2). PFTs are potent greenhouse gases, but they do not deplete stratospheric ozone. They are biologically benign and have been considered as blood substitutes and are used in eye surgery (3). PFTs make good tracers because of their physical characteristics and because they are present in the atmosphere at low levels. Background concentrations are several parts in 1015 (parts per quadrillion by volume, ppqv) so the release of small amounts of PFT results in unambiguous signals. The large numbers of fluorine atoms and the structure of of these molecules cause them to have high electron affinities, approximately 3 eV (4). They are detectable at femtogram * Corresponding author phone: (631) 344-4517; e-mail: twatson@ bnl.gov. 10.1021/es070940k CCC: $37.00 Published on Web 09/15/2007
2007 American Chemical Society
(10-15) levels using an electron capture detector (ECD) or using negative ionization chemical ionization mass spectrometry (NICI-MS) (5, 6). Because of their relatively low vapor pressures, PFTs can be concentrated on adsorbents so the sampling process can be used to increase the PFT signal. PFTs typically used as tracers are given in Table 1. PTFs have been used since the early 1980s to study atmospheric transport and dispersion on local, regional, and continental scales. These studies have led to increased understanding of the movement of pollutants and other hazardous substances in the atmosphere and to improvement and validation of atmospheric transport models. These research programs have application to issues of critical national and global importance such as homeland security, air quality, and climate change. Local scale transport and dispersion field programs using PFTs include the Atmospheric Studies in Complex Terrain (ASCOT), Brush Creek Valley study in 1984 (7); the Metropolitan Tracer Experiment (METREX), conducted in the Washington, D.C. area in 1984 (8); the TransAlp program carried out in Switzerland in 1989, 1990, and 1991 (9); the Vertical Transport and Mixing (VTMX) study carried out in Salt Lake City Utah in 2000 (10); the Urban Dispersion Program (UDP) studies carried out New York City in 2005, (11, 12); and Ameriflux Tower Footprint studies conducted near Gainesville, FL in 2002, 2004, and 2006 (13, 14). Regional scale studies include the Cross Appalachian Tracer Experiment (CAPTEX) (15); Project MOHAVE (16, 17); and the Big Bend Regional Aerosol and Observational Study (BRAVO) (18, 19). Continental scale studies include Across North America Tracer Experiment (ANATEX) (20, 21) and the European Tracer Experiment (ETEX), (22). Selected regional and long-range transport and dispersion experiments are summarized in Table S1, Supporting Information. Other uses of PFTs as tracers include research on building infiltration (23), detection of leaks of dielectric fluid in underground electrical transmission lines and leaks in gas lines (24, 25), tracking flows in oil fields (26), tracking and monitoring movement of material in geological formations intended for carbon sequestration (27, 28), and determining the integrity of hazardous material containment structures (29, 30). PFTs have been proposed as markers to detect leaks in radioactive storage containers and as tags for identifying explosives and detecting smuggled currency. PFTs have no biological effects and do not deplete stratospheric ozone, but they are powerful greenhouse gases (4). There are environmental concerns about releasing them into the atmosphere because the same characteristics that make them good atmospheric tracers cause them to have atmospheric lifetimes of thousands of years (2) and strong infrared absorption features (4). The greenhouse warming potential (GWP), is defined as the time-integrated radiative forcing of a trace substance relative to the same mass of reference gas (31). The lifetime and GWP have not been determined directly for the specific compounds commonly used as atmospheric tracers; however, they share characteristics with other perfluoro compounds that have been extensively studied. Examples include SF6 with a predicted lifetime of 3200 years; CF4 with a lifetime of 50 000 years; and other similar compounds with lifetimes greater than 3000 years such as perfluorocyclobutane, c-C4F8 (2, 31, 32, 33). The relative global warming impact of PFTs can be estimated from the radiative efficiencies and the contribution to radiative forcing of similar compounds. These quantities VOL. 41, NO. 20, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. PFTs Commonly Used as Tracers, Acronyms, IUPAC Names, Chemical Formulas, Molecular Weights, and Boiling Points acronym
chemical name
formula
molecular weight(g mol-1)
boiling point (°C)
PDCB PMCP PMCH o-PDCH PECH i-PPCH PTCH
perfluorodimethylcyclobutane perfluoromethylcyclopentane perfluoromethylcyclohexane perfluoro-1,2-dimethylcyclohexane perfluoroethylcyclohexane perfluoroisopropylcyclohexane perfluorotrimethylcyclohexane
C6F12 C6F12 C7F14 C8F16 C8F16 C9F18 C9F18
300 300 350 400 400 450 450
45.0 48.1 76 102 102 130 125
were determined using radiative transfer models that take into account the vertical distribution of these compounds in the atmosphere and the variation of solar radiation with altitude (31). A conservative estimate of total PFT radiative efficiency is 1 w m-2 ppb-1, twice as much as SF6. The PFT abundance is 3 orders of magnitude less than SF6, so the PFT radiative forcing will be at least 2 orders of magnitude less than that of SF6 or approximately 2 × 10-5 Wm-2. PFTs are thousands of times more effective as greenhouse gases than CO2, but the low atmospheric concentrations and release rates make their greenhouse contribution nearly insignificant when compared to the effect of CO2.
Experimental Section Atmospheric samples of PFTs are collected by passing air through activated charcoal. Samples used in this study were taken using the Brookhaven Atmospheric Tracer Sampler (BATS) and the Sequential Air Sampler (SAS). The BATS consists of a base unit containing a pump, timer, control electronics, and a lid containing. 22 or 23, 1/8 in. outside diameter stainless steel tubes packed with Ambersorb (Rhom and Hass, Philadelphia, PA). The lid also contains a multiport valve that places the active sampling tube in the sample stream. The nominal pumping rate is 50 mL min-1. Samples were collected on BATS for 30 min concentrating the material in approximately 1.5 liters of air on each tube. The SAS units collect samples on 20, 1/4 in. outside diameter glass tubes containing Ambersorb The flow rate for the SAS samplers is nominally 500 mL min-1. These units collected samples over 6 min, resulting in a sampled volume of 3 L, and over 30 min, resulting in a sampled volume of 15 L. Analysis of the BATS Lids and CATS tubes was performed using gas chromatography with an electron capture detector (ECD). The quantity of PFT available for analysis is determined by the volume of air that is sampled. The ECD has the sensitivity to quantify background levels of PFT if the material in 1.5 liters of ambient air is collected. However, in ambient air samples, there are many other compounds, including SF6, nitrogen oxides, and chlorofluorocarbons (CFCs) such as Freon, present in higher quantities than the PFTs that can potentially interfere with PFT detection. The first step in the analytical process is desorption of the sample from the collection tube by ballistically heating the tube to 400 °C driving the concentrated PFTs along with other compounds from the adsorbent into a 1% hydrogen in nitrogen carrier gas stream. The sample is mixed with oxygen and passes through an oxidizing catalyst and dryer and is concentrated on a trap packed with Florisil (Supelco, Inc., Bellefonte, PA) adsorbent. The focusing trap is heated, desorbing the sample into the carrier gas flow and onto a carbon layer open tubular (CLOT) precut column where it undergoes chromatographic separation. Flow from this column can be directed through a vent to the atmosphere or through a heated Pd reducing catalyst onto a second trap packed with Florisil. By switching the sample stream between the vent and the trap, the eluting PFTs can be directed onto the trap while interfering compounds are vented. The Pd catalyst combined with the 1% H2 in the carrier gas reduces 6910
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TABLE 2. LOD and LOQ (ppqv) from Standards Based on the Standard Deviation of 2 µL Standard Assuming a 1.5 L Sample Volume PDCB
PMCP PMCH ocPDCH iPPCH
standard 0.88 1.46 1.83 deviation LOD 3 5 6 LOQ 10 15 18
1.03 3 10
1.70 6 17
PTCH
ptPDCH
2.65
1.21
9 26
4 12
TABLE 3. LOD and LOQ (ppqv) from Measurements of Background Tracer Levels During NYC UDP PDCB
PMCP PMCH ocPDCH iPPCH
standard 0.88 1.16 1.27 deviation LOD 3 3 4 LOQ 10 12 12
0.37 1 4
1.01 3 10
PTCH
ptPDCH
1.26
1.17
4 13
4 12
the compounds that coelute with the PFTs to forms that are not detected by the ECD. After the PFTs have been collected on the trap, the precut column is back-flushed sweeping any compounds still on the column out the vent and preparing it for the next desorption cycle. The sample on the trap is desorbed, and collected compounds transferred to the main column, which is the same composition as the precut column. The PFTs are separated on the main column and delivered to the detector. The ECD signal is recorded using data acquisition software that integrates peaks and records the raw and processed data. Analysis of each sample takes approximately 15 min. Calibration of the gas chromatograph is accomplished using standards introduced into the analytical system in sample tubes. The tubes are loaded by injecting measured quantities of a standard mixture, determined using volumetric syringes, into a stream of ultrahigh purity (UHP) N2 flowing through the tube. Standards are run with each set of samples which allows variations in instrument performance to be quickly identified and corrected. The limit of detection (LOD) is three times noise and the limit of quantitation (LOQ) is 10 times noise. The confidence limits at the limit of quantitation are (30% (34). The standard deviation of injections of the smallest volume calibration level was used as the noise value used to determine the LOD and LOQ for the analysis method (Table 2). These values represent the uncertainty in an individual 1.5 L sample using this analytical method. These limits can be improved by collecting larger samples. The sample volume is determined by using the perfluorocarbon, ptPDCH as an internal standard (21). Since this compound is not released as a tracer, the background level can be determined and the amount of ptPDCH in an individual sample used to determine the actual volume of air that was passed through the adsorbent bed See suporting Infromation for details of the calculation.
TABLE 4. Background Measurement Statistics from NYC UDP: Mean, Standard Deviation (sdev), and Standard Deviation of the Mean (sdom) in Units of ppqv PDCB
PMCP
PMCH
ocPDCH
iPPCH
PTCH
ptPDCH
mean 2 7 7 1 1 2 7 stdev 0.88 1.16 1.27 0.37 1.01 1.26 1.17 sdom 0.03 0.03 0.04 0.01 0.03 0.05 0.03 N 680 679 704 704 703 704 1720
TABLE 5. Background Measurement Statistics Determined from Samples Taken in Suburban Long Island in Units of ppqv mean stdev sdom N
ppqv
ppqv
ppqv
ppqv
ppqv
ppqv
ppqv
2 0.62 0.09 50
8 1.93 0.32 36
8 1.53 0.22 50
1 0.30 0.05 37
1 0.33 0.05 37
1 0.84 0.16 26
7 0.30 0.05 37
TABLE 6. Background Measurement Statistics Determined from Samples Taken in Rural Florida in Units of ppqv mean stdev sdom N
PDCB
PMCP
PMCH
ocPDCH
iPPCH
PTCH
ptPDCH
3 0.4 0.07 36
8 0.6 0.09 36
8 0.9 0.14 36
1 0.4 0.07 36
0 0.2 0.03 36
0 0.2 0.03 36
8 0.8 0.10 36
Results and Discussion The New York City (NYC) Urban Dispersion Program (UDP) was designed to improve the ability of NYC’s emergency management teams and first response personnel to protect the public during releases of hazardous materials (11). The second NYC UDP field program was conducted in August 2005 in the midtown area of Manhattan. The sampling domain was 2 × 2 km that encompassed a complex urban environment containing many important commercial, cultural, and transportation centers as well as high traffic tourist destinations. PFTs were released at the surface and in a subway station. Samples were taken at 62 street level locations arrayed in a grid pattern over the domain and at eight subway stations and thirteen building rooftops. Six intensive observation periods (IOPs) were conducted. The limit of detection and quantitation for the combined sampling and analysis method using the standard deviation of the background tracer measurements as the noise level is given in Table 3. These are in good agreement with the same quantities determined from the calibration standard (Table 3). These numbers represent the uncertainty in an individual measurement.
Tracer background levels during NYC UDP (Table 4) were determined from the average of the results of the BATS samplers upwind of the tracer releases that showed no sign of elevated tracer concentration. Since these values were determined from the mean of a number of measurements, they can be accurate below the detection limit with the uncertainty given by the standard deviation of the mean. The Patchogue/Bayport background samples were collected near the cities of Patchogue and Bayport, on the south side of Long Island about 100 km east of New York City. Samples were collected using SAS samplers and were analyzed with the samples of the UDP as a quality check of the analytical process. Both 3 and 15 liter samples were collected (Table 5). The Florida perfluorocarbon tracer field experiments were conducted at the AmeriFlux site, located in a managed slash pine forest covering flat terrain 10 km northeast of Gainesville, Florida, in, 2002, 2004, and 2006 (13, 14). The experiments were designed to collect data for improvement and validation of canopy dispersion models. The background measurements were made about 3 km upwind of the site during the 2006 field program. Measurements were made using BATS samplers (Table 6). Most of the current PFT background is the result of Manhattan Project era weapons research. PFT background data are available, beginning in 1986, for five of the six PFTs in general use as atmospheric tracers and for the reference tracer, ptPDCH (Table 7). There are some obvious discrepancies in the values reported by different laboratories. Some possible reasons for these differences, including variations in calibration standards, problems in peak identification because of coelution of PFTs with unknown contaminants, and possibile misidentification of peaks have been previously presented (35). The background measurements in the UDP and Florida programs were made with samples of 1.5 liters of air. Improvements in signal-to-noise ratio and, therefore, detection limits can be made by collecting larger samples and by directly measuring sample volumes. Despite the difficulties of measuring these compounds at background levels the data show that the concentrations in the atmosphere have been increasing (Figures 1 and 2). The correlation coefficients for the linear fits are low, but the trends of increasing atmospheric concentration are unmistakable. It is likely that the rate of increase over the last 20 years is not linear. As the quantities of these compounds in the atmosphere are low, a release of several tons of the PFTs would cause a detectable increase in background levels. A release of this size could occur in a single event. The current atmospheric burden and estimates of average annual release rate based on the linear fits are given in Table 8.
TABLE 7. Background PFT Levels from Studies over the Last 20 Years in Units of ppqv, (1 σ. 1986 (36) ANATEX 1987 (21) MOHAVE 1992 (16) ETEX, Austria, 1994 (37) ETEX, Europe, 1994 (38) 1996, New Jersey, Rural (39) 2001, Korea (40) 2001, Ireland, UK (35) 2001, Europe urban (6) 2001, Europe remote (6) BRAVO, 2001, suburban (41) this study 2005, NY, Urban 2005, NY, Suburban 2006, Florida, Remote
PDCB
PMCP
PMCH
oPDCH
PTCH
ptPDCH
0.34 ( 0.01
3.22 ( 0.03 2.09 ( 0.43 6.29 ( 0.59 5.24 ( 1.03 4.6 ( 0.3 4.15 ( 0.05
4.6 ( 0.05 3.6 ( 0.05 4.91 ( 0.30 5.9 ( .64 4.6 ( 0.8 3.84 ( 0.10 7.4 ( 0.80 5.5 ( 0.2 6.3 ( 1.1 5.2 ( 1.3
0.30 ( 0.10 0.4 ( 0.03 0.56 ( 0.06 0.98 ( 0.48 0.6 ( 0.31 0.34 ( 0.10 1.12 ( 0.25 0.7 ( 0.1
0.07 0.45 ( 0.28 0.64 ( 0.62 0.59 ( 0.60
3.4 ( 1.0 4.34 ( 0.32
0.63 ( 0.18
2.7 ( 0.2 3.0 ( 1.5 2.5 ( 0.4 2 ( 0.4
6.3 ( 0.2 8.1 ( 1.8 6.8 ( 1.0
2 ( 0.9 2 ( 0.6 3 ( 0.4
7 ( 1.2 8 ( 1.9 8 ( 0.6
7 ( 1.3 7 ( 1.5 8 ( 0.9
0.07 ( 0.10
4.88 ( 0.03
0.6 ( 0.1 1 ( 0.4 1 ( 0.3 1 ( 0.4
5.37 ( 1.85 6.1 ( 0.8
7 ( 1.2 2 ( 1.3 1 ( 0.9 1 ( 0.9
7 ( 1.2 7 ( 0.3 8 ( 0.8
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TABLE 8. Current PFT Background Levels, Estimated Rates of Increase, and Estimated Annual Emissions as Well as Estimates from Ref 35 Presented for Comparison PDCB PMCP PMCH oPDCH ptPDCH background (ppqv) atmosphere burden (metric tons) atmosphere burden (metric tons) (35) rate of increase (ppqv year-1) estimated emission rate (metric tons year-1) estimated emission (metric tons year-1) (35)
CO2
3
8
8
1
7
140
430
500
71
500
370 (ppmv) 3.0 × 1012
148
344
351
53
356
NA
0.1
0.3
0.2
0.04
0.2
8
16
11
3
11
2 (ppmv) 1.6 × 1010
9
12
4
2
8
NA
The cumulative rate of increase of all PFTs, at current release rates, is less than 1 ppqv per year. As long as the PFT levels remain in the low ppqv range, their contribution to global radiative forcing will remain negligible relative to most other greenhouse emissions. However, because they are such potent greenhouse gases, the growth rates in atmospheric abundance must be monitored carefully. There is the potential that these compounds will be used as tracers to monitor CO2 sequestration in underground storage and that their use and subsequent release will increase dramatically. A long-term program to monitor the global atmospheric background of PFTs is necessary to track global trends. There also should be a program to establish a certified calibration standard and to conduct inter-comparisons between laboratories and between the GC/ECD and NICI-MS analytical methods.
Acknowledgments We acknowledge the contributions of sponsors and participants in the New York City Urban Dispersion Program particularly Jerry Allwine (Pacific Northwest National Laboratory) who served as the UDP science lead. We thank the Department of Energy Terrestrial Carbon Program for support of the Florida Tower Footprint studies. We also thank our colleagues Ernest Lewis and Terrance Sullivan for their helpful comments on the manuscript. This manuscript has been authored by employees of Brookhaven Science Associates, LLC under Contract no. DE-AC02-98CH10886 with the U.S. Department of Energy.
Supporting Information Available
FIGURE 1. Time series of PFT background levels for measurements made from 1982 to present. PDCB (blue) y ) 0.15x - 295, r2 ) 0.82. PMCP (pink) y ) 0.26x - 523, r2 ) 0.75. PMCH (turquoise) y ) 0.18x - 351, r2 ) 0.65
FIGURE 2. Time series of PFT background levels for measurements made from 1982 to present. oPDCH (blue) y ) 0.04x - 71, r2 ) 0.63. ptPDCH (pink) y ) 0.16x - 325, r2 ) 0.75. The source of the rate of increase of the PFTs is not known. It is puzzling because the current data on manufacture and uses of these compounds do not support these growth rates. It is particularly puzzling for the compound ptPDCH, for which there is no known use or manufacturing source other than the 1 or 2% present in oPDCH as a byproduct. This discrepancy could be explained if the values reported in 1986 are biased to show concentrations lower than the contemporary background level, or if there is an unknown coeluting compound causing the results of the current study to report unrealistically high concentrations. 6912
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A summary of selected regional and long-range PFT tracer experiments in Table S1 as well as details of the method and calculations to determine sample volume from the quantity of ptPDCH measured in samples. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review April 20, 2007. Revised manuscript received July 12, 2007. Accepted July 24, 2007. ES070940K
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