Environ. Sci. Technol. 2003, 37, 4358-4361
Atmospheric Perfluorocarbons M . A S L A M K . K H A L I L , * ,† REINHOLD A. RASMUSSEN,‡ JOHN A. CULBERTSON,§ JOHN M. PRINS,§ ERIC P. GRIMSRUD,§ AND MARTHA J. SHEARER† Department of Physics, Portland State University, P.O. Box 751, Portland, Oregon 97207, Department of Environmental and Biomolecular Systems, OGI School of Science & Engineering, Oregon Health & Science University, 20000 NW Walker Road, Beaverton, Oregon 97006, and Department of Chemistry, Montana State University, Bozeman, Montana 59717
Collectively, man-made emissions of a few greenhouse gases may cause about the same amount of global warming as increasing carbon dioxide. Among the most potent of these non-CO2 greenhouse gases are the perfluorocarbons that have extraordinarily long atmospheric lifetimes of 10 000 to more than 50 000 yr. We report atmospheric concentrations over two decades, between 1978 and 1997, of the three most abundant perfluorocarbonssCF4, C2F6, and C3F8sand delineate the sources that account for the present abundances and trends. We show that C2F6 and C3F8 are present at only 2.9 and 0.2 pptv, respectively. CF4 is the most abundant perfluorocarbon at 74 pptv (in 1997) of which about 40 pptv are from natural emissions, 33 pptv from aluminum manufacturing, and 1 pptv from the semiconductor industry. The increasing trend of CF4 has slowed in recent years due to the major reductions in the emission rate per ton of aluminum produced. The effect of the falling emission factor is partially offset by increased production and increasing use by the semiconductor industry.
Introduction Carbon tetrafluoride (CF4) is the most abundant perfluorocarbon in the earth’s atmosphere and one of the most potent greenhouse gases. It is estimated that a molecule of CF4 is as effective as 10 000 molecules of CO2 for causing global warming (1). Because of this, the perfluorocarbons are included in the Kyoto Protocol aimed at preventing global warming from anthropogenic emissions of various greenhouse gases. By analyzing stored air, we have reconstructed 2 decades of atmospheric concentrations of CF4; a companion molecule, perfluoroethane (C2F6); and to complete the suite, the very rare perfluoropropane (C3F8). The rate of increase of CF4 has slowed because during the past decade manmade emissions have been reduced considerably. The trend of C2F6 has increased however, but the concentrations remain very small. By using the estimated emission factors from man-made sources, we show that the global budgets of these gases explain quantitatively the observed concentrations and trends. * Corresponding author telephone: (503)725-8396; fax: (503)7258550; e-mail:
[email protected]. † Portland State University. ‡ Oregon Health & Science University. § Montana State University. 4358
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 19, 2003
TABLE 1. Annually Averaged Measured Concentrations of CF4 and C2F6 in Parts per Trillion (10-12) and of C3F8 in Parts per Quadrillion (10-15)a C3F8 (ppqv)
SE (ppqv)
Pt. Barrow, AK 2.80 0.02 2.99 0.05
203 232
3 20
Cape Meares, OR 1.43 0.07 1.35 0.07 1.45 0.05 1.52 0.03 1.63 0.04 1.58 0.03 1.70 0.04 1.87 0.08 1.89 0.08 1.96 0.04 1.93 2.12 0.03 2.35 0.04 2.48 0.03 2.33 0.18 2.37 0.02 2.62 0.06 2.81 0.04 2.69 0.09 2.92 0.06
77 72 80 86 87 83 91 94 108 117 141 129 137 155 139 158 176 196 195 219
4 1 5 7 5 2 3 2 7 5 7 3 17 10 10 6 12 12 7
158 160 175 193
5 3 2 2
year
CF4 (pptv)
SE (pptv)
1996 1997
73.6 74.5
0.2 0.2
1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997
56.1 58.8 60.2 61.6 62.7 61.6 63.2 64.9 66.5 65.2 67.7 67.3 67.8 69.5 71.4 70.0 73.7 73.4 74.1 74.2
1.0 1.3 0.8 0.9 1.0 0.5 0.9 0.5 0.8 1.4 1.1 0.8 0.4 0.7 0.9 1.2 1.8 1.0 1.0 0.0
1994 1995 1996 1997
69.8 72.4 72.7 72.6
a
C2F6 (pptv)
SE (pptv)
Palmer Station, Antarctica 1.1 2.49 0.05 0.4 2.64 0.03 0.2 2.64 0.03 0.3 2.75 0.02
The standard errors (SE) are reported in adjacent columns.
Atmospheric Measurements and Results Since 1978, we have stored clean air from Cape Meares, OR (45.5° N, 124° W), and over shorter times from Palmer Station in Antarctica (64.9° S, 64° W) and Barrow, AK (71.3° N, 156.6° W). We analyzed about 350 samples to determine the concentrations of CF4, C2F6, and C3F8 using standard gas chromatography coupled with mass spectrometry (GC/MS) (2-4). There were 200 samples from Oregon spanning 19781997, 90 from Palmer (1994-1997), and 60 from Barrow (1996-1997). The results are reported in Table 1 as annually averaged concentrations. Previous work has shown that there are natural sources of CF4 from the rocks and soils resulting in a background atmospheric concentration of about 40 pptv but none for C2F6 (5, 6). The present levels of CF4, C2F6, and C3F8 are 74 ( 2, 2.9 ( 0.1, and 0.22 ( 0.01 pptv, respectively (1997 values in Table 1). We will show that for CF4 and C2F6 the concentrations above the background levels are due to two known industrial sources, namely, aluminum manufacturing and electronic chip production. In separating aluminum from ore by electrochemical processes, CF4 and C2F6 are formed as a byproduct during “anode effect” episodes (7-10). For the last several years, the International Aluminum Institute (IAI) has embarked on a program to evaluate the emissions from primary aluminum production. They have taken into account the differences of emissions from the four major production processes and used direct measurements of emissions to evaluate the emission factors (9, 10). We have used these data to create 10.1021/es030327a CCC: $25.00
2003 American Chemical Society Published on Web 08/30/2003
a composite annual emission factor for aluminum production for each year, as described below. Data on global aluminum production from 1900 to the present were taken from industry and government sources (11-14). The basic process used in this production has remained the same over this time, so we can estimate the emissions of these perfluorocarbons from 1900 onward by taking a weighted product of the emission factor and the amount of aluminum produced. The IAI has described four technologies by which aluminum is extracted from the ore: Horizontal Stud Soderberg (HSS), Vertical Stud Soderberg (VSS), Side Worked Prebake (SWPB), and Center Worked Prebake (CWPB). The emissions of CF4 and C2F6 are quite different among these processes. The IAI has also estimated how much aluminum is produced by each process among the companies reporting the data. We have extended the same proportions to global aluminum production to obtain the amount of aluminum produced globally by each process. The composite emission factor is calculated as ∑EF(i)P(i)/∑P(i) where i indexes the four processes, EF(i) is the emission factor for the ith process, and P(i) is the global aluminum production using process i. The results for each year between 1985 and 1997 are EF (1985, 1986, ..., 1997) ) (1.06, 1.06, 0.92, 0.8, 0.7, 0.6, 0.56, 0.56, 0.47, 0.4, 0.39, 0.42, and 0.38). We see that there have been substantial reductions in the emissions factors of CF4 from about 1 kg/t of Al production in the 1980s and earlier to about 0.4 kg/t of Al production in 1997-2000. The emissions of C2F6 occur at the same time as CF4 but are only about 10-12% by mass of CF4, so similar reductions in emission factors have occurred. In the manufacturing of semiconductor chips, industrially produced CF4 and C2F6 are used in wafer etching and for cleaning chemical vapor deposition. There are various known industrial uses of C3F8 including semiconductor manufacturing, but the actual emissions have not been estimated. For this source, we have taken estimates of CF4 and C2F6 emissions from the EDGAR database (15, 16). The results show generally increasing emission rates, which are small still as compared with the aluminum manufacturing source for CF4, but quite sizable, especially during recent years for C2F6 because aluminum manufacturing is a relatively small source for this gas. It is noteworthy that the emissions of C2F6 increased at about 33 Gg/yr between 1980 and 1990 and at about 120 Gg/yr since then, with some slowing in the past few years. Therefore, most of the contribution of the semiconductor source would be in the recent years. For completeness, we list the estimated emissions (rounded) in Giga grams of CF4 per year from aluminum manufacturing for 1940-2000 as follows: 0.8, 1.1, 1.5, 2.1, 1.8, 0.9, 0.8, 1.1, 1.3, 1.4, 1.6, 1.9, 2.2, 2.6, 3.0, 3.3, 3.6, 3.6, 3.7, 4.3, 4.8, 5.0, 5.4, 5.6, 6.3, 6.7, 7.3, 8.0, 8.5, 9.5, 10.2, 10.9, 11.7, 12.8, 14.0, 12.8, 13.4, 14.6, 14.9, 15.5, 16.3, 16.0, 14.2, 14.7, 16.6, 16.3, 16.3, 15.2, 14.8, 13.2,11.7, 11.0, 11.0, 9.3, 7.7, 7.7, 8.8, 8.3, 8.7, 9.1, and 9.2, and from the semiconductor industry from 1980 to 2000 the estimated emissions are 0.0, 0.1, 0.1, 0.2, 0.2, 0.2, 0.3, 0.3, 0.3, 0.3, 0.4, 0.4, 0.6, 0.7, 0.9, 1.1, 1.2, 1.3, 1.3, 1.4, and 1.4. The annual emissions of C2F6 can be estimated from the CF4 data.
Theoretical Model Both CF4 and C2F6 are extraordinarily stable compounds with hardly any known destruction processes of consequence. Perhaps most of the destruction takes place as air passes through high-temperature combustion zones in power plants or automobiles (17, 18), giving effective atmospheric lifetimes for CF4 and C2F6 of ∼50 000 and ∼10 000 yr, respectively. Natural destruction processes seem to occur high in the atmosphere at altitudes of 100 km or more. For the short data set we are considering, whatever the exact lifetimes are, it does not lead to observable differences in expected
concentrations unless the actual lifetimes are less than 3000 yr. A mass balance can be written as
dCN/dt ) S(t) - CN(t)/τ(t) - [CN(t) - CS(t)]/τT
(1)
dCS/dt )- CS(t)/τ(t) + [CN(t) - CS(t)]/τT
(2)
where CN and CS are the average mixing ratios in the Northern and Southern Hemispheres, respectively; τ is the atmospheric lifetime; and τT is the inter-hemispheric transport time. We have assumed that all the emissions are in the Northern Hemisphere. The inter-hemispheric transport time is ∼1.6 yr. The sources are converted to pptv/yr in the entire atmosphere so for CF4 1 pptv/yr )14.7 Gg/yr and for C2F6 1 pptv/yr ) 23 Gg/yr. A natural source has been added to CF4, although this does not affect the calculations for the time span of interest here (∼20-100 yr), but it creates a baseline pre-industrial concentration for CF4. These equations are solved exactly for each yearly increment representing the resolution of the emissions data. The solutions are
CN(t + 1) ) C+(t + 1) + C-(t + 1) CS(t + 1) ) C+(t + 1) - C-(t + 1)
(3a)
C+(t + 1) ) C+(t) exp(-1/τ) + S(t)τ[1 - exp(-1/τ)] C-(t + 1) ) C-(t) exp(-1/τ - 2/τT) + S(t)/(1/τ + 2/τT)[1 - exp(-1/τ - 2/τT)] (3b) We can use our data in Table 1 to determine the natural background concentration of CF4 and the fraction of C2F6 emitted from aluminum manufacturing relative to CF4. The global average mixing ratio (or the sum of eqs 1 and 2) for the period of measurements can be approximated as Cn+1 ) Cn + Sn for each year n. Recognizing that the source of C2F6 is some fixed fraction (λ) of the emissions of CF4 and that the background concentration of CF4 is CCF4b, we see that
CCF4 ) CCF4b + λCC2F6
(4)
Therefore, if we plot the measured concentrations of CF4 against C2F6 as in Figure 1, the intercept will be the natural concentration of CCF4b, and the slope will be the fraction of C2F6 emitted relative to CF4. A regression of these data for 1978-1990 gives the background concentration of 43.8 pptv and λ ) 0.11. This is a period during which the main source of these gases was aluminum manufacturing so that the constancy of λ is a good assumption. Both these values (CCF4b and λ) are very close to independent measurements of CF4 in ice cores of about 40 pptv (6) and perfluorocarbon emission data from IAI showing that about 10% as much C2F6 is emitted as CF4 from aluminum manufacturing. After 1990, the semiconductor industry has played an increasing role in the concentrations of CF4 and particularly C2F6, so the fraction λ reflects a composite effect of the two industrial processes and is not necessarily constant in time. Using the emissions estimated earlier, we applied eq 3 to calculate the expected amounts of CF4 and C2F6 in the atmosphere over the last 100 yr and compared them with the current measurements spanning 1978-1998. The background concentration was taken to be 40.6 pptv for CF4 and 0 for C2F6 as determined from ice core analyses (6). The results in Figure 2 show that the present concentrations of these gases are fully explained by the known industrial sources and the natural background for CF4. In these figures, we have also included two other published time series of concentrationssall are consistent with the calculations (18, 19). VOL. 37, NO. 19, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4359
FIGURE 1. Relationship between CF4 and C2F6 when we took simultaneous measurements between 1978 and 1997 as discussed in the text. Based entirely on atmospheric measurements and without invoking any knowledge of specific emissions from industrial processes except that both gases come from aluminum manufacturing, we have used the relationship shown in this figure to deduce the natural background mixing ratio of CF4 (the intercept) and the fraction of C2F6 to CF4 emitted by aluminum manufacturing (the slope), as explained in the text. There are differences in the absolute calibration between the various published results. We have accepted the Harnisch Scale given in ref 20 as the most reliable present absolute calibration. The scale for absolute concentrations for our study has been adjusted to this scale by a constant multiplicative factor of 0.8. The relationships between this scale (20) and the other scales in published papers are as follows: For CF4, multiply the data in ref 21 by 0.88 and multiply the data in ref 19 by 0.9785. For C2F6, multiply the data in ref 2 by 0.36 to be compatible with the scale in ref 19.
Discussion The contribution of the semiconductor industry is small for CF4 and can even be neglected in view of the present variability in the observed concentrations. Of the 74 pptv CF4 present in the atmosphere, we attribute 40 pptv to natural sources, 33 pptv to aluminum manufacturing, and about 1 pptv to the semiconductor industry based on the mass balance calculations. The calculated trends also compare very well with observations. We divided the trends between two periods: 1978-1986 and 1986-1997. The first of these periods reflects a relatively high emission factor from aluminum manufacturing, and the second period spans the times when significant reductions in emissions were made by the aluminum industry. The measured trend of CF4 for 1978-1986 was 1.1 ( 0.2 pptv/yr, and the calculated trend is 1.07 pptv/yr. For the period 1986-1997, the measured trend was 0.85 ( 0.15 pptv/yr, and the calculated trend is 0.8 pptv/yr. Although there have been dramatic reductions in the emission factors, some of the gain is offset by increasing aluminum production that is sustaining the present rate of increase. The calculated inter-hemispheric ratio is 1.017 ( 0.002, which is very close to the measured ratio of 1.019 during 1995-1998. The Palmer data are unusually low during 1994; if these are included, the average inter-hemispheric ratio is 1.028 ( 0.01. These agreements may be improved with better estimates of emissions from the semiconductor industry. For C2F6, however, the aluminum manufacturing source is insufficient to explain a substantial fraction of the most 4360
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 19, 2003
FIGURE 2. Annual average atmospheric mixing ratios of (a) CF4 and (b) C2F6. The graphs show the agreement on trends between various independent measurements and the close agreement between measured and calculated concentrations. Cape Meares and Palmer are our measurements for this work, and the other data shown are from published studies (18, 19). The pre-industrial concentration is taken at 40.6 pptv based on ice core data (not shown on the graph) (5). The solid lines are the model results with the higher line representing the Northern Hemisphere, and the lower line representing the Southern Hemisphere. The theoretical calculations tend toward the ice core results for pre-industrial times. For CF4, the emissions are dominated by aluminum manufacturing, but during the later period the semiconductor manufacturing source is needed to achieve a better mass balance. For C2F6, the later emissions are driven by the semiconductor source. In panel b, the solid black line shows the expected global concentrations if the semiconductor source is not included. recent concentrations (taken to be 11.5% of the source of CF4). The inclusion of the estimated source from the electronics industry is needed to balance the budget and explain the observations (taken to be 70% of the CF4 emissions from this source). Of the 3.2 pptv of C2F6 in the present atmosphere about 2.6 pptv can be attributed to aluminum manufacturing, and the remaining 0.6 pptv (19%) can be attributed to the semiconductor industry. The trends of measured concentrations of C2F6 are 0.07 ( 0.01 pptv/yr for 1978-1986 and 0.09 ( 0.015 pptv/yr for 1987-1997. The calculated trends for these periods with the sources described earlier are 0.09 pptv/yr for both periods. Without the semiconductor source, the calculated trends in the second period would be less than 0.05 pptv/yr, which is significantly
lower than the measured values. The calculated interhemispheric ratio is 1.05, which is the same as the measured ratio of 1.05 ( 0.01 (1994-1997). For C3F8, the trends are 3.4 ( 1.1 ppqv/yr (parts per quadrillion per year) from 1978 to 1986 and have increased to 9 ( 2 ppqv/y. The inter-hemispheric ratio is 1.15 ( 0.03 (1994-1997). Both these patterns, the rapid increase of trend and the relatively large inter-hemispheric ratio, are consistent with the rapidly increasing use of such gases in semiconductor manufacturing. With the presumption of a long atmospheric lifetime, the emissions needed to account for these trends are 0.1 and 0.3 Gg/yr in the two periods mentioned above. The concentrations still remain well below 1 pptv, suggesting that present levels are not environmentally significant.
Acknowledgments This work was supported in part by a grant from the International Aluminum Institute to Andarz Co. We have benefited from discussions with Jerry Marks (consultant), Bud Lieber (Kaiser Aluminum), and Chris Butenhoff (PSUPhysics). We also thank Mr. Mark Tarver of IPAI (later IAI) for initiating this project.
Literature Cited (1) Houghton, J. T.; et al. In Climate Change 1995, The Science of Climate Change; Houghton, J. T., et al., Eds.; Cambridge University Press: Cambridge, U.K., 1996; pp 12-49. (2) Penkett, S.; Prosser, N.; Rasmussen, R.; Khalil, A. J. Geophys. Res. 1981, 86, 5172-5178. (3) Culbertson, J.; Prins, J.; Grimsrud, E. J. Chromatogr. 2000, 903, 261-265 (4) Culbertson, J.; Prins, J.; Grimsrud, E.; Rasmussen, R.; Khalil, A. Chemosphere (in press). (5) Harnisch, J.; Borchers, R.; Fabian, P.; Ga¨ggeler, H.; Schotterer, U. Nature 1996, 384, 32.
(6) Harnisch, J.; Eisenhauer, A. Geophys. Res. Lett. 1998, 25, 24012404. (7) Holiday, R.; Henry, J. Ind. Eng. Chem. 1959, 51, 1289-1292. (8) Tabereaux, A. JOM 1994, November, 30-34. (9) IAI (formerly IPAI). Anode Effect and Perfluorocarbon Compounds Emission Survey 1990-1993; International Aluminum Institute Report: London, 1996; pp 9-99. (10) IAI. Anode Effect Survey 1994-1997 and Perfluorocarbon Compounds Emissions Survey 1990-1997; International Aluminum Institute: London, 2000; pp 2-28. (11) IAI. Primary Aluminium Production, Current IAI Statistics; International Aluminum Institute: London, 2002; www.worldaluminum.org. (12) AMM. Production of Primary Aluminum. Metal Statistics, The Statistical Guide to North American Metals; American Metal Market: New York, 1963-1998. (13) Metallgesellschaft Aktiengesellschaft. Metal Statistics, 55th and 65th eds.; Metallgesellschaft Aktiengesellschaft: Frankfurt, Germany, 1968 and 1978. (14) U.S. Geological Survey. Minerals Yearbook; USGS: Washington, DC, 1994-2000; minerals.usgs.gov/minerals/pubs/commodity/ aluminum/#pubs. (15) Olivier, J.; et al. RIVM Report 773301001; RIVM: Bilthoven, The Netherlands, 2001. (16) Olivier, J.; et al. EDGAR Database, 2002; arch.rivm.nl/env/int/ coredata/edgar/data33_pfc.html. (17) Cicerone, R. Science 1979, 206, 59-61. (18) Ravishankara, A.; Solomon, S.; Turnipseed, A.; Warren, R. Science 1993, 259, 194-199. (19) Harnisch, J.; Borchers, R.; Fabian, P.; Maiss, M. Geophys. Res. Lett. 1996, 23, 1099-1102. (20) Harnisch, J.; Borchers, R.; Fabian, P.; Maiss, M. Geophys. Res. Lett. 1999, 26, 295-298. (21) Khalil, A.; Rasmussen, R. Geophys. Res. Lett. 1985, 12, 671-672.
Received for review January 21, 2003. Revised manuscript received July 3, 2003. Accepted July 10, 2003. ES030327A
VOL. 37, NO. 19, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4361
