Rare trace gases at the South Pole
Rare trace gases at the South Pole R. A. RASMUSSEN and M. A. K. KHALIL Department of Environmental Science Oregon Graduate Center Beaverton, Oregon 97006
The growth of population and industry has led to the accumulation of dozens of exotic gases in the Earth's atmosphere. All are present in extremely small quantities, each at concentrations less than a few tenths of parts per billion. Most of these compounds have long atmospheric lifetimes, ranging from a decade to perhaps centuries or more, thus creating the potential for continued accumulation and long-term perturbations of the Earth's environment. At present CC1 3F and CC12F2 are considered to be the most significant contributors to a possible future depletion of the stratospheric ozone layer, and increasing carbon dioxide (CO,) is regarded as the principal contributor to an anticipated significant warming of the Earth over the next 20-50 years, (Gribbin 1978; National Research Council 1979). The theory of stratospheric ozone depletion by chlorofluorocarbons has led to intensive research on atmospheric trace gases for nearly a decade now. Measurements of fluorocar-
bons CCI3F (F-Il) and CCL 2F2 (F-12) made at the South Pole have added considerably to the present knowledge of their lifetimes, dispersion, and sources (Chang and Penner 1978; Fink and Klais 1978; Khalil and Rasmussen 1981; Rasmussen, Khalil, and Dalluge 1981; Rowland et al. 1982). Most of these data have been described in earlier publications of Rasmussen (1978), Rasmussen and Khalil (1982), and Rasmussen, Khalil, and Dalluge (1980). Our antarctic research has shown that a number of much rarer manmade gases, including fluorocarbons CHC1F 2 (F-22) and C2C13F3 (F-113) and sulfur hexafluoride (SF,), have also reached the South Pole. At present little is known about the annual emissions or atmospheric removal processes of these trace gases. It is likely that they are very long-lived, with lifetimes of several decades. In this paper, we show the rise of CHC1F 2 (F-22), C2C11F1 (F-113), and SF, at the South Pole. For contrast, concentrations of these gases at midnorthern latitudes (45°N, Oregon) are reported as well. To demonstrate the unique atmospheric behavior of long-lived manmade gases, we have also included measurements of chloroform (CHC1 1 ), which has both natural and anthropogenic sources, and of methyliodide (CH 3I), which is believed to be entirely natural. Each of the gases discussed here, natural or anthropogenic, is present at less than 100 parts per trillion by volume, thus making them among the rarest trace gases ever measured in the Earth's atmosphere. The information we have obtained thus far is summarized in the table and figures 1 and 2.
Rare natural and anthropogenic trace gases at the South Polea
Co (pptv) (in percent 1979 per year) CHCIF2 (F-22) s (90°S) PNW (45°N) Global C2C13F3 (F-113) SP (90S) PNW (45°N) Global SF, SP (90°S) PNW (45°N) Global
Sources
Sinks
40.2 12.0 ± 0.9 52.4 10.1 ± 1.3 46.3 10.9 ± 3.2
Anthropogenic: propellent, refrigerant, plastics and chemical manufacturing
Reaction with tropospheric OH; photo decomposition in stratosphere (T > 10 yrs)
12.2 15.8 14.0
11.2 ± 2.3 11.7 ± 2.6 11.4 ± 2.4
Anthropogenic: refrigerant, foaming agent, solvent
Reaction with 0(D), photo decomposition in stratosphere (T > 10 yrs)
0.36 0.50 0.43
12,9 ± 2.6 8.6 ± 2.3 10.5 ± 2.4
Anth ropogenic: electrical equipment, atmospheric tracer
Stratosphere
Anthropogenic: F-22 production, pharmaceutical products, dyes, pesticides Natural: oceanic
Reaction with OH, short-lived ('r < 1 yr)
Natural: oceanic
Reaction with sunlight, shortlived (T days)
(T
> 10 yrs)
C (1979-1983) CHCI3 SP (90°S) PNW (45°N) Global CH3I si' (90°S) PNW (45°N) Global a
16 ± 2 45 ± 5 26
0.6 1.2
C represents concentrations; ± values are 90 percent confidence limits; "s' denotes South Pole; "PNW" denotes Pacific northwest (Oregon). CH3I concentrations vary from year to year by over a factor of 5. For F-22, F-113, and SF, concentrations, the formula C = C 0exp (0t) was employed to estimate the relative rate of change. "b' denotes 1003 to convert to percentage per year. "r" denotes atmospheric lifetime. '-" denotes no trend detected.
250
ANTARCTIC JOURNAL
C5
1
70 71
>
50
0 > 61 40.
0
30 Z 0 H
5
—J —J
F°
Cr
H
ct
41
LU
2
(/) H IN
11 Z
979 1980 1981 1982 1983
(-I-) Z 2( 0
TIME (yrs) Figure 2. Average concentrations of chloroform (CHCi 3) and methyliodide (CH 3 1) at the South Pole (sr) and Pacific northwest (PNw). For CHCI 31 PNW values are averaged over the winter, and the vertical bars represent 90 percent confidence limits. For CH 3I 1 concentrations were found to be low but variable over a factor of 5. The solid lines are to group data. ("pptv" denotes parts per trillion by volume.)
H Cr H Z LLJK 0 Z 0 OIc
0.c.
979 1980 1981 1982 1983
TIME (yrs) Figure 1. Monthly average concentrations of CHCIF 21 C 2C13 F3 , and SF, at the South Pole (s p ) and the Pacific northwest ( p Nw). The vertical bars indicate 90 percent confidence limits of the average concentration. For sulfur hexafluoride (SF.) these limits are smaller than the size of the circles used to represent the average value. The solid lines are C = C. exp (3t). Fluorocarbons (F-22 and F-113). These exotic fluorocarbons are thought to be entirely manmade. At present rates of increase their doubling time in the atmosphere is about 6 years, which
1983 REVIEW
may be regarded as rapid accumulation. If the concentrations of F-22 and F-113 become large enough, they may affect the global environment by the same mechanisms as proposed for the fluorocarbons CC1 3F (F-li) and CC12F2 (F-12); or the cumulative effects of the exotic fluorocarbons such as F-22 and F-113 may become significant even if each gas by itself is innocuous. Sulfur hexafluoride. SF6 is present at concentrations of less than one part per trillion by volume, and based on what we know now, it is unlikely to affect the global environment. It can, however, be used as a tracer of air motions on a global scale. Chloroform (CHC1 3) and met hyliodide (CHI). As shown in figure 2, CHC13 and CH3I behave differently from the other gases. Both are much shorter-lived and thus more variable in concentration. Because of this variability, any long-term trends, if they exist at all, are not yet detectable. The very existence of these gases at the South Pole indicates their natural origins. Since much of the southern part of the southern hemisphere is oceanic, it is likely to be the source of CHC1 3 and CH3I. These results are particularly significant for the case of chloroform since it is generally thought to be primarily anthropogenic (National Research Council 1978). Its global lifetime is estimated to be only about 6 months, since it reacts rapidly with tropospheric hydroxyl (OH) radicals. If CHC1 3 is 251
entirely anthropogenic, much of it which reaches the South Pole would have to be transported through the tropics, where its lifetime is even shorter due to a greater abundance of OH in that region. Thus, it should not even be detectable at the South Pole (Khalil, Rasmussen, and Hoyt in press). On the other hand, the sizable difference of concentration between Oregon and the South Pole suggests that a significant portion of it may be anthropogenic. Mass balance calculations based on the data shown here suggest that the oceans and anthropogenic sources contribute about equal amounts of CHC1 3 to the atmosphere. This work was supported in part by National Science Foundation grant DPP 81-08684. We thank the National Oceanic and Atmospheric Administration/Global Monitoring for Climate Change personnel for collecting air samples at the South Pole, and Rohith Gunawardena, Bob Dalluge, Trish Quinn, Don Stearns, and Bob Watkins for laboratory work at Oregon Graduate Center. Additional support for this work was provided by the Biospherics Research Corporation and the Andarz Company.
References Chang, J . S., and J . E. Penner. 1978. Analysis of global budgets of halocarbons. Atmospheric Environment, 12, 1867-1873. Fink, H. J . , and 0. Klais. 1978. Global distribution of fluorocarbons. Berzchte der Bunsengesellschaft fuer Physikalische Chemie, 82, 1147-1150.
Measurement of the column densities of water, nitric acid (HNOj, fluorocarbons (CF2 Cl2 and CFCI 3) and ozone during the austral winter DAVID G. MURCRAY University of Denver Department of Physics Denver, Colorado 80208
A grating spectro-radiometer is being prepared for operation at South Pole Station during the 1984-1985 austral winter. This instrument will measure the column densities of trace gases by spectral analysis of the thermal radiation from the atmosphere. The concentration of the gases to be measured may be in parts per billion or less. As a result, the thermal emissions associated with these gases are quite small, that is, on the order of 10 to 10 watt per square centimeter, per micrometer per steradium. It is important, therefore, that the instrument parameters are optimum.
252
Gribbin, J . (Ed.). 1978. Climatic Change. Cambridge: Cambridge University Press. Khalil, M. A. K., and R. A. Rasmussen. 1981. Decline in the atmospheric accumulation rates of CC1 3F (F-li), CC1 2F2 (F-12) and CH3CC11 . Journal of the Air Pollution Control Association, 31, 1274-1275. Khalil, M. A. K., R. A. Rasmussen, and S. D. Hoyt. In press. Atmospheric chloroform (CHCI 1 ): Ocean-air exchange and global mass balance. Tellus. National Research Council. 1978. Chloroform, Carbon tetrachlorjde and Other Halomethanes: An Environmental Assessment, Washington, D.C.: National Academy of Sciences. National Research Council. 1979. Stratospheric Ozone Depletion by Halocarbons: Chemistry and Transport, Washington, D.C.: National Academy of Sciences. Rasmussen, R. A. 1978. Halocarbons and N 2 0 analysis in Antarctica. Antarctic Journal of the U.S., 13(4), 191-193. Rasmussen, R. A., and M. A. K. Khalil. 1982. Atmospheric fluorocarbons and methyl chloroform at the South Pole. Antarctic Journal of the U. S., 17(5), 203-205. Rasmussen, R. A., M. A. K. Khalil, and R. W. Dalluge. 1980. Halocarbons and other trace gases in the antarctic atmosphere. Antarctic Journal of the U.S., 15(5), 177-179. Rasmussen, R. A., M. A. K. Khalil, and R. W. Dalluge. 1981. Atmospheric trace gases in Antarctica. Science, 211, 285-287. Rasmussen, R. A., M. A. K. Khalil, R. Gunawardena, and S. D. Hoyt. 1982. Atmospheric methyl iodide (CH,). Journal of Geophysical Research, 87, 3086-3090. Rowland, F S., S. C. Tyler, D. C. Montague, and Y. Makide. 1982. Dichlorodifluoromethane (C1 2F 2 ) in the earth's atmosphere. Geophysical Research Letters, 9, 481-484.
Computer simulation of the expected emission spectrum can be, and has been, used as a guide in the instrument development. These calculations, however, assume an ideal atmosphere: the effects of atmospheric particulates (i.e., ice crystals, cirrus clouds, etc.) are ignored. Because much of the winter data may be taken under less than ideal conditions, measurements under various atmospheric conditions at South Pole Station were desirable as an aid in final determination of design parameters. A prototype instrument was installed at South Pole Station in November 1982. Data was taken under conditions ranging from totally clear skies to overcast with visibilities down to 0.5 mile. Observations were made on a total of 8 days during the period from 20 November through 30 November. F. H. Murcray, F. G. Fernald, and J . Gillis participated in the field program. Analysis of the data obtained suggest that a spectral bandwidth of 0.5 wave number instead of the 1.5-2 wave number employed in the prototype would greatly facilitate the computation of column densities, particularly when light cirrus clouds are present. This spectral bandwidth can be realized while maintaining adequate energy throughput with a spectrometer utilizing larger optics than the prototype. This instrument is therefore being prepared for operation at South Pole Station during the 1984-4985 winter. This work is supported by National Science Foundation grant DPP 81-18005.
ANTARCTIC JOURNAL
The growth of population and industry has led to the accumulation of dozens of exotic gases in the Earth's atmosphere. All are present in extremely small quantities, each at concentrations less than a few tenths of parts per billion. Most of these compounds have long atmospheric lifetimes, ranging from a decade to perhaps centuries or more, thus creating the potential for continued accumulation and long-term perturbations of the Earth's environment. At present CC1 3F and CC12F2 are considered to be the most significant contributors to a possible future depletion of the stratospheric ozone layer, and increasing carbon dioxide (CO,) is regarded as the principal contributor to an anticipated significant warming of the Earth over the next 20-50 years, (Gribbin 1978; National Research Council 1979). The theory of stratospheric ozone depletion by chlorofluorocarbons has led to intensive research on atmospheric trace gases for nearly a decade now. Measurements of fluorocar-
bons CCI3F (F-Il) and CCL 2F2 (F-12) made at the South Pole have added considerably to the present knowledge of their lifetimes, dispersion, and sources (Chang and Penner 1978; Fink and Klais 1978; Khalil and Rasmussen 1981; Rasmussen, Khalil, and Dalluge 1981; Rowland et al. 1982). Most of these data have been described in earlier publications of Rasmussen (1978), Rasmussen and Khalil (1982), and Rasmussen, Khalil, and Dalluge (1980). Our antarctic research has shown that a number of much rarer manmade gases, including fluorocarbons CHC1F 2 (F-22) and C2C13F3 (F-113) and sulfur hexafluoride (SF,), have also reached the South Pole. At present little is known about the annual emissions or atmospheric removal processes of these trace gases. It is likely that they are very long-lived, with lifetimes of several decades. In this paper, we show the rise of CHC1F 2 (F-22), C2C11F1 (F-113), and SF, at the South Pole. For contrast, concentrations of these gases at midnorthern latitudes (45°N, Oregon) are reported as well. To demonstrate the unique atmospheric behavior of long-lived manmade gases, we have also included measurements of chloroform (CHC1 1 ), which has both natural and anthropogenic sources, and of methyliodide (CH 3I), which is believed to be entirely natural. Each of the gases discussed here, natural or anthropogenic, is present at less than 100 parts per trillion by volume, thus making them among the rarest trace gases ever measured in the Earth's atmosphere. The information we have obtained thus far is summarized in the table and figures 1 and 2.
Rare natural and anthropogenic trace gases at the South Polea
Co (pptv) (in percent 1979 per year) CHCIF2 (F-22) s (90°S) PNW (45°N) Global C2C13F3 (F-113) SP (90S) PNW (45°N) Global SF, SP (90°S) PNW (45°N) Global
Sources
Sinks
40.2 12.0 ± 0.9 52.4 10.1 ± 1.3 46.3 10.9 ± 3.2
Anthropogenic: propellent, refrigerant, plastics and chemical manufacturing
Reaction with tropospheric OH; photo decomposition in stratosphere (T > 10 yrs)
12.2 15.8 14.0
11.2 ± 2.3 11.7 ± 2.6 11.4 ± 2.4
Anthropogenic: refrigerant, foaming agent, solvent
Reaction with 0(D), photo decomposition in stratosphere (T > 10 yrs)
0.36 0.50 0.43
12,9 ± 2.6 8.6 ± 2.3 10.5 ± 2.4
Anth ropogenic: electrical equipment, atmospheric tracer
Stratosphere
Anthropogenic: F-22 production, pharmaceutical products, dyes, pesticides Natural: oceanic
Reaction with OH, short-lived ('r < 1 yr)
Natural: oceanic
Reaction with sunlight, shortlived (T days)
(T
> 10 yrs)
C (1979-1983) CHCI3 SP (90°S) PNW (45°N) Global CH3I si' (90°S) PNW (45°N) Global a
16 ± 2 45 ± 5 26
0.6 1.2
C represents concentrations; ± values are 90 percent confidence limits; "s' denotes South Pole; "PNW" denotes Pacific northwest (Oregon). CH3I concentrations vary from year to year by over a factor of 5. For F-22, F-113, and SF, concentrations, the formula C = C 0exp (0t) was employed to estimate the relative rate of change. "b' denotes 1003 to convert to percentage per year. "r" denotes atmospheric lifetime. '-" denotes no trend detected.
250
ANTARCTIC JOURNAL
C5
1
70 71
>
50
0 > 61 40.
0
30 Z 0 H
5
—J —J
F°
Cr
H
ct
41
LU
2
(/) H IN
11 Z
979 1980 1981 1982 1983
(-I-) Z 2( 0
TIME (yrs) Figure 2. Average concentrations of chloroform (CHCi 3) and methyliodide (CH 3 1) at the South Pole (sr) and Pacific northwest (PNw). For CHCI 31 PNW values are averaged over the winter, and the vertical bars represent 90 percent confidence limits. For CH 3I 1 concentrations were found to be low but variable over a factor of 5. The solid lines are to group data. ("pptv" denotes parts per trillion by volume.)
H Cr H Z LLJK 0 Z 0 OIc
0.c.
979 1980 1981 1982 1983
TIME (yrs) Figure 1. Monthly average concentrations of CHCIF 21 C 2C13 F3 , and SF, at the South Pole (s p ) and the Pacific northwest ( p Nw). The vertical bars indicate 90 percent confidence limits of the average concentration. For sulfur hexafluoride (SF.) these limits are smaller than the size of the circles used to represent the average value. The solid lines are C = C. exp (3t). Fluorocarbons (F-22 and F-113). These exotic fluorocarbons are thought to be entirely manmade. At present rates of increase their doubling time in the atmosphere is about 6 years, which
1983 REVIEW
may be regarded as rapid accumulation. If the concentrations of F-22 and F-113 become large enough, they may affect the global environment by the same mechanisms as proposed for the fluorocarbons CC1 3F (F-li) and CC12F2 (F-12); or the cumulative effects of the exotic fluorocarbons such as F-22 and F-113 may become significant even if each gas by itself is innocuous. Sulfur hexafluoride. SF6 is present at concentrations of less than one part per trillion by volume, and based on what we know now, it is unlikely to affect the global environment. It can, however, be used as a tracer of air motions on a global scale. Chloroform (CHC1 3) and met hyliodide (CHI). As shown in figure 2, CHC13 and CH3I behave differently from the other gases. Both are much shorter-lived and thus more variable in concentration. Because of this variability, any long-term trends, if they exist at all, are not yet detectable. The very existence of these gases at the South Pole indicates their natural origins. Since much of the southern part of the southern hemisphere is oceanic, it is likely to be the source of CHC1 3 and CH3I. These results are particularly significant for the case of chloroform since it is generally thought to be primarily anthropogenic (National Research Council 1978). Its global lifetime is estimated to be only about 6 months, since it reacts rapidly with tropospheric hydroxyl (OH) radicals. If CHC1 3 is 251
entirely anthropogenic, much of it which reaches the South Pole would have to be transported through the tropics, where its lifetime is even shorter due to a greater abundance of OH in that region. Thus, it should not even be detectable at the South Pole (Khalil, Rasmussen, and Hoyt in press). On the other hand, the sizable difference of concentration between Oregon and the South Pole suggests that a significant portion of it may be anthropogenic. Mass balance calculations based on the data shown here suggest that the oceans and anthropogenic sources contribute about equal amounts of CHC1 3 to the atmosphere. This work was supported in part by National Science Foundation grant DPP 81-08684. We thank the National Oceanic and Atmospheric Administration/Global Monitoring for Climate Change personnel for collecting air samples at the South Pole, and Rohith Gunawardena, Bob Dalluge, Trish Quinn, Don Stearns, and Bob Watkins for laboratory work at Oregon Graduate Center. Additional support for this work was provided by the Biospherics Research Corporation and the Andarz Company.
References Chang, J . S., and J . E. Penner. 1978. Analysis of global budgets of halocarbons. Atmospheric Environment, 12, 1867-1873. Fink, H. J . , and 0. Klais. 1978. Global distribution of fluorocarbons. Berzchte der Bunsengesellschaft fuer Physikalische Chemie, 82, 1147-1150.
Measurement of the column densities of water, nitric acid (HNOj, fluorocarbons (CF2 Cl2 and CFCI 3) and ozone during the austral winter DAVID G. MURCRAY University of Denver Department of Physics Denver, Colorado 80208
A grating spectro-radiometer is being prepared for operation at South Pole Station during the 1984-1985 austral winter. This instrument will measure the column densities of trace gases by spectral analysis of the thermal radiation from the atmosphere. The concentration of the gases to be measured may be in parts per billion or less. As a result, the thermal emissions associated with these gases are quite small, that is, on the order of 10 to 10 watt per square centimeter, per micrometer per steradium. It is important, therefore, that the instrument parameters are optimum.
252
Gribbin, J . (Ed.). 1978. Climatic Change. Cambridge: Cambridge University Press. Khalil, M. A. K., and R. A. Rasmussen. 1981. Decline in the atmospheric accumulation rates of CC1 3F (F-li), CC1 2F2 (F-12) and CH3CC11 . Journal of the Air Pollution Control Association, 31, 1274-1275. Khalil, M. A. K., R. A. Rasmussen, and S. D. Hoyt. In press. Atmospheric chloroform (CHCI 1 ): Ocean-air exchange and global mass balance. Tellus. National Research Council. 1978. Chloroform, Carbon tetrachlorjde and Other Halomethanes: An Environmental Assessment, Washington, D.C.: National Academy of Sciences. National Research Council. 1979. Stratospheric Ozone Depletion by Halocarbons: Chemistry and Transport, Washington, D.C.: National Academy of Sciences. Rasmussen, R. A. 1978. Halocarbons and N 2 0 analysis in Antarctica. Antarctic Journal of the U.S., 13(4), 191-193. Rasmussen, R. A., and M. A. K. Khalil. 1982. Atmospheric fluorocarbons and methyl chloroform at the South Pole. Antarctic Journal of the U. S., 17(5), 203-205. Rasmussen, R. A., M. A. K. Khalil, and R. W. Dalluge. 1980. Halocarbons and other trace gases in the antarctic atmosphere. Antarctic Journal of the U.S., 15(5), 177-179. Rasmussen, R. A., M. A. K. Khalil, and R. W. Dalluge. 1981. Atmospheric trace gases in Antarctica. Science, 211, 285-287. Rasmussen, R. A., M. A. K. Khalil, R. Gunawardena, and S. D. Hoyt. 1982. Atmospheric methyl iodide (CH,). Journal of Geophysical Research, 87, 3086-3090. Rowland, F S., S. C. Tyler, D. C. Montague, and Y. Makide. 1982. Dichlorodifluoromethane (C1 2F 2 ) in the earth's atmosphere. Geophysical Research Letters, 9, 481-484.
Computer simulation of the expected emission spectrum can be, and has been, used as a guide in the instrument development. These calculations, however, assume an ideal atmosphere: the effects of atmospheric particulates (i.e., ice crystals, cirrus clouds, etc.) are ignored. Because much of the winter data may be taken under less than ideal conditions, measurements under various atmospheric conditions at South Pole Station were desirable as an aid in final determination of design parameters. A prototype instrument was installed at South Pole Station in November 1982. Data was taken under conditions ranging from totally clear skies to overcast with visibilities down to 0.5 mile. Observations were made on a total of 8 days during the period from 20 November through 30 November. F. H. Murcray, F. G. Fernald, and J . Gillis participated in the field program. Analysis of the data obtained suggest that a spectral bandwidth of 0.5 wave number instead of the 1.5-2 wave number employed in the prototype would greatly facilitate the computation of column densities, particularly when light cirrus clouds are present. This spectral bandwidth can be realized while maintaining adequate energy throughput with a spectrometer utilizing larger optics than the prototype. This instrument is therefore being prepared for operation at South Pole Station during the 1984-4985 winter. This work is supported by National Science Foundation grant DPP 81-18005.
ANTARCTIC JOURNAL
