Atmospheric fluorocarbons and methyl chloroform at the South Pole
131- ***
41-
DOME
I
• $
3- .s ••'• 2'. 0 -10 -20 -30 -40 -50 -60 -70 TEMPERATURE (C)
) -10 -20 -30 -40 -50 -60 -70 -80 TEMPERATURE (C)
Figure 3. Monthly mean values of wind speed plotted against temperature at automatic weather stations (Aws) at D-10 and Dome C, and at two longer term inland stations in Antarctica.
As expected and reported previously (Wendler, Gosink, and Poggi 1981), the wind at Dome C is light. In fact, wind speed there is by far the lowest of all inland stations in Antarctica (figure 2, bottom). The mean annual value is around 3 meters per second, and there appears to be no annual cycle. The wind speed at Dome C is not only the lowest of all inland stations, but also the lowest among all antarctic stations, including coastal and near-coastal stations. This is unique because a freely exposed station at a height of 3,280 meters on all other continents would have a higher wind speed than many coastal stations. This shows again the influence of the katabatic wind. The driving force of the katabatic wind is, of course, the difference in temperature between the air near the ground and the air at the same altitude further down the slope. This temperature difference is a function of inversion strength, which is best developed during the winter months when there is no
Atmospheric fluorocarbons and methyl chloroform at the South Pole R. A. RASMUSSEN and M. A. K. KHALIL Department of Environmental Science Oregon Graduate Center Beaverton, Oregon 97006
The ultimate aim of our research is to determine exactly how human activities are modifying the composition of the Earth's atmosphere and what effect these changes will eventually have on the global environment. It is feared that long-lived gases 1982 REVIEW
heating of the surface by solar radiation. This is also the coldest time of year. Hence, if a place is dominated by gravity flow, one would expect a relationship between temperature and wind speed such that the colder the temperature, the stronger the wind. Looking at two inland stations (Plateau and Byrd) (figure 3), we can see that this relationship is very, well established for the monthly mean values. This relationship also holds at D-10, although there is somewhat more scatter because these data are single monthly values, not long-term monthly means as reported for Byrd and Plateau. Dome C shows no relationship whatsoever between temperature and wind. This result, showing the absence of any gravity flow, is to be expected. The increase in wind speed with decreasing temperature makes the climate of Antarctica inhospitable. The opposite is true for the interior of Alaska; as the inversion builds up over flat areas, there is no gravity flow and the inversion suppresses transmission of the wind speed aloft to the surface. Hence, during cold spells the winds are absent or very weak, while stronger winds destroy the inversion at least partly and bring warmer air to the surface. Antarctica, in contrast, shows the worst possible combination for human occupation: the colder, the surface temperature, the stronger the winds become, resulting in extremely low "equivalent chill temperatures." This work is being supported by National Science Foundation grant DPP 81-00161. Allen Peterson's group of Stanford University designed the stations, and Charles Stearns and coworkers of the University of Wisconsin have refined the units, improved their performance, and provided for maintenance, operation, and preliminary data analysis. To these people, as well as to Expeditions Polaires Françaises, and especially to its director, Jean Vaugelade, we are extremely thankful. A. Schmidt and J. Gosink went to Antarctica, and F. Eaton reviewed this article. Our thanks are extended to all these people.
References Wendler, G., Gosink, J., and Poggi, A. 1981. Katabatic wind measurements in Antarctica. Antarctic Journal of the U.S., 16(5), 192. Wendler, G., and Poggi, A. 1980. Measurements of the katabatic wind in Antarctica. Antarctic Journal of the U.S., 15(5), 193-195.
such as carbon dioxide (CO 2) released into the atmosphere by industrial processes and other human activities eventually may increase the Earth's surface temperature by enhancing the natural greenhouse effect. Gases containing chlorine (and bromine)—much as CC13F (fluorocarbon-11; F-il) and CC1 2F2 (fluorocarbon-12; F-12)--may deplete the stratospheric ozone layer, thus allowing more ultraviolet radiation to reach the Earth's surface and harming living organisms (Barney 1980; National Academy of Sciences 1979). In this article we discuss changes in the atmospheric abundances of the manmade gases CC1 3F, CC12F2, CHC1F2 (fluorocarbon-22; F-fl), C2C13F3 (fluorocarbon-113; F-113), and CH3CC13 (methyl chloroform) over the past 8 years. These gases may not only deplete the ozone layer but may also add to global warming. The very presence of these gases at the South Pole, at concentrations not much less than those in the Northern 203
rapidly. The yearly increase in the global emissions of CH3CC13 has also slowed since 1975. A simple global mass balance model reveals that 13(t) = 11C dC/dt would not remain constant but would gradually decline. Therefore, we adopted the following model for 13(t):
340
320 200 300
(i/C) 4C= NO dt
80 2 0 I-
280 160
13 + &, 8 < 0,
or
Iz W U 2 140 0 U
260
C C0expQ3,t + 12f
2),
240
20 220
200 80
1/1975 1/1977 1/1979 1/1981 1/1983 1/1975 TIME (years)
1/1977 1/1979 1/1981 1/1983
Figure 1. Observed concentrations of fluorocarbon-11 (CCI SF) and fluorocarbon-12 (CC1 2F2) at the South Pole (lower lines) and In the Pacific Northwest (iiw; 45°N; upper lines). The solid line is the function C = C0 expW0 t +
Hemisphere, attests to their long atmospheric lifetimes and their great potential for accumulating in the environment. We are reporting, for the first time, steady increases of F-22 and F-113 at the South Pole; our latest observations on F-li, F-12, and CH3CC13 show that these gases are not increasing as rapidly as they did in earlier years. We started observing concentrations of F-Il and CH 3CC13 at the South Pole in January 1975, and the first measurements of F-12 were made in January 1976. We have continued these measurements every January since then. The 8 years of data comprise the longest systematic and internally consistent measurements currently available. It is being recognized that F-22 and F-113 may also be important environmental contaminants. Techniques for the atmospheric measurement of these extremely rare fluorocarbons have been developed only recently. The measurements of these gases at the South Pole since January 1979 are the first and only ones available. To provide a contrast for the antarctic measurements, the concentrations of these gases were also measured in the Pacific Northwest (in Oregon and Washington at about 45°N) during January of every year. Concentrations of these five gases are measured by electron capture gas chromatography techniques. Discussions of the early data on F-li, F-12, and CH3CC13 and of the analytical techniques can be found elsewhere (Khalil and Rasmussen 1981; Rasmussen 1978; Rasmussen and Khalil 1980; Rasmussen, Khalil, and Dalluge 1980; Rasmussen, Khalil, Penkett, and Prosser 1980). The results are shown in figures 1 and 2. The rate of change was determined by the formula (11C) dC/dt = 13(t), where C is concentration and t is time. This relative expression is used because it does not depend on knowing the absolute concentration of a trace gas (accuracy); only relative changes (precision) need be known. The precision of measurements for these gases is very high at 2 percent. It is believed that the annual global emissions of F-li and F-fl have not changed much since 1975, whereas before 1975 they had been increasing very 204
where C. is the concentration in January 1975, 13 is the rate of increase during 1975, and 8 is the gradual decline in the increase with time since 1975. The most suitable values of CO3 13 and 8 were determined by a nonlinear least squares criterion and are reported in the table. The results for F-li, F-12, and CH3CC13 derived from the second equation are shown as solid lines in figures 1 and 2. The results imply that F-li, F-12, and CH3CC13 are not increasing as rapidly as they did during the 1970's; this observation is consistent with current knowledge of global anthropogenic sources. Much less is known about the global sources and sinks of F-22 and F-113. It is certain, however, that their atmospheric concentrations are rising rapidly, as shown in figure 2 and in the table, and this is cause for concern about the future global environment (see also Khalil and Rasmussen 1981; Rasmussen, Khalil, Penkett, and Prosser 1980). In addition to the fluorocarbons and CH3CC13, concentrations of 15 other trace gases have been measured at the South Pole and in the pacific Northwest since 1979 or earlier, namely, CC14 (carbon tetrachloride), N 20 (nitrous oxide), CH 3C1 (chloromethane), SF,, (sulfur hexafluoride), CO (carbon monoxide), CH, (methane), C2H2 (acetylene), C 2H4 (ethylene), C2H6 (ethane), CH, (benzene), C 7H8 (toluene), C3H8 (propane), CH3I (methyliodide), CHC1 3 (chloroform), and C 2HC1 3 (tn-
I0
I I I I I I
80 CHCIF2(F-22) -I
-;
140 40 P5W
Z 120 0 I.4 100 Z
SP
llI
[110
LJ U
Z o 80
IE
[40
20
U
40V1 1/1975 1/1977 1/1979 1/1981 1/1983 1/1975 1/1977 1/1979 1/1981 1/1983 TIME (years)
Figure 2. Observed concentrations of fluorocarbon-22(CHCIF), fluorocarbon413 (C2CI,F3), and methyl chloroform (CH 3 CC13) at the South Pole (sP; lower lines) and in the Pacific Northwest (PNw; upper lines). The solid line Is the function C = C0exp ( 0 t + 1 8t2) for CH3CCI3 and C = C0exp 0 t for F-fl and F-113.
ANTARCTIC JOURNAL
Yearly Increase In halocarbon concentration and slowdown of atmospheric accumulation at the South Pole (Si'; 90°S) and In the Pacific Northwest (PNw; 450N) Average concentration in base montha (pptv)
Gas F-li (CCI3F) SP PNW
Global F-12 (CCl2F2) SP PNW
Global Methyl chloroform (CH3CCI3) SP PNW
Global F-22 (CHCIF2) SP PNW
Global F-113 (C2CI3F3) SP PNW
Global
P.
(% per year)
S (% per year, per year)
Coefficient of determination (r)
92 125 109
18 11 14
-2.4 -1.2 -1.8
0.997 0.999 0.999
8 8 8
170 201 186
13 12 12
-1.4 —1.6 -1.4
0.999 0.999 0.999
7 7 7
46 84 65
24 15 18
-2.8 -1.4 -1.8
0.997 0.991 0.999
8 8 8
40 53 46
13 10 11
0.999 0.996 0.996
4 4 4
12 16 14
9 10 10
0.974 0.986 0.991
4 4 4
Note. The rate of change of trace gas concentration is assumed to follow the equation() f = - t. 13,, = 100 3 (percent per year) and S = 100& Concentrations of F-1 1, F-12, and CH 3 CCI 3 started rising at $3,, percent per year around 1975, but this rate declined on the average by 5 percent per year per year since then. a For F-il, F-12, and methyl chloroform, base month was January 1975; for F-22 and F-113, base month was January 1979. In parts per thousand, by volume. b Number of years measurements were made.
chioromethane). Results of these measurements are reported elsewhere (Rasmussen and Khalil in preparation; Rasmussen et al. 1981.) This work was supported in part by National Science Foundation grant DPP 81-08684. We thank National Oceanic and Atmospheric Administration Geophysical Monitoring for Climate Change personnel, especially Lts. R. Williscroft and C. McFee, for collecting the air samples at the South Pole, and R. W. Dalluge, R. Gunawardena, J . Wiederholt, D. Stearns, and R. Watkins for laboratory work at the Oregon Graduate Center. References Barney, C. 0. (Study Director). 1980. The Global 2000 Report to the President of the U.S. New York: Pergamon Press. Khalil, M. A. K., Rasmussen, R. A. 1981. Increase of CHCIF, in the Earth's atmosphere. Nature, 292 (5826), 823-824.
1982 REVIEW
National Academy of Sciences. 1979. Stratospheric ozone depletion by halocarbons: Chemistry and transport. Washington, D.C.: Author. Rasmussen, R. A., 1978. Halocarbon and N 20 analyses in Antarctica. Antarctic Journal of the U.S., 13(4), 191-193. Rasmussen, R. A., and Khalil, M. A. K. 1980. Atmospheric halocarbons: Measurements and analyses of selected trace gases. In A. C. Aiken (Ed.), Proceedings of the NATO Advanced Study institute on Atmospheric Ozone: its Variation and Human influences. Washington, D.C.: U. S. Department of Transportation. Rasmussen, R. A., and Khalil, M. A. K. In preparation. Hydrocarbons in Antarctica.
Rasmussen, R. A., Khalil, M. A. K., and Dalluge, R. W. 1980. Halocar bons and other trace gases in the antarctic atmosphere. Antarctic Journal of the U.S., 15(5), 177-179. Rasmussen, R. A., Khalil, M. A. K., and Dalluge, R. W. 1981. Atmospheric trace gases in Antarctica. Science, 211, 285-287. Rasmussen, R. A., Khalil, M. A. K., Penkett, S. A., and Prosser, N. J. D. 1980. CHCIF2 (F-22) in the atmosphere. Geophysical Research Letters, 7, 809-812.
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