Document mBR3kGNO5ZNGd5e6k0ZZRBmBk

fL-o r^\ . Ots <. / ) GEOPHYSICAL RESEARCH LETTERS, VOL. 8, NO. 9, PAGES 1005-1007, SEPTEMBER 1981 W? m GLOBAL ATMOSPHERIC DISTRIBUTION AND TREND OF METHYLCHLOROF0RH (CH3CCI3) R. A. Rasmussen and M. A. K. Khalil tx/i, Department of Environmental Science, Oregon Graduate Center, Beaverton, Oregon 97006 j tT*.. Abstract. Over the last two years atmospheric measurements of methylchloroform (CH3CCI3) have been made at six locations using EC/GC techniques. The concentrations are reported, and it is shown that CH3CCI3 concentrations increased at rates of 3.6Z2,1Z per year at Alaska (*v70*N) between 8/79 'and 1/81, 6.6Z+3.2Z per year at Oregon (^45*N) be tween 1/79 and 1/81, 4.3%4Z per year at Hawaii (24*11) between 11/79 and 1/81, 4.1Z4.8Z per year at Samoa (14*S) between 2/80 and 1/81, 11.5Z1.5Z per year at Tasmania (42*S) between 1/79 and 6/80, and at 8.7Z5.7Z per year at the south pole be tween 1/79 and 1/81. These recent yearly in creases are smaller than earlier rates of increase but are consistent with a lifetime of a.7 years (6 years-10 years) and a decline in the rate of global emissions. Introduction Methylchloroform (CH3CCI3), manufactured pri marily for use as an industrial degreasing sol vent, lias been released into the earth's atmosphere in steadily increasing amounts since the Ifearly 1950's (Neely and Plonks, 1978). In 1974 Holina and Rowland proposed that the inert, man made fluorocarbons CCI3F and CC12F2 were being emitted in sufficient quantities that their con tinued use could release atomic chlorine in the stratosphere which could catalytically destroy enough of the earth's natural ozone layer to cause damage to human health by allowing more ultraviolet radiation, normally absorbed by ozone, to reach the earth's surface. It was also discovered that these man-made gases efficiently absorb infrared radiation so that their accumula tion in the troposphere could lead to a global warming, affecting the earth's climatic and agri cultural patterns (Ramanathan, 1975; Kellogg, 1980). It soon became clear that large and in creasing emissions of other man-made gases such as CH3CCI3 and CCI4 would have similar environ mental consequences (Crutzen et al,, 1978; Mc Connell and Schiff, 1978). Unlike CCI3F and CCI2F2, some of the methylchloroform is removed in the troposphere after reacting with atmos pheric hydroxyl radicals. Nevertheless, the growing atmospheric burden of CH3CCI3 is cause for concern about the earth's future environment (NAS, 1980). Atmospheric measurements of CH3CCI3 which can reveal its atmospheric behaviour and ultimate environmental role have been made over the last eight years, first by Lovelock et al. (1973) and systematically since 1975 by Rasmussen ^Supplement (data, cables) is available with entire rticle on microfiche. Order from American Seophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document L81-008; $1.00. Payment must accompany order. Copyright 1981 by the American Geophysical Union et al. (1981), as well as by several oth^&fygroups ^0 (see Rasmussen and Khalil, 1981). In this paper we discuss the results of globalf\ff> e/rs CH3CCI3 measurements representing Its concentra tions at Pt. Barrow and Poker Flats, Alaska (v70*N), Cape Meares, Oregon (45*N), Cape Kuma- kahi, Hawaii (24*N), Samoa (14*5), Cape Grim, Tasmania (42*S), and a few from the south pole (90*S). II. Atmospheric Concentrations and Trends Once a week three samples were collected in specially designed flasks at the sites in Alaska, Hawaii and Samoa. These were sent by airmail to the Oregon Graduate Center's Atmospheric Trace Gases Laboratory where CH3CCI3 concentrations were determined by electron capture-gas chromato graphic techniques (EC/GC). Similar measurements were made for flask samples from the south pole, but only during January of every year (Rasmussen et al., 1981). Automated EC/GC measurements made every Friday at the sites in Tasmania and Oregon have been selected for this report. Details of the flask sampling systems and EC/GC analysis sre given by Rasmussen and Khalil (1980). The results of the observations at the six lo cations are shown as monthly average concentra tions of CH3CCI3 in Figure 1. The data from which Figure 1 was derived have been included in the microfilm appendix. To determine the rates of increase of CII3CCI3 we used the model (l/C)dC/dt = $ and applied classical least squares_techniques to estimate the average 6 (denoted 6) which is insensitive to S to errors in absolute accuracy. _The res^ltjs are E.2 reported in Table 1 along with 65 where B6B gives the 90Z confidence limits for 6 using the_ t-statlstlc which is also used to test whether 8 Is significantly greater than zero at the a level of significance. A latitudinal profile can be constructed from these data as shown in Figure 2 for two different periods about a year apart (1980 and 1981). The latitudinal profile shows a bump at high north ern latitudes where most of the sources are. Taking these features into account, average con centrations over the northern and southern hemi spheres were calculated which showed that over the two years from 1/1979 to 1/1981 the global atmospheric concentration rose at 6.7Z+2.0Z per year. The calculated average concentrations for both hemispheres are also given in the appendix. Rates of increase calculated for the hemispheri- cally and globally averaged data are mean values ovei two years, whereas the rates of Increase calculated for each site span different and gen erally shorter times. Independent data (Khalil and Rasmussen, 19S1) verify that the atmospheric concentrations of CH3CCI3 increased by a smaller percentage during 1980 than during 1979. This is a part of a gen eral decline In the atmospheric rate of increase m fe8'* Paper number 1L1158. 0094-3276/81/001L-1I58S01.00 1005 SL 036745 Iu0& Rasmussen and Khalil; Global Trends of CH^CCl^ TIME (montht) Figure 1. Monthly average concentrations of CH3CCI3 at six sites, * Pt, Barrow and Poker Flats, Alaska (^70*N); *-- = Cape Meates, Oregon ('45*!0; X-------------------- X * Cape Kumakahi, Hawaii (24*N); A - - - A Samoa (14*S); O ----- ----- D Cape Grin, Tasmania (42S); - South Pole (90*S). since 1975 which can be explained by the reduc tion in the rate of Increase of emissions. For example, the global emissions of CH3CCI3 in creased exponentially at about 17% per year be tween 1956 and 1973, but at only 82 per year on the average between 1974 and 1979 and increased hardly at all over the past three years. There fore, the average global increase of 6.72 yr"1 observed over the last two years is a composite of faster increase during 1979 and a somewhat slower increase during 1980, The calculated rates of increase are'consistent within their limits of uncertainties. Regarding the global emissions as constant Sin (<= lotitude) Figure 2, Latitudinal variation in the concen tration of CH3CCI3 shown at two times about a year apart. since 1979, the average expected rate of increase using a global mass balance can be written as: B - | [ln(l + ~~ (e^-l)) - nTJ T nC0 (1) where (0-T) is the time over which the average rate of increase is calculated (here 2 years), C0 is the number of molecules of CH3CCI3 in the entire atmosphere at time zero, S0 are the (con stant) emissions in molecules per year and n ^ 1/t where t is the global lifetime. Putting 8 " 0.067+0.02 per year as calculated from the ob servations, C0 calculated from a in Table 1, and S0 corresponding to 1.05xl09 lbs/yr (Neely and Farber, 1980), we obtain a lifetime of 7 years. The ratio S0/C0 is uncertain enough (+102) that lifetimes (r) between 6 years and 10 years are also consistent with eqn. (1) and an increase rate of 6.72% per year. Here we want only to demonstrate that the atmospheric trends of CH3CCI3 as shown in Figure 1 and Table 1 are in Table 1. INCREASE OF ATMOSPHERIC CH3CCI3 AT SIX LOCATIONS ON THE EARTH'S SURFACE Location Alaska (70*N) Cape Meares (45*N) Hawaii (24N) Samoa (14'S) Tasmania (42S) South Pole (90*S) Northern Hemisphere Southern Hemisphere Global Average Duration of Sampling (Number of Months Samples Here Collected) 8/79-1/81 (18) 1/79-1/81 (24) 11/79-1/81 (13) 2/80-1/81 (9) 1/79-6/80 (18) 1/79-1/81 (3) 1/79-12/80 1/79-12/80 1/79-12/80 a (pptv) 150 146 135 112 92 94 130 99 115 6 t 68 (2 per yr) 3.82.1 6.63.2 4.34.0 4.14.8 11.5+1.5 8.7+5.7 6.73.0 6.7+1.6 6.72.0 8 > 07 Yes (a 0.005) Yes (n = 0.005) Yes (a = 0.05) Undetermined Yes (a 0.005) Yes (a " 0.05) Yes (a 0.005) Yes (a - 0.005) Yes (a - 0.005) a is the mixing ratio (pptv = 10~12) of concentrations at beginning of the sampling at each site calculated by least squares techniques. 8 ij> the average increase based on (l/C)dC/dt " 8 sod 8 68 give the 902 confidence limits of 8 based on the t-statlstic. SL 0367^6 Rasmussen and Khalil: Global Trends of CH^CCl^ 1007 accord with the known sources and lifetime of CH3CCI3 (see Jcsson, 1980), In time, as more data are obtained from these sites, an accurate determination of the atmos pheric lifetime will become possible. Coupled with information on the sources and specific global sinks of CH3CCI3, Its potential role in the earth's environment can then be determined. Acknowledgements We thank Jeff Wiederholt and Steve Crawford of the Oregon Graduate Center. Samples from Barrow, Alaska, and the south pole were collected by NQAA/ GMCC. This work was supported by the National Aeronautics and Space Administration (NSG 7457), the Rational Science Foundation (ATM-7806628), the Chemical Manufacturers Association and Bio spherics Research Laboratories. References Crutzen, F.J., I.S.A. Isaksen & J.R. McAfee, The impact of the chlorocarbon industry on the ozone layer, J, Geophya. Res.. 83, 5493-5500, 1978. lesson, J.P., Release of industrial halocarbons and tropospheric budget. In Proceedings of the NATO Advanced Study Institute of Atmospheric Ozone, A.C. Aikin, Ed. (U.S. Dept, of Transpor tation, Washington, D.C., 1980). Hogg, W., Modeling future climate, Ambio, 9, 216-221, 1980. Khalil, H.A.K., & R.A. Rasmussen, Decline in the atmospheric accumulation rates of CCI3F (F-ll), CCI2F2 (F~12) and CH3CCI3. Submitted for pub lication, 1981. Lovelock, J.E., R.J. Maggs 6 R.J. Wade, Halogenated hydrocarbons in and over the Atlantic, Nature. 241, 194, 1973. McConnell, J.C., & H.I. Schiff, Methyl chloroform: impact on stratospheric ozone. Science, 199, 194, 1978. Molina, M., & F.S. Rowland, Stratospheric sink for cblorofluoromethanes: chlorine atom-catalysed destruction of ozone. Nature. 249. 810-812, 1974. National Academy of Sciences, Stratospheric Ozone Depletion by Halocarbons: Chemistry and Trans port (NAS, Washington, D.C., 1979). Neely, W.B., & J.H. Plonka, Estimation of time averaged hydroxyl radical concentration in the troposphere, Env. Sci. & Technol.. 12, 317-321, 1978. Neely, W.B., & H. Farber, personal communication, 1980. Ramanathan, V., Greenhouse effect due to chlorofluorocarbons: climatic implications. Science, 190, 50-52, 1975. Rasmussen, R.A., & M.A.K. Khalil, Interlaboratory comparison of fluorocarbons 11, 12, methylchloroform and nitrous oxide measurements, Atmos. Environ.. 15. 1559, 1981. Rasmussen, R.A., 6 M.A.K. Khalil, Atmospheric halocarbons: measurements and analyses of se lected trace gases. In Proceedings of the NATO Advanced Study Institute on Atmospheric Ozone. A.C. Aikin, Ed., (U.S. Dept, of Transportation, Washington, D.C., 1980). Rasmussen, R.A., M.A.K. Khalil 4 R.W, Dalluge, Atmospheric trace gases in Antarctica, Sci ence, 211, 285-287, 1981. (Received April 20, 1981; accepted July 14, 19S1.) SL 036747