T H E
J O U R N A L
OF
PHYSICAL CHEMISTRY Registered in U S Patent OJJice 0 Copyright, 1978, by the American Chemical Society
VOLUME 82, NUMBER 1
JANUARY 12,1978
Stratospheric Photodissociation of Several Saturated Perhalo Chlorofluorocarbon Compounds in Current Technological Use (Fluorocarbons-13, -113, -114, and -115)’ C. C. Chou, R. J. Mllsteln, W. S. Smith, H. Vera Ruiz, Mario J. Molina, and F. S. Rowland” Department of Chemistty, University of California, Irvine, California 927 17 (Received December 28, 1976) Publication costs assisted by the Division of Basic Energy Science, U.S. Department of Energy
Photon absorption cross sections have been measured in the stratospherically important 1849-2273-A range for CClF3(fluorocarbon-13), CC12FCClFz (FC-113),CClFzCClF2(FC-1141,and CClFzCF3(FC-115). Atmospheric residence times have been calculated for these four compounds, and for CC13F (FC-11) and CC12Fz(FC-121, with the assumption that the molecules are removed only by direct solar photolysis or by reaction with O(’D) atoms. Four different eddy diffusion models were used for stratospheric mixing. Direct photolysis accounts for >90% of the removal of FC-11, >8O% for FC-12 and FC-113, and 30 years as the average atmospheric lifetime implies that the stratospheric sinks account for at least 40% (eddy diffusion coefficient of Huten) to 75% (coefficients of Crutzen and of Wofsy) of the total decomposition of CC13F.18 There is no experimental evidence to date which indicates that nonstratospheric sinks (e.g., dissolution in the ocean) remove more than a negligible percentage of CCl3F from the a t m ~ s p h e r e . ~ , ~ ~ The available atmospheric data on CClzFz concentrations do not permit a reliable estimate of minimum atmospheric lifetime because of uncertainties such as the time delay for the release into the atmosphere of FC-12
Chou et al.
used as a refrigerant. Tropospheric measurements have now been reported for CClF&ClF2 (FC-114), in approximate agreement with the expected atmospheric conc e n t r a t i o n ~ The . ~ ~ presence ~~~ of CC12FCClF2(FC-113) has also been reported although there appear to be some problems with this molecule in avoiding contamination. No data are available for the atmospheric concentrations of the other two perhalo molecules discussed here, although all are more volatile than, and chemically at least as stable as, CC13F and are consequently unlikely to have much more effective tropospheric sinks than CC13F for which no important sink other than stratospheric photodissociation has been found. As the estimated lifetimes get longer, competition of other potential sinks must again be considered. For example, a tropospheric sink with a partial lifetime of 100 years would be unimportant for CC13F and dominant for CC1F3or CC1F2CF3. Photodissociation has been experimentally demonstrated to be an important (and probably the only major) sink for CC13F,and presumably also for CC12Fzand CC12FCC1F2. The absence of general experimentation with CC1F3, CC1F2CC1F2,and CClFzCF3leaves open the possibility that sinks important on their time scales might be discovered. No such processes come obviously to mind, however, and the probability seems high that stratospheric dissociation by direct photolysis and/or attack by O(lD) atoms are important terrestrial removal mechanisms for these molecules as well.
References and Notes This research was supported by ERDA Contract No. AT-(04-3)-34, Project Agreement No. 126. I t was presented in part at the 12th International Symposium on Free Radical Chemistry, Laguna Beach, Calif., January, 1976. M. J. Molina and F. S.Rowland, Nafure(London), 249, 810 (1974). F. S. Rowland and M. J. Molina, Rev. Geophys. Space Phys., 13, 1 (1975). R. S. Stolarski and R. J. Cicerone, Can. J. Chem., 52, 1582 (1974). F. S. Rowland, New Scientist, 64, 717 (1974). "Fluorocarbons and the Environment", Report on Federal Task Force on Inadvertent Modification of the Stratosphere (IMOS), June, 1975. GPO Stock No. 038-000-00226-1, US. Government Printing Office, Washington, D.C. "Halocarbons: Effects on Stratospheric Ozone", Panel on Atmospheric Chemistry, National Academy of Sciences, Washington, D.C., Sept, 1976. "Preliminary Economic Impact Assessment of Possible Regulatory Action to Control Atmospheric Emissions of Selected Halocarbons", K. H. Lloyd, Project Officer, A. D. Little, Inc., Cambridge, Mass., Sept, 1975. ADL NO. 76072-80. C. C. Chou, W. S.Smith, H. Vera Ruiz, K. Moe, G. Crescentini, M. J. Molina, and F. S. Rowland, J. Phys. Chem., 81, 286 (1977). R. K. M. Jayanty, R. Simonaitis, and J. Heicklen, J. Phofochem., 4, 203 (1975). W. S. Smith, C. C. Chou, and F. S. Rowland, unpublished results. J. N. Pitts, H. L. Sandoval, and R. Atkinson, Chem. Phys. Lett., 29, 31 (1974). The stratospheric data for CClg are measurements for samples taken in September, 1973, by L. E. Heidt and D. H. Ehhalt, and analyzed by F. S.C. Lee, M. J. Molina, and F. S.Rowland. A. L. Schmeltekopf, private communication. J. A. Davidson, C. M. Sadowski, H. I.Schiff, G. E. Streit, C. J. Howard, D. A. Jennings, and A. L. Schmeltekopf, J. Chem. Phys., 64, 57 (1976). G. E. Streit, C. J. Howard, and A. L. Schdeltekopf, J. Chem. Pbys., 65, 4761 (1976). The independent measurement of 2.8 X lo-'' cm3 molecule-' s-' has been since renormalized to the new value for O('D) 4- N20 as 1.4 X 10" cm3 molecule-' s-l and Is quoted in Table IV as 1.8 X 10-l~cm3 molecule-' s-l. We have retained the okl value as illustrative of the utility of Figure 4. F. S. Rowland and M. J. Molina, J. Phys. Chem., 80, 2049 (1976). J. P. Jesson, P. Meakin, and L. C. Glasgow, Afm. Envifon., 11, 499 (1977). P. W. Krey, R. J. Lagomarsino, M. Schonberg, and J. J. Frey, H. A.S.L.-298, US. Energy Research and Development Administratlon, 1976, pp 1-83; P. W. Krey and R. J. Lagomarsino, H.A.S.L.-294, ERDA, 1975, pp 97-123. D. R. Crohn, R. A. Rasmussen, and E. Robinson, "Measurement of Tropospheric Halocarbons by Gas Chromatography-Mass Spectrometry", Interim report submitted to EnvironmentalProtection
The Journal of Physical Chemistry, Vol. 82, No. 1, 1978 7
BrONO, and Its Stratospheric Significance
(24) H. B. Singh, L. J. Salas, J. Shiegeshi, and L. Cavanagh, "Atmospheric Fates of Halogenated Compounds". First year summary report to Environmental ProtectionAgency, Stanford Research Institute, Menlo Park, Calif., 1976.
Agency, Washington State University, 1976; R. Rasmussen, data presented to EPA meeting, Research Triangle Park, N.C., Feb, 1977. (22) V. Ramanathan, Science, 190, 50 (1975). (23) C. E. Junge, Z . Naturforsch. A , 31, 482 (1976).
Bromine Nitrate and I t s Stratospheric Significance John E. Spencer and F. S. Rowland" Department of Chemistry, University of California, Irvine, California 927 17 (Received May 19, 1977) Publication costs assisted by the U.S. Department of Energy
Bromine nitrate, BrON02, has been synthesized and purified, and its ultraviolet and infrared spectra have been measured. The stratospheric photolysis of BrONOz is about twenty times more rapid than that of the analogous ClONO2. However, since HBr is not as important a sink in the Br0,-catalytic chain for removal of stratospheric ozone as HC1 is for the C10, chain, as much an 10-20% of the Br may be present as BrON02. Measurements of the rate of its formation from BrO + NOz + M are necessary for accurate estimate of the stratospheric importance of BrON02.
Introduction The discovery that chlorine atoms released in the stratosphere by solar UV photolysis of chlorofluorocarbon compounds can remove ozone by the C10, catalytic chain's2 (reactions 1 and 2) has focussed attention on the strato-
c1+ 0 , +0
c10
-+
-+
c10 + 0 ,
c1+ 0 ,
spheric reactions of other halogenated species as weL3s4 The reactions of stratospheric bromine atoms have been discussed by Crutzen? Watson: and by Wofsy et al.7 and the BrO, catalytic chain of (3) and (4) is also very effective Br
+ 0,
-+
BrO
+ 0,
BrO t 0 -+ Br t 0,
(3) (4)
in removing ozone. Indeed, Br atoms are less easily removed from the BrO, chain than are C1 atoms from the C10, chain because C1 can be diverted from the chain through formation of HC1 by reaction with CH4, H2, or HOz while Br does not react with CH4 or H2. Since the abstraction reactions from these major sources are endothermic, Br is converted to HBr only by reaction with less abundant stratospheric species such as H02,and HBr plays a less important role for the BrO, chain than HC1 in the C10, chain. Consequently, the injection of large quantities of bromine atoms into the stratosphere would also be expected to result in significant depletion of the natural levels of stratospheric ozone. While methyl bromide is used as a soil fumigant, its C-H bonds permit rapid reaction with tropospheric OH radicals, and only a small fraction of the Br atoms from its decomposition is released in the ~tratosphere.~ On the other hand, some bromofluorocarbon compounds (e.g., CBrF,; CBrF2CBrF2)are extensively used as flame retardants, and will frequently be ultimately released to the atmosphere. Moreover, these compounds are apparently not subject to any tropospheric removal processes, and will closely parallel the atmospheric chemistry of chlorine molecules such as CC13F and CC12F2.s Consequently, all (or a major fraction) of the bromine atoms from these compounds can be expected to be released eventually in the stratosphere. Bromine has recently been detected in the stratosphere 0022-3654/78/2082-0007$0 1 .OO/O
(as Br-) by aircraft- and balloon-borne experiments analogous to those used for HC1 detection for several year~.~JO Chlorine nitrate, C10N02, can be formed in the stratosphere by the reaction of C10 with NO2, and its photolytic lifetime in the lower stratosphere (e30km) is long enough that detectable amounts are expected a t these a1tit~des.ll-l~Since bromine nitrate, BrON02, can be expected from the analogous reaction 5 , we have invesBrO
+ NO, + M
+
BrONO,
+M
(5)
tigated its ultraviolet and visible absorption characteristics, and have used these for estimates of its stratospheric lifetimes at various altitudes. In general bromine nitrate has about a 20-fold greater weighted absorption coefficient than chlorine nitrate in the lower stratosphere, and therefore should have a 20-fold shorter lifetime than its chlorine counterpart. However, the fraction of bromine as BrO should usually be larger than that of chlorine as C10 so that BrON02may play a fractional role in the BrO, chain not much less than that of CIONOz in the C10, chain. The literature on BrON02as a pure compound is sparse. Its synthesis and some of its reactions were first reported by Schmeisser and Taglinger in 1961.15J6 More recently Schack and Christie have studied some of its reactions in the condensed phase,17and Schack also has reported an incomplete infrared spectrum.ls
Experimental Section Bromine nitrate was synthesized by the reaction of BrCl with C10N02.15 Our preparation of chlorine nitrate has been discussed earlier.11J4 Typically, several milliliters of CION02 were mixed with an equal amount of BrCl and the mixture was allowed to remain at -50 to -70 "C for as long as 1week. The unreacted starting materials were then distilled off a t -78 "C, leaving behind a small amount of BrON02. The unreacted mixture was then allowed to continue to react and the process repeated. Sufficient quantities of BrONOz for subsequent experiments were produced by collecting the yields from several batches. Yields were always low and appeared to be little affected by varying reaction conditions. @ 1978 American Chemical Society
