Environ. Sci. Technol. 1994, 28, 1198-1200
The Stratospheric Fate of CF30H Tlmothy J. Walllngton' and Wllllarn F. Schneider' Ford Research Laboratory, Ford Motor Company, P.O. Box 2053, Mail Drop 3083/SRL, Dearborn, Michigan 48121-2053
Introduction By international agreement, chlorofluorocarbons (CFCs) are being replaced with environmentally acceptable alternatives. Hydrofluorocarbons (HFCs) are one class of CFC substitute. For example, HFC-134a is a replacement for CFC-12 in automotive air conditioning systems. Prior to large-scale industrial use, the environmental impact of the atmospheric release of HFCs requires consideration. CF3 radicals are produced during the oxidation of HFC23 ( l ) ,HFC-125 (2-4), and HFC-134a (5, 6 ) . In the atmosphere, CF3 radicals react with 0 2 to give CF302 radicals which, in turn, react rapidly with NO to form CF30 radicals (7, 8 ) :
-
+ 0, + M CF302+ NO
CF,
+M
(1)
CF30 + NO,
(2)
CF30,
The atmospheric fate of CF30 radicals is uncertain and the subject of a significant current research effort. It has been shown that CF30 radicals react with NO (9-12), organic compounds (10,13-16), and H20 (17). There has been speculation that CF30, (CF30 and CF302) radicals could participate in catalytic ozone destruction cycles in the stratosphere. As discussed recently by KOet al. (18), many potentially catalytic ozone destruction cycles involving CF30, radicals are possible. Two examples appear below:
Scheme 1
+ 0, CF30 + 0,
CF,O,
-
-
CF30 + 20,
(3)
+ 0,
(4)
CF,O,
and
Scheme 2
-
+ 0, CF30 + C10 + M CF300C1+ hv CF30,
CF30 + 20, CF,OOCl CF,O,
c1+0,-c10+0,
+M
+ C1
Envlron. Sci. Technol., Vol. 28, No. 6, 1994
CF30 + NO CF30 + CH,
-
FNO + COF,
(8)
CF30H + CH,
(9)
However, as previously discussed (181, uncertainties remain as to the effectiveness of reactions 8 and 9 in removing CF30 radicals in the stratosphere. First, although the rate and products of reaction 8 have been measured a t ambient temperature, no kinetic or mechanistic data are available for the lower temperatures applicable to the stratosphere. Hence, while it is clear that reaction 8 is rapid and leads to the destruction of the CF3 group at ambient temperature, it is possible that this is not the case at lower temperatures. Second, the stratospheric fate of CF30H is uncertain. As pointed out by KOet al. (18),if CF30radicals can be regenerated from CF3OH by some mechanism (for example, by OH radical attack),then reaction 9 will not constitute a sink for CF30, radicals. Instead, CF30H will act as a temporary reservoir for CF30, radicals. To assess fully the potential efficiency of catalytic ozone destruction cycles involving CF30, radicals and thereby to place discussions of the environmental impact of the atmospheric release of HFCs on a sound technical basis require further information regarding reaction 8 and the stratospheric fate of CF3OH. Work performed recently in our laboratory ( 17,24)bears upon the likely fate of CF30H in the stratosphere. In particular, the results from our previous work can be used to deduce that CF3OH will reform CF30 radicals in the stratosphere at a negligibly slow rate. We comment here upon the likely fate of CF3OH.
+ CF&H
-
+ HzO
(3)
Reaction: OH
(5)
The reaction of CF3OH with OH (reaction 10) has not been studied directly in our laboratory. However, an upper limit for the rate of the reverse reaction (-10) has been determined (17):
(6)
(7)
The ability of cycles such as Schemes 1 and 2 to destroy stratospheric ozone depends upon the balance between the efficiency of chain-propagation reactions and chaintermination reactions. The reaction of CF30 and CF302 radicals with ozone are important chain-propagation reactions in these cycles. Biggs et al. (29)have presented indirect experimental evidence suggesting that the reaction of CF30radicals with ozone proceeds with a rate constant Iz4 of 1x 10-12 cm3 molecule-' s-l at ambient temperature. In contrast, more direct experimental studies of reaction 4 have detected no (20-22) or much slower (23)reactions between CF30 radicals and 03. Upper limits for k4 reported by these workers lie in the range 3-6 X lo-', cm3 molecule-1 s-1 a t ambient temperature. In addition, an 1198
upper limit O f Iz3 < 5 X cm3molecule-'s-l was derived for the reaction of CF302radicals with ozone (20,22,23). At present, two chain-termination reactions have been identified as being important in the stratosphere. These are the reactions of CF30 radicals with NO and CH,:
-
CF,OH f OH CF,O
+ H,O
CFdl
+ H,O
(10)
CF30H + OH
(-10)
CF30
In addition, estimates of the enthalpy change in reaction -10 have been provided (24). From the calculated thermodynamic information, we can estimate the minimum likely equilibrium constant for reaction -10 at stratospheric temperatures. Similarly, from an estimate of the activation barrier, we can obtain the maximum likely rate constant of reaction -10 at stratospheric temperatures. By combining these two pieces of information, an upper limit for the rate of reaction 10 can be obtained. We consider first the equilibrium constant. Calculations performed in our laboratory and by Dixon and Fernandez 0013-936X/94/0928-1198$04.50/0
0 1994 American Chemical Society
(25) have shown that the strength of the CF30-H bond is approximately the same as that of water, or 119.5 kcal mol-’. The enthalpy change A H O - 1 0 is calculated to be -0.5 f 2.0 kcal mol-1 at 298.15 K (24) and to a very good approximation will be invariant down to a typical stratospheric temperature of 220 K. The entropy change ASo-lo is expected to be slightly negative due to the conversion of one rotational degree of freedom into a vibrational degree of freedom. This entropy change can be determined to good accuracy from the calculated MP2/6-31G**molecular structures, rotational constants, and vibrational spectra in combination with standard statistical mechanical formulas (26). At 220 K, TASo-lois calculated to be -0.6 f 0.2 kcal mol-l. From this we estimate AGO-10 to be 0.1 f 2.0 kcal mol-l, also a t 220 K. AGO-10 can be used to determine the equilibrium constant from K , = exp(-AGo/ RT). The bounds on Kp,-1oare thus calculated to be 10-(o.lo * 2.00) or 0.008 < K,,-lo < 80. We next consider the rate of reaction -10. The rate constant k-10 has been previously determined to have an upper limit of 4 X cm3 molecule-l s-l a t room temperature ( 1 7), but its temperature dependence has not been studied. The activation energy for reaction -10 can be estimated by considering the closely related reaction 9, for which E , has been determined to be 2.8 kcal/mol (27). Reaction 9 is 15 kcal/mol more exothermic than reaction -10, so that the activation energy of reaction -10 is almost certainly greater than that of 9. We conservatively estimate the activation energy of reaction -10 to be 3 kcal/mol, which when combined with the Arrhenius rate expression yields an upper limit for k l o a t 220 K of 7 X 10-17 cm3 molecule-1 s-l. The available data do not permit an accurate estimate of klo. However, an upper limit for the rate constant can be obtained by combining the upper limit for k-10 with the lower limit for K,,-to through the relationship K,,-lo = k-lo/klo. From this relationship, the upper limit for the rate constant k10 is 9 X 10-lb cm3 molecule-l s-l. It is important to emphasize that this estimate is an upper bound and that the likely value is considerably less. As a way of verifying our calculations, an estimate for klo can also be derived by considering the available kinetic database for the reaction of OH radicals with CH30H:
CH30H
+ OH
-+
CH30
+ H,O
(11)
CF30 (e.g., Schemes 1 or 2) must compete with the reformation of CF30H via reaction 9 or similar reactions with other hydrogen donors. For instance, assuming a rate constant of 3 X 10-15 cm3 molecule-l s-1 (27) and a - ~the methane concentration of 2 X lo1, molecule ~ m (291, lifetime of CF30 radicals with respect to reaction with CH4 is 160 s. Thus, the lifetime of CF30 radicals in the stratosphere is short. We conclude that CF30H is an effective sink for CF30, radicals in the stratosphere.
Other Possible Loss Mechanisms for CF30H Having established that the OH radical attack is not a significant loss of CF30H, the obvious question is “What happens to CF30H in the stratosphere?”. Any loss process for CF30H in the stratosphere must compete with the physical transport into the troposphere, which takes typically 2-3 years (18). Once in the troposphere, CF30H is expected to be incorporated into cloudwater-rainseawater, where hydrolysis will proceed to give HF and COz. With regard to potential loss processes for CF30H within the stratosphere, possibilities include (i) reaction with reactive free radicals such as Cl,O(3P),0(1D), or C10 radicals; (ii) photolysis; (iii) homogeneous decomposition into COFz and HF; and (iv) loss on stratospheric aerosol particles. With regard to the first possibility, many reactive freeradical species are found in the stratosphere that could potentially react with CF3OH. However, only three are present a t sufficiently high concentrations to be considered as potentially important scavengers of CF30H. They include O(3P) atoms, C1 atoms, and C10 radicals. Using the available thermodynamic data for these species together with the CF3O-H bond strength of 119 kcal mol-l, hydrogen abstraction by these species to regenerate CF30 is calculated to be endothermic by 17, 16, and 24 kcal mol-l, respectively. Consequently, the contributions of these reactions to the removal of CF3OH will be insignifican t. With regard to the second possibility, alcohols only absorb UV light of very short wavelengths. For example, the onset of significant absorption in methanol occurs at approximately 160 nm. Substitution of hydrogen by fluorine is not expected to lead to any significant red shift in the spectrum. Thus, photolysis of CF3OH can be excluded as a significant decomposition mechanism in the stratosphere. With regard to the third possibility, the activation barrier for the unimolecular decomposition reaction 12 has been calculated to be 47.9 kcal mol-l (30):
Lorenz et al. have reported k l l = 2.1 X 10-l1 exp(1640/T) cm3 molecule-l s-1 (28). At 220 K, this expression yields kll = 1.2 X 10-14 cm3 molecule-l s-1, The 0 - H bond in CF30H is 15 kcal mol-’ stronger than in CH30H. Hence, klo should be substantially less than k11, giving k10 < 1.2 CF30H HF COF, (12) X 10-14 cm3 molecule-l s-l a t 220 K, consistent with that derived above. More recent work suggests that this barrier may be The upper limit for klo can be used to estimate the overestimated (241, although it is still likely to be quite lifetime of CF30H with respect to reaction 10 in the high. In laboratory smog chamber experiments a t ambient stratosphere. The 24-h averaged OH radical concentration temperature, CF3OH is observed to decompose to give in the stratosphere is approximately 1X lo6molecule ~ m - ~ COFz and HF with a lifetime of the order of minutes to (29). Combining this concentration with the upper limit hours (14, 15). A substantial fraction, if not all, of this for k1o yields a minimum lifetime for CF3OH with respect decomposition has been ascribed to heterogeneous proto the reaction with OH of 3.6 years. The residence time cesses however (15). The contribution of homogeneous of air in the stratosphere is typically 2-3 years (18). Thus, unimolecular decomposition of CF3OH is still uncertain. even assuming the maximum possible rate for reaction 10, Finally, with regard to the fourth possibility, we must during its stratospheric residence time a typical CF30H consider interaction of CF3OH with aerosols. By analogy molecule reacts via reaction 10 statistically about once a t to the observed decomposition of CF30H in glass smog most. Any catalytic ozone destruction reactions involving chambers, it seems reasonable to suppose that if CF3OH +
+
Environ. Sci. Technol., Vol. 28, No. 6, 1994
1100
interacts with stratospheric aerosol particles, it will primarily result in the catalyzed decomposition of CF3OH into CFzO and HF. Whether homogeneous or heterogeneous, this decomposition results in the removal of a CF3 group from the stratosphere and precludes its further involvement in CF30, chemistry. Based upon the above arguments, we conclude that CF30H formed in the stratosphere via reaction of CF30 radicals with H-containing species will either be transported to the troposphere or decompose unimolecularly or on aerosols. Regeneration of CF30 radicals occurs only very slowly via reaction with OH radicals. Conclusion
CF30 radicals in the stratosphere react fairly rapidly with H-donors (e.g., HOz radicals or CH4) to generate CF30H. Regeneration of CF30 radicals is possible only via reaction with OH radicals. We have shown that this regeneration process is very slow and, thus, that CF30H is an effective sink for CF@, radicals. In model evaluations of the ozone depletion potentials of HFCs containing CF3 groups (e.g.,HFC-l34a),any reactions forming CF30H should be treated as an effectively permanent loss of CF30, radicals from the system. Acknowledgments
We thank Malcolm KO (Atmospheric and Environmental Research, Inc.) and Steve Japar (Ford) for discussions regarding catalytic cycles involving CF30radicals; Ole John Nielsen and Jens Sehested (Riser National Laboratory, Denmark) for ongoing discussions regarding CF30, chemistry; and Howard Sidebottom (University College,Dublin) for discussions regarding klo. Literature Cited Nielsen, 0. J.; Ellermann, T.; Sehested, J.; Bartkiewicz, E.; T' J'; Hurley, M' Int' J ' Kinet' lgg2f 24, 1009-1021. Edney, E. 0.;Driscoll, D. J. Znt. J . Chem. Kinet. 1992,24, 1067-1081. Tuazon, E. C.; Atkinson, R. J. Atmos. Chem. 1993,17,179199.
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