1
8122 J. Phys. Chem., Vol. 100, No. 46, 1996
Wallington et al.
the ratio k5/k4, absolute values for the rate constants could also
References and Notes
be derived (although with much less certainty). Bednarek et
(
(
1) Molina, M. J.; Rowland, F. S. Nature 1974, 249, 810.
2) Farman, J. D.; Gardiner, B. G.; Shanklin, J. D. Nature 1985, 315,
21
-15
3
-1 -1
al. quote values of k4 ) 2.7 × 10
cm molecule
s
and
4
-1
k5 ) 1.8 × 10 s in 38 Torr of O2 at 296 K. The ratio of
207.
(3) Solomon, S. Nature 1990, 347, 6291 and references therein.
18
-3
these values (k5/k4 ) 6.7 × 10 molecule cm ) is in excellent
agreement with the results from the present work shown in
Figure 2. While there have been no other direct sudies of k4,
(
4) McCulloch, A. EnViron. Monitor. Assess. 1994, 31, 167.
(5) Montzka, S. A.; Myers, R. C.; Butler, J. H.; Elkins, J. W.; Lock,
L. T.; Clarke, A. D. Geophys. Res. Lett. 1996, 23, 169.
(6) Ravishankara, A. R.; Turnipseed, A. A.; Jensen, N. R.; Barone, S.;
Mills, M.; Howard, C. J.; Solomon, S. Science 1994, 263, 71.
3
6
Wu and Carr derived a rate constant of approximately 1 ×
-16
1
0
for the analogous reaction of CFCl2CH2O with O2 at 298
(7) Wallington, T. J.; Schneider, W. F.; Sehested, J.; Nielsen, O. J. J.
K. It appears that reactions of fluorinated ethoxy radicals with
O2 proceed with rate constants which are 1-2 orders of
magnitude slower than the C2H5O + O2 reaction, which has a
Chem. Soc., Faraday Discuss. 1996, 100, 55.
(8) Pinnock, S.; Shine, K. P.; Smyth, T. J.; Hurley, M. D.; Wallington,
T. J. J. Geophys. Res. 1995, 100, 23227.
(9) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.;
-
14
at 298 K.9
rate constant of 1 × 10
Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina,
M. J. JPL Publication 94-26, 1994.
4
1
In the flash photolysis study of Maricq and Szente,
a
(10) Wallington, T. J.; Nielsen, O. J. Chem. Phys. Lett. 1991, 187, 33.
(11) Bhatnagar, A.; Carr, R. W. Chem. Phys. Lett. 1995, 238, 9.
(12) Peeters, J.; Pultau, V. Proceedings of CEC/EUROTRAC Workshop
transient absorption due to RO radicals was tentatively identi-
fied, and its time dependence was used to derive a value for k5.
For reasons which are unknown, the temperature dependence
of k5 derived by Maricq and Szente is inconsistent with the
results from the chamber studies.
on “Chemical Mechanisms Describing Tropospheric Processes”; Peeters,
J., Ed., Sept 1992.
(13) Wallington, T. J.; Schneider, W. F.; Worsnop, D. R.; Nielsen, O.
J.; Sehested, J.; DeBruyn, W. J.; Shorter, J. A. EnViron. Sci. Technol. 1994,
Finally, the thermochemistry of the decomposition reaction
2
8, 320A.
42
5
can be related to other alkoxy radicals. Atkinson and Carter
(14) Alternative Fluorocarbon Environmental Acceptability Study, W.
M. O. Global Ozone Research and Monitoring Project, Report 20; Scientific
Assessment of Stratospheric Ozone, Vol. 2, 1989.
have developed a formulation for the expected behavior of
organic alkoxy radicals in air at ambient temperature, relating
the tendency to decompose to the difference in enthalpy between
the O2 reaction and the decomposition reaction. According to
(15) Ingle, L. M. Proc. West Virginia Acad. Sci. 1968, 40, 1.
(16) Tromp, T. K.; Ko, M. K. W.; Rodriguez, J. M.; Sze, N. D. Nature
995, 376, 327.
1
(
17) Visscher, P. T.; Culbertson, C. W.; Oremland, R. S. Nature 1994,
43
the latest calculations of Schneider, the decomposition reaction
is approximately thermoneutral, while the O2 reaction is
3
69, 729.
(18) Wallington, T. J.; Hurley, M. D.; Ball, J. C.; Kaiser, E. W. EnViron.
Sci. Technol. 1992, 26, 1318.
-
1
exothermic by approximately 35-40 kcal mol . Thus, the
alkoxy radical from HFC-134a would be expected to exhibit
competition between O2 reaction and decomposition at room
temperature, as observed in all studies.
(19) Edney, E. O.; Driscoll, D. J. Int. J. Chem. Kinet. 1992, 24, 1067.
(20) Tuazon, E. C.; Atkinson, R. J. Atmos. Chem. 1993, 16, 301.
(21) Bednarek, G.; Breil, M.; Hoffmann, A.; Kohlmann, J. P.; M o¨ rs,
V.; Zellner, R. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 528.
22) Rattigan, O. V.; Rowley, D. M.; Wild, O.; Jones, R. L.; Cox, R.
A. J. Chem. Soc., Faraday Trans. 1994, 90, 1819.
23) Meller, R.; Boglu, D.; Moortgat, G. K. EUR 16171 EN, Becker,
(
Implications for Atmospheric Chemistry. All previous
assessments of the CF3C(O)F, and hence CF3COOH, yields in
the atmospheric degradation of HFC-134a are based upon the
assumption that reaction 3 produces CF3CFHO radicals which
are rapidly thermalized. In contrast, our experimental data show
that a substantial fraction of the CF3CFHO radicals formed in
reaction 3 undergo prompt decomposition and are not thermal-
ized. The fraction of CF3CFHO radicals which are thermalized
in the atmosphere, R, is 0.40 ( 0.15. Current assessments of
CF3COOH formation are too high by a factor of 1/R ) 1.8-
(
K. H., Ed., Tropospheric Oxidation Mechanisms (Joint EC/EuroTrac/GDCU
Workshop, LACTOZ-HALIPP, Leipzig, Sept 20-22, 1994).
(24) Dixon, D. A.; Fernandez, R. Proceedings of the STEP-HALOC-
SIDE/AFEAS Workshop; University College Dublin: Ireland, March 1993;
p 189.
(25) Lightfoot, P. D.; Cox, R. A.; Crowley, J. N.; Destriau, M.; Hayman,
G. D.; Jenkin, M. E.; Moortgat, G. K.; Zabel, F. Atmos. EnViron. 1992,
2
6A, 1806.
(26) Wallington, T. J.; Gierczak, C. A.; Ball, J. C.; Japar, S. M. Int. J.
Chem. Kinet. 1989, 21, 1077.
(27) Shetter, R. E.; Davidson, J. A.; Cantrell, C. A.; Calvert, J. G. ReV
Sci. Instrum. 1987, 58, 1427.
.0. Kanakidou et al.44 have recently reported the results of a
4
three-dimensional global modeling of HFC-134a chemistry and
find the CF3COOH yield from the atmospheric oxidation of
HFC-134a to be 29-36%. Incorporation of the results from
the present work revises this range downward to 7-20%. The
current consensus in the scientific community is that it is
unlikely that formation of CF3COOH is a significant environ-
(
28) Sehested, J.; Wallington, T. J. EnViron. Sci. Technol. 1993, 27,
146.
(29) Troe, J. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 161.
30) Gilbert, R. G.; Luther, K.; Troe, J. Ber. Bunsen-Ges. Phys. Chem.
983, 87, 169.
31) Wallington, T. J.; Orlando, J. J.; Tyndall, G. S. J. Phys. Chem.
(
1
(
1995, 99, 9437.
18,44
(32) Orlando, J. J.; Tyndall, G. S.; Wallington, T. J. J. Phys. Chem.
mental problem.
The results from the present work provide
1
996, 100, 7026.
33) Sehested, J.; Nielsen, O. J. Chem. Phys. Lett. 1993, 206, 369.
(34) Wallington, T. J.; Hurley, M. D. Chem. Phys. Lett. 1992, 189, 437.
35) Tyndall, G. S.; Orlando, J. J.; Kegley-Owen, C. S. J. Chem. Soc.,
Faraday Trans. 1 1995, 91, 3055.
further justification for this view.
(
To the best of our knowledge, this is the first instance where
it has been demonstrated that chemical activation of an alkoxy
radical plays an important role in its atmospheric fate. The
atmospheric oxidation of all organic compounds proceeds via
the formation of peroxy radicals. Chemical activation of alkoxy
radicals formed in reactions of peroxy radicals with NO may
be an important consideration in the degradation mechanism
of other organic compounds where two competing channels exist
for the alkoxy radical (e.g., for alkenes).
(
(
36) Wu, F. X.; Carr, R. W. J. Phys. Chem. 1996, 100, 9352.
(37) Møgelberg, T. E.; Nielsen, O. J.; Sehested, J.; Wallington, T. J.;
Hurley, M. D.; Schneider, W. F. Chem. Phys. Lett. 1994, 225, 375.
(
38) Barker, J. R., private communication, 1996.
(39) Br u¨ hlmann, U.; Dubs, M.; Huber, J. R. J. Chem. Phys. 1987, 86,
1
249.
(40) Lahmani, F.; Lardeux, C.; Solgadi, D. Chem. Phys. Lett. 1983, 102,
23.
5
(
(
(
41) Maricq, M. M.; Szente, J. J. J. Phys. Chem. 1992, 96, 10864.
42) Atkinson, R.; Carter, W. P. L. J. Atmos. Chem. 1991, 13, 195.
43) Schneider, W. F., private communication, 1996.
Acknowledgment. We thank Steve Japar and Bill Schneider
(
both Ford Motor Co., USA), John Barker (University of
(44) Kanakidou, M.; Dentener, F. J.; Crutzen, P. J. J. Geophys. Res.
995, 100, 18781.
1
Michigan), and Susan Solomon and Brian Ridley (both NCAR)
for helpful discussions and Reinhard Zellner (Universit a¨ t Essen)
for a preprint of ref 21.
JP9624764