Carbon Monoxide in Lower Atmosphere Reactions SIR: Numerous laboratory efforts to identify a homogeneous gas-phase chemical mechanism for removal of CO from the atmosphere-consistent with likely atmospheric concentrations-have been frustrated by the extremely slow reactions involved. The recent apparent exception is the work of Dimitriades and Whisman [ES&T, 5, 219 (1971)l who derived a lifetime of 0.2-0.3 year, consistent with the carbon dating lifetime of Weinstock (1969). The oxidizing agent operative in the Dimitriades and Whisman study was not identified (0and 0 3 were specifically excluded as possibilities), but it was suggested that O(lD), 02(lZg), or OZ('Ag) might be possibilities. However, the work of Fisher and McCarty (1966) specifically excludes the molecular oxygen singlet states as far too unreactive with CO, and the quenching rate constant for O(lD) by Nz ( = 8 X lo-" ccjsec) (Zipf, 1969) is far too rapid to permit a substantial concentration of O(lD) to accumulate. A reasonable upper limit based on ozone photolysis as an O(lD) source can be estimated from the work of Kummler et al. (1969, 1970) to be ljcc which precludes the possibility of O(lD) involvement in a CO sink even if it reacts with CO on every collision. Since the OH radical concentration is dependent on the O(3P) or O(lD) concentrations, only in a Los Angeles photochemical environment can OH substantially diminish the CO concentration, and turbulent transport effectively prevents even that possibility by diluting the reactants much faster (days to weeks) than reaction can occur. As illustrated in Table I, the rate constants and the upper limits for all known oxidizing agents preclude homogeneous gas-phase reaction on a scale necessary to explain atmospheric CO loss. Moreover, it is highly unlikely in the diverse experiments of Dimitriades and Whisman that even an un-
1. 2. 3. 4. 5.b 6. 7. 8.'
9. 10.
11. a h c d
known species would remain at constant concentration from experiment to experiment and throughout any given experiment thereby permitting an apparent first-order reaction to be observed. Even if an excited state of NOz or a hydrocarbon (of which we know little) were significant, it is unlikely that the excited state could remain constant throughout the consumption of the ground-state molecule. There is a much more plausible explanation for the results of Dimitriades. It is more consistent with first-order kinetics to assume that the reaction partner is adsorbed on the walls of the reaction vessel where a heterogeneous catalytic reaction occurs. As noted in the Federal Air Quality Criteria document for CO, there is at least one example of heterogeneous CO conversion rapid enough on some surfaces to explain the atmospheric CO conversion. While the global distribution of such surfaces is unknown, and hence extrapolations of global lifetimes from laboratory measurements are unwise, it is probable that the rate is rapid enough to explain laboratory experiments of the magnitude reported by Dimitriades and Whisman. This can be verified, of course, by the standard technique normally applied to such slow reactions-varying the surface to volume ratio and the surface type and determining the effect on the apparent first-order rate constant. Without such evidence, it is equally probable that the agreement with Weinstock is largely fortuitous. If interpreted in the same fashion as the Dimitriades and Whisman data, for example, the work of Fisher and McCarty (1966) in a much different reaction vessel would result in a minimum lifetime of over 30 years. In conclusion, it is highly unlikely that homogeneous gasphase kinetics at ground level can explain the CO sink anomaly. It is possible that heterogeneous chemistry is operative;
Table I. Reactions Consuming COG Rate constant, concentrations Reactions in rnolecules/cc CO+OH+COz+H 1.1 X 10-12e-520/T CO O(lD) M COz M 1 x 106 2 x 10' 1 x 1020 1 x 101~/[N0dZBdl 1 x 1010 7 x 1013 7 x 10-1*2 2 x 105 >3 x 10'0 >2 x 10s
Rate constants are taken from Schofield (1967) and typical concentrations are taken from Leighton (1961) and Bortner et al. (1971). Estimate. Extrapolation from Strickland-Constable (1938; 1953), Bawn (1936), Smith and Mooi (1955), and Krause (1961). This rate constants for the surface reaction is first order in CO, zero order in NzO.
1140 Environmental Science & Technology
but it is more probable that a biological sink is required to convert CO to C o n , such as the fungi suggested by Inman (1971). Literature Cited Bawn, C., Trans. Faraday SOC.,31,461 (1936). Bortner, M., Kummler, R., Jaffe, L. “CO Sink Anomaly,” (in press) 1971. SCI.TECHNOL..5. Dimitriades. B.. Whisman. M.. ENVIRON. 219 (1971j. ’ Fisher E., McCarty, M., J. Chem. Phys., 45,781 (1966). Inman, R.. Chem. Ena. News. May 10.1971. Krause, A:, Bull. Acaa. Pol. Sci. Ser. Sci. Chem., 9, 5 (1961). Kummler, R., Bortner, M., Ann. N.Y. Acad. Sci., 171, 273 (1970). Kummler, R., Bortner, M., Baurer, T., ENVIRON. SCI.TECHNOL.,3,248 (1969). Leighton, P.. “Air Pollution Photochemistry.” Academic Fress, New’York, N.Y., 1961. Madley, D., Strickland-Constable, R., Trans. Faraday SOC., 49, 1312 (1953). ,
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Schofield, P., Planet. Space. Sci., 15, 643 (1967). Smith, R., Mooi, J., J . Phys. Chem., 59, 814 (1955). Strickland-Constable, R., Trans. Faraday SOC.34, 137 (1938). Zipf, E., Can. J. Chem., 47,1863 (1969).
R. H. Kummler‘ Wayne State University Detroit, Mich. 48202 M. H. Bortner
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SIR: We agree with Messrs. Kummler, Bortner, and Jaffee that heterogeneous reactions may have a role in the oxidation of CO in our experimental system, but we disagree with the contention that heterogeneous catalysis provides a “much more plausible explanation” of our results. It is true, as Kummler et al. observe, that certain metal oxides are capable of effecting relatively rapid oxidation of CO at room temperature. However, no such effect was observed in the Pyrex glass reactor of our study, as can be deduced from our data on CO oxidation in the dark. Evidently, solar irradiation is prerequisite for initiating the oxidation process. A reaction-vessel effect, while conceivable, certainly is not evidenced by the comparison of the Fisher-McCarty data and our data. The Fisher-McCarty data did suggest a much slower oxidation of CO by oxygen under irradiation, but this difference could have been caused by difference in the type of radiation used. Fisher and McCarty used radiation in the visible and infrared region, well outside the near ultraviolet region used by us. In fact, in the few tests by Fisher and McCarty where the radiation spectrum extended into the near ultraviolet, they did observe faster oxidation of COand consequently shorter CO lifetime-which they attributed to the presence of nitrogen oxide impurity. The requirement that the unknown oxidizing species be present at constant concentration during each and every one of our tests in order to explain the observed kinetics is in-
Space Sciences Laboratory General Electric Co. Philadelphia, Pa. 19101
L. S. Jaffe George Washington University Washington, D . C . 20006 1
To whom correspondence should be addressed.
deed a difficult one to accept, as Kummler et al. reasonably observe. However, it is doubtful that this difficulty can be removed by assuming that heterogeneous, rather than homogeneous, chemistry is operative. This is because for heterogeneous oxidation of CO to follow the observed first-order kinetics, the oxidant species must have no effect on the reaction rate. This, in turn, requires that the unknown oxidant species be at a sufficiently high partial pressure to displace CO from, and saturate the catalytic sites in, the reactor’s surface. Such a required condition, however, is unlikely to exist because available evidence precludes any of the abundantly present species as being the oxidant. Thus, molecular oxygen at the ground or activated states must be precluded on the basis of the Fisher-McCarty and our studies, and, if the estimates of Kummler et al. concerning the hydroxyl radical role are correct, then hydroxyl is precluded also. In conclusion, the oxidant species must be present at extremely low partial pressures where first-order kinetics would not be likely unless the oxidant is present at constant concentration. B. Dimitriades’ M. Whisman Bartlesville Energy Research Center Bureau of Mines Bartlesville, Okla. 74003 To whom correspondence should be addressed.
Volume 5, Number 11, November 1971 1141
