Atmospheric Chemistry -- A Mysterious Isotope Effect In A Well-Known AtmosphericPollutant Discovered

July 23, 1998

The rate of chemical reactions as a rule depends on the mass of the atoms or molecules involved, and consequently it changes slightly for isotopically substituted but otherwise identical molecules. Therefore, in the case of oxygen which has three stable isotopes (16O, 17O, 18O), rate variations due to 17O are usually half as big as the accompanying variations due to 18O. In recent years, however, a new class of isotope fractionation effects has been discovered in which this simple rule of mass dependency is violated. The prime example of this so-called mass-independent fractionation is ozone formation, in which 17O and 18O are enriched almost equally (rather than in a 1:2 ratio). In the July 24,vol.281, 1998 issue of "Science", T. Röckmann, C.A.M. Brenninkmeijer, G. Saueressig, P. Bergamaschi, J.N. Crowley, H. Fischer and P.J. Crutzen, all at the Atmospheric Chemistry Division of the Max Planck Institute for Chemistry, report the presence of mass-independent fractionation in another reaction of primary importance to atmospheric chemistry, namely the reaction of carbon monoxide (CO) with the hydroxyl radical (OH).

The discovery was made in an effort to explain the anomalous excess of the 17O isotope in atmospheric CO, which has been observed at various air monitoring stations like Spitsbergen and the Canary Islands. The seasonal variation of the excess 17O signal gave the important indication that the origin of the effect might not be associated with one of the various CO sources, but rather with its removal mechanism, reaction with OH. The observed anticorrelation between atmospheric CO concentrations and the 17O isotope excess over the year could be explained if the fractionation associated with the reaction CO + OH ( CO2 + H is mass independent. Therefore, this dominant atmospheric CO removal reaction was investigated in laboratory experiments, and the results unambiguously confirmed mass-independent fractionation: During CO oxidization by OH under various conditions, the remaining CO portion gradually obtained an excess of the 17O isotope. The extent of the mass-independent fractionation signal measured in the laboratory indeed is of the right magnitude to explain the atmospheric observations.

The recently discovered isotope effect in CO has consequences of practical and more fundamental nature. The self-cleansing capacity of the atmosphere is based largely on the availability of OH radicals, which are responsible for the removal of CO, a major pollutant, and also of methane (CH4), a greenhouse gas. Yet, OH belongs to the most elusive of all atmospheric species with a lifetime of only one second. Nevertheless, the mass-independent fractionation in the reaction of CO with OH induces a clear isotope signal for the degree to which CO has been exposed to the removal reaction by OH. Thus, better insight into the atmospheric budget of CO, and a better understanding of the OH distribution in the atmosphere may be obtained.

Besides these useful applications of the discovered isotope effect, fundamental questions concerning reaction mechanisms in the gas phase are raised. At present it is impossible to explain the cause of the mass-independent isotope fractionation in the reaction of CO with OH, even though this reaction happens to be one of the most intensively studied gas phase reactions, of importance in the atmosphere as well as in combustion processes. Thus, further investigation of this atmospheric reaction may well contribute to finally unravel the mysterious causes of mass-independent fractionation processes.


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