Unusual oxygen isotope enrichments in atmospheric species: Chemical reaction dynamics and atmospheric implications

Observations of stratospheric O3 and CO2 have revealed unusual oxygen isotopic enrichments that cannot be explained by standard isotope effect theories. Attempts to explain these observations have invoked unusual dynamic (non-statistical) behavior of the reaction intermediates in both the ozone formation reaction and the O(1D)+CO2 reaction, O3* and CO3*, respectively, but, previous to this work, there was no direct experimental information on the dynamics of these reactions and/or intermediates. Here, the dynamics of O3* and CO3* are examined directly in two oxygen isotope exchange reactions O18P 3+O232 →O3*→O 16P3 +O234 O18D 1+C44O2 →CO3*→O 16P3 +C46O2 using crossed-beam scattering experiments. The results provide new information on isotope effects, product branching ratios, reaction mechanisms, and non-statistical (dynamic) behavior in chemical reactions and their influence on the isotopic compositions of atmospheric species.; The O+O2 experiments demonstrate directly the occurrence of non-statistical behavior in the formation and/or dissociation of O3* complexes. The experimental results are compared to predictions of quantum statistical and dynamic (quasi-classical trajectory) theories, showing that neither are adequate for describing the dynamics of O3* and highlighting the need for improvement in theoretical treatments of O3* dynamics. The scattering results provide both an important benchmark to guide these improvements and insight into non-statistical behavior in chemical reactions in general.; In contrast, the O(1D)+CO2 experiments revealed purely statistical behavior and no evidence for any unusual, dynamically-driven isotope effects. In addition, these experiments revealed the existence of a previously unconsidered isotope exchange channel that occurs without electronic quenching: O18D 1+C44O2 →CO3*→O 16D1 +C46O 2 To determine whether standard isotope effects in this "non-quenching" reaction affect CO2 isotopic compositions in the stratosphere and/or laboratory, a kinetics model was developed that included isotope-specific rate coefficients calculated by statistical theories guided by the scattering results. The model results indicate that the CO 2 isotopic composition is indeed sensitive to "non-quenching" isotope effects. The model was also used to explore the sensitivity of the CO 2 isotopic composition to any dynamically-driven isotope effects that might exist in the O(1D)+CO2 reactions, despite the experimental determination of overall statistical behavior, and to conclude that any dynamical effects would result in at most a relatively small change in CO2 isotopic compositions.