The non-mass-dependent isotopic composition of ozone and its photochemical transfer to stratospheric CO2: Unexpected variations in stratospheric CO2 and the unusual role of collisional quenching efficiencies in photochemistry experiments and kinetics modeling
Atypically large and non-mass-dependent kinetic isotope effects (KIEs) in the three-body ozone formation reaction, O(3P) + O2 → O3* + M → O3 + M*, lead to a non-mass-dependent oxygen isotopic composition for O3 observed in both the laboratory and the atmosphere. Theoretical work has suggested that a dynamically-driven, quantum symmetry isotope effect in the lifetime of the excited ozone complex O3* or its collisional stabilization is responsible, although the underlying chemical physics has remained mysterious. Stratospheric CO2 also has a non-mass-dependent oxygen isotopic composition that is thought to be transferred from ozone by photolysis to form O(1D) followed by the O(1D) + CO2 isotope exchange reaction. However, the non-mass-dependent isotopic compositions of CO2 measured either in UV photochemistry experiments or in stratospheric air samples could not easily be explained by isotope effects in ozone formation, leading some to claim that additional anomalous isotope effects must exist in ozone photolysis or in the O(1D) + CO2 isotope exchange reaction. In the research results presented here, I detail several significant advances in the understanding of the non-mass-dependent isotopic composition of ozone and its transfer to stratospheric CO2. I made these advances through new measurements and kinetics modeling of the isotopic composition of O3 and CO2 in photochemistry experiments in which mixtures of O2, CO2, and other bath gases were irradiated with UV light from a mercury lamp as well as comparisons of these results with the latitude, altitude, and seasonal dependence of the isotopic composition of stratospheric CO2.
For application to the non-mass-dependent isotopic composition of stratospheric CO2, I show using a kinetics model that the non-mass-dependent isotope effects in ozone formation alone can quantitatively account for the non-mass-dependent isotopic composition of CO2 in laboratory measurements of UV-irradiated mixtures of O2 and CO2 at atmospheric mixing ratios. I then used the kinetics model to provide a conceptual framework for understanding the significant differences in the non-mass-dependent isotopic composition of CO2 between the laboratory experiments and the stratosphere and between different regions of the stratosphere that I discovered in the atmospheric measurements. Based on model sensitivities to the temperature dependence of the ozone KIEs and mass-dependent isotope effects in ozone photolysis, differences in temperature and in the relative rate of ozone photolysis are found to be the likely sources of the differences in the non-mass-dependent isotopic composition of CO2 between the laboratory and the stratosphere and between different regions of the stratosphere.
Having accounted for the non-mass-dependent isotopic composition of CO2 at an atmospheric O2/CO2 mixing ratio, I performed additional laboratory measurements of the non-mass-dependent isotopic composition of CO2 as a function of the O2/CO2 mixing ratio to explore the dramatic decrease in the non-mass-dependent 17O and 18O enrichments in CO2 as the O2/CO2 mixing ratio decreases found in previous experiments. Kinetics modeling shows that expected changes in the non-mass-dependent KIEs in ozone formation as O2/CO2 decreases cannot explain the O2/CO2 dependence of the non-mass-dependent enrichments in CO2, so a number of different potential chemical mechanisms with non-mass-dependent isotope effects were tested using the model. Of the mechanisms tested, only inclusion of non-thermal rate coefficients for the reactions of 16O(1D), 17O(1D), and 18O(1D) with O2, CO2, and O3 led to any significant decrease in the non-mass-dependent isotopic composition of CO2 as the O2/CO2 mixing ratio is decreased in the model. These non-thermal rate coefficients were derived from non-thermal kinetic energy distributions for 16O(1D), 17O(1D), and 18O(1D) that were calculated using the hard sphere approximation for collisional energy transfer between O(1D) and O2 and between O(1D) and CO2 and the initial energy distributions from O3 photolysis. While the inclusion of the non-thermal rate coefficients in the model produced an O2/CO2 mixing ratio dependence that is still approximately 5 times smaller than the experimentally observed O2/CO2 mixing ratio dependence (i.e. -5 / instead of -50 / from high to low O2/CO2), that the non-thermal reactions involving 16O(1D), 17O(1D), and 18O(1D) could produce non-mass-dependent isotopic compositions even though the corresponding thermal reactions are mass-dependent is novel and, to our knowledge, has not been explored thoroughly in any previous work.
Because of the role that collisional energy plays in the isotope exchange between O(1D) and CO2, I also conducted measurements of the isotopic composition of O3 formed in different bath gases, M, to test how the efficiency of collisional energy transfer between O3* and M in ozone formation affects the non-mass-dependent isotopic composition of the resulting O3. New measurements and kinetics modeling of the isotopic composition of O3 formed in an air-like mixture of O2/N2 show statistically significant differences between the non-mass-dependent isotopic composition of O3 formed in pure O2 and in an air-like mixture of O2 and N2. Using a kinetics model, I explore possible origins for these differences in in these experiments and in experiments involving O3 photolysis or O3 formation in SF6. The combined results comparing the model results with the measurements suggest that mass-dependent KIEs in the O2(1Σ) + O3 reaction can likely be ruled out and that the radical complex mechanism for O3 formation (as opposed to the energy transfer mechanism) may indeed play a role in generating the differences in the isotopic composition of O3 formed in different bath gases.