UC Berkeley

Whether formed in the atmosphere or laboratory, ozone has a highly unusual isotopic composition that deviates from predictions of traditional chemical kinetics models and from usual patterns of ‘mass-dependent’ fractionation. Previous research indicates that the anomalous mass-independent isotopic composition of ozone originates in the energy-transfer mechanism for ozone formation (O + O2 + M → O3* + M → O3 + M#), due to a unique, and not fully understood, difference in the behavior of isotopically symmetric versus asymmetric metastable ozone (O3*).

In this dissertation, I present photochemistry experiments that provide insight into the origin of the mass-independent isotopic composition of ozone (called the 17O anomaly). In these experiments, ozone was formed in different pressures of various bath gases, M, each with unique energy transfer properties. The product ozone was isolated, and its isotopic composition measured by isotope ratio mass spectrometry. The results of these experiments show that there is an exponential dependence on the 17O anomaly on , the average energy transferred per collision of O3* with M, and indicate that the unusual symmetric/asymmetric differences in ozone are most prominent at energies of O3* near the dissociation threshold.

These experimental findings led me to design a modified master equation model that produces results in line with experimental bath gas dependent, pressure dependent, and temperature dependent measurements of the isotopic composition of ozone. This model demonstrates that all three of the aforementioned dependencies may be explained by mass-independent isotope effects in ozone that exist at energies up to merely ~100 cm-1 above the dissociation threshold. The model thus gives a plausible explanation for an array of previously unexplained experimental trends in the 17O anomaly in ozone. Moreover, the creation of this model allows relative isotope-specific rate coefficients in ozone to be calculated under the wide range of relevant atmospheric conditions. These kinetic values produced by the model can aid in the interpretation of the 17O anomaly in the many substances in the atmosphere and in and other parts of the Earth system that have acquired a 17O anomaly through reaction or isotope exchange with ozone.

Overall, the results of the experiments and modeling presented in this dissertation provide key insights into the origin of the 17O anomaly in ozone, as well as findings relevant for applications of the unique ozone isotope anomaly to atmospheric and Earth science studies.