UC Berkeley

This dissertation presents a series of experiments and observations using the isotopic composition of trace gases to inform atmospheric chemistry and transport ranging from Earth’s troposphere to the mesosphere. These include (1) the investigation of the isotopic composition of nitrous oxide (N2O) produced in plasma discharges relevant to atmospheric processes, (2) analysis of radiocarbon in carbon dioxide (CO2) sampled in the upper troposphere and lower stratosphere at high latitudes in the northern and southern hemispheres, and (3) a review of the chemical physics of isotope effects relevant for atmospheric chemistry.

First, because N2O is both a long-lived greenhouse gas and an ozone depleting substance, the processes responsible for the dramatic rise in atmospheric N2O concentrations continues to warrant investigation. Recent observations from aircraft and satellites suggest that there are potential in-situ atmospheric sources from lightning in the troposphere and high-energy events in the upper atmosphere that are not well constrained; although the global source strengths are likely small, their transient nature may confound analyses of atmospheric observations of N2O unless they are quantified and understood. In principle, the source of N2O can be inferred by measuring its isotopic composition, but that of N2O produced by these non-equilibrium plasmas is not known. In this study, N2O produced in four different types of plasma discharges (e.g., corona, pin-to-plane, plasma flashlight, and surface microdischarge) and under different conditions was analyzed by isotope ratio mass spectrometry, yielding distinctly different isotopic compositions. Kinetics modeling was performed to understand the importance of different reactions to the production of N2O and its isotopic composition. This work provides new constraints on the kinetics and chemical mechanisms of N2O production and destruction pathways in low-temperature, non-equilibrium plasmas, to which isotope compositions have not been applied before, as well as the ‘isotopic fingerprints’ of plasma-generated N2O – distinct from the microbial and combustion sources of N2O to the atmosphere – to compare with aircraft and satellite observations.

Second, vertical profiles are presented of Δ14C-CO2 measured on whole air samples collected across the tropopause at high latitudes in the northern and southern hemispheres by the NASA DC-8 aircraft during the fourth Atmospheric Tomography Mission (ATom-4) deployment in May 2018. These profiles not only provide important new constraints for inverse models of the carbon cycle but also show interesting differences between the hemispheres. These differences could result from differences in the amounts of isentropic transport from the subtropical troposphere into the lowermost stratosphere, the magnitudes of downwelling from the stratospheric overworld, tropospheric radiocarbon levels, or some combination of these factors. These data will be used to provide guidance for selecting the timing and location of future aircraft sampling missions needed to deconvolve the influence of these dynamical factors on the composition of the lowermost stratosphere. In addition, inferred cosmogenic 14C production rates are calculated and compared with past aircraft and ballooning missions from 1997 to 2013. The ATom-4 production rates are skewed high likely due to substantial isentropic mixing in of new air from the troposphere into the lowermost stratosphere; these results indicate the importance of sampling in the stratosphere above 15 km for studies using 14CO2 levels as a tracer of stratospheric residence times.

Third, a review of the chemical physics of isotope effects relevant for atmospheric chemistry is included that focuses on recent developments in the field and their historical context. The discussion includes non-mass dependent isotope effects in oxygen-containing species and an overview of the recent growth of “clumped isotope” measurements at natural isotopic abundances – i.e., of doubly-isotopically substituted species. This review lays the groundwork for introducing chemistry researchers to the field and summarizing the outstanding research questions to which experimental and theoretical physical chemists can contribute.