This dissertation presents a series of research projects designed and carried out to elucidate the physical chemistry and assess the atmospheric relevance of (1) carbonyl oxide radicals (i.e., Criegee intermediates) produced in alkene ozonolysis and (2) nitrous oxide (N2O) produced in lightning-induced corona discharges. The results provide UV absorption spectra and reaction rate coefficients for Criegee intermediates that will help constrain the formation and loss pathways of aerosol nucleation precursors such as H2SO4 and oxidized volatile organic compounds, and the isotopic signature of N2O formed in lightning that can help distinguish various N2O sources in atmospheric measurements.

Criegee intermediates are byproducts of the reaction of alkenes with ozone. Bimolecular reactions of Criegee intermediates can lead to the production of low-volatility organic compounds and acids in the atmosphere, which in turn play a role in determining the concentration, size, and optical properties of aerosols. Recently, a novel method for producing measurable quantities of stabilized Criegee intermediates in the laboratory paved the way for the development of new experimental techniques to study their chemical properties and predict their importance in the atmosphere. For this dissertation, a unique apparatus combining time-resolved UV absorption in a flow cell with laser depletion in a molecular beam was adapted to obtain the absolute absorption spectrum of CH3CHOO with high resolution and accuracy relative to previous spectral measurements by other groups. The resulting absorption cross sections imply a photolysis lifetime of about seven seconds in the atmosphere, long enough for CH3CHOO to participate in unimolecular and bimolecular reactions. The broad absorption band with weak structure in the long-wavelength region of the spectrum represents a “spectral fingerprint” for identifying CH3CHOO in future studies, and the cross sections provide valuable benchmarks for theory to characterize electronically excited states of CH3CHOO.

The fast reaction of CH2OO with water dimer is thought to dominate CH2OO removal in the atmosphere. However, reaction rates can vary considerably under different conditions of temperature, humidity, and pressure. A temperature-controlled flow cell was designed to measure the transient absorption of CH2OO and obtain rate coefficients for its reaction with water dimer from 283 to 324 K. The rate of the reaction of CH2OO with water dimer was found to exhibit a strong negative temperature dependence, pointing to the participation of a hydrogen-bonded pre-reactive complex between CH2OO and two water molecules. Due to the strong temperature dependence, and shifting competition between water dimer and water monomer (which has a positive temperature dependence), the effective loss rate of CH2OO by reaction with water vapor is highly sensitive to atmospheric conditions. The role played by the stable pre-reactive complex suggests that similar complexes could form between water dimer and other larger Criegee intermediates, and that the stability and relative energy of these complexes control the reaction rate with water and its temperature dependence.

Effective loss rates of Criegee intermediates due to bimolecular reactions in the atmosphere are limited by their rates of unimolecular decomposition. The rates of decomposition depend strongly on the molecular geometry, which affects the accessible isomerization pathways and dissociation products. (CH3)2COO is the main product of tetramethylethylene ozonolysis, and has been found to react slowly with water dimer and rapidly with SO2. While CH2OO decomposes slowly via isomerization to dioxirane, (CH3)2COO may decompose faster via intramolecular hydrogen transfer to form vinyl hydroperoxide. Fast (CH3)2COO decomposition could affect the significance of the Criegee intermediate H2SO4 source, as well as the non-photolytic production of OH radicals. In this dissertation, measurements of the transient absorption of (CH3)2COO to obtain thermal decomposition rate coefficients from 283 to 323 K by extrapolating the observed loss rate to zero concentration are reported. The rate of unimolecular decomposition is ~400 s-1 at 298 K and varies by nearly an order of magnitude within the studied temperature range. The effective loss rate of (CH3)2COO in the atmosphere due to thermal decomposition is thus competitive with its loss due to reaction with water vapor and with SO2, suggesting that the unimolecular decomposition pathway is a significant sink for (CH3)2COO and possibly other di-substituted Criegee intermediates, and should be included in models of Criegee chemistry in the atmosphere as well as in kinetic models of tetramethylethylene ozonolysis.

N2O is the third most important greenhouse gas after CO2 and methane, and is mainly emitted to the atmosphere as a byproduct of microbial activity in soils. The expanding use of nitrogen-containing fertilizers in agriculture has led to an increase in N2O atmospheric concentrations since preindustrial times. Isotopic measurements are a valuable tool to distinguish the influence of different sources of N2O, but the isotopic composition of N2O formed from corona discharge in lightning has not previously been measured. Here, a corona discharge cell apparatus was used to generate a corona discharge in flowing or static zero air, and the N2O formed at discharge cell pressures from ~0.1 to 10 Torr and discharge voltages from 0.25 to 5 kV was collected and measured with isotope ratio mass spectrometry to determine its isotopic composition. The results show enrichments in 15N of N2O up to 32‰ relative to the reactant N2, and even larger enrichments in 15N of up to 77‰ at the central nitrogen atom. Large depletions in 18O as large as -71‰ relative to reactant O2 were also measured. The isotopic composition measured here may help to elucidate the chemical mechanisms leading to N2O formation and destruction in a corona discharge. Furthermore, the isotope-isotope relationships of the N2O produced in the corona discharge experiments are distinct from those of N2O from other sources, implying that isotopic measurements can be used to determine whether local variations in the atmospheric concentration of N2O – e.g., the enhanced N2O levels recently measured in the upper tropical and subtropical troposphere – are due to lightning activity, soil emissions, or biomass burning.

Measurements and analysis of the mixing ratios and isotopic compositions of carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4), and molecular hydrogen (H2) from whole air samples collected during several NASA suborbital missions are presented. In each case, the global annual mean net isotope flux from the stratosphere to the troposphere, estimated empirically using the correlation between measured isotopic composition and N2O mixing ratio, is large enough to influence tropospheric isotopic compositions on hemispheric and potentially regional scales, and hence needs to be considered when making “top-down” inferences about the sources and sinks of the gas, including the impact of human activities, from isotope measurements at the surface. For CO2, in particular, I present and analyze 150 new measurements of Δ14C of CO2 collected from aircraft missions in 1997, 2000, 2004, 2012, and 2013, including the first hemispheric-scale 14CO2 vertical profiles that extend from the lower or middle troposphere into the lower stratosphere over a wide range of latitudes in the Northern Hemisphere for 2013. To within the uncertainties, the global annual mean net 14CO2 fluxes, which are approximately half the global 14C production rates, have not changed between 1997 and 2013, suggesting that stratospheric age spectra are wide enough to damp out variations in the cosmogenic 14C production rate associated with the 11-year solar cycle. This apparent lack of dependence of measured stratospheric 14CO2 on the solar cycle simplifies the application of stratospheric 14CO2 measurements as a model diagnostic to better understand rates and features of the stratospheric circulation and how they may be shifting over time as climate changes; an acceleration of the stratospheric circulation is predicted by climate models but is not, as of yet, confirmed by observations such as mean ages derived from the time lag between tropospheric and stratospheric concentrations of the gases CO2 and SF6 which are increasing in the troposphere. Additionally, these empirical 14CO2 isotope fluxes from the stratosphere to the troposphere are needed for carbon cycle studies. For 14CO2, as well as for the stable isotope compositions of CO2, N2O, CH4 and H2, the global annual mean net isotope flux is large enough that stratospheric chemistry, which enriches each gas in its rare, heavy isotopes, and subsequent transport from the stratosphere to the troposphere can interfere with determining the magnitudes of natural and anthropogenic emissions of these gases from surface observations. Overall, the measurements and net isotope fluxes presented here provide much needed constraints on the isotopic compositions of CO2, N2O, CH4 and H2 and the transport of these gases, necessary for studies that seek to (1) diagnose changes in the stratospheric circulation and residence times over time, (2) quantify sources and sinks in the biogeochemical cycles of these gases, and/or (3) interpret the large number of isotopic composition measurements made at Earth’s surface (e.g., to infer regional and global fossil fuel emissions from Δ14CO2).

In the field of reaction dynamics, the O(1D) + Xe electronic quenching reaction was investigated in a crossed beam experiment. Unexpected large-scale oscillations in the differential cross sections, whose shape and relative phase depend strongly on collision energy, were observed for the inelastic scattering products, O(3P) and Xe. Comparison of the experimental results with time-independent scattering calculations shows qualitatively that this behavior is caused by Stueckelberg interferences, for which the quantum phases of the multiple reaction pathways accessible during electronic quenching constructively and destructively interfere. This investigation into the quantum nature of O(1D) → O(3P) quenching by Xe provides sensitive new experimental constraints on the Xe–O potential energy curves and their couplings, as well as benchmarks for theoretical treatments of quantum scattering in general.

In the research reported in this dissertation, experiments were designed and performed to investigate the interactions of atmospheric gases with high-energy photons and/or electrons, which can produce highly reactive ions and radicals via ionization and dissociation, and which in turn may result in previously unexplored isotope effects or in formation of aerosols with optical properties that are difficult to predict and measure. Knowledge of both isotope effects and the optical properties of aerosols formed by UV photolysis of precursor gases are highly relevant for interpreting observations of and understanding chemical and physical processes occurring in a wide variety of planetary atmospheres. Here, photoionization efficiency spectra of isotopologues of N2 (14N2, 15N14N, and 15N2) and CO2 (12C16O2, 13C16O2, 12C16O18O, 13C16O18O, 12C18O2, and 13C18O2) using synchrotron radiation at the Advanced Light Source were measured, the polarization and intensity of laser light scattered by photochemically-generated aerosols suspended in the gas phase as a function of scattering angle were investigated in situ using a newly designed and built computer-controlled custom polarimeter, and the isotopic composition of N2O produced in a newly designed and constructed corona discharge apparatus was measured.

Isotope effects in the photoionization of N2 and CO2 may be important in determining the isotopic composition in planetary atmospheres, such as those on Earth and Titan in the case of N2 and on Mars and Venus in the case of CO2, and provide new data to address uncertainties in spectral peak assignments in the photoionization spectra, and yet have not been previously measured. For example, for N2, the measured differences in photoinization efficiencies between 14N2 and 15N14N, may help resolve differences in the isotopic composition of N2 versus that for HCN observed on Titan. In addition, the spectral assignment for the feature at 15.677 eV in the 14N2 photoionization spectrum has remained controversial despite decades of research. The measured isotope shifts for this peak are compared with isotope shifts predicted using Herzberg equations for the isotopic differences in harmonic oscillator energy levels plus the first anharmonic correction for the three proposed assignments. The measured isotope shifts for this peak relative to 14N2 are 0.015 +/- 0.001 eV for 15N2 and 0.008 +/- 0.001 eV for 15N14N (reported here for the first time), which match most closely with the isotope shifts predicted for transitions to the (A 2-Pi-u v=2)4-s-sigma-g 1-Pi-u state of 0.0143 eV for 15N2 and 0.0071 eV for 15N14N, and thus assignment to this transition is favored. For CO2, the measured isotope effects in photoionization yield ratios of photoionization rate coefficients, J (i.e., photoionization cross-sections convolved with the solar spectrum and integrated over all photoionization energies), that are less than 1: J(13C16O2)/J(12C16O2) = 0.97 +/- 0.02, J(12C16O18O)/J(12C16O2) = 0.97 +/- 0.02, J(13C16O18O)/J(12C16O2) = 0.97 +/- 0.02, J(12C18O2)/J(12C16O2) = 0.99 +/- 0.02, and J(13C18O2)/J(12C16O2) = 0.98 +/- 0.02. These isotope effects in photoionization rate coefficients are likely large enough to contribute to (if not dominate) enrichments in 13C in CO2 in the martian atmosphere, which have largely been attributed to atmospheric escape over billions of years, and may also be important in the atmospheres of Venus and Earth, thus warranting inclusion in models of the isotopic composition of CO2 in planetary atmospheres.

Aerosols generated by UV photolysis of precursor gases are present in a number of planetary atmospheres, including Titan, and most likely, early Earth and early Mars, and are expected to have a profound effect on atmospheric radiative transfer, yet the number of investigations of aerosol optical properties suspended in the gas phase is extremely limited. In order to measure the intensity and polarization state of light scattered by photochemically-generated aerosols as a function of scattering angle, a custom polarimeter consisting of a quarter-wave plate mounted in a computer-controlled rotating stage and a linear polarizer was designed and built. The polarimeter was placed into the 13 L reaction chamber to measure the intensity and polarization of light scattered by photochemically-generated aerosol in situ, and the parameters of the Stokes vector were calculated at a number of scattering angles. The results demonstrate that this technique can be used to measure the polarization and angular dependence (phase function) of light scattered by aerosol particles in situ while still suspended in the gas phase, with the ultimate goal of using these measurements to attain the size distribution and index of refraction of the aerosol particles for applications to radiative transfer in planetary atmospheres, such as early Earth and Titan.

Measurements of the isotopic composition of N2O can be used to infer its sources and sinks, and understanding the isotopic composition of N2O formed by corona discharge in air (a process which can occur, for example, in thunderstorms) may be important for understanding atmospheric observations of N2O if the isotope effects in N2O formation are large. To measure the isotopic composition of N2O formed by corona discharge, a new apparatus was designed and built to produce N2O by corona discharge and isolate and collect the N2O cryogenically for subsequent analysis by continuous flow isotope ratio mass spectrometry. Results from the first measurements of isotopic composition (reported as delta-15 N, delta-18O, and the "site-specific" delta-15N values, delta-15N-alpha and delta-15N-beta) indicate that, under some conditions, the isotopic composition of N2O formed by corona discharge is significantly different from the reactant N2 and O2 and from background tropospheric N2O, although none of the measurements reported here show fractionations larger than 4% (i.e., 40 per mil) relative to the starting N2 or O2 or tropospheric N2O. Additional testing under other experimental conditions (e.g., pressure, discharge residence time, discharge current) is warranted to assess whether fractionations might be large enough to include in atmospheric models.

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.

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.

Aerosols in planetary atmospheres play a critical role in radiative transfer and thus also in determining the penetration depth of UV radiation and atmospheric temperatures. For early Mars and an anoxic early Earth, aerosols may have significantly influenced the stability of liquid water at the surface, as well as climate and habitability, yet the study of aerosol formation remains poorly constrained by models and experiments, making conclusions difficult. Significant progress can be made in reducing these uncertainties by providing additional laboratory constraints and tests for the photochemical and microphysical models used to generate the greatly varying predictions about aerosol formation in terrestrial-like atmospheres. Photochemistry experiments measuring gas- and condensed-phase species were conducted to determine (1) the extent to which a photochemical haze could have formed in Mars' and Earth's early atmospheres and (2) whether such aerosols might be depleted in carbon-13, providing a potentially false biosignature in organic matter in the Martian and terrestrial rock and meteorite records.

To gain a greater understanding of the fundamentals of aerosol formation in the CO₂-rich atmospheres of early Earth and Mars, methane (CH₄) and mixtures of CH4 and carbon dioxide (CO₂) were irradiated with UV light in a static stainless steel reaction chamber, leading to the formation of aerosols. HeNe laser scattering was used to detect in situ the presence and relative amounts of aerosol produced, as well as the induction time required to form aerosols from the irradiation of the gases. Online mass spectrometry measurements were used to monitor the time evolution of gas phase species and, by difference, the relative amounts of carbon in the aerosol formed during an experiment. For comparison, particle samples were also collected during the irradiation experiments for "offline" analysis by elemental analyzer-isotope ratio mass spectrometry (EA-IRMS); the IRMS peak areas were used to quantify the relative amounts of carbon in the aerosols. From these three types of measurements, particle production was observed to be higher as the pressure of initial reactant CH₄ was increased from 5 to 200 Torr, but little sensitivity to the CH₄/CO₂ ratio between 0.34 and 5 for a given CH₄ pressure was observed. This behavior is in contrast to what has been predicted by photochemical models, which suggest that aerosol production rates decrease as CO₂ levels increase. Furthermore, aerosol formation was detectable at a CH₄/CO₂ ratio down to 0.25 by in situ HeNe light scattering and down to 0.34 by offline EA-IRMS analysis of collected particles. These experimental ratios are significantly lower than the threshold ratios of 0.6 to 1 predicted by photochemical models for particle production. This suggests that aerosol production in CO₂-rich atmospheres with feasible sources of CH₄ from mantle outgassing or methanogenic bacteria was likely to have been much more favorable than predicted by models. Further, more sophisticated photochemical models of early Earth's atmosphere will need to include reactions of organic species with oxygen in order to better predict the minimum CH₄/CO₂ threshold at which particles form as a function of CH₄/CO₂.

In addition to affecting atmospheric radiative transfer, organic aerosols that formed photochemically in the atmosphere could have deposited on the surface, and would thus have been incorporated in the rock record. If such aerosols were significantly lighter in carbon-13 than the CH₄ from which they formed, such a light isotopic signature might potentially be mistaken as a biomarker. To test this hypothesis, aerosols from the UV irradiation of CH₄ and mixtures of CH₄ and CO₂ were collected and analyzed for the carbon-13 isotopic composition by EA-IRMS. Compared to the isotopic composition of the reactant CH₄, the particle samples were depleted in ¹³C by 6±3 ‰. While this is likely too small to account for the large isotopic depletions in ¹³C observed between 2.5 to 3.0 billion years ago (which have been interpreted as the rise of methanotrophic bacteria), experimentation at lower temperatures is recommended to address whether the overall kinetic isotope effects in photochemical aerosol production might be larger at the temperatures in the upper atmosphere where CH₄ photolysis would have occurred.

Nitrous oxide (N₂O) and molecular nitrogen (N₂) isotopic compositions and aerosol optical properties were investigated through experiments and observations to elucidate their roles in atmospheric radiative transfer and chemistry. In Earth's atmosphere, the isotopic composition of N₂O, a potent greenhouse gas, is a useful tool for investigating its sources and sinks. N₂ is the main component of the atmospheres of Earth and Titan, and isotope effects in its photoionization may lead to isotopic fractionation. The optical properties of aerosols, a component of most planetary atmospheres, determine how they affect radiative transfer. A polarimeter was developed to measure aerosol optical properties in situ in an existing apparatus.

Three sets of measurements of N₂O isotopic composition provide new insight into its budget. First, a time-series from 1940 to 2005 from firn and archived air samples is consistent with the observed N₂O increase being largely due to isotopically light N₂O emissions from agriculture and reveals seasonal cycles due to the seasonally-varying influences of multiple N₂O sources and stratosphere-troposphere exchange. Second, measurements from the tropical free troposphere show coherent vertical variations in N₂O isotopic compositions consistent with the persistent influence of a regional surface source, most likely the ocean. Third, samples from the marine boundary layer with anomalously high N₂O mixing ratios and perturbed isotopic compositions were used to deduce a source isotopic composition that is perhaps representative of N₂O emitted from the South Atlantic Ocean.

Isotope effects in the non-dissociative photoionization of N₂ -- investigated by measuring the photoionization efficiency spectrum for its three isotopologues -- clarify peak identities and show that these previously ignored isotope effects may be important in planetary atmospheres. The shifts in peak energy due to isotopic substitution show that the controversial peak at 15.68 eV for ¹⁴N₂ is most likely due to a Rydberg state converging to the v'=2 level of the A²π_u N₂⁺ state. A model of Titan's atmosphere shows that isotopic self-shielding in ¹⁴N₂ photoionization may cause isotopic fractionation between N₂ and other N-bearing molecules distinct from that caused by N₂ photodissociation, providing a possible mechanism for determining the relative importance of ion versus neutral photochemistry.

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.