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Open Access Publications from the University of California

Scholarly Works (8 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2013)

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.

    Cover page: 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
    • Article
    • Peer Reviewed
    LBL Publications (2018)
    We examine in a simple organic aerosol the transition between heterogeneous chemistry under well-mixed conditions to chemistry under interfacial confinement. A single reaction mechanism, shown to reproduce observed OH oxidation chemistry for liquid and semisolid C30H62, is used in reaction-diffusion simulations to explore reactivity over a broad viscosity range. The results show that when internal mixing of the aerosol is fast and the particle interface is enriched in C-H groups, ketone and alcohol products, formed via peroxy radical disproportionation, predominate. As viscosity increases the reactions become confined to a shell at the gas-aerosol interface. The confinement is accompanied by emergence of acyloxy reaction pathways that are particularly active when the shell is 1 nm or less. We quantify this trend using a reaction-diffusion index, allowing the parts of the mechanism that control reactivity as viscosity increases to be identified.
      Cover page: Changes in Reactivity as Chemistry Becomes Confined to an Interface. The Case of Free Radical Oxidation of C30H62 Alkane by OH
      • Article
      • Peer Reviewed
      LBL Publications (2018)
      To fully utilize the results of laboratory-based studies of the chemistry of model atmospheric aerosol reactions, it is important to understand how to relate them to the conditions found in nature. In this study, we have taken a validated reaction-diffusion mechanism for oxidation of C30H62 aerosol by OH under flow tube conditions and examined its predictions for another experimental regime (continuous flow stirred tank reactor) and for the atmosphere, spanning alkane aerosol viscosities from liquid to semisolid. The results show that under OH-concentration-limited and aerosol-mixing-limited conditions, it should be possible to select laboratory experimental conditions where many aspects of the particle phase and volatile product chemistry under atmospheric conditions can be revealed. If the OH collision and organic diffusion rates are comparable, however, reactivity is highly sensitive to the details of both OH concentration and internal mixing. The characteristics of the transition between limiting conditions provide key insights into which parts of the reaction mechanism dominate in the various kinetic regimes.
        Cover page: Predicting Aerosol Reactivity Across Scales: from the Laboratory to the Atmosphere
        • Article
        • Peer Reviewed
        LBL Publications (2017)
        A key uncertainty in the heterogeneous oxidation of carboxylic acids by hydroxyl radicals (OH) in aqueous-phase aerosol is how the free-radical reaction pathways might be altered by acid-base chemistry. In particular, if acid-base reactions occur concurrently with acyloxy radical formation and unimolecular decomposition of alkoxy radicals, there is a possibility that differences in reaction pathways impact the partitioning of organic carbon between the gas and aqueous phases. To examine these questions, a kinetic model is developed for the OH-initiated oxidation of citric acid aerosol at high relative humidity. The reaction scheme, containing both free-radical and acid-base elementary reaction steps with physically validated rate coefficients, accurately predicts the experimentally observed molecular composition, particle size, and average elemental composition of the aerosol upon oxidation. The difference between the two reaction channels centers on the reactivity of carboxylic acid groups. Free-radical reactions mainly add functional groups to the carbon skeleton of neutral citric acid, because carboxylic acid moieties deactivate the unimolecular fragmentation of alkoxy radicals. In contrast, the conjugate carboxylate groups originating from acid-base equilibria activate both acyloxy radical formation and carbon-carbon bond scission of alkoxy radicals, leading to the formation of low molecular weight, highly oxidized products such as oxalic and mesoxalic acid. Subsequent hydration of carbonyl groups in the oxidized products increases the aerosol hygroscopicity and accelerates the substantial water uptake and volume growth that accompany oxidation. These results frame the oxidative lifecycle of atmospheric aerosol: it is governed by feedbacks between reactions that first increase the particle oxidation state, then eventually promote water uptake and acid-base chemistry. When coupled to free-radical reactions, acid-base channels lead to formation of low molecular weight gas-phase reaction products and decreasing particle size.
          Cover page: Aerosol Fragmentation Driven by Coupling of Acid–Base and Free-Radical Chemistry in the Heterogeneous Oxidation of Aqueous Citric Acid by OH Radicals
          • Article
          • Peer Reviewed
          LBL Publications (2016)
            Cover page: Accelerated free radical chemistry in the heterogeneous oxidation of semisolid organic aerosolCreative Commons 'BY-NC-ND' version 4.0 license
            • Article
            • Peer Reviewed
            LBL Publications (2019)
            For aerosol particles that exist in highly viscous, diffusion-limited states, steep chemical gradients are expected to form during photochemical aging in the atmosphere. Under these conditions, species at the aerosol surface are more rapidly transformed than molecules residing in the particle interior. To examine the formation and evolution of chemical gradients at aerosol interfaces, the heterogeneous reaction of hydroxyl radicals (OH) on ∼200 nm particles of pure squalane (a branched, liquid hydrocarbon) and octacosane (a linear, solid hydrocarbon) and binary mixtures of the two are used to understand how diffusion limitations and phase separation impact the particle reactivity. Aerosol mass spectrometry is used to measure the effective heterogeneous OH uptake coefficient (γeff) and oxidation kinetics in the bulk, which are compared with the elemental composition of the surface obtained using X-ray photoemission. When diffusion rates are fast relative to the reaction frequency, as is the case for squalane and low-viscosity squalane-octacosane mixtures, the reaction is efficient (γeff ∼ 0.3) and only limited by the arrival of OH to the interface. However, for cases, where the diffusion rates are slower than reaction rates, as in pure octacosane and higher-viscosity squalane-octacosane mixtures, the heterogeneous reaction occurs in a mixing-limited regime and is ∼10× slower (γeff ∼ 0.03). This is in contrast to carbon and oxygen K edge X-ray absorption measurements that show that the octacosane interface is oxidized much more rapidly than that of pure squalane particles. The O/C ratio of the surface (estimated to be the top 6-8 nm of the interface) is measured to change with rate constants of (3.0 ± 0.9) × 10-13 and (8.6 ± 1.2) × 10-13 cm3 molecule-1 s-1 for squalane and octacosane particles, respectively. The differences in surface oxidation rates are analyzed using a previously published reaction-diffusion model, which suggests that a 1-2 nm highly oxidized crust forms on octacosane particles, whereas in pure squalane, the reaction products are homogeneously mixed within the aerosol. This work illustrates how diffusion limitations can form particles with highly oxidized surfaces even at relatively low oxidant exposures, which is in turn expected to influence their microphysics in the atmosphere.
              Cover page: Using Nanoparticle X‑ray Spectroscopy to Probe the Formation of Reactive Chemical Gradients in Diffusion-Limited Aerosols
              • Article
              • Peer Reviewed
              LBL Publications (2017)
              Multiphase chemical reactions (gas + solid/liquid) involve a complex interplay between bulk and interface chemistry, diffusion, evaporation, and condensation. Reactions of atmospheric aerosols are an important example of this type of chemistry: the rich array of particle phase states and multiphase transformation pathways produce diverse but poorly understood interactions between chemistry and transport. Their chemistry is of intrinsic interest because of their role in controlling climate. Their characteristics also make them useful models for the study of principles of reactivity of condensed materials under confined conditions. In previous work, we have reported a computational study of the oxidation chemistry of a liquid aliphatic aerosol. In this study, we extend the calculations to investigate nearly the same reactions at a semisolid gas-aerosol interface. A reaction-diffusion model for heterogeneous oxidation of triacontane by hydroxyl radicals (OH) is described, and its predictions are compared to measurements of aerosol size and composition, which evolve continuously during oxidation. These results are also explicitly compared to those obtained for the corresponding liquid system, squalane, to pinpoint salient elements controlling reactivity. The diffusive confinement of the free radical intermediates at the interface results in enhanced importance of a few specific chemical processes such as the involvement of aldehydes in fragmentation and evaporation, and a significant role of radical-radical reactions in product formation. The simulations show that under typical laboratory conditions semisolid aerosols have highly oxidized nanometer-scale interfaces that encapsulate an unreacted core and may confer distinct optical properties and enhanced hygroscopicity. This highly oxidized layer dynamically evolves with reaction, which we propose to result in plasticization. The validated model is used to predict chemistry under atmospheric conditions, where the OH radical concentration is much lower. The oxidation reactions are more strongly influenced by diffusion in the particle, resulting in a more liquid-like character.
                Cover page: Diffusive confinement of free radical intermediates in the OH radical oxidation of semisolid aerosols
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2013)
                We report observations of stratospheric CO2 that reveal surprisingly large anomalous enrichments in (17)O that vary systematically with latitude, altitude, and season. The triple isotope slopes reached 1.95 ± 0.05(1σ) in the middle stratosphere and 2.22 ± 0.07 in the Arctic vortex versus 1.71 ± 0.03 from previous observations and a remarkable factor of 4 larger than the mass-dependent value of 0.52. Kinetics modeling of laboratory measurements of photochemical ozone-CO2 isotope exchange demonstrates that non-mass-dependent isotope effects in ozone formation alone quantitatively account for the (17)O anomaly in CO2 in the laboratory, resolving long-standing discrepancies between models and laboratory measurements. Model sensitivities to hypothetical mass-dependent isotope effects in reactions involving O3, O((1)D), or CO2 and to an empirically derived temperature dependence of the anomalous kinetic isotope effects in ozone formation then provide a conceptual framework for understanding the differences in the isotopic composition and the triple isotope slopes between the laboratory and the stratosphere and between different regions of the stratosphere. This understanding in turn provides a firmer foundation for the diverse biogeochemical and paleoclimate applications of (17)O anomalies in tropospheric CO2, O2, mineral sulfates, and fossil bones and teeth, which all derive from stratospheric CO2.
                  Cover page: Unexpected variations in the triple oxygen isotope composition of stratospheric carbon dioxide.
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