Doctor of Philosophy / Department of Agronomy / Ganga M. Hettiarachchi / Soil carbon (C) sequestration has been recognized as one of the most effective potential mitigation options for climate change. Underlying mechanisms of soil C sequestration/preservation is poorly understood, even after decades of soil C research. The main research objectives of this dissertation were three-fold: (1) enhancing our understanding in mineralogical and physicochemical mechanisms of soil C sequestration in microaggregates, (2) understanding the chemistry of organic C sequestered in soil aggregates, and (3) to determine the resilience of C to different temperature-moisture regimes and physical disturbance in a six-month incubation. An integrated approach was used in obtaining a better picture on mechanisms of C preservation. Two long-term agroecosystems located at the North Agronomy Farm, Manhattan, KS (Mollisols) and the Center of Experimentation and Research Fundacep in Cruz Alta-RS, Brazil (Oxisols) were used. Main plots of both systems were till and no-till. Mollisols consisted of three fertilizer treatments; control, manure/compost and urea. Oxisols had three different crop rotations; simple, intermediate, and complex. Submicron level information gathered by spectromicroscopy approaches, identified the direct preservation of OC structures with the original morphology; suggesting that the preservation of OC is a primary mechanism of C sequestration in these soils. Physical protection and organo-mineral associations seemed to also be involved in OC preservation. Manure/compost addition and no-till favored labile C preservation in aggregates of Mollisols. Significant associations observed between reactive minerals and C pools in Mollisols indicated the significance of organo-mineral associations in OC preservation. Large microaggregates exerted strong C preservation through physical protection and organo-mineral associations. Unlike in Mollisols, Oxisols showed a poor correlation between reactive mineral fraction and organic C which indicated the significance of physical protection over organo-mineral associations. Resilience of sequestred C was significantly affected by temperature across both temperate and tropical soil ecosystems, directly and indirectly. High temperature influenced soil acidity and reactive minerals, ultimately affecting organo-mineral associations. Macromolecular propeties of humic acid fraction showed changes after six months. Overall, direct and indirect evidence from this study suggested that the preservation of SOC is an ecosystem property supporting the newly proposed theories in soil C dynamics.

The demand for electrical energy is steadily increasing. Highly efficient organic solar cells based on mixed, strongly absorbing organic molecules convert sunlight into electricity and, thus, have the potential to contribute to the worlds energy production. The continuous development of new materials during the last decades lead to a swift increase of power conversion efficiencies (PCE) of organic solar cells, recently reaching 12%. Despite these breakthroughs, the usage of highly complex organic molecules blended together to form a self-organised absorber layer results in complicated morphologies that are poorly understood. However, the morphology has a tremendous impact on the photon-to-electron conversion, affecting all processes ranging from light absorption to charge carrier extraction. This dissertation studies the role of phase-separation of the self-organised thin film blend layers utilized in organic solar cells. On the molecular scale, we manipulate the phase-separation, using different molecule combinations ranging from the well-known ZnPc:C 60 blend layers to highly efficient oligothiophene:C60 blend layers. On the macroscopic scale, we shape the morphology by depositing the aforementioned blend layers on differently heated substrates (in-vacuo substrate temperature, Tsub). To characterise the manufactured blend layers, we utilize high resolution microscopy techniques such as photoconductive atomic force microscopy, different electron microscopic techniques, X-ray microscopy etc., and various established and newly developed computational simulations to rationalise the experimental findings. This multi-technique, multi-scale approach fulfils the demands of several scientific articles to analyse a wide range of length scales to understand the underlying optoelectronic processes. Varying the mixing ratio of a ZnPc:C60 blend layer from 2:1 to 6:1 at fixed in vacuo substrate temperature results in a continuous increase of surface roughness, decrease of short-circuit current, and decrease of crystallinity. Additionally performed density functional theory calculations and 3D drift-diffusion simulations explain the observed crystalline ZnPc nanorod formation by the presence of C60 in the bulk volume and the in turn lowered recombination at crystalline ZnPc nanorods. Moving to oligothiophene:C60 blend layers used in highly efficient organic solar cells deposited at elevated substrate temperatures, we find an increase of phase-separation, surface roughness, decrease of oligothiophene-C60 contacts, and reduced disorder upon increasing Tsub from RT (PCE=4.5%) to 80 °C (PCE=6.8%). At Tsub =140 °C, we observe the formation of micrometer-sized aggregates on the surface resulting in inhomogeneous light absorption and charge carrier extraction, which in turn massively lowers the power conversion efficiency to 1.9%. Subtly changing the molecular structure of the oligothiophene molecule by attaching two additional methyl side chains affects the thin film growth, which is also dependent on the substrate type. In conclusion, the utilized highly sensitive characterisation methods are suitable to study the impact of the morphology on the device performance of all kinds of organic electronic devices, as we demonstrate for organic blend layers. At the prototypical ZnPc:C60 blend, we discovered a way to grow ZnPc nanorods from the blend layer. These nanorods are highly crystalline and facilitate a lowered charge carrier recombination which is highly desirable in organic solar cells. The obtained results at oligothiophene: C60 blends clearly demonstrate the universality of the multi-technique approach for an in-depth understanding of the fragile interplay between phase-separation and phase-connectivity in efficient organic solar cells. Overall, we can conclude that both molecular structure and external processing parameters affect the morphology in manifold ways and, thus, need to be considered already at the synthesis of new materials.

<p dir="ltr">Atmospheric aerosols are solid and liquid particles emitted from a range of natural and anthropogenic sources, and that impact Earth’s climate directly by interacting with solar radiation, as well as indirectly through modifications to the properties and lifecycles of clouds. Furthermore, atmospheric particles yield substantial implications on air quality, visibility, and human health. While the impact of aerosols on the planet has been broadly defined, accurate forecasting of atmospheric particle processes remains challenging due to their complex physicochemical properties. Highly variable aerosol characteristics include size, morphology, viscosity, elemental and molecular composition, hygroscopicity, mixing state, and light-absorption. Moreover, aerosols experience transformations as they evolve during transport downwind of the emission source. Aerosol evolution is dictated (between many other factors) by ambient conditions, such as relative humidity, temperature, and sunlight activity. This dissertation aims at providing a comprehensive characterization of atmospheric particles both at the bulk and single-particle level by implementing a unique combination of offline and online instrumentation.</p><p dir="ltr">The first chapter of this dissertation describes sources of atmospheric particles, as well as aerosol properties frequently examined to quantify their impact on our planet, such as chemical composition and light absorption. The second chapter delves into the wide range of techniques implemented in this study to characterize laboratory-generated and field-sampled aerosols. Notably, online measurements of chemical composition and optical properties were acquired with aircraft-deployed mass spectrometers and a particle-into-liquid sampler. These were frequently used to complement single-particle analysis employed with offline electron and X-ray microscopy techniques.</p><p dir="ltr">The third chapter describes a systematic approach to infer the viscosity of organic particles based on their morphology. Specifically, particles deform upon impacting a rigid surface during sampling, and the degree of deformation is highly influenced by the viscoelastic properties. Highly viscous and solid particles will retain their shape after sampling, whereas liquid-like particles will flatten drastically. Hence, we expanded on a semi-quantitative approach to infer the viscosity of particles based on their measured height-to-width aspect ratios. The fourth chapter discussed bulk measurements of chemical composition of smoke plumes emitted during wildfires in Western United States. An aerosol mass spectrometer was employed to quantify the mass concentration of key chemical species and their subsequent evolution during plume transport. Analyzed samples corresponded to daytime and nighttime particulate, which provided valuable insights on the impact of photochemical reactions on the composition and evolution of biomass burning particles. The fifth chapter serves as a follow-up study for that discussed in Chapter 4. Biomass burning particles were deposited onto substrates and taken for further chemical imaging. Scanning electron microscopy coupled with X-ray microanalysis provided single-particle information on the size, morphology, and elemental composition of aerosols sampled at different locations of the smoke plume. More detailed chemical information was acquired using synchrotron-based X-ray microscopy coupled with near-edge X-ray absorption fine structure spectroscopy. This technique distinguished between organic carbon, soot, and inorganic species, while also determining the contribution of functional groups, including alkenes, aliphatic, and carboxyl groups. Chemical imaging measurements were examined with respect to real-time optical data acquired onboard research aircraft. This facilitated correlating the chemical and light-absorbing properties of particles. The sixth chapter discusses a multi-modal, novel approach to distinguish between sources of soot-containing particles. Atomic force microscopy, integrated with Raman spectroscopy, was implemented for a screening of the morphological and spectral features of individual particles. Subsequently, automated μ-Raman was used to acquire the spectra of large ensembles of particles that are considered representative of the whole particle population. Emission sources of soot particles were then determined following two curve-fitting approaches previously established.</p><p dir="ltr">Overall, the studies discussed in this dissertation provide a comprehensive understanding of aerosol characteristics at the single-particle level, which is often overlooked by atmospheric model when predicting the impact of atmospheric particles on climate.</p>