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ItemUnderstanding bioaerosols atmospheric lifecycle, abundance variability and impacts(Georgia Institute of Technology, 2019-12-18) Negron, Arnaldo AndresBioaerosols are ubiquitous in the atmosphere and may have important impacts on human health, cloud formation, the hydrological cycles and biogeochemical cycles. Measuring and characterizing bioaerosols remains a challenge owing to their low atmospheric concentration. During this thesis, we have developed an approach to collect large amounts of bioaerosols (e.g., on ground-based or airborne platforms) in a liquid suspension over the sub-hour to multiple hour timescale using a modified high-volume wet cyclone. The bioaerosols are then subsequently characterized using flow cytometry and other biology tools, results leading to robust quantifications of bioaerosol populations. Together with the observations from rapid autofluorescence detection techniques, they can provide powerful insights on the concentration, composition, and activity of bioaerosol with rapid time resolution. The new characterization approach was applied to study bioaerosol populations in multiple, distinct environments: i) an urban environment in the Southeast United States surrounded by heavy forestation (Atlanta, GA), ii) the marine boundary layer, free troposphere, terrestrial environments near California during the BOAS 2015 aircraft campaign, and, iii) the remote Eastern Mediterranean sea influenced by the European continental outflow and Saharan dust events. In the Southeast United States, we observed that the bioaerosol population is highly dynamic and driven by the prevailing meteorology. We detect high concentrations of large bioaerosol population rich in nucleic acid (consistent with wet-ejected fungal spores) during humid and warm days after rain events, while other days are characterized by smaller bioaerosol (consistent with bacteria) that are low in nucleic acid content. During the airborne deployment at the California Coast, small bacterialike particles that are low in nucleic acid content are ubiquitous and tend to be enhanced in the marine free troposphere compared to the boundary layer thought to be the source. Concentrations of microbes in the marine boundary layer are about 10 times less than those found in the airmasses characterized by terrestrial emissions, while the cell types from flow cytometry and light induced fluorescence indicate very different populations. In the Eastern Mediterranean, bioaerosol is dominated by small bioaerosol with low nucleic acid content (consistent with bacterial cells). Interestingly, the highest concentration is not observed during periods where continental outflow airmasses are sampled, but during dust events. The observations carried out during this thesis show that bioaerosol associated with air masses influenced by terrestrial (and especially dust) emissions carry the largest bioaerosol concentrations. We also see that smaller bioaerosol consistent with microbes (with a diameter ~ 1 μm and low nucleic content) are ubiquitous at concentrations ranging between 104 m-3 and 105 m-3. Microbes in the marine boundary layer off the coast of California are about 10 times lower than that observed in terrestrial environments (103 m-3 to 104 m-3), although in the Eastern Mediterranean, bioaerosol concentrations can be as high as in terrestrial environments. Occasionally, we observe concentrations of larger nucleic acid-rich particles (consistent with fungal spores), especially after rain events. The extent to which the fungal spores travel is surprisingly large – given that they are observed at the remote Eastern Mediterranean, hundreds (and maybe thousands) of kilometers away from their terrestrial origin. The impacts of these concentrations and types of bioaerosol in all the environments sampled can be significant. We estimate for example that the phosphorous delivery from bioaerosol to the Eastern Mediterranean Sea, although much lower than recent model estimates, can still explain the concentrations that are associated with background levels of atmospheric phosphorus. In terms of their impacts on clouds, the concentration of marine bioaerosol is high enough to potentially influence ice nucleation in warm mixed-phase cloud, especially given that secondary ice processes are favored and can promote any initial low levels of primary ice. The above mentioned potential impacts of bioaerosol, however, may be modulated by atmospheric processing – very few studies of which exist. Towards this, we studied the response of microbes to simulated atmospheric acidification (a process that occurs everywhere in the atmosphere) by quantifying their cultivability and ability to express ice nucleation capacity as a function of pH levels observed for micron-sized particles in the atmosphere. For this, a droplet freezing assay was developed and used to study the effect of aerosol pH on an ice active P. syringae strain. Surprisingly, the microbes could resist considerable levels of acidification, as they retain their cultivability and ice nucleation capability to pH levels as low as 4. Upon increased acidification, however, (e.g., pH=2.5 or less), the ice active P. syringae lost cultivability and reduced their ice nucleation temperature close to -15oC, approaching the properties of Arizona test dust. Repeated freezing-thawing cycles over the same strains exhibit repeatable ice nucleation results. These results show that models of ice nucleation that consider the effects of bioaerosol need to consider the impacts of atmospheric acidification; the smooth dependence of ice nucleating characteristics (freezing temperature vs. pH) suggests that such effects can be parameterized using the approach developed during this thesis. The methods and scientific results produced during this thesis show that the simple yet powerful methods developed here can be readily used to sample bioaerosol, characterize their population characteristics, metabolic state, ice nucleation activity, and response to a variety of atmospheric stressors.
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ItemTransforming the preparation of physics graduate teaching assistants(Georgia Institute of Technology, 2019-12-02) Alicea-Munoz, EmilyGraduate Teaching Assistants (GTAs) are key partners in the education of undergraduate students. In large-enrollment intro physics classes, students spend roughly half of their in-class hours in labs and recitations under the supervision of GTAs. Since GTAs can have a large impact on their students' learning, it is important to provide them with appropriate preparation for teaching. But GTAs are also students themselves -- they have many demands on their time, and not all of them want to become professors after grad school. Therefore, it is crucial that GTA preparation not be a burden but rather be fully integrated into their professional development. The School of Physics at Georgia Tech has been offering a GTA prep course for first-year Ph.D. students since 2013. The majority of these first-time GTAs have no prior teaching experience but consider teaching to be an important part of their professional development as physicists. Through a cycle of implementation and revision, and guided by the 3P Framework we developed (Pedagogy, Physics, Professional Development), the course has evolved into a robust and comprehensive professional development program that is well-received by physics graduate students. We assessed the effectiveness of the course with a combination of surveys, pre/post tests, and student evaluations. We found that GTAs feel better prepared for teaching after going through the Orientation. GTAs consider most useful the course activities in which they can practice and get feedback on their teaching ("Microteaching", "Lab Simulation") and the lessons in which we discuss the pedagogical content knowledge necessary to teach intro physics labs and recitations ("Teaching Physics"). GTAs who participate in the GTA prep course adopt more learner-centered teaching approaches and increase their pedagogical knowledge. They also receive higher end-of-semester student evaluations than GTAs whose first teaching experience predated the establishment of the GTA prep course.
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ItemTransport in low-dimensional interacting systems(Georgia Institute of Technology, 2019-11-20) Sharma, KamalOne-dimensional quantum many-body systems have been an interesting area of theoretical research since last 90 years. However, advances in fabrication technologies has led to influx of real materials and devices that are one-dimensional or quasi one-dimensional. These devices have brought back a renewed interest in understating the physics of such systems. However, the established Luttinger liquid theory has some limitations due to absence of scattering processes at finite temperatures. Further, any arbitrarily weak interaction potential between Luttinger liquid bosons leads to divergences already in the first order calculation. We adopt the low energy one-dimensional Wigner crystal as the strongly interacting regime of Luttinger liquid. We show that the violation of the Wiedemann-Franz law can be demonstrated by calculating correction to thermal conductance of a non-linear interacting Wigner crystal.
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ItemUnderstanding ocean iron dynamics and impacts on marine ecosystems(Georgia Institute of Technology, 2019-11-12) Pham, Anh Le-DuyFe is one of the most important nutrients for phytoplankton growth in the ocean, making it a crucial element in the regulation of the ocean carbon balance and biogeochemical cycles. Atmospheric deposition of dissolved Fe (dFe) to the ocean has increased over the last decades partly due to human activities, which can significantly alter marine ecosystems. Thus, a comprehensive understanding of how the ocean Fe cycling operates and how it will respond to human perturbations is urgently needed. In this work, I first significantly improve the Fe parameterization in a global ocean biogeochemistry model, constrained by new high-quality ocean Fe datasets. Then, I identify key mechanisms that control the ocean Fe cycle in various ocean basins and examine the responses of marine phytoplankton to an increasing Fe deposition through a suite of model simulations. These simulations are performed in an ocean biogeochemistry and an ecosystem models, which incorporate the newly improved Fe scheme. The refinement of model Fe parameterization and its evaluation are undertaken in chapters two to four. In these chapters, I show that my newly developed Fe scheme displays a remarkable improvement in reproducing observations over the old scheme. Through a suite of model simulations, I reveal the crucial role of the concurrent release of dFe and ligands from sinking organic particles in forming and maintaining the subsurface dFe maxima observed in many ocean transects. Moreover, the inclusion of spatially varying ligand classes with different binding strengths in the model is important to explain the strong vertical dFe gradient observed in the upper ocean. I also identify the relative roles of different external dFe sources in different ocean basins. While atmospheric deposition is an important source of dFe in the Atlantic and Indian Oceans, sedimentary and hydrothermal dFe inputs are more important in the Pacific Ocean. The relative contributions of external sources and ocean interior processes on regulating the upper ocean dFe pattern are explored in chapter five. This task is done by analyzing the dFe budget and the dFe distribution field simulated in different ocean Fe models, using an unsupervised classification technique. The results show that the upper ocean dFe patterns are largely controlled by interior ocean processes and that without an appropriate representation of these processes, Fe models cannot reproduce observations, even with a correct magnitude of the external fluxes. In chapter six, I explore the impact of an increasing dFe atmospheric deposition on the Indian Ocean phytoplankton and carbon balance by using an ocean ecosystem model, which incorporates the newly improved Fe scheme. I found that while diatom growth and export organic carbon flux are enhanced south of 40 degree S, they decrease in some regions in the northern Indian Ocean, compensated by increases in coccolithophores growth and carbonate carbon export. These changes lead to a decrease in the carbon dioxide uptake over the Indian Ocean.
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ItemMechanics of hierarchical, filamentous tissues(Georgia Institute of Technology, 2019-11-12) Michel, Jonathan A.Structural hierarchy, the property of possessing spatial organization on multiple, distinct length scales, is omnipresent in biological tissues, and is increasingly popular as a means of pursuing designer properties of human-made materials. Hierarchy can offer economy of material, resilience against fracture, and novel mechanical response; however, the apparent opportunity for errors in assembly at multiple stages na\"ively seems to present an imposing obstacle to the evolution of hierarchical tissue. Nonetheless, many organisms, from many evolutionary lineages, exhibit structural hierarchy. In this work, we build upon previous efforts to model tissue as spring networks. We create networks with a nested, self-similar structure, whose geometrical attributes can be independently varied at each scale. Following previous researchers, we focus upon the mean coordination number, which gives the typical number of nearest neighbors to which a vertex in a network is connected, as a parameter for controlling the elastic properties of structures. We extend this idea, defining separate coordination numbers for the network architecture, and find a simple scaling law relating a material's stiffness to its structural attributes at each length scale. We validate this scaling law with simulations, and find it to hold for structures derived from crystalline lattices and triangulations of random point sets. From this scaling law, we predict that the variability in the stiffness of a network resulting from variability in its structural attributes at each length scale diminishes with increasing levels of hierarchy, up to some threshold. Our results suggest that robustness to errors in assembly may be a generic benefit of a modular assembly process. Finally, we elucidate the role of large-scale and small-scale structural attributes. We find the small scale structure sets the vibrational density of states of our model systems at large frequency, while the large-scale structure is important in coordinating a system-wide, percolating force network to stiffen the material.
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ItemEcological Community Assembly in the Face of Anthropogenic Environmental Changes(Georgia Institute of Technology, 2019-11-11) Yang, XianAnthropogenic environmental changes, such as increased nitrogen (N) deposition, changes in precipitation regimes, and habitat loss and fragmentation, are known to affect Earth’s ecosystems. Understanding mechanisms regulating the assembly of ecological communities in the face of anthropogenic environmental changes is one of the primary goals of contemporary ecology. In this dissertation, I present four studies addressing questions on community assembly under anthropogenic environmental changes. First, I conducted an experiment in a semi-arid grassland to examine how anthropogenic environmental changes, in the form of resource addition, influence phylogenetic alpha- and beta-diversity of the communities. I found N and water addition influenced different aspects of grassland community structure. N addition altered plant community phylogenetic structure, driving communities towards phylogenetic overdispersion; water addition promoted phylogenetic convergence, driving communities to converge towards a more similar phylogenetic structure over time. Next, I used bacterivorous ciliated protists as model organisms to explore how the loss of a keystone local community affects metacommunity biodiversity and ecosystem functions. I found that local communities with distinct environmental conditions supported endemic species, and had greater impact on regional-scale diversity than other local communities, therefore qualifying them as keystone communities. These keystone communities also had significant impacts on ecosystem functions, including biomass production and particulate organic matter decomposition. Finally, I investigated the drivers of variation in the phyllosphere microbial community composition in a fragmented subtropical forest on the islands of the Thousand-Island Lake, China. I found that stochastic processes, rather than deterministic processes, played a prominent role in shaping phyllosphere bacterial and fungal communities in the context of habitat fragmentation. Taken together, these findings further our understanding of community assembly processes in the face of anthropogenic environmental changes.
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Item6-connected graphs are two-three linked(Georgia Institute of Technology, 2019-11-11) Xie, ShijieLet $G$ be a graph and $a_0, a_1, a_2, b_1,$ and $b_2$ be distinct vertices of $G$. Motivated by their work on Four Color Theorem, Hadwiger's conjecture for $K_6$, and J\o rgensen's conjecture, Robertson and Seymour asked when does $G$ contain disjoint connected subgraphs $G_1, G_2$, such that $\{a_0, a_1, a_2\}\subseteq V(G_1)$ and $\{b_1, b_2\}\subseteq V(G_2)$. We prove that if $G$ is 6-connected then such $G_1,G_2$ exist. Joint work with Robin Thomas and Xingxing Yu.
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ItemAtlantic meridional overturning circulation variability since the last glaciation: Insights from a novel multiproxy approach(Georgia Institute of Technology, 2019-11-11) Valley, Shannon GabrielleAtlantic Meridional Overturning Circulation (AMOC) transports warm surface water northward across the equator, carrying heat from the Southern to the Northern Hemisphere. AMOC plays a central role in the global redistribution of heat and precipitation during both abrupt and longer-term climate shifts. Over the next century, AMOC is projected to weaken due to greenhouse gas warming, though projecting the variability of its future behavior is dependent on a better understanding of how AMOC changes are forced, including the evolving states of its constituent water masses. To address this, I analyzed several water mass tracer records from Florida Straits intermediate water- a water mass that forms part of AMOC’s upper branch. To investigate AMOC variability during the Younger Dryas (YD) and Heinrich Stadial 1 (HS1)- climatological periods associated with ice sheet melt- I generated a new, high-resolution record of benthic seawater Cd (Cdw) from a Florida Straits sediment core at 546 m water depth. This record provides additional evidence for lower Cdw relative to modern during both the YD and HS1. Lower Cdw values are interpreted as AMOC weakening, reflecting a decreased northward transport of southern-sourced higher nutrient intermediate waters by the surface return flow of AMOC. Comparison of this new Cdw record with previously published neodymium isotope and δ18O records from the same core shows synchronous transitions, further illustrating the connection between Cdw levels and AMOC strength in the Florida Straits. An increase in Cdw near 16 ka bolsters existing evidence for a resumption of upper branch AMOC strength approximately midway through Heinrich Stadial 1. The Cdw record was extended to ∼35,000 yr before present, including all of Marine Isotope Stages (MIS) 2 and part of MIS 3, and temperature and oxygen isotopic composition of seawater were also reconstructed from the same core in order to provide a fuller picture of water mass properties and circulation at this location. During the Last Glacial Maximum (LGM, 19-23 kyr before present), Cdw levels were generally low. A novel Mg/Li-derived temperature record reveals persistently cold glacial temperatures (∼3.6 ℃, two to three degrees colder than during the Holocene or MIS3). My published study is one of the first to make use of this promising new temperature proxy. In contrast to the YD and HS1, there is no indication of AMOC variability over Heinrich Stadials 2 and 3, consistent with previous studies that conclude the AMOC is less sensitive to freshwater forcing during glaciations than during the last deglaciation. While there is some inconsistency between proxies, Cdw increases over some MIS3 Dansgaard-Oeschger interstadials, providing qualified support for a strengthening in AMOC. This study highlights the distinct nature of water masses and circulation during the LGM, relative to the stadial (weak AMOC) periods of the deglaciation and Late MIS3. Seeking to resolve an apparent contradiction of AMOC trends from paleorecords of the more recent past, I applied the Cdw Florida Straits transport characterization to infer upper AMOC change over the last ~1,000 years. In combined core records from intermediate water depth with high resolution over the Common Era, there is little evidence of AMOC reduction over the Little Ice Age relative to that during the YD stadial. Limited data since 1850 CE prohibits comparison of this record to other AMOC proxies in the modern era. However, from the Medieval Warm Period through the Little Ice Age, the newly generated Cdw and Mg/Li-derived temperature records are consistent with other indicators of AMOC variability that were reconstructed from further north in the Atlantic. This agreement supports evidence for a meridionally consistent AMOC on decadal and greater timescales.
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ItemCrosslinked Polytriazoles toward Chemically Stable Separations Membranes(Georgia Institute of Technology, 2019-11-08) Hamlett, Breanne LeighMembranes have emerged as energy efficient molecular separation platforms relative to traditional energy-intensive processes in the fields of hydrocarbon separations and water desalination. However, membranes are seldom used in large scale industrial separations because they suffer from (1) high cost and (2) difficulty in tunability between different separation processes. Ultra-thin membranes can decrease operation costs and as well as decrease polymer cost by using less material. However, current ultra-thin polyamide membranes are not capable of withstanding the harsh cleaning and pretreatment conditions used in water desalination. To achieve the goal of chemical resistant membranes, the copper-catalyzed azide alkyne cycloaddition (CuAAC) reaction was used to create highly stable and chemical resistant triazole linkages throughout the polymer membrane. Additionally, triazoles were used to crosslink polymers of intrinsic microporosity (PIMs), to add to PIMs already exceptional gas properties and address PIMs deficiencies in liquid separations. Furthermore, interfacial polymerization was used to create ultra-thin polytriazole membranes. Lastly, CuO hollow fiber membranes were used as a macroporous support to simultaneously add mechanical stability to the polytriazole membranes and catalyze the CuAAC reaction on the surface, creating a polytriazole composite hollow fiber membrane.
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ItemCoherent structures in incompressible fluid flows(Georgia Institute of Technology, 2019-11-08) Short, Kimberly YovelThe work is broadly related to the transition to turbulence in pipe at intermediate Reynolds numbers and includes a discussion of two classes of structures observed during the transition to turbulence: numerically-extracted solutions of the Navier-Stokes equations (the ``exact'') and localized/patterned turbulent spots that are not themselves solutions of the Navier-Stokes equation but are nonetheless pervasive during the transition (the ``inexact''). High-dimensional descriptions of turbulence is predicted by periodic orbit theory (POT) which expects to describe turbulence exactly, as opposed to approximately. The search for relative periodic orbits and traveling waves in pipe---the constituents solutions to periodic orbit theory---are discussed. The successful search for relative periodic orbits at transitional Reynolds numbers gave a catalogue of invariant solutions; many of these solutions were continued in parameter space to find that solutions coexist with the transitional regime. In addition to collecting solutions to eventually test periodic orbit theory, an investigation into the Barkley's pipe model---a system that successfully models the transition to turbulence in pipe---was undertaken.