Matemaattis-luonnontieteellinen tiedekunta

 

Recent Submissions

  • Kangasluoma, Juha (Helsingin yliopisto, 2015)
    Atmospheric aerosols exhibit the largest uncertainties in the radiative forcings of the Earth s climate. A large fraction of the uncertainties originates from the aerosol-cloud in-teractions, a process which is not understood completely. Understanding the processes related to aerosol formation and growth will help to narrow down these uncertainties and understand the anthropogenic fraction of the radiative forcings. Atmospheric new particle formation creates a large fraction of the particles present in the atmosphere. To understand the process thoroughly, direct measurements of the par-ticle concentration and chemical composition are needed. This need has pushed the in-strument development forward, allowing measurements which were not possible five years ago: a direct measurement of the concentration and composition of sub-3 nm particles. The purpose of this thesis was to develop methods to calibrate the instruments to reliably measure sub-3 nm particles. We developed an experimental setup to reproducibly generate well-characterized sub-3 nm particles in the laboratory, with the particular aim to characterize particle counters. During the course of the work the particle generation meth-ods were modified to provide test aerosols smaller than 3 nm, of which chemical composi-tion was monitored simultaneously with a mass spectrometer. Together with the modifica-tion of the generators, commercial fine particle counters were shown to be capable of sub-2 nm particle detection Controlling the properties of the generated particles was found to be extremely important in the measurement of sub-3 nm particles. With the various test particles we studied the importance of the particle size, charging state and chemical composition on their detection, as well as the importance of the particle counter working fluid on the de-tection. The chemical composition of the particles was the most important factor determin-ing the smallest detectable size in the sub-3 nm size range. One of the particle counters, the Particle Size Magnifier (PSM) was characterized more in depth at low pressure conditions and various operation temperatures, providing in depth understanding on the instrument operation. The results provided key aspects on how the instrument should be operated in field experiments in order to obtain reliable data. In laboratory conditions the inversion routine of the particle PSM was experimentally verified against a high resolution differential mobility analyzer and an electrometer. The results of this thesis provide basic knowledge on sub-3 nm particle generation and detection. The utilization of several generation techniques and test aerosols of differ-ent chemical composition, the operation of sub-3 nm particle detectors was improved along with a more in-depth understanding on the parameters governing the particle - vapor interactions. The laboratory instrumentation used in this thesis offer tools for chemical (composition) and physical (electrical mobility, charging state) characterization of the sub-3 nm particles. Overall this thesis improved the methods dealing with sub-3 nm particles from their generation to characterization, and to their detection. The methods are directly applicable to other fields of nanomaterial production and characterization, as well as envi-ronmental and industrial nanoparticle monitoring.
  • Jokinen, Tuija (Helsingin yliopisto, 2015)
    We live in a world full of aerosols and witness their existence constantly. Changes in visibility, road dust and pollen filling the air in the spring time and even dosing some medicines are all related to aerosols. The most important aspect for this thesis is however, the formation of aerosol precursor molecules and clusters and their possible effects on aerosol properties. Different types of aerosols e. g. organic and inorganic, ice and pollen, biogenic and anthropogenic, when acting as cloud condensation nuclei (CCN), can change the optical properties of clouds and thus have different climate effects via changes in precipitation or cloud cover. Also the mechanism how the small cloud seeds are formed can have a major effect on the cloud properties. Clouds reflect and scatter radiation cooling the atmosphere but to this day aerosol effects still form the largest uncertainty in estimates of the climate of the future. Low-volatility compounds in the ambient air are the most important components in both aerosol formation and their growth to sizes that can affect cloud properties such as their reflectivity. The vapours in the atmosphere form around half of the first precursors of aerosol particles via nucleation, still molecular in size. The rest is released directly into the air e.g. by the traffic or from the oceans as sea salt. Neutral molecular size precursor substituents are difficult to detect because they lack electric charge and their concentrations rarely exceed the detection limits of the used measurement instruments. This is one of the reasons why the first steps of nucleation process are still partly unsolved after decades of research. This thesis concentrates first in development of selective methods and ultrasensitive instrumentation for the detection of acidic aerosol precursor molecules and clusters. These compounds include sulphuric acid, which is known to be highly important precursor for new particle formation. This thesis presents the first ambient measurements with the new instrumentation and even though sulphuric acid was detected in relative high concentrations during a particle formation event, no ambient cluster formation was yet observed. The same instrumentation was further used in an ultraclean chamber experiment were sulphuric acid and dimethyl amine formed growing clusters and they were detected from the smallest clusters all the way up to ~2 nm size. These compounds are likely to dominate the new particle formation with low-volatility organic compounds. Tens of thousands of volatile organic compounds (VOC) are emitted in the atmosphere by terrestrial vegetation, marine environments and anthropogenic sources, making the search for the meaningful compounds for new particle formation extremely difficult. In this thesis a new group of extremely low-volatility organic compounds (ELVOC) from terpene oxidation are introduced. They form a large source of secondary organic aerosol (SOA) and might help explaining the former big gap between the measured and the modelled SOA loads in the atmosphere. Formation of these compounds from RO2 radicals via rapid autoxidation process to stable molecules is also studied in details in laboratory experiments. This thesis also utilizes a global modelling framework, where the measurement results are applied. According to this thesis, ELVOC in particular are in crucial role when estimating SOA and CCN formation in the air. All together this thesis is a comprehensive study of both organic and inorganic low-volatility precursor molecules and clusters in the atmosphere, from their origin, formation mechanisms, and measurement techniques to their possible effects on climate now and in the future.
  • Kaasalainen, Touko (Helsingin yliopisto, 2015)
    The number of computed tomography (CT) examinations has increased in recent years due to developments in scanner technology and the increased diagnostic capabilities of CT. Nowadays, CT has become a major contributor to accumulated radiation doses from radiological examinations, accounting for approximately 60% of the overall medical radiation dose in Western countries. Ionizing radiation is generally considered harmful to health, and current knowledge suggests that the risk for stochastic effects increases linearly with radiation dose. Minimizing patient doses in CT requires effective optimization practices, including both technical and clinical approaches. CT optimization aims to reduce patients exposure to radiation without compromising image quality for diagnosis. The aim of this dissertation was to explore the feasibility of using anthropomorphic phantoms and metal-oxide-semiconductor field-effect transistors (MOSFETs) in CT optimization and patient dose measurements, and to study CT optimization in versatile clinical situations. Specifically, this thesis focused on studying the effects of patient centering on the CT scanner isocenter by determining changes in patient dose and image quality. Additionally, as a part of this thesis, we constructed and optimized ultralow-dose CT protocols for craniosynostosis imaging, and explored different optimization methods for reducing radiation exposure to eye lenses. Moreover, fetal radiation doses were assessed in the most typical CT examinations of a pregnant woman which also place the fetus at the highest risk for ionizing radiation-induced health detriments. Anthropomorphic phantoms and MOSFET dosimeters proved feasible in CT optimization even with the use of ultralow-dose levels. Patient vertical off-centering posed a common and serious problem in chest CT, as a majority of the scanned patients were positioned below the isocenter of the CT scanner, which significantly affected both radiation dose and image quality. This exposes the radiosensitive anterior surface tissues, including the breasts and thyroid gland, to greater risk. Special attention should focus on pediatric patients in particular, as they were typically miscentered lower than adults were. The use of constructed ultralow-dose CT protocols with model-based iterative reconstruction can enable craniosynostosis CT imaging with sufficient image quality for diagnosis with an effective dose of less than 20 μSv for the patient. This dose level was approximately 85% lower than the level used in routine CT protocols in the hospital for craniosynostosis, and was comparable to the radiation exposure of a plain-skull radiography examination. The most efficient method for reducing the dose to the eye lens proved to be gantry tilting, which leaves the eye lenses outside the primary radiation beam, thereby reducing the absorbed dose up to 75%. However, measurements with two different anthropomorphic head phantoms showed that patient geometry significantly affects dose-reduction capabilities. If lenses can only partially be cropped outside the primary beam, organ-based tube current modulation or bismuth shields may also be used for reducing the dose to the lenses. Based on the measured absorbed doses in this thesis, the radiation dose to the fetus poses no obstacle to an optimized CT examination with a medically necessary indication. The volumetric CT dose index (CTDIvol) provides a rough estimate of the fetal dose when the uterus is in the primary radiation beam, although the extent of the scan range has a substantial effect on the fetal dose. The results support the conception that when the fetus or uterus is not in the scan range, the fetal dose is affected mainly by the distance from the scan range.
  • Soto, Tomás (Helsingin yliopisto, 2015)
    The homogeneous Besov spaces B(s,p,q) and the homogeneous Triebel-Lizorkin spaces F(s,p,q) are two scales of function spaces on the Euclidean space that include the standard and fractional-order Sobolev spaces, and arise naturally in the contexts of e.g. partial differential equations and interpolation theory. In the first article presented in this thesis (joint work with H. Koch, P. Koskela and E. Saksman), we characterize the homeomorphisms of the Euclidean space that induce bounded composition operators on the Besov spaces whose norms are invariant under a scaling change of variable, i.e. where sp equals the dimension of the underlying space, for most indices in the smoothness range 0 < s < 1. We also establish similar results in the setting of certain more general metric measure spaces. In the second article (joint work with M. Bonk and E. Saksman), we present a new characterization of the Triebel-Lizorkin spaces for most indices in the smoothness range 0 < s < 1 in the setting of doubling metric measure spaces, and use the characterization to expand on an earlier result by Koskela, Yang and Zhou concerning the bounded composition operators on the scaling invariant Triebel-Lizorkin spaces. We also establish results concerning general Sobolev-type embeddings and complex interpolation for these function spaces in the generality of doubling metric measure spaces. In the third article, we establish several characterizations of certain larger scales of function spaces on the Euclidean space introduced by Yang and Yuan. One of these characterizations can in particular be applied in the setting of more general metric measure spaces.
  • Niinimäki, Teppo (Helsingin yliopisto, 2015)
    Bayesian networks are probabilistic graphical models, which can compactly represent complex probabilistic dependencies between a set of variables. Once learned from data or constructed by some other means, they can both give insight into the modeled domain and be used for probabilistic reasoning tasks, such as prediction of future data points. Learning a Bayesian network consists of two tasks: discovering a graphical dependency structure on variables, and finding the numerical parameters of a conditional distribution for each variable. Structure discovery has attracted considerable interest in the recent decades. Attention has mostly been paid to finding a structure that best fits the data under certain criterion. The optimization approach can lead to noisy and partly arbitrary results due to the uncertainty caused by a small amount of data. The so-called full Bayesian approach addresses this shortcoming by learning the posterior distribution of structures. In practice, the posterior distribution is summarized by constructing a representative sample of structures, or by computing marginal posterior probabilities of individual arcs or other substructures. This thesis presents algorithms for the full Bayesian approach to structure learning in Bayesian networks. Because the existing exact algorithms only scale to small networks of up to about 25 variables, we investigate sampling based, Monte Carlo methods. The state-of-the-art sampling algorithms draw orderings of variables along a Markov chain. We propose several improvements to this algorithm. First, we show that sampling partial orders instead of linear orders can lead to radically improved mixing of the Markov chain and consequently better estimates. Second, we suggest replacing Markov chain Monte Carlo by annealed importance sampling. This can further improve the accuracy of estimates and has also other advantages such as independent samples and easy parallelization. Third, we propose a way to correct the bias that is caused by sampling orderings of variables instead of structures. Fourth, we present an algorithm that can significantly speed up per-sample computations via approximation. In addition, the thesis proposes a new algorithm for so-called local learning of the Bayesian network structure. In local learning the task is to discover the neighborhood of a given target variable. In contrast to previous algorithms that are based on conditional independence tests between variables, our algorithm gives scores to larger substructures. This approach often leads to more accurate results.
  • Hänninen, Timo (Helsingin yliopisto, 2015)
    Controlling integral operators by dyadic model operators, and studying the boundedness of dyadic operators on Lebesgue spaces are central themes in dyadic harmonic analysis. This dissertation consists of an introductory part and five articles contributing to these themes. Many operators of harmonic analysis can be dominated by positive dyadic operators by using Lerner's median oscillation decomposition. In the first and fifth article, we extend this decomposition to Banach space valued functions and non-doubling measures. Dyadic shifts and paraproducts are dyadic model operators for Calderón-Zygmund operators. In the second article, we study the boundedness of these operators on unweighted Lebesgue spaces in an abstract operator-valued setting. We prove that operator-valued dyadic shifts are bounded, and we characterize the boundedness of operator-valued dyadic paraproducts. Furthermore, we extend Hytönen's dyadic representation theorem, which states that every Calderón-Zygmund operator can be represented by dyadic shifts and paraproducts, to the operator-valued setting. In the third article, we characterize the boundedness of linear and bilinear positive dyadic operators from a weighted Lebesgue space to another. We consider the case that the Lebesgue exponent of the range side is strictly less than the Lebesgue exponent of the domain side. We show that, in this range of the exponents, the Sawyer testing condition is insufficient for the boundedness. We introduce a sequential testing condition, of which the Sawyer testing condition can be viewed as an endpoint case, and prove that this testing condition is both sufficient and necessary for the boundedness. No characterization in the bilinear case was available until this article. Furthermore, we show that the sequential testing condition is necessary for the boundedness of any positive linear or bilinear operator, and hence it may be helpful in charactering the boundedness of other operators as well. In the fourth article, we characterize the boundedness of positive dyadic operators from a weighted Lebesgue space to another in an abstract operator-valued setting. The purpose is to understand which kind of testing condition is needed in this setting. We prove that an operator-valued positive dyadic operator is bounded if and only if the operator and its adjoint are each bounded on the class of all functions localized on dyadic cubes and taking values on a unit sphere. Furthermore, we show that the boundedness on unweighted Lebesgue spaces is characterized by an endpoint case of this condition. We work directly with Lebesgue spaces, without using interpolation between endpoint spaces. In the second article, we give new (in our opinion simple) proofs for the key tools that we use: decoupling inequality for martingale differences and a variant of Pythagoras' theorem for Lebesgue spaces.
  • Kajos, Maija (Helsingin yliopisto, 2015)
    Vast amount of volatile organic compounds (VOCs) are emitted into the atmosphere from various natural and manmade sources. VOCs have an important role in the atmospheric chemistry. They participate in ozone production in the planetary boundary layer and affect the oxidation capacity of the atmosphere. VOCs also contribute to the formation and growth processes of atmospheric aerosol particles, which, once large enough, can act as a cloud condensation nuclei (CCN) and influence the climate by altering the properties of clouds. Globally, VOC emissions from forest vegetation are dominating over the other sources. The circumpolar boreal forests cover more than 35% of the Earth s total forested area, making it one of the biggest biomes on planet. This thesis focuses on the biogenic VOCs in the boreal forests with regard to their shoot scale emissions to their role in the atmosphere. First, the VOC emissions of two different Larix species, L. cajanderi and L. sibirica, were measured and reported quantitatively for the first time. Larix species are the predominant trees in large parts of the Siberian forests, where the climate is too harsh for other tree species to grow. The emissions of both examined Larix species were dominated by monoterpenes similarly to other tree species with comparable emission potentials. Second, a protocol for proton transfer reaction mass spectrometer (PTR-MS) was developed for calibration and data processing of long-term and stand-alone VOC measurements. The reliability of this protocol was tested by comparing simultaneous VOC measurements of two PTR-MS and two gas chromatograph mass spectrometers (GC-MS). The detection of five compounds was analyzed in depth and strengths and weaknesses of the measurements were highlighted. Third, the increase in biogenic VOC and CCN concentrations was investigated in connection with the global warming. This was done by analyzing long-term data of concentrations and compositions of aerosol particles and their biogenic precursor VOCs in different environments. A negative aerosol-climate feedback, driven by the increase of BVOC emissions due to climate warming, was hypothesized and found.
  • Franti, Lasse (Helsingin yliopisto, 2015)
    The gauge/gravity duality connects the dynamics of gravity theories in the bulk with the dynamics of field theories on the boundary. In this thesis we introduce two thermalization scenarios and investigate them using a suitable holographic description. We will first study the thermalization of equal-time correlators and entanglement entropy in a hyperscaling violating AdS-Lifshitz-Vaidya metric. This work verifies the agreement between numerical procedures and preceding analytical predictions and generalises the previous studies of thermalization in this kind of situations. In the latter part we will use the duality to describe the quark-qluon plasma created in heavy ion collisions. The anisotropic plasma is modelled by introducing anisotropies into the source on the gravity side and letting them evolve according to the equations of motion. The boundary dynamics is extracted by finding the boundary stress-energy tensor. The results agree with the conventional models. The situations considered here are rather simple but this work demonstrates the applicability of holography in the anisotropic case.
  • Leino, Aleksi (Helsingin yliopisto, 2015)
    Ions in the keV energy range are regularly used in the semiconductor industry for device fabrication. Irradiation with ions of higher energies can also induce favorable structural changes in the irradiated samples. Among these, irradiation effects of the so-called swift heavy ions (SHIs, heavy ions with specific energies in the 1 MeV / amu range) in electrically insulating materials are particularly interesting. Despite the wide range of existing applications (filters, printed circuit boards and geological dating) and application potential (fuel cells, cell mimicking membranes) of SHI irradiation, the mechanisms by which SHIs interact with insulators are still under debate. Modelling of SHIs is a very challenging task as, contrary to ions with lower energies, they mostly interact with electrons, inducing lots of electronic excitations. Incorporating the latter with atomistic dynamics is especially difficult in insulators, and the methods have not yet been fully established. SHIs can induce a cylindrical region of structural transformation known as an ion track. In crystalline silicon dioxide, a track consists of an amorphized region that is typically several microns long and has a radius of less than ten nanometers. Furthermore, it was recently found out that SHI irradiation can be used to induce a shape transformation in metal nanoclusters (NCs) that are embedded in amorphous silicon dioxide. Spherical NCs (radius 1-50 nm) elongate along the ion beam direction and are shaped into nanorods or prolate spheroids. The phenomenon can be exploited to produce large arrays of equally aligned nanoclusters within a solid substrate, which is difficult to achieve otherwise. In this thesis, ion track formation and the elongation of gold nanoclusters in silicon dioxide are studied using so called two-temperature molecular dynamics simulations. The structure of the tracks is studied and a mechanism is proposed for the nanoparticle elongation effect. The work presented here is a step towards the understanding of SHI related effects in a broader range of insulating materials for the SHI based applications.
  • Kangas, Kaisa (Helsingin yliopisto, 2015)
    The starting point for this dissertation is whether the concept of Zariski geometry, introduced by Hrushovski and Zilber, could be generalized to the context of non-elementary classes. This leads to the axiomatization of Zariski-like structures. As our main result, we prove that if the canonical pregeometry of a Zariski-like structure is non locally modular, then the structure interprets either an algebraically closed field or a non-classical group. This is a counterpart to the result by Hrushovski and Zilber which states that an algebraically closed field can be found in a non locally modular Zariski geometry. It demonstrates that the concept of a Zariski-like structure captures one of the most essential features of a Zariski geometry. Finally, we give a non-trivial example by showing that the cover of the multiplicative group of an algebraically closed field of characteristic zero is Zariski-like. We define a Zariski-like structure as a quasiminimal pregeometry structure that has certain properties. Instead of assuming underlying topologies, we formulate the axioms for a countable collection C of Galois definable sets that have some of the properties of irreducible closed sets from the Zariski geometry context. Quasiminimal classes are abstract elementary classes (AECs) that arise from a quasiminimal pregeometry structure. They are uncountably categorical, and have both the amalgamation property (AP) and the joint embedding property (JEP), and thus also a model homogeneous universal monster model, which we denote by M. To adapt Hrushovski's and Zilber's proof to our setting, we first generalize Hrushovski's Group Configuration Theorem to the context of quasiminimal classes. For this, we develop an independence calculus that has all the usual properties of non-forking and works in our context. We then prove the group configuration theorem and apply it to find a 1-dimensional group, assuming that the canonical pregeometry obtained from the bounded closure operator is non-trivial. A field can be found under the further assumptions that M does not interpret a non-classical group and the canonical pregeometry is non locally modular. Finally, we show that the cover of the multiplicative group of an algebraically closed field, studied by e.g. Boris Zilber and Lucy Burton, provides a non-trivial example of a Zariski-like structure. Burton obtained a topology on the cover by taking sets definable by positive, quantifier-free first order formulae as the basic closed sets. This is called the PQF-topology, and the sets that are closed with respect to it are called the PQF-closed sets. We show that the cover becomes Zariski-like after adding names for a countable number of elements to the language. The axioms for a Zariski-like structure are then satisfied if the collection C is taken to consist of the PQF-closed sets that are definable over the empty set.
  • Tolppanen, Sakari (Helsingin yliopisto, 2015)
    Chemistry plays a key role in dealing with several of the big environmental problems of the future, but yet, chemistry education is often seen as irrelevant by students. Therefore, it is evident that ways to make chemistry education more relevant are called for. Educational experts have argued that sustainable development is a context that would bring relevance to science education, including chemistry education, as it bridges the gap between science and society. However, research on students perspective on the relevance of sustainable development is scarce. This thesis examines sustainable development and its education from the students viewpoint. This is done by seeking to answer the research problem: What do international students find relevant in sustainable development and its education? To answer this research problem, this thesis breaks down the problem into four research questions. The first research question examines what type of questions students ask about sustainable development, particularly in the area climate change. The second research question examines the kind of actions students take to make the world a better place. The third research question examines students expectations when applying to a non-formal educational program focused on sustainable development. The last research question examines how these expectations were met through the non-formal educational program. To address the research problem, the thesis adopted a multi-method approach, consisting of descriptive research, case studies and elements of grounded theory. The data was collected before, during and after an international youth camp, the Millennium Youth Camp held in the summers of 2010-2014. The participants of the study were 16-19 -year old students from around the world who were interested in science. The thesis consists of six interconnected studies. The first study examines the type of questions students ask about sustainable development and the second study examines the type of questions students ask about climate change, specifically. The data for these two studies were collected through an online survey from the students applying to the international youth camp. The data were analyzed using content analysis. The results indicate that students ask a variety of academic, societal and moral questions related to sustainable development. These questions cover many relevant aspects of sustainable development, and climate change specifically, and build a premise for student-centered education. In the third study, students attending the international youth camp were interviewed on the type of actions they take to make the world a better place. The data was analyzed though inductive and deductive content analysis and the results show that student actions can be categorized into three distinct groups, namely, personal responsible actions, participatory actions and future oriented actions. The fourth study used quantitative methods to address what type of expectations students have in education for sustainable development. The data was collected from students applying to the non-formal education program. The results show that in addition to wanting more knowledge on specific scientific phenomena and the nature of science, students expect to learn about societal impacts of environmental issues and discuss related moral issues. Studies four, five and six examine how the aforementioned expectations of the students can be met through non-formal education. These studies examine what type of structures and programs in the camp made the educational experience relevant for the students. The thesis concludes by asserting that students questions, actions and expectations can be used to make education for sustainable development more relevant in a number of ways. The thesis discusses the possibilities of (i) moving towards more student-centered learning, in which students questions and actions are the foundation of education, (ii) increasing relevant social and societal discussion with peers and experts, and (iii) providing students with opportunities to work on projects that address student interest. The thesis takes examples from the non-formal educational program studied and discusses how these same methods could be implemented into other similar programs or formal education.
  • Keceli, Asli (Helsingin yliopisto, 2015)
    The Standard Model of particle physics (SM) is a gauge field theory that provides a very successful description of the electromagnetic, weak and strong interactions among the elementary particles. It is in very good agreement with the precision measurements and the list of all the fundamental particles predicted by the model was completed with the discovery of the last missing piece, the Higgs boson, at the LHC in 2012. However, it is believed to be valid up to a certain energy scale and widely considered as a low-scale approximation of a more fundamental theory due to some theoretical and phenomenological issues appearing in the model. Among many alternatives, supersymmetry is considered as the most prominent candidate for new physics beyond the SM. Supersymmetry relates two different classes of the particles known as fermions and bosons. The simplest straightforward supersymmetrization of the SM is named as minimal supersymmetric Standard Model (MSSM) where minimal set of new supersymmetric particles is introduced as superpartners of the Standard Model particles. It is the most studied low-scale supersymmetric model since it has very appealing features such as containing a dark matter candidate and providing a solution to the naturalness problem of the SM. After the Higgs discovery, the parameter space of the model has been investigated in great detail and it has been observed that the measured Higgs mass can be achieved only for the parameter regions which generate a severe fine-tuning. Such large fine-tuning can be alleviated by extending the minimal field content of the model via a singlet and/or a triplet. In this thesis, we discuss the triplet extension of the supersymmetric Standard Model where the MSSM field content is enlarged by introducing a triplet chiral superfield with zero hypercharge. The first part of the thesis contains an overview of the SM and the second part is dedicated to the general features of supersymmetry. After discussing aspects of the MSSM in the third part, we discuss the triplet extended supersymmetric Standard Model where we investigate the implications of the triplet on the Higgs phenomenology. We show that the measured mass of the Higgs boson can be achieved in this model without requiring heavy third generation squarks and/or large squark mixing parameters which reduce the amount of the required fine-tuning. Afterwards, we study the charged Higgs sector where a triplet scalar field with non-zero vacuum expectation value leads to h±iZW∓ coupling at tree level. We discuss how this coupling alters the charged Higgs decay and production channels at the LHC.
  • Patokoski, Johanna (Helsingin yliopisto, 2015)
    Volatile organic compounds (VOCs) are known to be key players in the atmospheric processes. They are emitted both from natural and non-biogenic sources. Although the biogenic sources are generally dominant compared with the anthropogenic sources in some circumstances the anthropogenic emissions can dominate e.g. in densely populated areas or during pollution plumes. VOCs are ambient trace gases including a vast group of compounds. Some of the VOCs are very reactive, participating in atmospheric transformation processes e.g. secondary organic aerosol formation and growth. Some VOCs are also known to be harmful air pollutants for humans. Thus VOCs can have direct effects on air quality and secondary effects on climate. In this thesis, the aim was to define sources of VOCs, determine influence of transport and transformation of trace gases and aerosols. VOCs were measured mainly by a real time proton transfer reaction mass spectrometer. In addition, gas and liquid chromatography were used. The measurements were conducted at the rural SMEAR II and the urban background SMEAR III sites. Concentrations of VOCs were observed to have inter-annual, seasonal and diurnal variations due to meteorological factors, photochemistry and different sources. The anthropogenic influence was dominant in winter at both sites, while the biogenic influence with enhanced photochemical reactions increased in spring. The mixing and dilution effect also increased in spring and it led to decreased levels of volume mixing ratios (VMRs) in daytime especially in short-lived compounds. The VMRs of aromatic compounds were lower in spring than in winter due to lesser sources and enhanced photochemistry at both sites. At the urban background site there were more local sources than rural site and thus more variation of the diurnal patterns; higher VMRs of VOCs were observed. At both sites long-range transport was found to be an important source for long-lived VOCs. The source areas of long-lived VOCs were studied with trajectory analysis. Eastern Europe including West Russia was found to be major source area for all studied VOCs. In addition, some of them had specific source areas of their own related e.g. to their use in the solvent industry. During measurements there were two different forest fire episodes in Eastern Europe and a one-day prescribed biomass burning experiment. VMRs of VOCs related to burning were observed to increase during these pollution plumes. Atmospheric oxidation of BVOCs (biogenic VOCs) revealed that monoterpenes and their oxidation products by ozone were limiting factors for nucleation particle growth. Study of total OH reactivity showed that there are a number of unmeasured biogenic compounds which may have effects on the local atmospheric chemical processes. Keywords: variations of VOC concentrations, source analysis, long-range transport, forest fires, atmospheric chemistry
  • Niemi, Esa (Helsingin yliopisto, 2015)
    Inverse problems arise, for example, from various imaging applications in medicine and physics. Their inherent property is ill-posedness; even a very small error in the measurement data can lead to a large error in the reconstruction. To overcome this difficulty, regularization is necessary for the inversion. In this work three new computational regularization methods for limited-data inverse problems are introduced and studied. The problems of special interest are stationary and dynamic X-ray tomography (CT) with sparsely sampled X-ray projection data and acoustic inverse scattering with limited-aperture data. In the first article of this thesis we develop a computational reconstruction algorithm for solving stationary sparse-data CT problems. Sparse-data cases arise e.g. from the need to minimize radiation dose in medical imaging. The new reconstruction algorithm is based on total variation regularization, it preserves sharp features of the target and is suitable for large-scale problems such as 3D CT. Its performance is illustrated by numerical results computed from both simulated and real X-ray data. In the second and third articles we introduce an inversion method for dynamic CT application making use of a few fixed X-ray sources and detectors. In this application the attainable temporal resolution is high while the CT data measured at a single time step is extremely sparse. The inversion method is motivated by level set methods and it regularizes the problem in space-time so that certain regularity is required both in spatial and temporal directions. Some of its important theoretical aspects are analyzed, and a computational implementation of the method is tested using both simulated and real X-ray data. The new methodology provides whole new possibilities e.g. for 4D angiographic imaging with high temporal resolution. In the fourth article a numerical implementation of the so-called enclosure method by Masaru Ikehata is introduced and studied using simulated test data. The enclosure method is suitable for limited-aperture obstacle scattering problems, where one uses only one incident wave and measures the far field pattern of the scattered field on some possibly limited aperture. The name of the method comes from the fact that it finds the convex hull of the obstacle, rather than its precise shape. Numerical evidence presented suggests that the method can approximately recover the shape and position of an obstacle from noisy limited-aperture far field data.
  • Rautio, Anne (Helsingin yliopisto, 2015)
    Groundwater (GW) and surface water (SW) have been studied as separate resources in most previous hydrological or hydrogeological studies in Finland. This traditional research approach has both underestimated the commonness of natural exchange between GW and SW systems and oversimplified the management of water resources. This thesis research investigated the previously poorly recognized GW−SW interactions in two boreal snow-type catchments, Lake Pyhäjärvi and River Vantaa, with physical methods, isotopic and chemical tracers and thermal methods, particularly low-altitude aerial infrared (AIR) surveys. The aim of the work was to identify the GW−SW interactions sites, to define the ubiquity of these interactions, to assess the applicability of the used methods, to provide baseline data on seasonal variation in isotopic and chemical tracers in the studied catchments and to provide new insights into more integrated water resources management. The field studies performed in Lake Pyhäjärvi revealed that the direct GW discharge areas are associated with the esker deposits and focused on the 10-km NE shoreline of lake. The direct GW discharge was spatially highly variable due to the heterogeneous substrate in lake shoreline area. The results from the various methods correlated and confirmed the GW discharge into Lake Pyhäjärvi at the NE shoreline. The baseline data on isotopic patterns and hydrogeochemistry in the hydrological cycle were provided by a one-year monitoring survey in the Lake Pyhäjärvi catchment. The results revealed strong seasonality in general water chemistry, stable isotopes and dissolved silica concentrations of different water bodies on the catchment scale that should be considered in GW SW interaction studies in northern high-latitude regions with snow-type hydrology. Samples taken during the spring thaw and high-precipitation events could be problematic in terms of both sampling and interpreting the results. In the River Vantaa and its tributaries (a 203-km-long river channel altogether), around 370 GW discharge sites were located with two catchment scale AIR surveys in two consecutive years. The identified interaction locations in the proximity of 12 municipal water intake plants during the low-flow seasons should be considered as potential risk areas for water supply during flood periods and/or with high pumping rates and taken under consideration in water resources management. This work revealed that the GW−SW interactions are a far more general phenomenon in the studied catchments than has thus far been acknowledged, and these two resources should be integrated as one entity in hydrological studies. The GW−SW interaction should be taken account in water resources management, especially in changing climate conditions. This work highlighted the importance of an integrated approach that applies detailed local field measurements combined with chemical and isotopic sampling, as well as AIR surveys on the catchment scale.