Matemaattis-luonnontieteellinen tiedekunta


Recent Submissions

  • Hemmilä, Marja (Helsingin yliopisto, 2020)
    Atmospheric aerosol particles are small, liquid or solid pieces that are floating in the air. They have a significant effect on air quality, human health and cloud formation. Sources of aerosols can be either primary or secondary, meaning that they can directly be emitted from the source to the air (e.g. sea salt, sand or pollen) or they can be formed from the precursor gases in the air. For example, sulphuric acid, ammonia, amines and oxidised organic vapours are gases that affect the nucleation process. Biogenic Volatile Organic Compounds (VOCs) are gases that are emitted by e.g. boreal forest, and they affect secondary organic aerosol (SOA) population by contributing to the production of oxidised organic vapours that participate in the formation and growth of secondary aerosol particles. In this thesis, thermal desorption inlet gas chromatograph coupled with mass spectrometer (TD-GC-MS) was used to determine how monoterpenes, which are one sub-group of the BVOCs, are emitted from Scots pine and Norway spruce trees. It was discovered that individual trees emit different amounts of various monoterpenes, even when the trees belong to the same species. We concluded that the emissions depend on the chemotype of the trees, which is an inherited property of the individual tree. Nitrogen containing gases, such as ammonia, amines and nitric acid can also take part in the aerosol formation and growth processes. Ammonia and amines stabilise sulphuric acid clusters, therefore helping the new aerosol particles to form. Another nitrogen contain gas, HONO, strongly affects atmospheric chemistry because it reacts with solar radiation and forms a OH• radical, which is one of the main radicals in the atmosphere. We measured the seasonal and diurnal variation of ammonia, nitric acid and HONO in the boreal forest with an instrument of Measuring AeRosols and Gases in Ambient air (MARGA), which is an online ion chromatograph with a sampling system. In this thesis, I developed a method for measuring aliphatic amines from the boreal forest air. I also coupled MARGA with a mass spectrometer (MARGA-MS) and used it to measure amine concentrations from the boreal forest air, observing the seasonal and diurnal variation of atmospheric amines. While I was measuring the atmospheric concentrations, the idea that amines could be emitted from the boreal forest floor and also melting snow and thawing ground, was born. To test this hypothesis, I measured with the MARGA-MS connected to a dynamic flow through chamber emissions from the boreal forest floor. I found that the boreal forest floor is indeed a source of amines.
  • Mattinen, Miika (Helsingin yliopisto, 2020)
    Two-dimensional (2D) materials rank among the most scientifically exciting materials of the early 21st century. Transition metal dichalcogenides (TMDCs) have emerged into the spotlight due to the semiconducting nature of many TMDCs, which is in contrast to the most actively studied 2D material, semimetallic graphene. Research on the basic properties of TMDCs has been very active and fruitful, resulting in unveiling of many new phenomena and properties. Furthermore, there is a strong drive to realize the technological potential of TMDCs. For use in practical applications, TMDCs need to be synthesized as uniform films of controlled thickness on large and complex substrates. In order to realize cost-effective industrial production, the synthesis needs to be done at low temperatures using methods that are highly controllable, scalable, and repeatable. Atomic layer deposition (ALD) is an advanced gas-phase thin film deposition technique capable of fulfilling the requirements of many demanding applications. ALD has already proven its industrial applicability in fields ranging from electroluminescent displays to microelectronics, photovoltaics, and corrosion protection. To realize the potential of ALD in the deposition of TMDCs, suitable ALD precursors possessing adequate reactivity, volatility, and thermal stability have to be identified and evaluated. In this thesis, 29 precursor candidates were tested for seven metals. Successful ALD processes were developed for five 2D sulfides: MoS2, SnS2, WS2, HfS2, and ZrS2. In addition, ALD processes were developed for oxides of molybdenum and tungsten. The oxides may be converted into the respective 2D sulfides. Furthermore, α MoO3 is a 2D material by itself. The sulfide processes varied in terms of their growth behavior and morphology of the films. All of the films crystallized in 2D structures. In the case of SnS2 and WS2, crystallization required mild post-deposition annealing, which preserves the smooth morphology of the as-deposited amorphous films and gives an additional degree of freedom in processing. Particular attention was paid to the role of the substrate in the growth of TMDCs in tuning the film growth, morphology, and crystallinity. HfS2, MoS2, SnS2, and ZrS2 films were observed to grow in a van der Waals epitaxial manner on mica, which is a promising approach to achieve high film quality under mild conditions. Once the deposition processes are developed, the produced films should be evaluated for the target applications. A major challenge is to improve the performance of large-area TMDC films grown under application-relevant conditions up to the level of TMDC flakes that have been manually exfoliated from bulk crystals. The possible applications of ALD TMDCs are comprehensively reviewed in the literature part of the thesis. In the experimental part, results on photodetector (HfS2, SnS2, and ZrS2), field-effect transistor (SnS2), and hydrogen gas sensor (MoOx) devices are shown. It is anticipated that the processes developed in this thesis can be used also for other applications. For example, the rough MoS2 and disordered WS2 films should be promising for energy storage and conversion applications.
  • Hella, Olli (Helsingin yliopisto, 2020)
    Central limit theorems are one of the most central theorems in the theory of probability. They have also been actively studied in the field of dynamical systems. In the first article of this thesis an adaptation of Stein's method, introduced by Charles Stein in 1972 is presented. Our adaptation gives new correlation-decay conditions for both univariate and multivariate observables under which central limit theorem holds for time-independent dynamical systems. When these conditions are satisfied, this adaptation also yields estimates for convergence rates without much extra work. We also present a scheme for checking these conditions and consider it in two example models. In the second article the scope of this adaptation is extended further to time-dependent dynamical systems. The applicability of this method is shown for time-dependent expanding circle maps model and also for quasistatic dynamical systems, which is a new research area introduced recently by Dobbs and Stenlund. The third article considers model of time-dependent compositions of Pomeau-Manneville-type intermittent maps. In this model we also establish central limit theorems with a rate of convergence. This article uses the results in the second article and earlier work of Juho Leppänen on the functional correlation bounds for Pomeau-Manneville maps with time-dependent parameters. Quasistatic systems are also further studied and we present general conditions under which multivariate CLT for quasistatic systems holds. In the fourth article we study random compositions of transformations. We prove a theorem for almost sure convergence of variance for normalized and fiberwise centered Birkhoff sums. This in combination with other results can be used to establish quenched central limit theorems with a rate of convergence for random dynamical systems. Two examples which use the theorem in the fourth article are proved in the second and third article.
  • Mieskolainen, Mikael (Helsingin yliopisto, 2020)
    High energy diffraction probes fundamental interactions, the vacuum, and quantum mechanically coherent matter waves at asymptotic energies. In this work, we algorithmize our abstract ideas and develop a set of rigid rules for diffraction. To get spin under control, we construct a new Monte Carlo simulation engine, GRANIITTI. It is the first event generator with custom spin-dependent scattering amplitudes for the glueball domain semi-exclusive diffraction, driven by fully multithreaded importance sampling and written in C++. Our simulations provide new computational evidence that the enigmatic glueball filter observable is a spin polarization filter for tensor resonances. For algorithmic spin studies, we automate the classic Laplace spherical harmonics inverse expansion, carefully define the geometric acceptance related phase space issues and study the harmonic mixing properties systematically in different Lorentz frames. To improve the big picture, we generalize the standard soft diffraction observables and definitions by developing a high dimensional probabilistic framework based on incidence algebras, Combinatorial Superstatistics, and solve also a new superposition inverse problem using the Möbius inversion theorem. For inverting stochastic autoconvolution integral equations or `inverting the proton', we develop a novel recursive inverse algorithm based on the Fast Fourier Transform and relative entropy minimization. The first algorithmic inverse results of the proton double multiplicity structure and multiparton interaction rates are obtained using the published LHC data, in agreement with standard phenomenology. For optimal inversion of the detector efficiency response, we build the first Deep Learning based solution working in higher phase space dimensions, DeepEfficiency, which inverts the detector response on an event-by-event basis and minimizes the event generator dependence. Using the ALICE experiment proton-proton data at the LHC at 13 TeV, we obtain the first unfolded fiducial measurement of the multidimensional combinatorial partial cross sections, the first multidimensional maximum likelihood fit of the effective soft pomeron intercept and the first multidimensional maximum likelihood fit of the single, double and non-diffractive component cross sections. Great care is taken with the fiducial and non-fiducial definitions. The second topic of measurements centers on semi-exclusive central diffractive production of hadron pairs, which we study with the ALICE data. We measure and fit the resonance spectra of identified pion and kaon pairs, which is crucial on the road towards solving the mysteries of glueballs, the proton structure fluctuations, and the pomeron.
  • Nikkola, Paavo (Helsingin yliopisto, 2020)
    This thesis provides insights into the petrogenesis of Iceland basalts via three subprojects. The first uses olivine macrocrysts as a proxy for mantle melting conditions below Iceland, the second utilizes petrological thermobarometry to resolve the crustal storage conditions of the most primitive basaltic rocks (ankaramites) of the Eyjafjallajökull volcano, and the third investigates basalt fractionation processes within the Hafnarhraun pāhoehoe lava lobe. The sub-Icelandic mantle is evidently heterogeneous in composition. Yet olivine major and minor element compositions in Iceland basalts typically concur with common mantle lherzolite as the source of magmas, with the only potential exceptions being the basalts of Eyjafjallajökull and Vestmannaeyjar volcanic systems in South Iceland. These South Iceland basalts have forsteriterich olivine with relatively high Ni and low Mn contents, together with low Sc and V and high Cr, Ti, Zn, Cu and Li contents. Elevated Ni and low Mn in olivine have been attributed to olivine-free pyroxenitic mantle source; however, the South Iceland olivine compositions are best explained by the effect of comparatively high-pressure (Pfinal>1.4 GPa) and high-temperature melting of somewhat enriched olivine-bearing mantle. I conclude this because (i) elevated Ni and low Mn in olivine can also indicate deep, high-temperature, mantle melting, (ii) the abundances of Sc, V, Ti and Zn in the South Iceland olivine are compatible with low-degree partial melts of olivine-rich mantle, and (iii) melts of olivine-free pyroxenite are, according to recent models, easily consumed in reactions with subsolidus mantle peridotite and thus unlikely to migrate to the crust and crystallize olivine. The identified high-Ni/low-Mn olivine macrocrysts suggest final mantle equilibration depths greater than 45 km for South Iceland magmas, and imply effective mantle-to-surface magma transport. Two Eyjafjallajökull ankaramite outcrops (Hvammsmúli and Brattaskjól), rich in olivine (Fo81–90) and clinopyroxene (Mg#cpx 78–90) macrocrysts (~30 vol%) in near equal amounts, have a specifically prominent “deep mantle source signature” (high-Ni/low-Mn) in olivine. To investigate the crustal storage conditions of these and other Eyjafjallajökull basaltic magmas, I analyzed olivine, clinopyroxene, spinel and melt inclusion compositions from these volcanic units. These analyzes revealed that the olivine-hosted spinel inclusions have exceptionally high Cr#spl (52–80) and TiO2 (1–3 wt%) and low Al2O3 (8–22 wt%) compared to typical chromian spinels in Iceland, in line with the postulated deep and enriched mantle source of the parental magmas. According to olivine-spinel oxybarometry, these spinels crystallized under a moderate oxygen fugacity (ΔlogFMQ 0–0.5). Furthermore, jadeite-in-clinopyroxene barometry indicates clinopyroxene crystallization at a rather low pressure (1.7–4.2 kbar; external precision ±1.4 kbar), implying a magma storage depth of 10.7±5 km. Additionally, clinopyroxe-liquid, olivine-liquid and liquid only thermometry gives varying crystallization temperatures of 1120–1195 °C, 1136–1213 °C and 1155–1222 °C, respectively, for the compositionally diverse macrocrysts. The scarcity of macrocryst plagioclase and trends in clinopyroxene compositions indicate that the mid-crustal crystallizing assemblage was olivine and clinopyroxene, and plagioclase fractionated later. Diffusive re-equilibration in Brattaskjól olivine grains suggests that this crystal assemblage mobilized and erupted from its storage within a few weeks. To conclude, the Brattaskjól and Hvammsmúli crystal cargoes are agitated wehrlitic or plagioclase-wehrlitic mushes from the mid-crust that ascended to the surface relatively rapidly. Basaltic lavas are practically never primitive mantle melts owing to fractional crystallization in the crust, which, at low pressure, may be aided by volatile exsolution. Deciphering magma fractionation processes from solidified crustal intrusions is hampered by their often complex emplacement history. The emplacement of pāhoehoe lavas, however, is simpler and well understood, and hence I investigated the mechanisms of basalt fractionation from a differentiated pāhoehoe lava lobe in Hafnarhraun lava flow field. Here, volatile exsolution had facilitated separation of basaltic residual melts to form three types of melt segregations: vesicle cylinders (VC) in the core of the lobe and two types of horizontal vesicle sheets (HVS1 and HVS2) in the upper part of the lobe. Interestingly, the VC do not match chemically with the modelled residual melts of the lobe, and their formation seems to have included two stages: volatile-aid-ed melt separation from crystallizing base of the lobe and later contamination by primitive macro-and microphenocrysts in the lava core. HVS1, which resemble VC, were formed as the ascending VC diapirs accumulated to the upper solidification front of the lava lobe. HVS2, in turn, are distinctly evolved in compositions compared to other units in the lobe and were formed as highly fractionated residual melts seeped to voids in the upper crust of the lobe. Processes analogous to segregation formation at Hafnarhraun may contribute to genesis of evolved basalts and silicic rocks in shallow magmatic systems. Overall, my work highlights the exceptional nature of South Iceland among other volcanically active regions in Iceland. Furthermore, analyses of the Hafnarhraun pāhoehoe lava reveal the processes of melt segregation formation in pāhoehoe lava lobes. I hope future research will expand on these findings, further resolving the nature of mantle melting below South Iceland and the significance of volatile-aided processes in crustal magma differentiation.
  • Korhonen, Natalia (Helsingin yliopisto, 2020)
    Climate change refers to a change in the mean state of the climate that persists for an extended period, typically 30 years or longer. The natural inter-annual variability of climate refers to internal variation of the climate system in shorter time-scales. In this thesis I have studied the climate of the last glacial and its impact on human population sizes in Europe during the end of the last glaciation, the change in forest fire danger and strong winds in Europe under the on-going human-induced climate change, and the relationship between the stratospheric winds and the phase of the Arctic Oscillation in present climate. A regression model was developed to downscale low-resolution dynamical EMIC simulations to regional scale. The regression model was calibrated by gridded data of regional scale resolution observations of present day climate and simulations of glacial climate. The downscaled climate was used in estimating the size of human population in Europe during the end of the last glaciation, between 30,000 and 13,000 years ago. The simulated changes in human population size correlated significantly with an independent archeological data of changes in human population size. The change in the forest fire danger in Europe was investigated by ERA-Interim and ERA-40 reanalysis. The forest fire danger was found to have increased in Southern and Eastern Europe during the period 1980–2012, whilst no significant trend was found elsewhere in Europe. The projected changes in the geostrophic wind speeds under human-induced climate change in Northern Europe during the current century were explored from simulations of nine general circulation models. According to the simulations, the changes in mean and extreme wind speeds are going to be small; in parts of northwestern Russia and southern Baltic Sea the winds might increase by 2-4% and over the Norwegian Sea the winds might decrease by 2-8%. In this thesis the connection between the stratospheric winds and surface Arctic Oscillation was studied statistically. The found stratospheric connection was applied in post-processing the European Centre for Medium-Range Weather Forecasts two-week mean temperature reforecasts for weeks 3–4 and weeks 5–6 in Northern Europe during boreal winter, and the skill scores of those weeks were slightly improved.
  • Hyvönen, Jaakko (Finnish Botanical Publishing Board, 1989)
    Acta Botanica Fennica
    The genus Pogonatum P. Beauv. is characterized by a mammillose exo- thecium, 32 compound peristome teeth and by the absence of stomata. As so delimited die genus comprises 52 species including those species formerly assigned to Neopogonatum Xu & Xiong, Pseudatrichum Reim., Plagioracelopus Smith Merrill and Racelopus Dozy & Molk. The relationship of Pogonatum to other genera of the Polytrichaceae is evaluated and a cladogram of the genus is presented. Several monophyletic groups within the genus are recognized. The genus is divided into four subgenera. Subg. Alienum Hyvönen includes only Pogonatum volvatum (C. Mull.) Par. Subg. Dendroidea (Schimp.) Hyvönen, comb. nov. consists of three species formerly assigned to the sections Cephalotrichum (C. Miill.) Besch. and Dendroidea. Subg. Catharinella (C. Miill.) Hyvönen, comb. nov. contains 31 species including the nine species formerly accommodated in sect. Racelopus Touw. These species form a monophyletic group but distinction at sectional level is not supported by the cladistic analysis. Subg. Pogonatum comprises 17 species. The taxonomy of all species is revised including 395 specific and infraspecific combinations in the genera Neopogonatum, Plagioracelopus, Pogonatum, Pseudatrichum and Racelopus. 130 new synonyms are presented along with the selection of 13 new lectotypes. Pogonatum norrisii Hyvönen is described as new to science, and short diagnoses of other species are given with notes on phylogeny and ecology. Distribution of all species is illustrated by maps and diagnostic characters by line drawings. Citations of relevant illustrations are given. A key for the genus is presented. Discussion of morphology and anatomy is restricted to those characters used to infer the phylogeny of the genus.
  • Abera, Temesgen (Helsingin yliopisto, 2020)
    The climate system responds to changes in the structure and physiology of vegetation. These changes can be induced by seasonal growing cycles, anthropogenic land cover changes (LCCs), and precipitation extremes. The extent to which vegetation changes impact the climate depends on the type of ecosystem, the season, and the intensity of perturbations from LCCs and precipitation extremes. Under the growing impacts of climate change and human modification of natural vegetation cover, understanding and monitoring the underlying biogeophysical processes through which vegetation affects the climate are central to the development and implementation of effective land use plans and mitigation measures. In Eastern Africa (EA) the vegetation is characterized by multiple growing cycles and affected by agricultural expansion as well as recurrent and severe drought events. Nonetheless, the degrees to which vegetation changes affect the surface energy budget and land surface temperature (LST) remain uncertain. Moreover, the relative contributions of various biogeophysical mechanisms to land surface warming or cooling across biomes, seasons, and scales (regional to local) are unknown. The objective of this thesis was to analyze and quantify the climatic impacts of land changes induced by vegetation seasonal dynamics, agricultural expansion, and precipitation extremes in EA. In particular, this thesis investigated these impacts across biomes and spatio-temporal scales. To address this objective, satellite observation and meteorological data were utilized along with empirical models, observation-based metrics, and statistical methods. The results showed that rainfall–vegetation interaction had a strong impact on LST seasonality across ecoregions and rainfall modality patterns. Furthermore, seasonal LST dynamics were largely controlled by evapotranspiration (ET) changes that offset the albedo impact on the surface radiation balance. Forest loss disturbed the LST dynamics and increased local LST consistently and notably during dry seasons, whereas during the wet season its impact was limited because of strong rainfall–vegetation interaction. Moreover, drought events affected LST anomalies; however, the impact of droughts on temperature anomalies was highly regulated by vegetation greening. In addition, the conversion of forest to cropland generated the highest net warming (1.3 K) compared with other conversion types (savanna, shrubland, grassland, and cropland). Warming from the reduction of ET and surface roughness was up to ~10 times stronger than the cooling effect from albedo increases (−0.12 K). Furthermore, large scale analysis revealed a comparable warming magnitude during bushland-to-cropland conversion associated with the dominant impact of latent heat (LE) flux reduction, which outweighed the albedo effect by up to ~5 times. A similar mechanism dominated the surface feedback during precipitation extremes; where LE flux anomalies dominated the energy exchange causing the strongest LST anomaly in grassland, followed by savanna. By contrast, the impact was negligible in forest ecosystems. In conclusion, the results of this thesis clarify the mechanics and magnitude of the impacts of vegetation dynamics on LST across biomes and seasons. These results are crucial for guiding land use planning and climate change mitigation efforts in EA. The methods and results of this thesis can assist in the development of ecosystem-based mitigation strategies that are tailored to EA biomes. Moreover, they can be used for assessing the performance of climate models and observation-based global scale studies that focus on the biogeophysical impacts of LCCs. Keywords: LST seasonality; Land cover change; Bushland (Acacia-Commiphora); Biophysical effects; Precipitation extremes; Satellite observation.
  • Häkkinen, Riina (Helsingin yliopisto, 2020)
    The utilization of renewable biomass and development of greener technologies are in high demand. Especially, cellulose is a desired component in many applications, but the insolubility in most solvents limits its use. The currently used methods have also environmental issues and other concerns. To overcome these problems, ionic liquids were discovered to be able to dissolve cellulose and other renewable biomass fairly effectively. However, ionic liquids cannot be considered as “innocent” solvents, as their synthesis is not green, and they are often toxic and expensive. Deep eutectic solvents are recognized as promising green alternatives for ionic liquids and petroleum-based solvents, and are now widely used in biomass processing. Generally, deep eutectic solvents are binary mixtures formed by mixing cheap components together: a hydrogen bond donor with a hydrogen bond acceptor. The strong hydrogen bonding between the components is believed to be responsible of the reduced melting point of the mixture. As deep eutectic solvents were introduced quite recently, these novel solvents are still lacking the knowledge and understanding of their fundamentals. In this thesis, the introduction section covers the background on basic carbohydrate chemistry, ionic liquids which are able to dissolve cellulose, and deep eutectic solvents with their applications in biomass treatment. The characteristics of ionic liquids and deep eutectic solvents are compared. Generally, ionic liquids have better dissolution capability towards polysaccharides than deep eutectic solvents, but very little research has been done to explain why. The aim of this thesis is to understand and explain the fundamentals of deep eutectic solvents. Therefore, the properties affecting carbohydrate solubility, including cellulose (publication I), physicochemical properties of a new DES (publication III), solvent-solute interactions (publication I, II) and phase behavior of different alcoholic solutes in a DES (publication II) are studied in more detail. Furthermore, the unique properties of DESs are utilized in a novel way: as a green binder for Lithium-ion batteries (publication IV).
  • Mäkelä, Jarmo (Helsingin yliopisto, 2020)
    How important are different uncertainty sources when simulating the future state of the forest ecosystem in Finland? In this thesis, we examine this question and provide some answers to this broad topic by simulating 21st century ecosystem conditions with a land-ecosystem model called JSBACH and compare the results to similar simulations performed by another model called PREBAS. We consider four different sources of uncertainty that are related to 1) the model that is used to generate the future ecosystem conditions; 2) climate used to drive the model, represented by an ensemble of CMIP5 simulations; 3) RCP scenarios that depict the rising atmospheric CO2 concentration and; 4) forest management actions. Before running the simulations described above, we calibrated and validated the JSBACH model extensively on different temporal resolutions and with multiple model modifications in order to improve the site-level model representation of transpiration, evaporation and carbon assimilation of boreal forests. These hindcasting calibrations were performed with two Bayesian approaches: the adaptive Metropolis algorithm and the adaptive population importance sampler. The resulting parameter values were compared to literature and the model performance was validated with distinct data sets and independent validation sites. We then generated a suitable model setup and parameter distributions to represent the JSBACH model uncertainty in the 21st century simulations. Canonical correlation analysis and redundancy indices were used to gleam the impact of the different uncertainty sources on multiple groups of ecosystem variables. Overall, forest management actions and RCP scenarios tend to dominate the uncertainties towards the end of the century, but the effect of climate models and parameters should not be overlooked especially since a more detailed examination revealed that their impact was not fully captured.
  • Leppä-aho, Janne (Helsingin yliopisto, 2020)
    Probabilistic graphical models provide a general framework for modeling relationships between multiple random variables. The main tool in this framework is a mathematical object called graph which visualizes the assertions of conditional independence between the variables. This thesis investigates methods for learning these graphs from observational data. Regarding undirected graphical models, we propose a new scoring criterion for learning a dependence structure of a Gaussian graphical model. The scoring criterion is derived as an approximation to often intractable Bayesian marginal likelihood. We prove that the scoring criterion is consistent and demonstrate its applicability to high-dimensional problems when combined with an efficient search algorithm. Secondly, we present a non-parametric method for learning undirected graphs from continuous data. The method combines a conditional mutual information estimator with a permutation test in order to perform conditional independence testing without assuming any specific parametric distributions for the involved random variables. Accompanying this test with a constraint-based structure learning algorithm creates a method which performs well in numerical experiments when the data generating mechanisms involve non-linearities. For directed graphical models, we propose a new scoring criterion for learning Bayesian network structures from discrete data. The criterion approximates a hard-to-compute quantity called the normalized maximum likelihood. We study the theoretical properties of the score and compare it experimentally to popular alternatives. Experiments show that the proposed criterion provides a robust and safe choice for structure learning and prediction over a wide variety of different settings. Finally, as an application of directed graphical models, we derive a closed form expression for Bayesian network Fisher kernel. This provides us with a similarity measure over discrete data vectors, capable of taking into account the dependence structure between the components. We illustrate the similarity measured by this kernel with an example where we use it to seek sets of observations that are important and representative of the underlying Bayesian network model.
  • Paajanen, Antti (Helsingin yliopisto, 2020)
    Cellulose, the major component of plant matter, has a complex hierarchical structure that extends from the scale of cells down to the molecular level. Knowledge of the structural fundamentals of cellulose is relevant, not only for an understanding of plant life, but also for numerous technologies that use it as a raw material. The methods of computational physics are increasingly used to support experimental efforts in cellulose research. This thesis reports molecular and fluid dynamics simulations that address questions related to the pyrolytic degradation of cellulose and the aggregation and deaggregation of cellulose microfibrils. Cellulose pyrolysis involves hundreds of chemical reactions and volatile products, the description of which remains a formidable challenge. Here, we demonstrate the use of reactive force field methods for predicting mechanisms and kinetics of cellulose pyrolysis. We show that reactive molecular dynamics simulations can reproduce essential features of the degradation process, most notably its onset via glycosidic bond cleavage, and thus offer a means to complement quantum chemistry methods and experimental analytics. The aggregation of microfibrils is fundamental to the structural hierarchy of native cellulose and has direct implications for its processing into nanostructured forms. Here, we use atomistic simulations to elaborate on the effects of chemical modification on microfibril interactions. Our simulations reveal the sensitivity of the interaction to non-uniform substitution patterns, a feature that is not captured by continuous theoretical models. Our findings suggest a connection between uneven charge distribution and heterogeneity observed in disintegration experiments. We also investigate the structure of microfibril bundles, and their relationship to the bound water of the cell wall, using molecular dynamics simulations. The simulations predict the spontaneous formation of a twisted ribbon-like bundle with a twist rate compatible with recent experimental evidence. This also leads to a reasonable prediction for the amount of bound water, which consists of molecular water layers surrounding the fibrils, along with several other experimental indicators. Microfibril interactions also manifest themselves in the rheology of aqueous cellulose nanofibril suspensions. Here, we demonstrate the coordinated use of rheometry, printing experiments and computational fluid dynamics simulations in the development of cellulose-based hydrogels for wound dressing applications. One of our key findings is the inadequacy of rotational rheometry as a basis for models of printer head flow, and the consequent need for an alternative model building strategy.
  • Suikkanen, Einari (Helsingin yliopisto, 2020)
    Peralkaline syenites and granites form a small yet significant group of rocks within the A-type granite association worldwide. Although the Mid-Proterozoic Finnish rapakivi (A-type) granite complexes are voluminous in southern Finland, they only host minor quantities of peralkaline and marginally metaluminous syenitic rocks. Within the southeastern part of the subalkaline 1644-Ma Suomenniemi rapakivi granite complex (SE Finland), these alkali-feldspar rich syenitic rocks form numerous NW-oriented dike- and pod-like bodies, up to 5 meters in width and 100 meters in length. The Suomenniemi complex is thought to have formed by melting of granodioritic lower crust, whereas peralkaline syenites generally form by melting or fractionation of alkaline and transitional basalts, sourced in the subcontinental lithospheric mantle. Because of extensive fractionation and significant late fluid processes, peralkaline rocks are often associated with important mineralization. The origin and ore-forming potential of these rocks were studied using isotope geochemistry (single-grain zircon U-Pb and O, whole-rock Sm-Nd), mineral chemistry, whole-rock geochemistry, optical petrography and cathodoluminescence petrography. The data imply that these syenitic rocks formed in situ from the rapakivi granite either in post-magmatic (episyenites) or late-magmatic stage in the presence of a sodic fluid, and do not require a distinct magmatic source. Varying temperatures and differing fluid compositions produced geochemically and mineralogically diverse syenitic rocks, connected by relatively sodic and oxidized mineralogy and significant Si-depletion. The mechanism for the loss of Si from the rapakivi granite (or granite magma) is an elusive issue. In some, if not all, of the syenitic rocks it likely results from quartz dissolution and transport (episyenitization) after the transition from lithostatic to hydrostatically pressurized magmatic-hydrothermal system. Some of the syenitic rocks include hypersolvus feldspar and record ductile deformation, suggesting relatively high alteration temperature and pressure; if these rocks did not form in the subsolidus, a late-magmatic filter-pressing process during magmatic shearing resulted in loss of interstitial Si-rich magma and caused accumulation of K-feldspar. The ambiguous (magmatic/post-magmatic) textures of these rocks emphasize the nontrivial distinction between magmatic and (high-temperature) metasomatic processes. While episyenites are related to significant uranium and tin deposits worldwide, the economic potential of the syenitic rocks found in the Suomenniemi complex area is probably insignificant. The possible occurrence of these rocks beyond the Suomenniemi rapakivi complex, as well as the exact timing of alteration, should be constrained by further field work and radiogenic isotope dating.
  • Hyväkkö, Uula (Helsingin yliopisto, 2020)
    This thesis explores novel applications which utilize ionic liquids and electrolyte solutions in treatment of biomass for obtaining high-quality value-added products, such as cellulose, hemicelluloses and lignin as relatively pure fractions, which can be further processed into useful materials and chemicals. Albeit ionic liquids have been described as chemically and thermally highly stable solvents, the potential drawbacks of ionic liquids and electrolyte solutions regarding to their possible degradation in common processing temperatures are discussed in detail in this thesis. The potential of a highly hydrophilic aqueous organic electrolyte solution tetra-n-butylphosphonium hydroxide [P4444][OH] was studied in fractionation of wheat straw. Lignin from the biomass was extracted with varying concentations of [P4444][OH] in water. The carbohydrate rich fractions were isolated and finally all collected fractions were analyzed. The results showed that 40 w/w% aqueous solutions of [P4444][OH] were found stable. The irreversible decomposition kinetics were assessed at for 60 w/w% solution and it was not possible to obtain 70 w/w% concentrations or higher because of the decomposition at during evaporation at 25 °C. The next project was to assess the hydrolytic stability of 1,5-diazabicyclo[4.3.0]non-5-enium acetate [DBNH][OAc] using HPLC. The [DBNH][OAc] can rapidly dissolve large quantities of cellulose and can be utilized in the IONCELL-F process to convert cellulose pulp into strong cellulosic man-made fibers. In addition, synthesis routes were studied for the superbase precursors 9-methyl-1,5-diazabicyclo(4.3.0)non-5-ene (9-mDBN) which was assumed to lead to increased hydrolytic stabilities of the final ionic liquid. In the last research, aqueous solution of triethylammonium hydrogensulphate [TEAH][HSO4] was studied for fractionation of wheat straw and aspen in comparison with non-sulfate NaOH pulping and various recently discovered pulping methods utilizing a microwave reactor in all procedures. All fractions were analyzed with GPC and spectroscopic methods in order to evaluate their potential for further refining.
  • Todorov, Aleksandar (Helsingin yliopisto, 2020)
    Many pharmaceuticals, corrosion inhibitors, colorants, materials for LEDs and OLEDs, as well pH and fluorescence indicators contain as a key core structure the quinoline or quinolone motive. The chemical modifications of the quinoline and quinolone core are as important as their synthesis for the development of new materials. In order to achieve the desirable diversity the protection group chemistry, as well the functional group transformations, play a significant role in the quinoline and quinolone chemical modifications. Additionally, in many cases the tautomeric state of the hydroxyquinoline is of paramount importance for the correct functioning of the corresponding pharmaceuticals. The literature review part of the thesis deals with the quinoline and quinolone syntheses and their chemical modifications. In addition to this, the rudimentary concept of tautomerism is introduced, together with the tautomerism in hydroxyquinolines. Moreover, the basic concept of visible-light photocatalysis is presented with highlights of the reductive quenching cycles. The experimental results presented in this thesis have been published in three peer-reviewed journals. The first one covers tautomeric locking and switching and the second and third ones report visible-light photoreductions of functionalized quinolines. The design and synthesis of hydroxyquinolines equipped with two side arms with hydrogen bond accepting/donating properties and or metal chelating properties, allow us to achieve a fragile tautomeric equilibrium. This equilibrium was subject to controllable adjustment to one or the other side by metal chelation and locking or solvent addition. Deprotection of O-benzylated quinolines to the corresponding quinolones was achieved by visible-light photoreduction. The described method merges the tautomeric abilities of the masked quinolones and modern photoredox catalysis, allowing us to achieve regio- and chemoselectivity. Reduction of nitroquinolines to the corresponding aminoquinolines was achieved by visible-light photoreduction. The described method uses ascorbic acid as a sacrificial reductant and hydrogen source, which make it particularly useful. The method is green and has broad functional group tolerance.
  • Saponaro, Giulia (Helsingin yliopisto, 2020)
    Clouds play a vital role in Earth's energy balance by modulating atmospheric processes, thus it is crucial to have accurate information on their spatial and temporal variability. Furthermore, clouds are relevant in those processes involved in aerosol-cloud-radiation interactions. The work conducted and presented herein concentrates on the retrievals of cloud properties, as well as their application for climate studies. While remote sensing observation systems have been used to analyze the atmosphere and observe its changes for the last decades, climate models predict how climate will change in the future. Altogether, these sources of observations are needed to better understand cloud processes and their impact on climate. In this thesis aerosol and cloud properties from the three above mentioned sources are applied to evaluate their potential in representing cloud properties and applicability in climate studies on local, regional and global scales. One aim of this thesis focuses on evaluating cloud parameters from ground-based remote-sensing sensors and from climate models using the MODerate Imaging Spectroradiometer (MODIS) data as a reference dataset. It is found that ground-based measurements of liquid clouds are in good agreement with MODIS cloud droplet size while poor correlation is found in the amount of cloud liquid water due to the management of drizzle. The comparison of the cloud diagnostic from three climate models with MODIS data, enabled through the application of a satellite simulator, helped to understand discrepancies among models as well as discover deficiencies in their simulation processes. These findings are important to further improve the parametrization of atmospheric constituents in climate models, therefore enhancing the accuracy of climate projections. In this thesis it is also assessed the impact of aerosol particles on clouds. Satellite data can be used to derive climatically crucial quantities that are otherwise not directly retrieved (such as aerosol index and cloud droplet number concentration) which can be used to infer the sensitivity of clouds to aerosols changes. Results on the local and regional scales show that contrasting aerosol backgrounds indicate a higher sensitivity of clouds to aerosol changes in cleaner ambient air and a lower sensitivity in polluted areas, further corroborating the notion that anthropogenic emission modify clouds. On the global scale, the estimates of the aerosol-cloud interactions present, overall, a good agreement between the satellite- and model-based values which are in line with the results from other models.
  • Björkqvist, Jan-Victor (Helsingin yliopisto, 2020)
    Waves are important for both the leisure and safety of the human population. Open-sea waves have been studied since the 1940’s and their central properties are known. The wave field is described by the so called wave spectrum, which is a decomposition of the wave energy with respect to the wave frequency. In practice, the wave field is still often reduced to a few parameters, most importantly the dominant frequency (so called peak frequency) and the significant wave height. These parameters, however, does not sufficiently describe an archipelago wave field, but waves in archipelagos have still received relatively little attention from the scientific community. This thesis focuses on waves in archipelagos, and the study was carried out by using both numerical models and instrumental observations from the Helsinki archipelago and the Archipelago Sea in the Baltic Sea. Waves in archipelagos are heavily affected by the numerous small islands; they attenuate long waves arriving from the open sea, while also defining new fetches for local waves. As a result, the wave spectrum has a wide frequency range where the energy is practically constant. The existence of this energy carrying range is in contrast to open sea measurements where the energy is concentrated around one dominant frequency. This study proposed a characteristic frequency that quantified the centre of the energy carrying range. For a traditional open sea spectrum the characteristic frequency closely resembled the dominant frequency, thus making it suitable for a wide range of wave conditions. The height of single waves in the archipelago were lower relative to the significant wave height. As a consequence, there was a large (10-15%) discrepancy between two definitions of the significant wave height; in the open sea this discrepancy is typically only 7-8%. The three numerical models of this study simulated the archipelago wave field well. The largest discrepancy with the observations was found in an area just outside the archipelago that was sheltered by a peninsula. Inside the archipelago the models disagreed slightly on the energy distribution within the energy carrying range. These small differences strongly affected the dominant frequency in a way that was not representative of the good model performance. The differences were inconsequential for the significant wave height. During certain conditions the energy of the shortest waves were underestimated when using more advanced methods to calculate the energy transfer from the wind to the waves, most probably because a too small friction velocity. A simple older method to determine the friction velocity reproduced the shorter waves well. Coarse operational wind products were sufficient to force the high-resolution coastal wave models. Providing wind data only every third hour reduced the variability in the modelled wave field in the time scales between 2 and 10 hours. An hourly wind product captured all variations well, except for the statistical sampling variability in the measurements. Spatial properties of the wave field were inferred from high-frequency wave staff measurements taken by R/V Aranda. These measurements were used to form a new wave spectrum where the waves are decomposed according to their inverse phase-speed. The new spectrum agreed well with the spatial wavenumber spectrum for the shortest waves, while the frequency spectrum did not. The good agreement between the inverse phase-speed spectrum and the wavenumber spectrum meant that the effect of the Doppler shift was small. The reason for the disparate results of the frequency domain were attributed to wave non-linearities. Using direct measurements to determining the waves as a function of their phase speed can be useful when studying the interaction between the wind and the waves, since no additional current measurements are needed to quantify the real wave speed relative to the wind.
  • Leppänen, Leena (Helsingin yliopisto, 2019)
    Information on snow water equivalent (SWE) of seasonal snow is used for various purposes, including long-term climate monitoring and river discharge forecasting. Global monitoring of SWE is made feasible through remote sensing. Currently, passive microwave observations are utilized for SWE retrievals. The main challenges in the interpretation of microwave observations include the spatial variability of snow characteristics and the inaccurate characterization of snow microstructure in retrieval algorithms. Even a minor variability in snow microstructure has a notable impact to microwave emission from the snowpack. This thesis work aims to improve snow microstructure modelling and measurement methods, and understanding the influence of snow microstructure to passive microwave observations, in order to enable a more accurate SWE estimation from remote sensing observations. The thesis work applies two types of models: physical snow models and radiative transfer models that simulate microwave emission. The physical snow models use meteorological driving data to simulate physical snow characteristics, such as SWE and snow microstructure. Models are used for different purposes such as hydrological simulations and avalanche forecasting. On the other hand, microwave emission models use physical snow characteristics for predicting microwave emission from a snowpack. Microwave emission models are applied for the interpretation of spaceborne passive microwave remote sensing observations, for example. In this study, physical snow model simulations and microwave emission model simulations are compared with field observations to investigate problems in characterizing snow for microwave emission models. An extensive set of manual field measurements of snow characteristics is used for the comparisons. The measurements are collected from taiga snow in Sodankylä, northern Finland. The representativeness of the measurements is defined by investigating the spatial and temporal variability of snow characteristics. The work includes studies on microwave emission modelling from natural snowpacks and from excavated snow slabs. Radiometric observations of microwave emission from natural snowpacks are compared with simulations from three microwave emission models coupled with three physical snow models. Additionally, homogenous snow samples are excavated from the natural snowpack during the Arctic Snow Microstructure Experiment, and the incident snow characteristics and microwave emission characteristics are measured with an experimental set-up developed for this study. Predictions of two microwave emission models are compared with the radiometric observations of collected snow samples. The results indicate that none of the model configurations can accurately simulate the microwave emission from natural snowpack or snow samples. The results also suggest that the characterization of microstructure in the applied microwave emission models is not adequate.
  • Herranen, Jaana (Helsingin yliopisto, 2019)
    As the world is constantly changing, and there are concerns over a sustainable future, educating teachers for sustainability is crucial, as education is one of the most effective means to improve sustainability. Science, such as chemistry, plays a significant role in addressing sustainability issues, because chemistry can contribute to both solving as well as causing the challenges through knowledge and products that chemistry produces. Science and sustainability are inherently connected, as are the discussions over their education. On both these fields, discussions over the role of the students have emerged. In science education there has been a growing interest to educate scientifically literate students who can use scientific thinking in their own lives and in the society. This requires active participation of the students in their own learning. Sustainability education has been advocating transformative learning so that students could take action in their own lives towards sustainability. Moreover, teacher education could be developed in a direction in which the student teachers would be given possibilities to make decisions concerning the learning and teaching methods used and contents chosen, and develop their actioncompetence through active participation. However, in order to reach sustainability, all citizens should be considered as learners, not only students in schools and universities. Discussion over the learners’ roles has led to the using of terms, such as learner-centred and learner-driven learning. What these terms actually entail is, however, not always clear. In science education, learner-driven approaches are usually practiced in the form of open inquiry – an inquiry that starts with the students’ questions. Addressing and using the students’ questions is important in science education, but also in sustainability education to activate learners to think and act for sustainability. The aim of this thesis is to understand the possibilities and challenges of learner-centred and learner-driven science teacher education for sustainability. The research questions are: i) Which possibilities do learner-centred and learner-driven science teacher education for sustainability offer? and ii) What are the challenges for learner-centred and learner-driven science teacher education for sustainability? For this purpose, two types of approaches are studied: inquiry-based education as a typical approach in science teacher education from the point of view of learner-centred and learner-driven inquiry, and sustainability education as a part of science teacher education for sustainability from the viewpoint of learnercentred and learner-driven sustainability education. This is a qualitative multi-method research with one systematic review and three case studies applying grounded theory and discourse analysis. The thesis consists of four articles: i) Inquiry as a context-based practice – A case study of pre-service teachers’ beliefs and implementation of inquiry in contextbased science teaching ii) Student-question-based inquiry in science education, iii) From learner-centred to learner-driven sustainability education, and iv) Challenges and tensions in collaborative planning of a student-led course on sustainability education. Data for the studies was derived from three sources including higher education student groups and peer-reviewed articles. Study I utilised data from five student teachers who participated in a course “inquiry-based chemistry teaching” in 2015. Their beliefs about inquiry were studied by interviewing them, and their implementations of inquiry were studied from their reports. Data in study II consisted of 30 articles reviewed using systematic review. In studies III and IV, the research data consisted of a planning process of higher education students (student teachers and students interested in teaching) who planned and ran a course “sustainable development in education” in 2015. Their planning meetings and two semi-structured interviews were analysed using discourse analysis and grounded theory. As a result, understanding on the differences between learner-centred and learner-driven sustainability education was obtained. This thesis reveals that learner-driven and learnercentred education are different constructs, especially related to the learners’ roles. Studentled planning on sustainability education was studied to be challenging, as the students had to discuss several interrelated issues on sustainability and sustainability education, as well as their own roles and ways to work as a group. However, the challenges in learner-driven approaches can sometimes be viewed as part of the process. In addition, possibilities for learner-centred and learner-driven practices were revealed on how to use students’ questions in inquiries and contexts-based inquiry as a humanistic approach. For science education, a student-question-based inquiry model was created, which the teacher can use to support students in their question asking. The study also revealed challenges related to the ownership of students’ questions. The results from this thesis are relevant when planning teacher education for sustainability. This thesis points out that especially higher education has the potential to involve the students more in teaching by promoting action-competence among students through learner-driven education. Science teacher education could be focusing more on using learner-centred and learner-driven approaches, because the studied higher education students could plan and carry out teaching that mirror central aspects of science and sustainability education. Moreover, in order to be able to use learner-driven approaches, there is a need to use extra-situational knowledge, to improve students’ ownership of their own questions, to redefine expertise, and to work with non-predefined goals and with the whole community.
  • Sarnela, Nina (Helsingin yliopisto, 2019)
    Atmospheric aerosols are small liquid or solid particles suspended in the air. These small particles have significant effects to our climate and health. Approximately half of the particles that grow into cloud condensation nuclei −size are primary particles and emitted directly into the atmosphere, whereas the other half are secondary particles which are formed in the atmosphere. In new particle formation, molecular clusters form from atmospheric low-volatility vapors by condensation and/or chemical reactions. Atmospheric oxidation is a key phenomenon enhancing atmospheric particle formation since oxidized compounds condense easier due to their lower vapor pressure. So far two oxidation processes have been identified as relevant for new particle formation: the oxidation of sulfur dioxide to sulfuric acid and oxidation of volatile organic compounds to highly oxygenated compounds. The most significant atmospheric oxidants have previously thought to be ozone, hydroxyl radical and nitrate radical. Recently the importance of stabilized Criegee intermediates in atmospheric oxidation has been brought into discussion. In this thesis, we used Chemical Ionization Atmospheric Pressure interface Time of Flight mass spectrometer together with different particle measurements in order to widen the understanding of the first steps of new particle formation. We also developed new mass spectrometric measurement techniques to fill the gaps in our current methods. We developed an indirect method to measure non- OH oxidants of sulfur dioxide to better understand the role of stabilized Criegee intermediates and other non-OH oxidants of sulfur dioxide in sulfuric acid formation. We also developed a new technique to determine concentration of ambient dimethylamine at sub-pptV-level. We used both of these new techniques to measure the ambient concentrations in Boreal forest, at SMEAR II station (Station for Measuring Ecosystem-Atmosphere Relations II, Hyytiälä, Finland). Furthermore, we measured new particle formation in different environments and in a chamber study and tried to identify the condensing vapors. We studied the ozonolysis of α-pinene, the most abundant monoterpene in the atmosphere, in controlled chamber measurements in order to be able to follow the formation of highly oxygenated organics and oxidation of sulfur dioxide purely by stabilized Criegee intermediates and compare the results with kinetic model results. We studied the new particle formation near an oil refinery and found that significantly large fraction of the growth during the new particle formation events was due to sulfuric acid condensation. In our studies at the Atlantic coast, we identified the molecular steps involved in new particle formation at iodine-rich environment and could follow the growth of molecular clusters by subsequent addition of iodic acid molecules. We also did field measurements in Arctic and Antarctic sites and showed that the occurrence of high iodic acid concentration is not limited only to coastal areas with macro algae beds. Keywords: mass spectrometry, atmospheric aerosols, low-volatility vapors, ozonolysis, new particle formation

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