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  • Holmström, Eero (Helsingin yliopisto, 2012)
    Understanding radiation effects in silicon (Si) is of great technological importance. The material, being the basis of modern semiconductor electronics and photonics, is subjected to radiation already at the processing stage, and in many applications throughout the lifetime of the manufactured component. Despite decades of research, many fundamental questions on the subject are still not satisfactorily answered, and new ones arise constantly as device fabrication shifts towards the nanoscale. In this study, methods of computational physics are harnessed to tackle basic questions on the radiation response of bulk and nanostructured Si systems, as well as to explain atomic-scale phenomena underlying existing experimental results. Empirical potentials and quantum mechanical models are coupled with molecular dynamics simulations to model the response of Si to irradiation and to characterize the created crystal damage. The threshold displacement energy, i.e., the smallest recoil energy required to create a lattice defect, is determined in Si bulk and nanowires, in the latter system also as a function of mechanical strain. It is found that commonly used values for this quantity are drastically underestimated. Strain on the nanowire causes the threshold energy to drop, with an effect on defect production that is significantly higher than in an another nanostructure with similar dimensions, the carbon nanotube. Simulating ion irradiation of Si nanowires reveals that the large surface area to volume ratio of the nanostructure causes up to a three-fold enhancement in defect production as compared to bulk Si. Amorphous defect clusters created by energetic neutron bombardment are predicted, on the basis of their electronic structure and abundance, to cause a deleterious phenomenon called type inversion in Si strip detectors in high-energy physics experiments. The thinning of Si lamellae using a focused ion beam is studied in conjunction with experiment to unravel the cause for the failure of the thinning method for very thin samples. Simulations predict a mechanism of erosion of the structure which is observed as catastrophic shrinkage of the sample in experiment. The results of the thesis contribute to the understanding of fundamental questions of radiation effects in Si as well as provide explanations to known experimental conundrums. At the same time, the results unambiguously indicate that further experimental testing is needed in order to ultimately evaluate the accuracy of the theoretical predictions.
  • Järvi, Tommi (Helsingin yliopisto, 2009)
    Nanotechnology applications are entering the market in increasing numbers, nanoparticles being among the main classes of materials used. Particles can be used, e.g., for catalysing chemical reactions, such as is done in car exhaust catalysts today. They can also modify the optical and electronic properties of materials or be used as building blocks for thin film coatings on a variety of surfaces. To develop materials for specific applications, an intricate control of the particle properties, structure, size and shape is required. All these depend on a multitude of factors from methods of synthesis and deposition to post-processing. This thesis addresses the control of nanoparticle structure by low-energy cluster beam deposition and post-synthesis ion irradiation. Cluster deposition in high vacuum offers a method for obtaining precisely controlled cluster-assembled materials with minimal contamination. Due to the clusters small size, however, the cluster-surface interaction may drastically change the cluster properties on deposition. In this thesis, the deposition process of metal and alloy clusters on metallic surfaces is modelled using molecular dynamics simulations, and the mechanisms influencing cluster structure are identified. Two mechanisms, mechanical melting upon deposition and thermally activated dislocation motion, are shown to determine whether a deposited cluster will align epitaxially with its support. The semiconductor industry has used ion irradiation as a tool to modify material properties for decades. Irradiation can be used for doping, patterning surfaces, and inducing chemical ordering in alloys, just to give a few examples. The irradiation response of nanoparticles has, however, remained an almost uncharted territory. Although irradiation effects in nanoparticles embedded inside solid matrices have been studied, almost no work has been done on supported particles. In this thesis, the response of supported nanoparticles is studied systematically for heavy and light ion irradiation. The processes leading to damage production are identified and models are developed for both types of irradiation. In recent experiments, helium irradiation has been shown to induce a phase transformation from multiply twinned to single-crystalline nanoparticles in bimetallic alloys, but the nature of the transition has remained unknown. The alloys for which the effect has been observed are CuAu and FePt. It is shown in this thesis that transient amorphization leads to the observed transition and that while CuAu and FePt do not amorphize upon irradiation in bulk or as thin films, they readily do so as nanoparticles. This is the first time such an effect is demonstrated with supported particles, not embedded in a matrix where mixing is always an issue. An understanding of the above physical processes is essential, if nanoparticles are to be used in applications in an optimal way. This thesis clarifies the mechanisms which control particle morphology, and paves way for the synthesis of nanostructured materials tailored for specific applications.
  • Jansson, Ville (Helsingin yliopisto, 2013)
    Neutron irradiation induces structural nano-scale changes in steels that in the long term cause degradation of the mechanical properties of the materials. These processes are important to understand to e.g. ensure the integrity of the steel wall of the reactor pressure vessel during the operational life-time of a nuclear power plant. In this thesis, some of the irradiation defects have been studied by using as a model alloy the iron-carbon (Fe-C) system, as iron and carbon are the basic elements in any steel. The interactions between C and vacancy (V) clusters and between C and self-interstitial atom (SIA) clusters have been studied using Molecular Dynamics simulation techniques. This way C-V clusters, such as C2V and CV2, able to trap large SIA clusters, have been identified and characterized. Using Object Kinetic Monte Carlo (OKMC), an model for the radiation-induced nanostructure evolution in Fe-C has been constructed. The model was validated by reproducing experimental data in terms of V and SIA cluster densities and mean sizes from irradiation experiments at low (340 K) and high operational temperature of light water reactors (560 K), as well as reproducing data from post-irradiation annealing up to 780 K. The new model has allowed a deeper understanding of the effect of carbon on the irradiation defect evolution. It was found that the effect of the immobile C-V complexes can be introduced using generic traps for SIA and vacancy clusters. These generic traps have a binding energy that depends on the size of the trapped cluster, which is supported by previously performed atomistic studies. Different trap regimes need to be used at low and high temperatures to account for the different populations of 1/2 <111> and <100> SIA loops at different temperatures, as observed in previous TEM studies. The traps are found to have an important function as nucleation points that promote the growth of larger clusters. The nanostructure evolution model, which is the main result of this thesis, is fully based on physical considerations and only uses a few parameters for calibration. The model is found to be capable of reproducing the experimental trends both at 340 K, 560 K and for annealing up to 700 K; thereby providing insight into the physical mechanisms of importance to determine the type of nanostructural evolution undergone by Fe alloys during irradiation.
  • Smolander, Sampo (Helsingin yliopisto, 2006)
    This work develops methods to account for shoot structure in models of coniferous canopy radiative transfer. Shoot structure, as it varies along the light gradient inside canopy, affects the efficiency of light interception per unit needle area, foliage biomass, or foliage nitrogen. The clumping of needles in the shoot volume also causes a notable amount of multiple scattering of light within coniferous shoots. The effect of shoot structure on light interception is treated in the context of canopy level photosynthesis and resource use models, and the phenomenon of within-shoot multiple scattering in the context of physical canopy reflectance models for remote sensing purposes. Light interception. A method for estimating the amount of PAR (Photosynthetically Active Radiation) intercepted by a conifer shoot is presented. The method combines modelling of the directional distribution of radiation above canopy, fish-eye photographs taken at shoot locations to measure canopy gap fraction, and geometrical measurements of shoot orientation and structure. Data on light availability, shoot and needle structure and nitrogen content has been collected from canopies of Pacific silver fir (Abies amabilis (Dougl.) Forbes) and Norway spruce (Picea abies (L.) Karst.). Shoot structure acclimated to light gradient inside canopy so that more shaded shoots have better light interception efficiency. Light interception efficiency of shoots varied about two-fold per needle area, about four-fold per needle dry mass, and about five-fold per nitrogen content. Comparison of fertilized and control stands of Norway spruce indicated that light interception efficiency is not greatly affected by fertilization. Light scattering. Structure of coniferous shoots gives rise to multiple scattering of light between the needles of the shoot. Using geometric models of shoots, multiple scattering was studied by photon tracing simulations. Based on simulation results, the dependence of the scattering coefficient of shoot from the scattering coefficient of needles is shown to follow a simple one-parameter model. The single parameter, termed the recollision probability, describes the level of clumping of the needles in the shoot, is wavelength independent, and can be connected to previously used clumping indices. By using the recollision probability to correct for the within-shoot multiple scattering, canopy radiative transfer models which have used leaves as basic elements can use shoots as basic elements, and thus be applied for coniferous forests. Preliminary testing of this approach seems to explain, at least partially, why coniferous forests appear darker than broadleaved forests in satellite data.
  • Lunttila, Tuomas (Helsingin yliopisto, 2012)
    Almost all information on astrophysical objects is obtained through observation of electromagnetic radiation. The observed radiation has been altered in interactions with matter, and understanding the transport of radiation is a key prerequisite for understanding the physical conditions in the observed objects. The transport of radiation is described by the radiative transfer equation. Owing to its complex nature, solving the radiative transfer equation is difficult, and it is usually necessary to resort to numerical calculations. In this thesis, the focus is on the modelling of radiation transport in interstellar clouds. The dense gas and dust in interstellar clouds scatter, absorb, and emit radiation, and understanding the radiative transfer effects is crucial in the interpretation of observations. Four of the five articles that are contained in this thesis concern various applications of radiative transfer modelling. Two articles focus on the modelling of spectral line radiation. We study the use of OH Zeeman splitting observations in the determination of magnetic field strengths in molecular clouds. The role of magnetic fields in the process of star formation is still largely an open question with two competing models: the turbulence dominated scenario where magnetic fields are weak, and the ambipolar diffusion driven model with stronger magnetic fields. By combining magneto-hydrodynamical calculations with radiative transfer simulations, we show that the turbulence dominated scenario is consistent with the observed magnetic field strengths. Two articles concern the dust radiative transfer. We study the dust density distribution and grain properties in the dust envelope surrounding the carbon star IRC +10216. By modelling the surface brightness distribution of the scattered light in the dust envelope, we can infer the mass-loss history of the star and improve models of newly formed dust grains. In another article we use magneto-hydrodynamical calculations and radiative transfer simulations to study the reliability of cloud core mass estimates. Observations of dust thermal emission at the far-infrared and sub-millimetre wavelengths are commonly used to determine the masses of molecular cloud cores. By constructing synthetic observations of a model cloud and comparing the estimated masses to the true masses that are obtained directly from the cloud model, we can determine the robustness of mass estimates. Instead of focusing on the applications of radiative transfer modelling, one article describes new numerical methods for efficient radiative transfer simulations. We describe new algorithms for radiative transfer on hierarchical grids. The new algorithms, in particular the use of sub-iterations, are faster by a factor of several compared to the old methods.
  • Niskanen, Antti (Helsingin yliopisto, 2006)
    Atomic Layer Deposition (ALD) is a chemical, gas-phase thin film deposition method. It is known for its ability for accurate and precise thickness control, and uniform and conformal film growth. One area where ALD has not yet excelled is film deposition at low temperatures. Also deposition of metals, besides the noble metals, has proven to be quite challenging. To alleviate these limitations, more aggressive reactants are required. One such group of reactants are radicals, which may be formed by dissociating gases. Dissociation is most conveniently done with a plasma source. For example, dissociating molecular oxygen or hydrogen, oxygen or hydrogen radicals are generated. The use of radicals in ALD may surmount some of the above limitations: oxide film deposition at low temperatures may become feasible if oxygen radicals are used as they are highly reactive. Also, as hydrogen radicals are very effective reducing agents, they may be used to deposit metals. In this work, a plasma source was incorporated in an existing ALD reactor for radical generation, and the reactor was used to study five different Radical Enhanced ALD processes. The modifications to the existing reactor and the different possibilities during the modification process are discussed. The studied materials include two metals, copper and silver, and three oxides, aluminium oxide, titanium dioxide and tantalum oxide. The materials were characterized and their properties were compared to other variations of the same process, utilizing the same metal precursor, to understand what kind of effect the non-metal precursor has on the film properties and growth characteristics. Both metals were deposited successfully, and silver for the first time by ALD. The films had low resistivity and grew conformally in the ALD mode, demonstrating that the REALD of metals is true ALD. The oxide films had exceptionally high growth rates, and aluminium oxide grew at room temperature with low cycle times and resulted in good quality films. Both aluminium oxide and titanium dioxide were deposited on natural fibres without damaging the fibre. Tantalum oxide was also deposited successfully, with good electrical properties, but at slightly higher temperature than the other two oxides, due to the evaporation temperature required by the metal precursor. Overall, the ability of REALD to deposit metallic and oxide films with high quality at low temperatures was demonstrated.
  • Service, Robert (Helsingin yliopisto, 2012)
    The thesis consists of four papers in the area of mathematical biology and probability theory. Mathematical biology is an field of research which seeks understanding of biological phenomena through the application of existing and new mathematical methods. The motivating biological problem addressed in the first two papers of the thesis falls into the area of the mathematical theory of evolution, where an ecological model described by a dynamical system is equipped with a further mechanism, under which one (or more) of the species represented in the model is able to undergo evolution through mutation and natural selection. The present work examines when the possibility of displacement of a resident phenotype by a mutant of another phenotype is described in simple terms by a so-called optimisation principle. An optimisation principle is a numerical function, defined for phenotypes, that allows one to compare the potential to invade all potential environments set by some currently present phenotype simultaneously. The main result of the first paper gives a set of necessary and sufficient conditions for when an optimization principle exists. The third and fourth papers in the thesis deal from different viewpoints with topics connected to Poisson point processes.
  • Riekki, Kirsi (Helsingin yliopisto, 2012)
    The self-organized growth of nanodots and size selection are studied using reaction kinetic model rate equations. Two independent numerical methods and a mesoscopic continuous model are used to solve and analytically predict the details of the stationary nanodot size distribution. The strongly reversible growth of kinetic origin is studied. The power-law distributions which are common in nature, display scaling of the size distribution with clearly defined scaling exponents. The stochastic simulation results and predictions of continuous model are in good agreement. The self assembly of nanodots, observed in experiments and enabling the industrial use of dots in electronics, arises from the strain in heteroepitaxial growth systems and leads to uniform size distributions. To model the size selection, the size dependent thermodynamical energy of the nanodot is included into the reaction kinetics. The resulting distribution is studied in detail to resolve the overshooting phenomenon in which the mean of the distribution exceeds the thermodynamically favored size. The physical origin of the overshooting is explained as a combination of the reaction kinetics and the thermodynamical energy. The skewness of the size distribution is found from the numerical data, and it is added into the continuous model as a parameter to obtain an analytical estimate of the mean size. The predictions of overshooting are calculated for two different types of growth; the 3D metal nanodots and semiconductor nanodots with double-well thermodynamical energy. The optimal, narrow size distributions are found, and external adatom flux from e.g. an external adatom source or ion beam assisted deposition improves the size selection by driving the size distribution to the narrowest location. Nucleation theory calculations of the thermodynamically stable distributions are performed, and the results are comparable to numerical and modelling results.
  • Lahtinen, Maarit (Helsingin yliopisto, 2013)
    Laccases (EC 1.10.3.2, benzenediol: oxygen oxidoreductase) are multicopper oxidases that can catalyze the oxidation of several, mainly different phenolic but also some inorganic substrates. Laccases selectively catalyze the one-electron oxidation of a phenolic substrate to a phenoxy radical, which can react further in non-enzymatic radical reactions. The phenolic subunits of lignin, one of the major components of wood, are natural substrates of laccases. In the presence of suitable small molecules, mediators, laccases can also catalyze the oxidation of the etherified (i.e. non-phenolic) subunits of lignin. The aim of this research is to increase the knowledge on the direct reactions of laccases and lignin, without mediators. Recently, this area has begun to garner increasing general interest as a result of the biorefinery concept, which aims to produce valuable raw material from sustainable resources. In addition, the most recent development of laccase mediators has been focusing on lignin-based phenolic molecules, which links these two areas, the laccase-mediator system and reactions without the mediator, directly to each other. Monomeric and dimeric lignin model compounds were used to evaluate the reactions and reactivity with laccases. Many of the model compounds represented the most common linkage-type in lignin, the beta-O-4 structure; thus, more efficient ways to synthesize these types of compounds were developed. Further, the oxidizabilities of the compounds, revealed by cyclic voltammetry, and the oxidation rates using the low- and the high-redox potential laccases from M. albomyces and T. hirsuta were compared in view of the theory that the reaction rate is dependent on the redox potential difference between the substrate and the laccase. However, it was found that the redox potential difference could not entirely explain the preferences of the studied laccases. The reaction products from the lignin model compounds were mainly formed as a result of 5-5 coupling and oxidation of the benzylic hydroxyl group to an aldehyde. The analysis was also performed as a function of time; for guaiacylic products the 5-5 coupling was the preferred reaction and these products were formed first. One model compound, vanillyl alcohol, was used to examine the effect of pH, enzyme dosage and temperature, all of which affected the product distribution. The observed predominating product was the 5-5 dimer, although according to computational evaluation, vanillin was the thermodynamically favored product, with a difference of 5.6 kcal mol-1. The transition states leading to the products seemed to affect the observed product distribution. In addition, the calculated pKa-values suggested that at the used pH range (4.5 7.5), rearomatization of the quinone intermediates could occur through deprotonation rather than through protonation. Finally, the M. albomyces laccase was tested in the presence of 1-allyl-3-methylimidazolium chloride, [Amim]Cl; an ionic liquid able to dissolve lignin. An expected decrease in enzyme activity was also found experimentally. The monolignol coniferyl alcohol was polymerized further, as expected, but the chemical structure of the formed dehydropolymer (DHP) was also affected by the presence of [Amim]Cl.
  • Lindfors, Anders (Helsingin yliopisto, 2007)
    Solar ultraviolet (UV) radiation has a broad range of effects concerning life on Earth. Soon after the mid-1980s, it was recognized that the stratospheric ozone content was declining over large areas of the globe. Because the stratospheric ozone layer protects life on Earth from harmful UV radiation, this lead to concern about possible changes in the UV radiation due to anthropogenic activity. Initiated by this concern, many stations for monitoring of the surface UV radiation were founded in the late 1980s and early 1990s. As a consequence, there is an apparent lack of information on UV radiation further in the past: measurements cannot tell us how the UV radiation levels have changed on time scales of, for instance, several decades. The aim of this thesis was to improve our understanding of past variations in the surface UV radiation by developing techniques for UV reconstruction. Such techniques utilize commonly available meteorological data together with measurements of the total ozone column for reconstructing, or estimating, the amount of UV radiation reaching Earth's surface in the past. Two different techniques for UV reconstruction were developed. Both are based on first calculating the clear-sky UV radiation using a radiative transfer model. The clear-sky value is then corrected for the effect of clouds based on either (i) sunshine duration or (ii) pyranometer measurements. Both techniques account also for the variations in the surface albedo caused by snow, whereas aerosols are included as a typical climatological aerosol load. Using these methods, long time series of reconstructed UV radiation were produced for five European locations, namely Sodankylä and Jokioinen in Finland, Bergen in Norway, Norrköping in Sweden, and Davos in Switzerland. Both UV reconstruction techniques developed in this thesis account for the greater part of the factors affecting the amount of UV radiation reaching the Earth's surface. Thus, they are considered reliable and trustworthy, as suggested also by the good performance of the methods. The pyranometer-based method shows better performance than the sunshine-based method, especially for daily values. For monthly values, the difference between the performances of the methods is smaller, indicating that the sunshine-based method is roughly as good as the pyranometer-based for assessing long-term changes in the surface UV radiation. The time series of reconstructed UV radiation produced in this thesis provide new insight into the past UV radiation climate and how the UV radiation has varied throughout the years. Especially the sunshine-based UV time series, extending back to 1926 and 1950 at Davos and Sodankylä, respectively, also put the recent changes driven by the ozone decline observed over the last few decades into perspective. At Davos, the reconstructed UV over the period 1926-2003 shows considerable variation throughout the entire period, with high values in the mid-1940s, early 1960s, and in the 1990s. Moreover, the variations prior to 1980 were found to be caused primarily by variations in the cloudiness, while the increase of 4.5 %/decade over the period 1979-1999 was supported by both the decline in the total ozone column and changes in the cloudiness. Of the other stations included in this work, both Sodankylä and Norrköping show a clear increase in the UV radiation since the early 1980s (3-4 %/decade), driven primarily by changes in the cloudiness, and to a lesser extent by the diminution of the total ozone. At Jokioinen, a weak increase was found, while at Bergen there was no considerable overall change in the UV radiation level.
  • Holopainen, Jari (Helsingin yliopisto, 2006)
    In this paper both documentary and natural proxy data have been used to improve the accuracy of palaeoclimatic knowledge in Finland since the 18th century. Early meteorological observations from Turku (1748-1800) were analyzed first as a potential source of climate variability. The reliability of the calculated mean temperatures was evaluated by comparing them with those of contemporary temperature records from Stockholm, St. Petersburg and Uppsala. The resulting monthly, seasonal and yearly mean temperatures from 1748 to 1800 were compared with the present day mean values (1961-1990): the comparison suggests that the winters of the period 1749-1800 were 0.8 ºC colder than today, while the summers were 0.4 ºC warmer. Over the same period, springs were 0.9 ºC and autumns 0.1 ºC colder than today. Despite their uncertainties when compared with modern meteorological data, early temperature measurements offer direct and daily information about the weather for all months of the year, in contrast with other proxies. Secondly, early meteorological observations from Tornio (1737-1749) and Ylitornio (1792-1838) were used to study the temporal behaviour of the climate-tree growth relationship during the past three centuries in northern Finland. Analyses showed that the correlations between ring widths and mid-summer (July) temperatures did not vary significantly as a function of time. Early (June) and late summer (August) mean temperatures were secondary to mid-summer temperatures in controlling the radial growth. According the dataset used, there was no clear signature of temporally reduced sensitivity of Scots pine ring widths to mid-summer temperatures over the periods of early and modern meteorological observations. Thirdly, plant phenological data with tree-rings from south-west Finland since 1750 were examined as a palaeoclimate indicator. The information from the fragmentary, partly overlapping, partly nonsystematically biased plant phenological records of 14 different phenomena were combined into one continuous time series of phenological indices. The indices were found to be reliable indicators of the February to June temperature variations. In contrast, there was no correlation between the phenological indices and the precipitation data. Moreover, the correlations between the studied tree-rings and spring temperatures varied as a function of time and hence, their use in palaeoclimate reconstruction is questionable. The use of present tree-ring datasets for palaeoclimate purposes may become possible after the application of more sophisticated calibration methods. Climate variability since the 18th century is perhaps best seen in the fourth paper study of the multiproxy spring temperature reconstruction of south-west Finland. With the help of transfer functions, an attempt has been made to utilize both documentary and natural proxies. The reconstruction was verified with statistics showing a high degree of validity between the reconstructed and observed temperatures. According to the proxies and modern meteorological observations from Turku, springs have become warmer and have featured a warming trend since around the 1850s. Over the period of 1750 to around 1850, springs featured larger multidecadal low-frequency variability, as well as a smaller range of annual temperature variations. The coldest springtimes occurred around the 1840s and 1850s and the first decade of the 19th century. Particularly warm periods occurred in the 1760s, 1790s, 1820s, 1930s, 1970s and from 1987 onwards, although in this period cold springs occurred, such as the springs of 1994 and 1996. On the basis of the available material, long-term temperature changes have been related to changes in the atmospheric circulation, such as the North Atlantic Oscillation (February-June).
  • Setälä, Harri (Helsingin yliopisto, 2008)
    Dimeric phenolic compounds lignans and dilignols form in the so-called oxidative coupling reaction of phenols. Enzymes such as peroxidases and lac-cases catalyze the reaction using hydrogen peroxide or oxygen respectively as oxidant generating phenoxy radicals which couple together according to certain rules. In this thesis, the effects of the structures of starting materials mono-lignols and the effects of reaction conditions such as pH and solvent system on this coupling mechanism and on its regio- and stereoselectivity have been studied. After the primary coupling of two phenoxy radicals a very reactive quinone me-thide intermediate is formed. This intermediate reacts quickly with a suitable nucleophile which can be, for example, an intramolecular hydroxyl group or another nucleophile such as water, methanol, or a phenolic compound in the reaction system. This reaction is catalyzed by acids. After the nucleophilic addi-tion to the quinone methide, other hydrolytic reactions, rearrangements, and elimination reactions occur leading finally to stable dimeric structures called lignans or dilignols. Similar reactions occur also in the so-called lignification process when monolignol (or dilignol) reacts with the growing lignin polymer. New kinds of structures have been observed in this thesis. The dimeric com-pounds with so-called spirodienone structure have been observed to form both in the dehydrodimerization of methyl sinapate and in the beta-1-type cross-coupling reaction of two different monolignols. This beta-1-type dilignol with a spirodienone structure was the first synthetized and published dilignol model compound, and at present, it has been observed to exist as a fundamental construction unit in lignins. The enantioselectivity of the oxidative coupling reaction was also studied for obtaining enantiopure lignans and dilignols. A rather good enantioselectivity was obtained in the oxidative coupling reaction of two monolignols with chiral auxiliary substituents using peroxidase/H2O2 as an oxidation system. This observation was published as one of the first enantioselective oxidative coupling reaction of phenols. Pure enantiomers of lignans were also obtained by using chiral cryogenic chromatography as a chiral resolution technique. This technique was shown to be an alternative route to prepare enantiopure lignans or lignin model compounds in a preparative scale.
  • Härkönen, Jari (Helsingin yliopisto, 2002)
  • Laitinen, Tiera (Helsingin yliopisto, 2007)
    Tässä väitöskirjassa perehdytään magneettisen rekonnektion ilmenemismuotoihin ja vaikutuksiin Maan magnetosfäärissä. Keskeisenä tutkimusvälineenä käytetään magnetohydrodynaamista (MHD) Gumics-magnetosfäärisimulaatiota. Työssä kehitetään myös uusia menetelmiä simulaatiossa ilmenevän rekonnektion tunnistamiseksi ja mittaamiseksi. MHD-simulaatio sopii suuren mittakaavan ilmiöiden tarkasteluun, joten kuvaa rekonnektiosta täydennetään pienen mittakaavan piirteiden osalta Cluster-satelliittien avulla. Tärkein tutkimuksen tuoma edistysaskel menetelmien saralla on rekonnektioviivan paikallistaminen topologisesti erityyppisten magneettikenttäviivojen alueiden liitoskohdassa olevana erottajaviivana neljän kentän tienoon menetelmää käyttäen. Tämä topologinen lähestymistapa on hyödyllinen erityisesti magnetopausilla, jonka monimutkainen geometria tekee magneettikentän paikalliseen käyttäytymiseen perustuvien rekonnektioviivan etsintätapojen soveltamisen hankalaksi. Topologisesti määritelty rekonnektioviiva on myös helppo tunnistaa magnetosfäärin globaalin konvektion solmukohdaksi. Magnetopausin rekonnektioviivan käyttäytyminen Gumicsissa noudattaa komponenttirekonnektio-olettamaan pohjautuvia teoreettisia ennusteita. Rekonnektion kvantitatiivinen tarkastelu Gumics-simulaatiossa perustuu energian muuntumiseen, joka lasketaan Poyntingin vektorin divergenssinä tai Poyntingin vuona valitun umpinaisen pinnan läpi. Rekonnektioon liittyvän energian muuntumisen jakautumista magnetopausilla tarkastellaan energian muuntumisen pintatiheyden avulla ja rekonnektion kokonaismäärää rekonnektiotehon avulla. Magnetopausin ja pyrstön rekonnektiotehot ovat simulaatiossa samaa suuruusluokkaa. Tärkeimmät magnetopausin rekonnektiotehoa säätelevät parametrit ovat aurinkotuulen nopeus ja aurinkotuulen magneettikentän suunta. Magnetopausin rekonnektio puolestaan säätelee energian ja aineen pääsyä magnetosfääriin, joskaan magnetopausin läpäisevät vuot eivät ole aivan suoraan verrannollisia rekonnektiotehoon. Pyrstön rekonnektioteho sen sijaan on suoraan verrannollinen magnetopausilta tulevaan energiavuohon; pyrstörekonnektio Gumicsissa on siis ulkoista pakotetta seuraava passiivinen energian käsittelijä. Simulaation tuottama rekonnektio on realistinen magnetosfäärin globaalissa mittakaavassa tarkasteltuna, mutta satelliittihavainnot paljastavat rekonnektiosta simulaation erottelukykyä pienimittakaavaisempia piirteitä. Havaintopuolella tämän väitöstutkimuksen tärkein löytö on protonien diffuusioalueen rakenteeseen kuuluvien Hallin kenttien kääntyminen pyrstön virtalevyn aaltoilun mukana.
  • Snellman, Jan (Helsingin yliopisto, 2015)
    The mathematical description of turbulence is one of the greatest unresolved problems of modern physics. Many targets of astrophysical research, such as stellar convection zones and accretion discs, are very turbulent. Especially, the understanding of stellar convection zones is important for the theory of stellar evolution. Therefore, it is necessary to use approximate descriptions for turbulence while modelling these objects. One approximate method for describing turbulence is to divide the quantities under study into mean and fluctuating parts, the latter of which represent small scale changes present in turbulence. This approach is known as the Reynolds decomposition, which makes it possible to derive equations for the mean quantities. The equations acquired depend on correlations of the fluctuating quantities, such as the correlations of the fluctuating velocity components known as the Reynolds stresses, and turbulent heat and passive scalar fluxes. A mathematically precise way of handling these correlations is to derive equations also for them, but the resultant equations will depend on new, higher order correlations. If one derives equations for these new correlations, a new set of even higher order correlations is involved, and the equation system will not be closed. This is called the closure problem. The closure problem can be circumvented by using approximations known as closure models, which work by replacing the higher order correlations with lower order ones, thereby creating a closed system. Second order closure models, in which the third order correlations have been replaced by relaxation terms of second order, are studied in this Thesis by comparing their results with those of direct numerical simulations (DNS). The two closure models studied are the minimal tau approximation (MTA) and the isotropising variable relaxation time (IVRT) closure. The physical phenomena, to which the closures were applied, included homogeneous isotropically forced turbulence with rotation and shear, compressible as well as homogeneous Boussinesq convection, decaying turbulence, and passive scalar transport. In the case of homogeneous isotropic turbulence it was found that MTA is capable of reproducing the DNS results with Strouhal numbers of about unity. It was also found that the Reynolds stress components, contributing to angular momentum transport in accretion discs, can change sign depending on rotation rate, which was seen in studies of compressible convection too, meaning that convection can potentially contribute to accretion of matter. Decaying turbulence studies indicated that the relaxation time scales occurring in the relaxation closures tend to constant values at high Reynolds numbers, and this was also observed when studying passive scalar transport. However, in studies concerning Boussinesq convection no asymptotic behaviour was found as a function of the Rayleigh and Taylor numbers. The correspondence of the closure models to direct numerical simulations is found to be generally achievable, but with varying quality depending on the physical situation. Given the asymptotic behaviour of the optimum closure parameters for forced turbulence, they can be considered universal in this case. For rotating Boussinesq convection the same conclusion cannot be drawn with respect to the Rayleigh and Taylor numbers.
  • Vainonen-Ahlgren, Elizaveta (Helsingin yliopisto, 2000)
  • Vehkalahti, Kimmo (Helsingin yliopisto, 2000)
  • Tuomenvirta, Heikki (Helsingin yliopisto, 2004)
  • Sundström, Anu-Maija (Helsingin yliopisto, 2014)
    Atmospheric aerosol particles affect public health, environment, weather and climate in various ways, and therefore the importance on obtaining information about their spatial and temporal variation is evident. Remote sensing measurements have particular capability to provide broad horizontal and/or vertical view on the ambient aerosol field from local to global scales. They also can provide observations over remote areas where carrying out in situ measurements is not possible. The aim of this Thesis, is to explore both ground-based and spaceborne remote sensing measurement techniques for monitoring aerosol particles, and their applications on air quality as well as climate studies. In the first part of this Thesis the potential of a ground-based ceilometer-type lidar to be used as an aerosol measurement device is investigated. Ceilometers are originally designed for observing cloud heights, and at the time of the study they were not commonly used to monitor aerosols. The results obtained in this study indicate that the absolute accuracy of a ceilometer-type lidar is sufficient for quantitative aerosol measurements in some applications. The first study using an improved version of the AATSR (Advanced Along-Track Scanning Radiometer) satellite algorithm shows that aerosol optical depth (AOD) can be retrieved with sufficient accuracy over Eastern China, where the aerosol conditions are highly variable and therefore challenging from the satellite remote sensing point of view. In addition, the improved version of the algorithm provides also valuable information about the fine mode particle contribution to the total AOD. The satellite based AOD data is also used to evaluate the performance of a coupled climate-aerosol model. The comparison of ECHAM5-HAM model and satellite-based AOD (from MODerate Imaging Spectroradiometer) showed that, with few exceptions, the model reproduced relatively well the spatiotemporal variation of AOD over India and China. In this Thesis it is also shown that satellite data can be used to derive such climatically relevant quantities that are not directly available in common retrieval products (such as e.g. AOD). By combining coincident observations from two different satellite instruments, an observation-based estimate of the clear-sky shortwave aerosol direct radiative effect ADRE (at the top of the atmosphere) can be established. Results of the case study over Eastern China show that, overall, the satellite-based estimates of ADRE, aerosol-free fluxes, and their spatial variation are in agreement with model-based values.