Browsing by Title

Sort by: Order: Results:

Now showing items 302-321 of 949
  • Laakso, Teemu (Helsingin yliopisto, 2011)
    Hamiltonian systems in stellar and planetary dynamics are typically near integrable. For example, Solar System planets are almost in two-body orbits, and in simulations of the Galaxy, the orbits of stars seem regular. For such systems, sophisticated numerical methods can be developed through integrable approximations. Following this theme, we discuss three distinct problems. We start by considering numerical integration techniques for planetary systems. Perturbation methods (that utilize the integrability of the two-body motion) are preferred over conventional "blind" integration schemes. We introduce perturbation methods formulated with Cartesian variables. In our numerical comparisons, these are superior to their conventional counterparts, but, by definition, lack the energy-preserving properties of symplectic integrators. However, they are exceptionally well suited for relatively short-term integrations in which moderately high positional accuracy is required. The next exercise falls into the category of stability questions in solar systems. Traditionally, the interest has been on the orbital stability of planets, which have been quantified, e.g., by Liapunov exponents. We offer a complementary aspect by considering the protective effect that massive gas giants, like Jupiter, can offer to Earth-like planets inside the habitable zone of a planetary system. Our method produces a single quantity, called the escape rate, which characterizes the system of giant planets. We obtain some interesting results by computing escape rates for the Solar System. Galaxy modelling is our third and final topic. Because of the sheer number of stars (about 10^11 in Milky Way) galaxies are often modelled as smooth potentials hosting distributions of stars. Unfortunately, only a handful of suitable potentials are integrable (harmonic oscillator, isochrone and Stäckel potential). This severely limits the possibilities of finding an integrable approximation for an observed galaxy. A solution to this problem is torus construction; a method for numerically creating a foliation of invariant phase-space tori corresponding to a given target Hamiltonian. Canonically, the invariant tori are constructed by deforming the tori of some existing integrable toy Hamiltonian. Our contribution is to demonstrate how this can be accomplished by using a Stäckel toy Hamiltonian in ellipsoidal coordinates.
  • Alves Antunes Soares, Joana Soares (2016)
    Atmospheric aerosols are subject to extensive research, due to their effect on air quality, human health and ecosystems, and hold a pivotal role in the Earth s climate. The first focus of this study is to improve the modelling of aerosol emissions and its dispersion in the atmosphere in both spatial and temporal scales and secondly, to integrate the dispersion modelling with population activity data that leads to exposure metrics. The mathematical models used in this study are fully or partially developed by the Finnish Meteorological Institute: a global-to -mesoscale chemical transport model, SILAM; a local-scale point/line-source dispersion model, UDM/CAR-FMI; and a human exposure and intake fraction assessment model, EXPAND. One of the outcomes of this work was the refinement of the emissions modelling for global-to-mesoscale dispersion model. Firstly, a new parameterisation for bubble-mediated sea salt emissions has been developed by combining and re-assessing widely used formulations and datasets. This parameterisation takes into account the effects of wind speed and seawater salinity and temperature, and can be applicable to particles with dry diameters raging between 0.01 and 10 µm. The parameterization is valid for low-to-moderate wind speed, seawater salinity ranging between 0 and 33 and seawater temperature ranging between -2 and 25 °C. Secondly, the near-real time fire estimation system, IS4FIRES, based on Fire Radiative Power (FRP) data from MODIS, was refined to reduce the overestimation of particulate matter (PM) emissions by including more vegetation types, improving the diurnal variation, removing highly-energetic sources and recalibrating the emission factors. Applying dynamic emission modelling brought more insight to the spatial distribution of these emissions, their contribution to the atmospheric budget, and possible impact on air quality and climate. The modelling shows that sea salt aerosol (SSA) can be transported far over land and contribute up to 6 µg m-3 to PM10 (at annual level), and indicate that the Mediterranean has sharp gradients of concentrations, becoming an interesting area to analyse for climate considerations. For fire, the simulations show the importance of meteorology and vegetation type for the intensity of the emissions. The simulations also show that MODIS FRP is accounting for highly energetic sources as a wildland fire, bringing up to an 80% overestimation in AOD, close to the misattributed sources. The second outcome is related to urban-scale modelling. The emissions for Helsinki Metropolitan Area (HMA) were revised to bring up-to-date the emissions for traffic and energy sectors in use for urban-scale modelling. The EXPAND model was revised to combine concentrations and activity data in order to compute parameters such as population exposure or intake fraction. EXPAND includes improvements of the associated urban emission and dispersion modelling system, time use of population, and infiltration coefficients from outdoor to indoor air. This refinement showed that PM2.5 in HMA is mainly originated from long-range transport, with the largest local contributors being vehicular emissions and shipping (at harbours and its vicinity). At annual level, the population living mostly indoors (home and work) is mainly exposed to PM2.5 with an acutely increased exposure while commuting.
  • Haapanala, Sami (Helsingin yliopisto, 2012)
    Volatile organic compounds (VOCs) are reactive trace gases abundant in the atmosphere. As strongly reactive constituents they have many impacts on the atmospheric chemistry. To properly understand the processes involved in VOC cycles within the biosphere and atmosphere, we must have comprehensive knowledge on the biogenic and anthropogenic emissions and atmospheric concentrations. In this thesis, the emissions of VOCs were measured in many scales from various Fennoscandian ecosystems, utilizing several methods. First, enclosure method was applied to study the branch scale VOC emission from mountain birch. The measurements were conducted during two successive growing seasons. Significant emissions of sesquiterpenes were observed in the first year but not in the second one. Second, relaxed eddy accumulation (REA) method was used to study the ecosystem scale emissions of light hydrocarbons from a wetland. In addition to methane, the only significant emission was that of isoprene. Isoprene emission potential was found to be about 680 μg m-2 h-1. The REA method was further developed by studying the possible stability dependency of the method. Another ecosystem scale flux measurement method, disjunct eddy accumulation (DEA), was applied to study monoterpene emissions from a timber felling area. The emissions of monoterpenes were very high for a few months after felling, peaking at 5200 μg m−2 h−1. Despite of the short period those emissions seem to be of great importance for the atmospheric VOC burden. Third, profiles of boundary layer concentrations of some biogenic and anthropogenic VOCs were measured using a hot air balloon as a platform. Landscape scale emissions were inferred using the concentration data. Overall, our understanding of the previously poorly known VOC emissions from some Fennoscandian ecosystems was increased. This study highlights the importance to quantify emission hotspots of even marginal ecosystems to get comprehensive understanding for emission inventories and future redictions. The correct choice of measuring methods and experiment setup is essential for reliable field data.
  • Kajos, Maija (Helsingin yliopisto, 2015)
    Vast amount of volatile organic compounds (VOCs) are emitted into the atmosphere from various natural and manmade sources. VOCs have an important role in the atmospheric chemistry. They participate in ozone production in the planetary boundary layer and affect the oxidation capacity of the atmosphere. VOCs also contribute to the formation and growth processes of atmospheric aerosol particles, which, once large enough, can act as a cloud condensation nuclei (CCN) and influence the climate by altering the properties of clouds. Globally, VOC emissions from forest vegetation are dominating over the other sources. The circumpolar boreal forests cover more than 35% of the Earth s total forested area, making it one of the biggest biomes on planet. This thesis focuses on the biogenic VOCs in the boreal forests with regard to their shoot scale emissions to their role in the atmosphere. First, the VOC emissions of two different Larix species, L. cajanderi and L. sibirica, were measured and reported quantitatively for the first time. Larix species are the predominant trees in large parts of the Siberian forests, where the climate is too harsh for other tree species to grow. The emissions of both examined Larix species were dominated by monoterpenes similarly to other tree species with comparable emission potentials. Second, a protocol for proton transfer reaction mass spectrometer (PTR-MS) was developed for calibration and data processing of long-term and stand-alone VOC measurements. The reliability of this protocol was tested by comparing simultaneous VOC measurements of two PTR-MS and two gas chromatograph mass spectrometers (GC-MS). The detection of five compounds was analyzed in depth and strengths and weaknesses of the measurements were highlighted. Third, the increase in biogenic VOC and CCN concentrations was investigated in connection with the global warming. This was done by analyzing long-term data of concentrations and compositions of aerosol particles and their biogenic precursor VOCs in different environments. A negative aerosol-climate feedback, driven by the increase of BVOC emissions due to climate warming, was hypothesized and found.
  • Suomalainen, Juha (Helsingin yliopisto, 2012)
    The reflectance factor is a quantity describing the efficiency of a surface to reflect light and affecting the observed brightness of reflected light. It is a complex property that varies with the view and illumination geometries as well as the wavelength and polarization of the light. The reflectance factor response is a peculiar property of each target surface. In optical remote sensing, the observed reflectance properties of natural surfaces are used directly for, e.g., classifying targets. Also, it is possible to extract target physical properties from observations, but generally this requires an understanding and modeling of the reflectance properties of the target. The most direct way to expand our understanding of the reflectance properties of natural surfaces is through empirical measurements. This thesis presents three original measurement setups for obtaining the reflectance properties of natural surfaces and some of the results acquired using them. The first instrument is the Finnish Geodetic Institute Field Goniospectrometer (FIGIFIGO); an instrument for measuring the view angle dependency of polarized hyperspectral reflectance factor on small targets. The second instrument is an unmanned aerial vehicle (UAV) setup with a consumer camera used for taking measurements. The procedure allows 2D-mapping of the reflectance factor view angle dependency over larger areas. The third instrument is a virtual hyperspectral LiDAR, i.e. a setup for acquiring laser scanner point clouds with 3D-referenced reflectance spectra ([x,y,z,R(λ)]). During the research period 2005 2011, the FIGIFIGO was used to measure the angular reflectance properties of nearly 400 remote sensing targets, making the acquired reflectance library one of the largest of its kind in the world. These data have been exploited in a number of studies, including studies dealing with the vicarious calibration of airborne remote sensing sensors and satellite imagery and the development and characterization of reflectance reference targets for airborne remote sensing sensors, and the reflectance measurements have been published as a means of increasing the general understanding of the scattering of selected targets. The two latter instrument prototypes demonstrate emerging technologies that are being used in a novel way in remote sensing. Both measurement concepts have shown promising results, indicating that, in some cases, it can be beneficial to use such a methodology in place of the traditional remote sensing methods. Thus, the author believes that such measurement concepts will be used more widely in the near future.
  • Polishchuk, Tatiana (Helsingin yliopisto, 2013)
    The quickly growing community of Internet users is requesting multiple applications and services. At the same time the structure of the network is changing. From the performance point of view, there is a tight interplay between the application and the network design. The network must be constructed to provide an adequate performance of the target application. In this thesis we consider how to improve the quality of users' experience concentrating on two popular and resource-consuming applications: bulk data transfer and real-time video streaming. We share our view on the techniques which enable feasibility and deployability of the network functionality leading to unquestionable performance improvement for the corresponding applications. Modern mobile devices, equipped with several network interfaces, as well as multihomed residential Internet hosts are capable of maintaining multiple simultaneous attachments to the network. We propose to enable simultaneous multipath data transmission in order to increase throughput and speed up such bandwidth-demanding applications as, for example, file download. We design an extension for Host Identity Protocol (mHIP), and propose a multipath data scheduling solution on a wedge layer between IP and transport, which effectively distributes packets from a TCP connection over available paths. We support our protocol with a congestion control scheme and prove its ability to compete in a friendly manner against the legacy network protocols. Moreover, applying game-theoretic analytical modelling we investigate how the multihomed HIP multipath-enabled hosts coexist in the shared network. The number of real-time applications grows quickly. Efficient and reliable transport of multimedia content is a critical issue of today's IP network design. In this thesis we solve scalability issues of the multicast dissemination trees controlled by the hybrid error correction. We propose a scalable multicast architecture for potentially large overlay networks. Our techniques address suboptimality of the adaptive hybrid error correction (AHEC) scheme in the multicast scenarios. A hierarchical multi-stage multicast tree topology is constructed in order to improve the performance of AHEC and guarantee QoS for the multicast clients. We choose an evolutionary networking approach that has the potential to lower the required resources for multimedia applications by utilizing the error-correction domain separation paradigm in combination with selective insertion of the supplementary data from parallel networks, when the corresponding content is available. Clearly both multipath data transmission and multicast content dissemination are the future Internet trends. We study multiple problems related to the deployment of these methods.
  • Anekallu, Chandrasekhar Reddy (Helsingin yliopisto, 2013)
    The Sun drives the near-Earth electrodynamics by supplying the needed energy through the continuous stream of plasma called the solar wind blowing away from the Sun. The solar wind energy enters the Earth's magnetosphere through the magnetopause and mechanisms such as magnetic reconnection, diffusion, impulsive penetration, etc., facilitate the entry. For example, magnetic reconnection between magnetosheath and magnetospheric fields efficiently converts energy from magnetic to kinetic forms. Quantifying the amount of energy converted at the magnetopause in the magnetic reconnection and its subsequent distribution within the magnetosphere ionosphere system is one of the most important questions in space physics. Our current understanding of the conversion process at the magnetopause is based on theory of magnetopause reconnection. When the interplanetary magnetic field (IMF) is directed southward, magnetic reconnection takes place equatorward of magnetospheric cusps and the magnetic tension accelerates the plasma converting magnetic energy into kinetic form, while in the tail magnetic energy increases at the expense of plasma kinetic energy. During northward IMF, reconnection moves behind the cusps and the field lines advect towards the dayside. Hence, for southward IMF, equatorward of cusps is an electromagnetic load whereas the tailward of cusps is a generator and vice versa for northward IMF. Magnetohydrodynamic (MHD) simulations confirm this theoretical description. However, observational verification of this understanding has not been addressed due to limitations associated with spacecraft observations and methodology to calculate energy conversion. The focus of this doctoral thesis is to investigate the magnetopause energy conversion using observations and to compare to previous simulation results on the subject. The final aim is to present the first statistical investigation of magnetopause energy conversion within the magnetopause using European Space Agency's Cluster spacecraft observations. The thesis is based on four articles including an introductory part. The introduction presents a review of the physics of the magnetopause, energy conversion process and the methods to investigate the subject observationally and compares the results to previous modeling results. The thesis ends with a discussion of energy conversion in the context of magnetopause reconnection and presents guidelines to address the topic in future investigations. In Paper I and II we estimated energy conversion across the Earth's magnetopause using Cluster observations and presented a comparison with the Finnish Meteorological Institute's GUMICS-4 global MHD simulations. Detailed data analysis and comparison with simulations indicated the existence of spatial variation in magnetopause energy conversion associated with IMF direction. These papers present the first observational evidence that the earlier simulation results may correctly reflect the nature of magnetic energy conversion within the magnetopause. In Paper III we investigated the usability of single spacecraft methods to calculate energy conversion instead of the more accurate multi spacecraft methods that can only be utilized during a limited periods of time when the inter-spacecraft distance is optimal. Paper III presents a comparison of magnetopause normal, velocity and energy conversion between multi and single spacecraft methods. Paper III also presents the uncertainties associated with single spacecraft methods in comparison to multi spacecraft methods. The paper concludes that single spacecraft methods consistently produce results similar to multi spacecraft methods while magnitude differences remain. In Paper IV we examine the spatial variation of magnetopause energy conversion and compare observations with simulations and with current theoretical understanding. A database of 4000 magnetopause crossings from Cluster spacecraft 1 was compiled from 2001-2008. Using single spacecraft methods, we estimated energy conversion and investigated magnetopause energy conversion as a function of solar wind parameters and the IMF. We found that the spatial pattern to some extent agrees with our current theoretical understanding with some disagreements. We interpret that the observed spatial pattern reflects the globally continuous and locally intermittent nature of magnetopause reconnection. The disagreements with simulations arise partly due to the local behaviour present in observations which is difficult to reproduce in global MHD simulations.
  • Clark, Barnaby (Helsingin yliopisto, 2010)
    The Taita Hills in southeastern Kenya form the northernmost part of Africa’s Eastern Arc Mountains, which have been identified by Conservation International as one of the top ten biodiversity hotspots on Earth. As with many areas of the developing world, over recent decades the Taita Hills have experienced significant population growth leading to associated major changes in land use and land cover (LULC), as well as escalating land degradation, particularly soil erosion. Multi-temporal medium resolution multispectral optical satellite data, such as imagery from the SPOT HRV, HRVIR, and HRG sensors, provides a valuable source of information for environmental monitoring and modelling at a landscape level at local and regional scales. However, utilization of multi-temporal SPOT data in quantitative remote sensing studies requires the removal of atmospheric effects and the derivation of surface reflectance factor. Furthermore, for areas of rugged terrain, such as the Taita Hills, topographic correction is necessary to derive comparable reflectance throughout a SPOT scene. Reliable monitoring of LULC change over time and modelling of land degradation and human population distribution and abundance are of crucial importance to sustainable development, natural resource management, biodiversity conservation, and understanding and mitigating climate change and its impacts. The main purpose of this thesis was to develop and validate enhanced processing of SPOT satellite imagery for use in environmental monitoring and modelling at a landscape level, in regions of the developing world with limited ancillary data availability. The Taita Hills formed the application study site, whilst the Helsinki metropolitan region was used as a control site for validation and assessment of the applied atmospheric correction techniques, where multiangular reflectance field measurements were taken and where horizontal visibility meteorological data concurrent with image acquisition were available. The proposed historical empirical line method (HELM) for absolute atmospheric correction was found to be the only applied technique that could derive surface reflectance factor within an RMSE of < 0.02 ps in the SPOT visible and near-infrared bands; an accuracy level identified as a benchmark for successful atmospheric correction. A multi-scale segmentation/object relationship modelling (MSS/ORM) approach was applied to map LULC in the Taita Hills from the multi-temporal SPOT imagery. This object-based procedure was shown to derive significant improvements over a uni-scale maximum-likelihood technique. The derived LULC data was used in combination with low cost GIS geospatial layers describing elevation, rainfall and soil type, to model degradation in the Taita Hills in the form of potential soil loss, utilizing the simple universal soil loss equation (USLE). Furthermore, human population distribution and abundance were modelled with satisfactory results using only SPOT and GIS derived data and non-Gaussian predictive modelling techniques. The SPOT derived LULC data was found to be unnecessary as a predictor because the first and second order image texture measurements had greater power to explain variation in dwelling unit occurrence and abundance. The ability of the procedures to be implemented locally in the developing world using low-cost or freely available data and software was considered. The techniques discussed in this thesis are considered equally applicable to other medium- and high-resolution optical satellite imagery, as well the utilized SPOT data.
  • Rissanen, Antti (Edita prima Oy, 2011)
    This research is connected with an education development project for the four-year-long officer education program at the National Defence University. In this curriculum physics was studied in two alternative course plans namely scientific and general. Observations connected to the later one e.g. student feedback and learning outcome gave indications that action was needed to support the course. The reform work was focused on the production of aligned course related instructional material. The learning material project produced a customized textbook set for the students of the general basic physics course. The research adapts phases that are typical in Design Based Research (DBR). The research analyses the feature requirements for physics textbook aimed at a specific sector and frames supporting instructional material development, and summarizes the experiences gained in the learning material project when the selected frames have been applied. The quality of instructional material is an essential part of qualified teaching. The goal of instructional material customization is to increase the product's customer centric nature and to enhance its function as a support media for the learning process. Textbooks are still one of the core elements in physics teaching. The idea of a textbook will remain but the form and appearance may change according to the prevailing technology. The work deals with substance connected frames (demands of a physics textbook according to the PER-viewpoint, quality thinking in educational material development), frames of university pedagogy and instructional material production processes. A wide knowledge and understanding of different frames are useful in development work, if they are to be utilized to aid inspiration without limiting new reasoning and new kinds of models. Applying customization even in the frame utilization supports creative and situation aware design and diminishes the gap between theory and practice. Generally, physics teachers produce their own supplementary instructional material. Even though customization thinking is not unknown the threshold to produce an entire textbook might be high. Even though the observations here are from the general physics course at the NDU, the research gives tools also for development in other discipline related educational contexts. This research is an example of an instructional material development work together the questions it uncovers, and presents thoughts when textbook customization is rewarding. At the same time, the research aims to further creative customization thinking in instruction and development. Key words: Physics textbook, PER (Physics Education Research), Instructional quality, Customization, Creativity
  • Hirvonen, Sami-Pekka (Helsingin yliopisto, 2015)
    Poly(benzimidazobenzophenanthroline) (BBL), which may be doped into a p- and n-type semiconductor, has been chemically modified to enhance its processability. In its pristine form BBL is processable only in some concentrated acids and nitromethane/Lewis acid solutions. The corrosiveness of these solvents and moisture sensitivity of the solutions severely limits the possibilities for the use of BBL as an electroactive material in organoelectronic devices and limit the possibilities for its post synthetic chemical modification. In this work processable BBL derivatives have been prepared using two different approaches i) preparation of block-copolymers by attachment of water soluble polymers on BBL functional chain ends ii) attaching bulky side groups on one of the monomers and using that as a starting material in the polymerization reaction. Short BBLs of DP 10 or 20 were prepared with functional chain ends and monofunctional poly(ethylene oxide) (PEO) or poly(N-isopropylacrylamide) of several different molecular weights were attached at the chain ends. Elemental analysis showed that the extent of chain end substitution varied quite a lot when PEO was used. It was also shown that thermogravimetric analysis (TGA) was not suitable to determine the fraction of PEO in BBL-PEO block-copolymers. However, the extent of chain end substitution using PNIPAM was evaluated using TGA. BBL-PEOs could be processed into aqueous dispersions at concentrations ≥ 1.5 mg/ml by means of ultrasonication, and these dispersions were proven to be colloidally very stable. The dispersions could be used to spin- or drop cast films. At higher concentrations BBL-PEO dispersions were shown to form gels. The reason for the gelling is the packing of the polymer into nano-wire structures which then aggregate into 3D-network. SEM- and cryo-TEM imaging confirmed the existence of wire-type structures. Wire structures and gelation were also observed for aqueous dispersions of pristine BBL. BBL-PNIPAM block-copolymers were found to be indispersible in water with PNIPAM fractions lower than 35 mass %. However, they gelled upon ultrasonication. With higher PNIPAM fractions the polymers were dispersible and did not form gels in any of the studied concentrations. Dispersibility in water and the colloidal stability of BBL-PEOs and BBL-PNIPAMs was attributed to electrostatic stabilization as the particles showed negative zeta-potentials. Dispersible BBL-PNIPAMs were more stable due the high amount of dissociative groups originating from the partial degradation of the amide moieties during the coupling to BBL. One of the monomers used to prepare BBL, 1,4,5,8-naphthalenetetracarboxylic acid 1,8-monoanhydride, was successfully mono- and dibrominated. The bromine was then used as a functional group and replaced either directly with 2-ethyl-1-hexylamine or substituted with azide and reacted with 1-hexyne. The modified monomers were used in preparation of BBL. BBLs bearing either 2-ethyl-1-hexylamine or 4-butyl-1,2,3-triazole groups were found to be sparingly soluble in dimethylsulphoxide and N-methyl-2-pyrrolidone.
  • Alenius, Teija (Helsingin yliopisto, 2007)
    This thesis discusses the prehistoric human disturbance during the Holocene by means of case studies using detailed high-resolution pollen analysis from lake sediment. The four lakes studied are situated between 61o 40' and 61o 50' latitudes in the Finnish Karelian inland area and vary between 2.4 and 28.8 ha in size. The existence of Early Metal Age population was one important question. Another study question concerned the development of grazing, and the relationship between slash-and-burn cultivation and permanent field cultivation. The results were presented as pollen percentages and pollen concentrations (grains cm 3). Accumulation values (grains cm 2 yr 1) were calculated for Lake Nautajärvi and Lake Orijärvi sediment, where the sediment accumulation rate was precisely determined. Sediment properties were determined using loss-on-ignition (LOI) and magnetic susceptibility (k). Dating methods used include both conventional and AMS 14C determinations, paleomagnetic dating and varve choronology. The isolation of Lake Kirjavalampi on the northern shore of Lake Ladoga took place ca. 1460 1300 BC. The long sediment cores from Finland, Lake Kirkkolampi and Lake Orijärvi in southeastern Finland and Lake Nautajärvi in south central Finland all extended back to the Early Holocene and were isolated from the Baltic basin ca. 9600 BC, 8600 BC and 7675 BC, respectively. In the long sediment cores, the expansion of Alnus was visible between 7200 - 6840 BC. The spread of Tilia was dated in Lake Kirkkolampi to 6600 BC, in Lake Orijärvi to 5000 BC and at Lake Nautajärvi to 4600 BC. Picea is present locally in Lake Kirkkolampi from 4340 BC, in Lake Orijärvi from 6520 BC and in Lake Nautajärvi from 3500 BC onwards. The first modifications in the pollen data, apparently connected to anthropogenic impacts, were dated to the beginning of the Early Metal Period, 1880 1600 BC. Anthropogenic activity became clear in all the study sites by the end of the Early Metal Period, between 500 BC AD 300. According to Secale pollen, slash-and-burn cultivation was practised around the eastern study lakes from AD 300 600 onwards, and at the study site in central Finland from AD 880 onwards. The overall human impact, however, remained low in the studied sites until the Late Iron Age. Increasing human activity, including an increase in fire frequency was detected from AD 800 900 onwards in the study sites in eastern Finland. In Lake Kirkkolampi, this included cultivation on permanent fields, but in Lake Orijärvi, permanent field cultivation became visible as late as AD 1220, even when the macrofossil data demonstrated the onset of cultivation on permanent fields as early as the 7th century AD. On the northern shore of Lake Ladoga, local activity became visible from ca. AD 1260 onwards and at Lake Nautajärvi, sediment the local occupation was traceable from 1420 AD onwards. The highest values of Secale pollen were recorded both in Lake Orijärvi and Lake Kirjavalampi between ca. AD 1700 1900, and could be associated with the most intensive period of slash-and-burn from AD 1750 to 1850 in eastern Finland.
  • Hjort, Jan (Helsingin yliopisto, 2006)
    Determination of the environmental factors controlling earth surface processes and landform patterns is one of the central themes in physical geography. However, the identification of the main drivers of the geomorphological phenomena is often challenging. Novel spatial analysis and modelling methods could provide new insights into the process-environment relationships. The objective of this research was to map and quantitatively analyse the occurrence of cryogenic phenomena in subarctic Finland. More precisely, utilising a grid-based approach the distribution and abundance of periglacial landforms were modelled to identify important landscape scale environmental factors. The study was performed using a comprehensive empirical data set of periglacial landforms from an area of 600 km2 at a 25-ha resolution. The utilised statistical methods were generalized linear modelling (GLM) and hierarchical partitioning (HP). GLMs were used to produce distribution and abundance models and HP to reveal independently the most likely causal variables. The GLM models were assessed utilising statistical evaluation measures, prediction maps, field observations and the results of HP analyses. A total of 40 different landform types and subtypes were identified. Topographical, soil property and vegetation variables were the primary correlates for the occurrence and cover of active periglacial landforms on the landscape scale. In the model evaluation, most of the GLMs were shown to be robust although the explanation power, prediction ability as well as the selected explanatory variables varied between the models. The great potential of the combination of a spatial grid system, terrain data and novel statistical techniques to map the occurrence of periglacial landforms was demonstrated in this study. GLM proved to be a useful modelling framework for testing the shapes of the response functions and significances of the environmental variables and the HP method helped to make better deductions of the important factors of earth surface processes. Hence, the numerical approach presented in this study can be a useful addition to the current range of techniques available to researchers to map and monitor different geographical phenomena.
  • Oksanen, Esko (Helsingin yliopisto, 2009)
    Inorganic pyrophosphatases (PPases, EC hydrolyse pyrophosphate in a reaction that provides the thermodynamic 'push' for many reactions in the cell, including DNA and protein synthesis. Soluble PPases can be classified into two families that differ completely in both sequence and structure. While Family I PPases are found in all kingdoms, family II PPases occur only in certain prokaryotes. The enzyme from baker's yeast (Saccharomyces cerevisiae) is very well characterised both kinetically and structurally, but the exact mechanism has remained elusive. The enzyme uses divalent cations as cofactors; in vivo the metal is magnesium. Two metals are permanently bound to the enzyme, while two come with the substrate. The reaction cycle involves the activation of the nucleophilic oxygen and allows different pathways for product release. In this thesis I have solved the crystal structures of wild type yeast PPase and seven active site variants in the presence of the native cofactor magnesium. These structures explain the effects of the mutations and have allowed me to describe each intermediate along the catalytic pathway with a structure. Although establishing the ʻchoreographyʼ of the heavy atoms is an important step in understanding the mechanism, hydrogen atoms are crucial for the mechanism. The most unambiguous method to determine the positions of these hydrogen atoms is neutron crystallography. In order to determine the neutron structure of yeast PPase I perdeuterated the enzyme and grew large crystals of it. Since the crystals were not stable at ambient temperature, a cooling device was developed to allow neutron data collection. In order to investigate the structural changes during the reaction in real time by time-resolved crystallography a photolysable substrate precursor is needed. I synthesised a candidate molecule and characterised its photolysis kinetics, but unfortunately it is hydrolysed by both yeast and Thermotoga maritima PPases. The mechanism of Family II PPases is subtly different from Family I. The native metal cofactor is manganese instead of magnesium, but the metal activation is more complex because the metal ions that arrive with the substrate are magnesium different from those permanently bound to the enzyme. I determined the crystal structures of wild type Bacillus subtilis PPase with the inhibitor imidodiphosphate and an inactive H98Q variant with the substrate pyrophosphate. These structures revealed a new trimetal site that activates the nucleophile. I also determined that the metal ion sites were partially occupied by manganese and iron using anomalous X- ray scattering.
  • Henriksson, Krister (Helsingin yliopisto, 2005)
  • Mäkelä, Hanna (Helsingin yliopisto, 2015)
    Roughly three-quarters of Finland s area is covered by forests. Any climatological changes influencing the danger of forest fire are important to evaluate and consider. The objective of this thesis is to study the long-term past and future changes in climatically-driven forest fire danger in Finland based on the summertime mean temperature and precipitation sum. The work is composed of two parts. In the first part, long-term gridded datasets of observed monthly mean temperatures and precipitation sums for Finland are developed. In the second part, these gridded datasets are used together with calculated values of the Finnish Forest Fire Index and probabilistic climate model simulations (from the ENSEMBLES project) to estimate the number of forest fire danger days during the summer season (June-August). The long-term variation of Finland s climatological forest fire danger is studied roughly for 100 years backwards and into the future. One of the main achievements of this thesis is that it explores the possibility of quantifying past and future fire-weather using a relatively limited database with regard to both weather variables and their spatial coverage. This enables a wider exploitation of scattered data series from earlier times and can also provide opportunities for projections using data with a low resolution. The climatological forest fire danger in Finland varies considerably from year to year. There have not been any significant increasing or decreasing trends in the number of fire danger days during the 20th century (1908-2011). On average, the highest probability of forest fire danger occurs in June and July, when a fire hazard exists on roughly 35-40% of all days. The intra-seasonal variation of fire danger has been large enough to enable the occurrence of conflagrations even though the fire danger for the season as a whole has been at an average level. Despite the projected increase in average summertime precipitation, the Finnish climate will provide more favourable conditions for the occurrence of forest fires in the future than today. This is due to increases in the mean temperature. The probability of an increase in the number of fire danger days is 56-75% in the near future (2010-2029) and 71-91% by the end of the current century (2080-2099), depending on the region. This would indicate an increase of 1-2 and 7-10 days, respectively. It is thus clearly important to further develop existing tools for the forecasting of fire danger, and to maintain the capabilities of the fire prevention, surveillance and suppression services. Future projections of all relevant meteorological variables (temperature, precipitation, humidity, evaporation and wind speed) at higher temporal and spatial resolutions, in addition to information on the type of the summertime precipitation and the length of the dry periods, would notably improve the assessment of the future climatological forest fire danger.
  • Viskari, Toni (Helsingin yliopisto, 2012)
    Atmospheric aerosol particles have several important effects on the environment and human society. The exact impact of aerosol particles is largely determined by their particle size distributions. However, no single instrument is able to measure the whole range of the particle size distribution. Estimating a particle size distribution from multiple simultaneous measurements remains a challenge in aerosol physical research. Current methods to combine different measurements require assumptions concerning the overlapping measurement ranges and have difficulties in accounting for measurement uncertainties. In this thesis, Extended Kalman Filter (EKF) is presented as a promising method to estimate particle number size distributions from multiple simultaneous measurements. The particle number size distribution estimated by EKF includes information from prior particle number size distributions as propagated by a dynamical model and is based on the reliabilities of the applied information sources. Known physical processes and dynamically evolving error covariances constrain the estimate both over time and particle size. The method was tested with measurements from Differential Mobility Particle Sizer (DMPS), Aerodynamic Particle Sizer (APS) and nephelometer. The particle number concentration was chosen as the state of interest. The initial EKF implementation presented here includes simplifications, yet the results are positive and the estimate successfully incorporated information from the chosen instruments. For particle sizes smaller than 4 micrometers, the estimate fits the available measurements and smooths the particle number size distribution over both time and particle diameter. The estimate has difficulties with particles larger than 4 micrometers due to issues with both measurements and the dynamical model in that particle size range. The EKF implementation appears to reduce the impact of measurement noise on the estimate, but has a delayed reaction to sudden large changes in size distribution.
  • Pushkina, Diana (Helsingfors universitet, 2007)
    Short and long term environmental changes, variations in climate and vegetation during the late Neogene shaped the geographical ranges of large terrestrial mammals by allowing origination, distribution and dispersal of certain species that make up faunas. Climatic fluctuations were intensified during the latest Neogene, Pleistocene (1.8-0.01 Ma), at the end of which also human presence became more conspicuous. Both climate and humans have been linked to extensive alteration and extinctions in mammalian faunas across the world. This dissertation consists of a set of papers that examine different periods of the Neogene and associated faunas in northern Eurasia. Major trends in changing environments and climate were studied by means of the tooth crown height (hypsodonty) and dietary structure in herbivorous terrestrial mammals or/and species commonness (locality coverage, abundance proxy). This study was also intended to bring to light a great deal of information contained in Russian literature to fill in the gap between the European literature and not translated Russian records. Since the middle Miocene (~15-11 Ma), central Asia has been the focal point of the transformation in Eurasia towards more open and dry environment. The drying of the central part of Eurasia hampered the spread of temperate or mesophilic species between western and eastern sides of the continent, and created conditions for origination of the cold and arid adapted grazing fauna in north-eastern Eurasia. Throughout the climatically unstable late middle and late Pleistocene, Europe that was more maritime during interglacials than Siberia, experienced the most drastic faunal alternations between the interglacial Palaeoloxodon antiquus and glacial Mammuthus primigenius assemblages that permanently inhabited the Mediterranean and Siberia, respectively. During more climatically equable middle part of the middle Pleistocene (Holsteinian interglacial) that was climatically similar to the current Holocene, the interglacial species could have spread eastwards. The origins, dispersal and cohesiveness of the Palaeoloxodon antiquus assemblage in Eurasia are examined. During the latest Weichselian Glaciation (Late Glacial, 15 000-10 500 yr BP, latest late Paleolithic) and Holocene (last 10 000 yr) a rapid warming initiated fragmentation of dry and cold tundra-steppes when increased temperature and humidity produced boggy tundra in the north and forests in the south of the most part of northern Eurasia. The most significant change took place in central Asia influencing the glacial mammoth fauna decline as is seen in southern Siberia from decreased mean hypsodonty and the shift in dietary preferences from grazing towards browsing in herbivorous ungulates along with decreased mean body size in large mammals. It is difficult to disentangle the role of humans from climate effect in large mammal extinctions in Eurasia at the Weichselian-Holocene boundary because they pretty much coincided. The study is consistent with the idea that Eurasian late Pleistocene extinctions were first climatically driven, after which the impact of rapidly expanding humans must have become more manifest and crucial either by direct hunting or via indirect activities. Only the data for the extinct steppe bison may indicate a disproportionate selection by humans although more sufficient and recently updated data are needed. Key words: Pleistocene, Neogene, Paleolithic, interglacial, glacial, large mammals, distribution, hypsodonty, aridity, precipitation, body size, commonness, extinction, human influence.
  • Pohjola, Mia (Helsingin yliopisto, 2006)
    This thesis contains three subject areas concerning particulate matter in urban area air quality: 1) Analysis of the measured concentrations of particulate matter mass concentrations in the Helsinki Metropolitan Area (HMA) in different locations in relation to traffic sources, and at different times of year and day. 2) The evolution of traffic exhaust originated particulate matter number concentrations and sizes in local street scale are studied by a combination of a dispersion model and an aerosol process model. 3) Some situations of high particulate matter concentrations are analysed with regard to their meteorological origins, especially temperature inversion situations, in the HMA and three other European cities. The prediction of the occurrence of meteorological conditions conducive to elevated particulate matter concentrations in the studied cities is examined. The performance of current numerical weather forecasting models in the case of air pollution episode situations is considered. The study of the ambient measurements revealed clear diurnal variation of the PM10 concentrations in the HMA measurement sites, irrespective of the year and the season of the year. The diurnal variation of local vehicular traffic flows seemed to have no substantial correlation with the PM2.5 concentrations, indicating that the PM10 concentrations were originated mainly from local vehicular traffic (direct emissions and suspension), while the PM2.5 concentrations were mostly of regionally and long-range transported origin. The modelling study of traffic exhaust dispersion and transformation showed that the number concentrations of particles originating from street traffic exhaust undergo a substantial change during the first tens of seconds after being emitted from the vehicle tailpipe. The dilution process was shown to dominate total number concentrations. Minimal effect of both condensation and coagulation was seen in the Aitken mode number concentrations. The included air pollution episodes were chosen on the basis of occurrence in either winter or spring, and having at least partly local origin. In the HMA, air pollution episodes were shown to be linked to predominantly stable atmospheric conditions with high atmospheric pressure and low wind speeds in conjunction with relatively low ambient temperatures. For the other European cities studied, the best meteorological predictors for the elevated concentrations of PM10 were shown to be temporal (hourly) evolutions of temperature inversions, stable atmospheric stability and in some cases, wind speed. Concerning the weather prediction during particulate matter related air pollution episodes, the use of the studied models were found to overpredict pollutant dispersion, leading to underprediction of pollutant concentration levels.
  • Kohonen, Jukka (Helsingin yliopisto, 2015)
    Clustering is a central task in computational statistics. Its aim is to divide observed data into groups of items, based on the similarity of their features. Among various approaches to clustering, Bayesian model-based clustering has recently gained popularity. Many existing works are based on stochastic sampling methods. This work is concerned with exact, exponential-time algorithms for the Bayesian model-based clustering task. In particular, we consider the exact computation of two summary statistics: the number of clusters, and pairwise incidence of items in the same cluster. We present an implemented algorithm for computing these statistics substantially faster than would be achieved by direct enumeration of the possible partitions. The method is practically applicable to data sets of up to approximately 25 items. We apply a variant of the exact inference method into graphical models where a given variable may have up to four parent variables. The parent variables can then have up to 16 value combinations, and the task is to cluster them and find combinations that lead to similar conditional probability tables. Further contributions of this work are related to number theory. We show that a novel combination of addition chains and additive bases provides the optimal arrangement of multiplications, when the task is to use repeated multiplication starting from a given number or entity, but only a certain kind of function of the successive powers is required. This arrangement speeds up the computation of the posterior distribution for the number of clusters. The same arrangement method can be applied to other multiplicative tasks, for example, in matrix multiplication. We also present new algorithmic results related to finding extremal additive bases. Before this work, the extremal additive bases were known up to length 23. We have computed them up to length 24 in the unrestricted case, and up to length 41 in the restricted case.