Matemaattis-luonnontieteellinen tiedekunta

 

Recent Submissions

  • Du, Mian (Helsingin yliopisto, 2017)
    The ongoing information explosion has a particular impact on business areas, involving corporate strategy and business decision-making. Business intelligence tools aim to help users to understand market trends, which is critical for their day-to-day operations. For example, it is a typical business intelligence task to effectively obtain accurate and relevant information about the competitor’s activity in the same industry sector. This thesis presents research on a natural language processing system, which aims to address the problem of information overload in the business domain. It uses document filtering, information extraction, and supervised and semi-supervised learning. Input to the system includes news documents from on-line news websites and company press pages. We first demonstrate that a combination of NLP techniques and frequent sequential pattern mining can be used for finding patterns from unstructured natural-language text, i.e., news articles. The patterns relate to a specific domain of news. Evaluation results show that scenario-based summarization can filter out irrelevant documents and also extract important sentences from relevant documents as summaries for pre-defined scenarios in a specific domain. For document-level filtering, this method achieves very high precision, while keeping quite high recall in our study. Next, we present experiments with supervised learning for labelling business-news documents with multiple industry sectors. The main contribution is that combining a named-entity-based rote classifier with the balanced classifiers yields better results than either classifier alone. This method also improves on the best score previously reported, while using the same amount of training data for the rote classifier, and considerably less for the statistical classifiers. We then explore the interplay between company news, social media visibility, and stock prices. Information extracted from on-line news by means of the deep linguistic analysis is used to construct queries to various social media platforms. The main results presented in the thesis demonstrate the interesting correlations between the mentions of a company in the news and the views of its page in Wikipedia. Based on the above research topics, the thesis also presents the design and architecture of a complete decision-support system. The system is an example of using the above research results to extract, analyze and organize information from plain-text news.
  • Alekseychik, Pavel (Helsingin yliopisto, 2017)
    North Eurasia (NE) is recognized as a region of high importance for the global climate change. Large and systematic shifts in temperature and precipitation, predicted for NE, are expected to cause irreversible disturbances in the ecosystem-atmosphere interactions. The region is dominated by natural boreal and arctic ecosystems, which are experiencing increasing levels of anthropogenic influence through environmental pollution (mainly agriculture- and mining-related) and land use change. However, despite the general consensus on the importance of environmental changes in NE, many aspects of the problem remain poorly understood. Due to the great extent of NE, many crucial areas remain inaccessible or lack the required infrastructure, thereby relevant surface-atmosphere exchange measurement data remains scarce. This thesis examines the surface budgets of carbon dioxide and energy of subarctic peatlands, which count among the most important ecosystems in NE owing to their large carbon storage and areal coverage, and their high sensitivity to climate and land use changes. Peatlands are also heterogeneous ecosystems, with large diversity found both internally and between the ecosystem subtypes. Peatland heterogeneity is notable on several characteristic scales, including the single plant, microsite, microtopography element and ecosystem type. Different scales of variation are covered with specific measurement techniques, i.e. plant-scale gas exchange, chamber and eddy-covariance (EC) technique. The latter is in the focus of this work. Aerodynamic roughness length (z0) and photosynthesis rate measured by EC were compared with the plant- and microsite-scale measurements of leaf area index (LAI) and photosynthesis rate. High correspondence between the estimates on different scales was found, indicating that the transition between the upscaled plant and ecosystem-scale estimates is possible, and thus adding credibility to both. In the fen Siikaneva-1, LAI showed a strong linear relationship with z0, while the EC-derived photosynthesis rate closely followed the upscaled plant chamber estimate. Ecosystem-scale EC measurements made in 2015 at a typical raised bog in West Siberian middle taiga (Mukhrino Field Station) revealed a high cumulative May-August net ecosystem exchange (NEE) of 202 gC m-2 and a mean Bowen ratio (ratio of sensible to latent heat flux) of 0.28, which was lower than expected. Inter-site differences in terms of energy balance were investigated for a selection of eight Fennoscandian peatland sites with different mire types and management histories. The site mean Bowen ratio ranged from 0.28 to 1.35, evapotranspiration (ET) from 1.2 to 2.8 mm/day, and the decoupling parameter (Ω, indicating the relative strength of radiation and surface conductance controls on evapotranspiration) from 0.11 to 0.48. Generally, the energy balance and ecosystem features were found to be tightly linked, corroborating the predictability of peatland energy balances across a wide spectrum of ecosystems. Finally, on a yet larger scale, the current measurement station network of the entire NE is discussed. Measurement gaps are identified based on ecosystem type and climate representation by the existing field stations, with the focus on peatlands. Tentative developments for the NE measurement network are proposed.It is established that Siberia is a region very sparsely covered by ecosystem and climate change monitoring sites, especially when state of the art techniques are concerned (e.g. EC).
  • Hippeläinen, Eero (Helsingin yliopisto, 2017)
    In radionuclide therapy, the patient is injected with relatively high amounts of therapeutic radiopharmaceutical which localises to target tissue and emits ionising radiation. Unfortunately, a perfectly targeting radiopharmaceutical has not been discovered and part of the radiopharmaceutical accumulates to healthy tissues, which are also thus irradiated. In order to ensure safe use, the absorbed dose of radiation-sensitive organs must be monitored. The focus of this thesis is on patient specific dosimetry of lutetium-177 (Lu-177) labelled somatostatin analogue Lu-177-DOTA-Tyr3-octreotate (Lu-177-DOTATATE) treatments and development of internal dosimetry software. Lu-177-DOTATATE is a radiopharmaceutical that binds to somatostatin receptors and is used to treat patients with metastatic neuroendocrine tumours. Recent studies have shown significant treatment outcome improvements with Lu-177-DOTATATE when compared to previously used somatostatin analogue treatments. However, the kidneys are the healthy organ which receives the highest amount of radiation dose from Lu-177-DOTATATE treatments and could be the organ that limits the number of treatments a patient can tolerate. In addition, absorbed dose to kidneys varies highly from patient to patient and thus patient specific dosimetry is recommended. Despite many years of dosimetry research and the existence of several published scientific dosimetry tools, there is no clinically validated kidney dosimetry software for Lu-177-DOTATATE treatments. The aim of this thesis was to study quantification accuracy of Lu-177 radionuclide using SPECT/CT imaging and to study mean absorbed doses to kidneys and dose distribution characteristics of Lu-177-DOTATATE. A streamlined voxel level absorbed dose software for clinical practice was developed and validated for kidney dosimetry of Lu-177-DOTATATE treatments. The effect of reconstruction methods on Lu-177 quantification accuracy was studied using an anthropomorphic phantom with known Lu-177 sources. Acquired data were reconstructed using different image compensation methods and results were compared with known source activities in the phantom. It was found that Monte Carlo simulation based scatter compensation and SPECT detector response compensation improved Lu-177 quantification accuracy considerably. Similar findings were also observed with data from patients treated with Lu-177-DOTATATE. A Monte Carlo simulation study was carried out to investigate absorbed dose distribution of Lu-177. Two main findings were that electrons emitted by Lu-177 can be assumed to absorb locally when the resolution of the imaging system is taken into account and the photon cross-irradiation can contribute significantly to total absorbed dose especially in the vicinity of highly active volumes. Using Lu-177-DOTATATE patient data, two different kidney absorbed dose calculation methods were compared. Comparing the mean kidney absorbed dose with the estimated maximum absorbed dose, it was observed that Lu-177-DOTATATE accumulates unevenly to kidney causing significantly heterogeneous dose distribution within kidneys. In addition, a simplified imaging protocol was found to be adequate for dosimetry purposes and was later adopted clinical practice. Combining previous findings new voxel level dosimetry software was developed. The clinical feasibility of the proposed software was tested with digital phantom simulations and reanalysing patient data from Lu-177-DOTATATE treatments. The software was found to be reliable and to speed up and simplify the dosimetry workflow.
  • Kokkonen, Tommi (Helsingin yliopisto, 2017)
    Research in science education has long been concerned with a problem that students acquire conceptions which are unsatisfactory from the scientific point of view. These conceptions are also often robust and slow to change. The learning process whereby the students’ conceptions undergo a change is often viewed from the point of view of conceptual change. In this thesis, this traditional problem of conceptual change is approached as a problem of concept learning, where concepts are complex structures and parts of a conceptual system. The approach is thus termed here the systemic view. It is influenced by recent cognitive science research on relational concepts, which are concepts characterized by their relational structure and/or the relations they bear to other concepts. Because scientific models can also be conceptualized as relational structures, relational structures are central from the viewpoint of scientific knowledge. The systemic view thus bridges the cognitive aspects of learning (students’ initial knowledge) and the target knowledge, thereby illuminating the learning process that leads from initial conceptions to advanced scientific knowledge. The articles presented in this thesis consist of two empirical studies (I and II), in which students’ conceptions about DC circuits are examined from the systemic view perspective. These studies develop and apply the directed graph model, which is a graphical representation of the different conceptual elements. It allows examining students’ conceptions and their change in detail. Such graphs also act as templates for computational modelling of the learning process reported in two other articles (IV and V). The computational models allow examining structural aspects of concepts and their context-dependent dynamics. Article III examines the role of models and modelling in concept learning and suggests how seeing models as relational categories clarifies the cognitive aspects related to model-based learning. The results of the thesis show that in learning advanced scientific knowledge, students’ ability to modify and revise relational knowledge is vital to the learning and acquisition of correct conceptions. A result of practical significance is the strong context and task dependence of these processes of modifications and revisions.
  • Myllys, Nanna (Helsingin yliopisto, 2017)
    Atmospheric aerosol particles affect the global climate and human health. A large fraction of atmospheric clusters is formed as a result of collisions and favourable interactions between molecules. However, the exact mechanisms and participating compounds are not fully resolved. The cluster formation mechanisms at the molecular-level are essential to understand what kind of effects aerosol particles have on climate change and health-related issues. Currently, aerosol particles provide the largest uncertainties in estimates of the future climate. In this thesis, potential cluster formation mechanisms between sulfuric acid and oxidized organic molecules with stabilizing compounds are studied using computational methods. Cluster stabilities must be determined accurately in order to provide trustworthy evaporation and formation rates in atmospheric conditions. This leads to the focus of this thesis: to evaluate the accuracy and applicability of different quantum chemical methods, and to find a robust methodology to study atmospheric cluster formation mechanisms and stabilities in the ambient air. Density functional theory is confirmed to be sufficient to optimize geometries and to calculate vibrational frequencies for molecular clusters. However, for binding energies high-level electronic structure calculations are necessary. The CCSD(T) method is known as the gold standard in quantum chemistry, but it is computationally too demanding for molecular clusters. Therefore, a domain-based local pair natural orbital (DLPNO) approximation is utilized. The DLPNO–CCSD(T) method allows highly accurate calculations for systems comprising more than hundred atoms. The formation energies can be calculated for atmospheric clusters containing up to ten molecules with an approach close to the CCSD(T) accuracy. Large clusters have previously been out of reach with highly accurate quantum chemical methods. The aim of the theoretical background in this thesis is to present an overview of quantum chemical methods. The introductory part of the thesis can be used as a handbook for problem solving related to molecular-level cluster formation mechanisms. The research presented here contributes significantly to the current knowledge of the participation of organic compounds in the first steps of aerosol particle formation. Additionally, this research suggests that some other mechanisms than clustering, or other chemical compounds are needed to bridge the gap between experimental and theoretical findings. Guidelines for future atmospheric cluster formation studies are given.
  • Feijó Barreira, Luís Miguel (Helsingin yliopisto, 2017)
    Volatile organic compounds (VOCs) comprise a large diversity of species that are emitted into the atmosphere from both biogenic and anthropogenic sources. These species play a key role in atmospheric photochemistry, due to their high reactivity with atmospheric oxidants, and in the formation and growth of secondary organic aerosols. The trace levels usually found in ambient air and the enormous heterogeneity of VOC sources and emissions around the Earth call for the development of novel analytical methodologies and portable devices, which provide reduced analytical steps and allow the measurement of these compounds at virtually any spatial location. The main goal of this doctoral thesis was to develop further and apply solid-phase microextraction (SPME) based analytical methods for the sampling and analysis of VOCs in the atmosphere. The SPME techniques used included conventional SPME fibers, needle trap microextraction (NTME) devices and a novel SPME Arrow system. Portable gas chromatography-mass spectrometry (GC-MS) was employed for the fast on-site measurement of atmospheric volatiles. Field measurements were performed at the Station for Measuring Ecosystem-Atmosphere Relations (SMEAR II) in Hyytiälä, Finland. Dynamic SPME collection combined with portable GC-MS allowed the rapid on-site measurement of the most abundant compounds present in the sampling site atmosphere with minimal analytical steps. The potential of NTME and portable GC-MS for the field measurement of biogenic and anthropogenic organic volatiles was also demonstrated, and the method developed was applied to clarify the effect of snow pack on the concentration of biogenic volatile organic compounds (BVOCs) in the air. SPME and portable GC-MS were used for the characterization of BVOCs emitted from chambers installed at the forest soil. A novel SPME Arrow system was also successfully characterized and employed for the sampling of atmospheric VOCs. The results demonstrated the great potential and versatility of SPME-based sampling techniques combined with portable GC-MS for the rapid on-site sampling and analysis of VOCs in the atmosphere.
  • Tenkanen, Henrikki (Helsingin yliopisto, 2017)
    Understanding the spatial patterns of accessibility and mobility are a key (factor) to comprehend the functioning of our societies. Hence, their analysis has become increasingly important for both scientific research and spatial planning. Spatial accessibility and mobility are closely related concepts, as accessibility describes the potential to move by modeling, whereas spatial mobility describes the realized movements of individuals. While both spatial accessibility and mobility have been widely studied, the understanding of how time and temporal change affects accessibility and mobility has been rather limited this far. In the era of ‘big data’, the wealth of temporally sensitive spatial data has made it possible, better than ever, to capture and understand the temporal realities of spatial accessibility and mobility, and hence start to understand better the dynamics of our societies and complex living environment. In this thesis, I aim to develop novel approaches and methods to study the spatio-temporal realities of our living environments via concepts of accessibility and mobility: How people can access places, how they actually move, and how they use space. I inspect these dynamics on several temporal granularities, covering hourly, daily, monthly, and yearly observations and analyses. With novel big data sources, the methodological development and careful assessment of the information extracted from them is extremely important as they are increasingly used to guide decision-making. Hence, I investigate the opportunities and pitfalls of different data sources and methodological approaches in this work. Contextually, I aim to reveal the role of time and the mode of transportation in relation to spatial accessibility and mobility, in both urban and rural environments, and discuss their role in spatial planning. I base my findings on five scientific articles on studies carried out in: Peruvian Amazonia; national parks of South Africa and Finland; Tallinn, Estonia; and Helsinki metropolitan area, Finland. I use and combine data from various sources to extract knowledge from them, including GPS devices; transportation schedules; mobile phones; social media; statistics; land-use data; and surveys. My results demonstrate that spatial accessibility and mobility are highly dependent on time, having clear diurnal and seasonal changes. Hence, it is important to consider temporality when analyzing accessibility, as people, transport and activities all fluctuate as a function of time that affects e.g. the spatial equality of reaching services. In addition, different transport modes should be considered as there are clear differences between them. Furthermore, I show that, in addition to the observed spatial population dynamics, also nature’s own dynamism affects accessibility and mobility on a regional level due to the seasonal variation in river-levels. Also, the visitation patterns in national parks vary significantly over time, as can be observed from social media. Methodologically, this work demonstrates that with a sophisticated fusion of methods and data, it is possible to assess; enrich; harmonize; and increase the spatial and temporal accuracy of data that can be used to better inform spatial planning and decision-making. Finally, I wish to emphasize the importance of bringing scientific knowledge and tools into practice. Hence, all the tools, analytical workflows, and data are openly available for everyone whenever possible. This approach has helped to bring the knowledge and tools into practice with relevant stakeholders in relation to spatial planning. Keywords: Accessibility; Spatial mobility; Spatio-temporal; Multimodal; Travel time; Open data; Social media; GIS; Data science; Data mining; Spatial planning; National parks; Finland; South Africa; Peruvian Amazonia; Helsinki Region; Tallinn
  • Päivärinta, Sanna-Mari (Helsingin yliopisto, 2017)
    Odd nitrogen (NOx = N + NO + NO2) in the polar regions is mainly produced in the upper atmosphere through ionization processes by solar extreme ultraviolet radiation, soft X-rays and high energy particles originating from the space. During periods of high geomagnetic activity, normally close to the solar maximum, energetic particle precipitation (EPP) provides an in-situ source of NOx also in the middle atmosphere. Understanding the behaviour of NOx in the middle atmosphere is of great importance due to its capability to act as a catalyst in chemical reaction cycles destroying ozone in the stratosphere. This work considers EPP in the form of solar proton events (SPEs). Atmospheric dynamics play an important role in determining the distributions of long-lived trace gases in the middle atmosphere. The main loss mechanism for NOx is photolysis at the upper stratospheric and mesospheric altitudes, leading to long photochemical lifetime of NOx during the dark polar winter. NOx in the middle atmosphere, also if produced in-situ due to SPEs, is therefore affected by atmospheric dynamics, and transported from the mesosphere-lower thermosphere (MLT) region down to the middle atmosphere. This descent phenomenon can be intensified in the aftermath of sudden stratospheric warmings (SSWs), which are dynamical phenomena able to affect a wide range of altitudes in the Northern polar region atmosphere. The enhanced downward transport of NOx can thus strengthen the NOx-ozone connection in the stratosphere. In this work we used both space born observations from several satellite instruments and a chemistry transport model in the examination of the SSW and SPE caused effects in the stratosphere and mesosphere. The scientific objectives of this work were to find out the individual and combined effects of SSWs and SPEs on the NOx and ozone balance in the Northern middle atmosphere, and assess the relative contributions of dynamics (SSWs) and in-situ production of NOx (SPEs) on ozone in the stratosphere. The results showed dramatic increases in NOx in the middle atmosphere, even by a factor of 50, following both periods of enhanced NOx descent in connection with SSWs and in-situ production of NOx due to SPEs. A clear long-term (order of months) decrease in stratospheric ozone (10-90 %), coinciding with the enhanced amounts of NOx, was evident and affected mostly by dynamics in the upper stratosphere. The results of this work emphasize the importance of in-situ production of NOx (SPEs) on the ozone balance in the upper stratosphere, but also the key role of dynamics (SSWs) in transporting the SPE effect to even lower altitudes and its capability to strengthen the effect.
  • Sammaljärvi, Juuso (Helsingin yliopisto, 2017)
    The C-14 polymethyl methacrylate (C-14-PMMA) impregnation technique is used to analyse the spatial distribution of porosity. As such, this work focuses on two interlinked aims: to further develop the C-14-PMMA impregnation technique, particularly the polymerisation and autoradiography steps; and to further develop its application extending it from granitic materials to fine-grained structures such as clays as well. To achieve these aims, method development focused on making the C-14-PMMA impregnation technique more flexible and suitable to a wide variety of conditions while maintaining its robustness. As such, we investigated the use of thermally degrading initiators and other impregnants. Thus, we examined the use of thermally degrading initiators as a way of completing the entire sample handling procedure in a laboratory without necessitating transportation to irradiation facilities, which are currently rare. Furthermore, we studied the use of a water-soluble comonomer, 2-hydroxyethyl methacrylate (HEMA), to allow for the possibility of impregnating water-saturated matrices in future, the focus of articles I and II. The second aim focused on the application-related development for the C-14-PMMA impregnation technique and tailoring the methodology to suit the different applications. Towards this end, we compared our results with those obtained via previously used methodologies and other measurement techniques. As such, the C-14-PMMA impregnation technique can be linked to multiple other techniques, providing comprehensive information on the materials studied. In this study, we examined the mineral porosities of rock at the centimetre scale using C-14-PMMA autoradiography (AG) and at the micrometre scale using scanning electron microscopy (SEM) imaging with the aim of studying the effect of the mineral structure on the spatial distribution of porosity. This is detailed in article III. We also studied the fixation of a contrast agent that does not require pressurised intrusion used to image the connected porosity of crystalline rock through SEM and X-ray tomography. In this case, impregnation with methyl methacrylate (MMA) served to fix the contrast agent in place. This research is described in article IV. Our results from the method development studies show that polymerisation can be reliably accomplished within crystalline rock using thermally degrading initiators with no notable drawbacks. We also found that clay rock could be impregnated and the impregnant could be polymerised with the same initiators as well. The two-monomer mixtures (100% C-14-labelled methyl methacrylate (C-14-MMA) and a mixture of 75% HEMA and 25% C-14-MMA) yielded fairly similar results and were consistent with reference values. The application development studies show that valuable information may be gained by combining the spatial distribution of the porosity with spatial information on the mineral structures. The contrast agent for SEM and X-ray tomography was successfully fixed in place by impregnation and polymerisation, and the contrast improved through both imaging techniques.
  • Salminen, Miia (Helsingin yliopisto, 2017)
    Monitoring of snow cover in northern hemisphere is highly important for climate research and for operational activities, such as those related to hydrology and weather forecasting. The appearance and melting of seasonal snow cover dominate the hydrological and climatic patterns in the boreal and arctic regions. Spatial variability (in particular during the spring and autumn transition months) and long-term trends in global snow cover distribution are strongly interconnected to changes in Earth System (ES). Satellite data based estimates on snow cover extent are utilized e.g. in near-real-time hydrological forecasting, water resource management and to construct long-term Climate Data Records (CDRs) essential for climate research. Information on the quantitative reliability of snow cover monitoring is urgently needed by these different applications as the usefulness of satellite data based results is strongly dependent on the quality of the interpretation. This doctoral dissertation investigates the factors affecting the reliability of snow cover monitoring using optical satellite data and focuses on boreal regions (zone characterized by seasonal snow cover). Based on the analysis of different factors relevant to snow mapping performance, the work introduces a methodology to assess the uncertainty of snow cover extent estimates, focusing on the retrieval of fractional snow cover (within a pixel) during the spring melt period. The results demonstrate that optical remote sensing is well suited for determining snow extent in the melting season and that the characterizing the uncertainty in snow estimates facilitates the improvement of the snow mapping algorithms. The overall message is that using a versatile accuracy analysis it is possible to develop uncertainty estimates for the optical remote sensing of snow cover, which is a considerable advance in remote sensing. The results of this work can also be utilized in the development of other interpretation algorithms. This thesis consists of five articles predominantly dealing with quantitative data analysis, while the summary chapter synthesizes the results mainly in the algorithm accuracy point of view. The first four articles determine the reflectance characteristics essential for the forward and inverse modeling of boreal landscapes (forward model describes the observations as a function of the investigated variable). The effects of snow, snow-free ground and boreal forest canopy on the observed satellite scene reflectance are specified. The effects of all the error components are clarified in the fifth article and a novel experimental method to analyze and quantify the amount of uncertainty is presented. The five articles employ different remote sensing and ground truth data sets measured and/or analyzed for this research, covering the region of Finland and also applied to boreal forest region in northern Europe.
  • Schallhart, Simon (Helsingin yliopisto, 2017)
    Volatile organic compounds (VOCs) are emitted in vast amounts from biogenic and anthropogenic sources. They influence air quality and thereby human health. In the atmosphere VOCs can be oxidized to form compounds with lower volatility and form aerosol particles, which can affect the climate. The basis of this thesis are VOC measurements with a proton transfer reaction time of flight (PTR-TOF) mass spectrometer. Its suitability for measuring the volatile organic compound spectra with 10 Hz resolution made it possible to calculate VOC exchange from different ecosystems with the eddy covariance method. The reliability of this method was determined by comparisons with other well-established ecosystem scale flux methods and upscaled emissions from leaf cuvettes. The measurements in this work resulted in the quantification of the total exchange in a broadleaf forest in Bosco Fontana, Italy and a conifer forest in Hyytiälä, Finland. By using a new automated method, 29 VOCs with exchange were measured in Bosco Fontana and 25 VOCs in Hyytiälä. These two ecosystems differ as the major terpene emissions are isoprene for the oak forest and monoterpenes for the Scots pine forest. Additional to isoprene and the monoterpenes, methanol, acetonitrile, acetaldehyde, acetone and acetic acid fluxes were measured at both sites. To identify the measured signals and determine error sources, fragmentation, possible losses and sources of different compounds were investigated. In a research stable in Switzerland, amine measurements and calibrations were performed to identify the source of trimethylamine. During measurements in Hyytiälä, the anthropogenic source of the measured butene was determined and a memory effect of acetic acid in our measurement setup was discovered. Overall, this thesis addresses the potential of concentration and ecosystem exchange measurements using a PTR-TOF and challenges which arise during the measurements and data analysis. The obtained results are useful insights into the precursors and amplifiers (amines) of new particle formation and aerosol growth. Furthermore, the recorded direct total ecosystem exchange measurements expand the limited data available and can be used to improve and validate emission models.
  • Hellas, Arto (Helsingin yliopisto, 2017)
    The introductory programming course is one of the very first courses that computer science students encounter. The course is challenging not only because of the content, but also due to the challenges related to finding a place in a new community. Many have little knowledge of what to expect from university studies, some struggle to adjust their study behavior to match the expected pace, and a few simply cannot attend instruction due to e.g. family or work constraints. As a consequence, a considerable number of students end up failing the course, or pass the course with substandard knowledge. This leads to students failing to proceed in their studies at a desirable pace, to students who struggle with the subsequent courses, and to students who completely drop out from their studies. This thesis explores the issue of retention in introductory programming courses through multiple viewpoints. We first analyze how the teaching approaches reported in literature affect introductory programming course pass rates. Then, changes on the retention at the University of Helsinki are studied using two separate approaches. The first approach is the use of a contemporary variant of Cognitive Apprenticeship called the Extreme Apprenticeship method, and the second approach is the use of a massive open online course (MOOC) in programming for recruiting students before they enter their university studies. Furthermore, data from an automatic assessment system implemented for the purposes of this thesis is studied to determine how novices write their first lines of code, and what factors contribute to the feeling of difficulty in learning programming. On average, the teaching approaches described in the literature improve the course pass rates by one third. However, the literature tends to neglect the effect of intervention on the subsequent courses. In both studies at the University of Helsinki, retention improved considerably, and the students on average also fare better in subsequent courses. Finally, the data that has been gathered with the automatic assessment system provides an excellent starting point for future research.
  • Liu, Xiaoli (Helsingin yliopisto, 2017)
    A community is a collection of populations of different species living in the same geographical area. Species interact with each other in the community and this interaction affects species distribution, abundance, and even evolution. Species interact in various ways, for instance through competition, predation, parasitism, mutualism, and commensalism. We have two focuses in this thesis. One focus is analyzing the dynamical behaviors of the discretization systems of the Lotka-Volterra predator-prey model. It is well known that the dynamics of the logistic map is more complex compared with logistic differential equation. Period doubling and the onset of chaos in the sense of Li-York occur for some values. Inspired by this, we analyze the dynamical behaviors of the discretization systems of the Lotka-Volterra predator-prey model (articles I and II). In article I, we show that the system undergoes fold bifurcation, flip bifurcation and Neimark-Sacker bifurcation, and has a stable invariant cycle in the interior of $R_+^2$ for some parameter values. In article II, we show that the unique positive equilibrium undergoes flip bifurcation and Neimark-Sacker bifurcation. Moreover, system displays much interesting dynamical behaviors, including period-5, 6, 9, 10, 14, 18, 20, 25 orbits, invariant cycles, cascade of period-doubling, quasi-period orbits and the chaotic sets. We emphasize that the discretization of continuous models (articles I and II are not acceptable as a derivation of discrete predator-prey models. A discrete predator-prey model is also formulated in Section 2. We analyze the dynamics (articles I and II) from the mathematical point of view instead of biological point of view. The other focus is disease-competition in an ecological system. We propose a model combining disease and competition and study how a disease affects the two competing species (article III). In our model, we assume that only one of the species is susceptible to an SI type disease with mass action incidence, and that infected individuals do not reproduce but suffer from additional disease induced death. We further assume that infection does not reduce the competitive ability of the infected. We show that infection of the superior competitor enables the inferior competitor to coexist, either as a stable steady state or limit cycle. In the case where two competing species coexist without the disease, the introduction of disease is partially determined by the basic reproduction number. If the reproduction number is less than 1, the disease free coexistence equilibrium is globally asymptotically stable. If the basic reproduction number is larger than 1, our system is uniformly persistent. The unique coexisting endemic disease equilibrium exists and is globally stable under certain conditions. However, infection of the inferior competitor does not change the outcome.
  • Keronen, Petri (Helsingin yliopisto, 2017)
    FLUX AND CONCENTRATION MEASUREMENTS OF CARBON DIOXIDE AND OZONE IN A FORESTED ENVIRONMENT This thesis compiles the setup and characteristics of the measurement systems for CO2, H2O vapour and O3 eddy-covariance (EC) flux and vertical concentration profile, and their calibration arrangements at the SMEAR II station of the University of Helsinki. A commercial chemiluminescence O3 analyser operating on a liquid reagent solution was modified to improve its suitability and reliability in long term EC flux measurements. Fluxes of CO2 and O3 were also determined with the flux-profile method relating the fluxes to the observed concentration profiles. The EC and the flux-profile methods were inter-compared and their performances were evaluated. A separate measurement system utilising a commercial, low cost Non-dispersive Infrared-absorption analyser and incorporating an automatic calibration system for measuring atmospheric CO2 mole fraction was developed in the thesis. The performance of the system was evaluated and its measurement accuracy was determined in comparisons to reference standards. The measured CO2 mole fraction data was compared with the results of an independent atmospheric transport simulation (MACC-II) for period November 2006–December 2011. The modifications made to the reagent liquid flow system improved the reliability of the EC O3 analyser. The fluxes measured with the eddy-covariance and flux-profile method agreed during daytime under unstable conditions. At night-time the flux-profile method estimated higher respiration of CO2 and stronger deposition of O3, but no apparent reason for over- or underestimation by either method was identified. Night-time eddy-covariance fluxes had a specious dependence on turbulence even after accounting for storage flux. Additionally the chemical sink strength was evaluated negligible also for O3. Accounting for vertical advection flux removed the dependencies. However, while accounting for the vertical advection flux resulted in invariance of both net ecosystem exchange (NEE) of CO2 and deposition rate of O3 on turbulence intensity, the importance for estimating e.g. NEE by different ways at other flux tower sites was stressed. The atmospheric CO2 mole fraction measurement system had an accuracy of 0.3–0.4 µmol mol-1, an operating ratio of 99 % and its data coverage was 95 % during the comparison period. The trend and phase of the modelled and measured atmospheric CO2 mole fraction data, on the average 390 µmol mol-1, were observed to generally agree and the average bias of the simulation, –0.2 µmol mol-1, was within the measurement accuracy. The instrumentation fulfilled the set requirements of accuracy and continuous operation, and the result of the comparison implied that the CO2 mole fraction data from the SMEAR II station would clearly have the potential to be assimilated in the MACC-II simulations and thus improving their accuracy.
  • Kaskela, Anu (Geological Survey of Finland, 2017)
    Marine ecosystems provide a wide range of ecosystem services to human society, including supporting, regulating, cultural, and provisioning services. The concept of Blue Growth even considers marine areas as potential drivers of the economy. However, despite the long tradition of ocean exploration, the realization of Blue Growth and effective marine spatial planning often suffer from incomplete and scattered marine data over large areas. This dissertation presents a GIS-based approach to analyzing and characterizing the geologic seabed environment of the Baltic Sea. It combines scattered geospatial data to produce spatial representations of the Baltic Sea in terms of the seabed geomorphic features, marine landscapes, and geodiversity. The broad scales of the analyses reflect the scale of the available datasets and the needs of transnational ecosystem-based management. Spatial analysis techniques enabled the identification of coherent geomorphic features and quantification of geodiversity patterns over the entire Baltic Sea region, within the limits of the input data resolution. Based on the results, the overall geological landscape of the Baltic Sea is characterized by plains and basins. Other geomorphic features, such as elevations and valleys, are characteristic of certain sub-regions. The seabed geodiversity generally increases from south to north and from the open sea to areas with a high shore density. The crystalline bedrock areas provide more diverse seabed environments than the sedimentary rock areas. Archipelagos, in particular, stand out as seabed areas with high geodiversity. The results underline the significance of the ongoing processes (erosion, sediment transport, and accumulation), the basement rock type, past glaciations, and certain geological events during the last deglaciation in shaping the seabed environment of the Baltic Sea. The dissertation presents new evidence that the geodiversity of the seafloor influences the distribution of the zoobenthic assemblages of the eastern Gulf of Finland. It is suggested that the high geodiversity and archipelago gradient could directly influence benthic assemblages and biodiversity by providing a multitude of habitats and indirectly by channeling water movement. Additionally, the potential key habitats, rocky reefs, were mapped with good accuracy in seabed areas with limited data, and the features were also recognized to have ecological value. These spatial datasets provide valuable background material for more detailed studies on the rocky reefs and the archipelago areas, as well as for monitoring their status. The study provides spatial information on the seabed characteristics of the Baltic Sea for scientists, marine spatial planners, and managers. The results emphasize that geodiversity should be acknowledged in the ecosystem-based management of marine areas, because it has intrinsic value, it provides several abiotic ecosystem services, and it is associated with the biodiversity and long-term conservation of the marine environment.