Browsing by Subject "physics"

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  • Franck, Anna (Helsingin yliopisto, 2022)
    The atmosphere surrounding our planet is vital for the existence of many living organisms, including humans. Although this layer is quite thin, there are numerous components interacting with each other with processes taking place across widely different spatial and temporal scales. No single instrument is able to cover all of these scales, and therefore, in order to advance our knowledge of atmospheric processes and composition, different instruments, methods and synergy of instruments have to be applied. Remote sensing techniques offer a variety of possibilities for atmospheric research. Satellite remote sensing is exploited to get a regional or global view on the problems, to verify climate models, as well as to reach locations which are not accessible for measurements otherwise. Ground-based remote sensing allows a continuous monitoring of the vertical structure of the atmosphere and, due to exploiting various wavelengths, the observation of atmospheric compounds of various sizes from gases to aerosol particles to snowflakes. In this thesis, several remote sensing techniques have been utilized to find new methods of utilizing existing observations as well as the application of known methods to new geographical locations. A novel method is proposed for retrieving convective boundary layer height during spring and summer months using insect echoes in radar returns. Observations from several different radar frequencies were analysed and the proposed method was proven applicable at all frequencies given some limitations. Moreover, this method can serve as a platform for future research in different geographical locations where insects might behave differently. The synergy of ground-based lidar and airborne in situ measurements were used to study elevated aerosol layers in Southern Finland. Based on two cases, a clear-sky and partly cloudy case, the temporal and spatial variability of aerosol particle number concentration in the boundary layer and several elevated layers were investigated. Nucleation mode particles (the smallest aerosol sizes) were also detected in one of the elevated layers, which was probably not mixing with the boundary layer during a new particle formation event. In addition to aerosol particles, some lidars have the capability to measure water vapor profiles. Several calibration methods for this type of lidar were analysed in order to find an alternative to the usual method of using a radiosonde launched close by, since radiosondes may not always be available at every site. Output from a weather forecast model, or a radiosonde profile which was 100 km away, were both found to be reliable, whereas the use of satellite products required more caution in the absence of other methods. The seasonal variability of water vapour profiles was also studied. Satellite remote sensing observations were probed to obtain proxies of nucleation mode aerosol particles, which are otherwise not seen from space. So far, the results were not very successful, however, some bottlenecks were identified with a potential to improve the proxies in the future.
  • Byggmästar, Jesper (Helsingin yliopisto, 2020)
    One of the key challenges to overcome when designing fusion reactors is choosing appropriate materials that can withstand the intense particle irradiation and heat loads inside the reactor. The current top candidates for different parts of the plasma-facing reactor walls are tungsten, beryllium, and various advanced steels. Understanding the effects of ion and neutron irradiation in these materials requires detailed studies of the radiation-induced atom-level changes in the crystal structure, a goal achievable by a combined effort of experimental measurements and computer modelling. This thesis uses the latter to advance the understanding of radiation damage in iron, tungsten, and beryllium. The main tool is molecular dynamics simulations, with which radiation damage can be studied with atomistic resolution. A major part of the thesis is devoted to the development of interatomic potentials to allow more accurate simulations. We demonstrate how improved analytical potentials tailored to radiation damage allow us to study radiation effects in more detail and with higher accuracy than before. In particular, we investigate the formation, evolution and transformation of defect clusters such as dislocation loops, voids, and the C15 Laves phase cluster in iron and tungsten. We focus on aspects of radiation damage in fusion reactor materials that have previously received little attention. These include effects of radiation-induced collision cascades overlapping with previous damage in iron and tungsten, the stochastic stress- and temperature-driven interaction between dislocations and voids in iron, and simulations of beryllium oxide. The latter is made possible by developing the first interatomic potential for beryllium-oxygen interactions. Furthermore, we show how the use of machine learning leads to significantly more accurate modelling of radiation damage compared to analytical potentials. Specifically, we train a machine-learning potential for tungsten that significantly outperforms existing analytical potentials and makes simulations of radiation damage with quantum-level accuracy possible.
  • Zhao, Junlei (Helsingin yliopisto, 2016)
    "There's Plenty of Room at the Bottom.", the lecture by Prof. Richard Feynman on December, 29th, 1959 at Caltech, USA, describes the field, which is "not quite the same as the others in that it will not tell us much of fundamental physics but it is more like solid-state physics in the sense that it might tell us much of great interest about the strange phenomena that occur in complex situations." This simple inspiring idea has often been referred to as the first "seed" of one of the most promising interdisciplinary branches of science, nanoscience. Nanoparticles (NPs), one of the primary building blocks for nanostructures and its application, have been incidentally synthesized and used by ancient Romans when manufacturing beautiful cups. Modern technology requires the synthesis of NPs to be precise for specific application. The composition, structure, morphology and size are four parameters which dominate the properties of NPs. How to develop a method which can control these parameters accurately and precisely is an essential question for the researchers of nanoscience. Among the wide range of existing synthesis methods, magnetron sputtering inert gas condensation has been commonly used during recent years. The method allows simultaneous control of composition, magnetron power, inert gas pressure, NP drift velocity, and aggregation zone length. To achieve a reliable control of the fabricated NPs, it is essential to understand how the nano-scale growth is influenced by these experimental conditions. In this thesis, the growth mechanisms of Si, NiCr and Fe nanoparticles are studied using multi-scale simulation methods. We investigate the effects of the macro-scaled experimental parameters on the structural properties of nanoparticles. The work presented here is a step towards the understanding of the growth process of NPs in inert gas condensation chambers and the precise control of NP properties.
  • Lasa Esquisabel, Ane (Helsingin yliopisto, 2014)
    The world's energy demand and harmful green-house gas emissions are continuously increasing, while the fossil fuel reservoir may soon end. Currently, there is no clear alternative to the traditional energy production methods for a safe and clean future. Fusion could be part of the solution offering a green-house gas free, virtually endless, safe and large scale energy production. A major challenge for fusion is, however, to produce more energy than needed to achieve and maintain the fusion reaction. The most feasible fusion reaction is based on two hydrogen isotopes: deuterium and tritium, which fuse to produce a helium atom and a neutron. For these atoms to fuse, they must overcome the repulsive interaction between them, requiring extreme temperatures. Thus, the particles ionize forming gas plasma. On Earth, this condition can only be met by isolating the plasma from its environment, for instance, by using closed magnetic fields to form a torus-like shaped plasma, also known as tokamak. However, the plasma particles will interact with the reactor walls as their confinement is never perfect, the exhausted plasma must leave the reactor and impurities are introduced in the plasma boundary to control its characteristics. The plasma-wall interactions are especially intense at the divertor, where the plasma is designed to touch the wall. Understanding these processes is essential to develop safe, long-lasting materials and to avoid contaminating the plasma fuel. The main candidates as first wall materials in future fusion reactors are beryllium for the main wall, and tungsten and carbon for the divertor. Also, the materials may mix due to wall erosion, transport of the eroded particles and their deposition in a new location. Plasma-wall interactions can be studied in current experimental reactors or in linear plasma devices. However, this work is often insufficient to understand the underlying mechanisms. Further, the effects of plasma-wall interactions in materials develop in a wide range of time and length scales. Multi scale modelling is a tool that allows to overcome these challenges, improving the predictions for future fusion reactors. In this thesis, the plasma wall interactions taking place in a fusion reactors divertor have been studied by computational means. The interaction of pure and mixed divertor materials, with plasma and impurity particles were modelled. The work was mainly based on atomistic scale calculations, and a Kinetic Monte Carlo algorithm has also been developed to extend the results to macroscopic scales, enabling a direct comparison with experiments. First, deuterium irradiation of various W-C composites has been modelled, focusing on deuterium implantation, variations of the substrate composition and C erosion mechanisms. Carbon was preferentially eroded, varying the substrate's composition throughout the irradiation. The presence of carbon also affected the D implantation characteristics. As carbon became less likely to be an ITER first wall material, the present work focused on the tungsten-beryllium-deuterium system. The tungsten-beryllium mixing showed a strong dependence on irradiation energy and angle. Further, the presence of Be led to higher fuel implantation and W erosion was suppressed by mixed layer formation. The obtained yields were compared to Binary Collision Approximation results, in order to improve the description of the latter method. Furthermore, an unexpected and possibly harmful phenomenon has been addressed in this thesis: porous nano-morphology formation in tungsten by helium plasma exposure. First, the main characteristics and active mechanisms in the system were identified by atomistic simulations. Then, the porous morphology growth was modelled by implementing these processes in a Kinetic Monte Carlo code, resulting in rates that agreed with experimental findings. A morphology growth model was derived where the time dependence is driven by the evolution of the surface roughness, which is a stochastic process and thus evolves as the square root of time.
  • Parviainen, Stefan (Helsingin yliopisto, 2014)
    Modern society runs largely on electricity, and where there is electricity there are electric fields. As the boundaries of technology are pushed forward, stronger and stronger electric fields are either required, or appear due to unwanted effects. Examples of such applications, where very high electric fields are utilised include particle accelerators and atom probes. To further be able to improve on such techniques, it is necessary to gain a good understanding of the processes that are involved. Because it is often difficult, if not impossible, to observe these processes with high resolution in experiments, one needs to consider the use of atomistic simulations instead. This thesis provides an extension to classical molecular dynamics by describing an implementation where several electronic effects are considered when dealing with metal surfaces under high electric fields. These effects include the charging of surface atoms, field electron emission and the resulting resistive heating, as well as field evaporation of both neutral and charged atoms. In addition to the implementation details, the thesis also contains a brief background of the physics involved in these processes. Using the implementation, it is observed that a surface protrusion may grow on an initially flat surface in the presence of a near-surface void when a strong external electric field is applied. The growth is very rapid, resulting finally catastrophic breakage. This mechanism may explain the appearance of field emitters on otherwise pristine samples, and the instability of measured field emission currents. Simulations also reveal that high aspect ratio protrusions are subject to Rayleigh instability due to the temperature rise caused by field electron emission currents. As a result a large fraction of the protrusion can break off. The model also allows for the study of the trajectories of field evaporated ions from a surface, as they are accelerated in the electric field. From the simulations we see that even changes in the surface morphology on the atomic scale may result in aberrations in atom probe tomography experiments.
  • Veira Canle, Daniel; Mäkinen, Joni Mikko Kristian; Blomqvist, Richard; Gritsevich, M.; Salmi, Ari; Haeggström, Edward (2021)
    The primary goal of this study is to localize a defect (cavity) in a curved geometry. Curved topologies exhibit multiple resonances and the presence of hotspots for acoustic waves. Launching acoustic waves along a specific direction e.g. by means of an extended laser source reduces the complexity of the scattering problem. We performed experiments to demonstrate the use of a laser line source and verified the experimental results in FEM simulations. In both cases, we could locate and determine the size of a pit in a steel hemisphere which allowed us to visualize the defect on a 3D model of the sample. Such an approach could benefit patients by enabling contactless inspection of acetabular cups.
  • 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.
  • 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.
  • Schobesberger, Siegfried (Helsingin yliopisto, 2014)
    Atmospheric aerosols have important effects on health and climate. An important source is the formation of aerosol particles from gas-phase precursors. In this thesis, the goal was to improve our understanding of how exactly this atmospheric particle formation proceeds. Attempts have been made to describe aerosol particle formation by classical nucleation theory. To test this theory, the heterogeneous nucleation of n-propanol vapor on 4 11 nm seed particles was investigated. The choice of seed particle material was found to determine if classical theories could be applied or not, probably because of material-specific inter-molecular interactions between the vapor and the seed particle. The classical theories fail to describe these interactions, which can be crucial in microscopic systems. The critical processes of atmospheric particle formation occur at sizes below 2 nm. In this thesis, novel techniques were employed to access this size range, primarily the atmospheric pressure interface time-of-flight (APi-TOF) mass spectrometer that can directly measure the composition of ions and ionic clusters up to a size of about 2 nm. APi-TOFs were employed at the CLOUD facility at CERN during experiments that focused on exploring particle formation from various systems of vapors. The results of the APi-TOF measurements were the key in revealing the detailed mechanisms of how clusters were initially formed by which vapors, and how these clusters grew to sizes > 2 nm. Clusters of sulfuric acid + ammonia and sulfuric acid + dimethylamine were shown to form and grow via strong hydrogen bonds. The APi-TOF measurements also showed that certain large monoterpene oxidation products, some of them very highly oxidized, can directly bind with bisulfate ions and with sulfuric acid molecules. The clusters then grow by the addition of more of these large oxidized organics and sulfuric acid molecules. Similarities with results from measurements in the boreal forest suggest that large oxidized organics indeed play a crucial role in ambient particle formation events. A light airplane was used to explore how the mechanisms of actual aerosol particle formation vary throughout the atmosphere above the boreal forest, from the canopy up into the free troposphere. They confirmed the extent of boundary layer new particle formation events, and showed indications of an important role of dynamical processes at the top of the boundary layer. Local enhancements of particle formation were observed in connection with clouds. This thesis goal was achieved chiefly by using state-of-the-art experimental techniques together with high-quality laboratory experiments as well as in the field, and by taking ambient measurements aloft. Hopes are that this work will prove to be an important contribution in advancing our knowledge of the detailed mechanisms of atmospheric aerosol particle formation.
  • Saressalo, Anton (Helsingin yliopisto, 2021)
    Electric discharge is present in various aspects of our everyday lives. Internal combustion engines rely on spark plugs for the running of the motor, fluorescent lighting functions by gas discharge and a lightning bolt strikes somewhere on earth every second. An electrical breakdown is an event where a voltage across two conductive electrodes, separated by an electrically insulating medium, becomes high enough for the insulating properties of the medium to be weakened, subsequently allowing an electric current to pass through the medium. A special type of such an event is a vacuum arc breakdown, where the electrodes are separated by a gap of void, which acts as a good insulator, but will still be breached under sufficiently high voltage. When controlled, the electric arcing can be used as a powerful tool to focus energy to a specific location. However, several applications are also hindered by the occurrence of breakdowns, including particle accelerators, vacuum interrupters and solar panels. A common factor in these applications is the aim to maximize the electric field strength to optimize the operational efficiency and ecological footprint of such a device. The breakdown phenomenon is at the crossroads of many fields of science, including plasma, materials and surface physics. Effort to explain the breakdown origin has been ongoing for more than a hundred years, and, despite of the constant progress, there are only hypotheses on the exact nature of the process. This work presents an experimental approach for studying the breakdown phenomenon between Cu electrodes, separated by a vacuum gap. The breakdowns are generated as a consequence of repeatedly applying high-voltage pulses across the gap. As a result, statistics, such as breakdown frequency, of the events are investigated and any effects on the surface analyzed. It was shown that cleaning the electrode surface, either by the electric pulsing or plasma treatment, improves the breakdown resistance of the system, whereas any idle time between the high-voltage pulses increases the breakdown probability. Furthermore, it was found that the breakdown events can be attributed to distinct classes, suggesting separate processes responsible for the breakdown generation. One set of processes were labeled extrinsic, as they are driven by the external factors responsible of the surface contamination of the electrode surface. The other processes were characterized as intrinsic, as they were defined by inherent material properties and continued affecting the breakdown frequency even when the effect of extrinsic processes was minimized by plasma cleaning of the surface. Understanding the formation mechanisms of a vacuum arc breakdown allows designing applications that can sustain higher electric fields without breakdown events. The results of this work provide insight on how improving the surface state of an electrode can increase its breakdown resistance. Additionally, an algorithm is presented for recovering the pulsing voltage after a previous breakdown to a high level in an optimal way, with a minimal probability of follow-up breakdowns.
  • Lolicato, Fabio (Helsingin yliopisto, 2020)
    The present thesis focuses on two interrelated research themes that deal with interactions of biological and synthetic nanocomplexes with cell membrane surfaces. Understanding the physical and chemical principles that regulate these interactions is crucial to figure out how native peripheral protein complexes function on the surface of cell membranes, and how man-made nanoparticles with the desired properties can be utilized in the vicinity of cell membranes. In order to provide the most accurate representation of these phenomena by molecular level resolution, the research presented in this thesis has been carried out using atomic and molecular simulation methods. The first part of the thesis focuses on the interaction of fibroblast growth factor II (FGF2) with the plasma membrane. We studied the entry point of FGF2 at the inner layer of the plasma membrane and the importance of PI(4,5)P2 lipids in recruitment and oligomerization of FGF2 at the membrane surface. Understanding how to regulate the secretion of FGF2 will pave the way for biomedical applications as to the development of drugs that can prevent tumor cells from secreting FGF2. The second part of this thesis concentrates on the interactions between gold nanoparticles and cell membranes. We first investigated the role of temperature and lipid composition in regulating the intake of monoprotected gold nanoparticles into model membranes. Understanding this process is critically important in the development of means to control the translocation of man-made nanoparticles into a cell, related to the design of novel drug delivery vehicles with reduced toxicity. Second, we studied how gold nanoparticles can be exploited in single-particle tracking measurements to understand nanoscale membrane dynamics with optimal temporal resolution. Altogether, this thesis work provides novel insight into the interplay between molecular complexes and cell membrane surfaces and underlines the added value that emerges from the linking of computer simulations and experimental techniques.
  • Franchin, Alessandro (Helsingin yliopisto, 2015)
    This thesis focuses on the experimental characterization of secondary atmospheric nanoparticles and ions during their formation. This work was developed in two distinct and complementary levels: a scientific level, aimed to advance the understanding of particle formation and a more technical level, dedicated to instrument development and characterization. Understanding and characterizing aerosol formation, is important, as formation of aerosol particles from precursor gases is one of the main sources of atmospheric aerosols. Elucidating how aerosol formation proceeds in detail is critical to better quantify aerosol contribution to the Earth's radiation budget. Experimentally characterizing the first steps of aerosol formation is the key to understanding this phenomenon. Developing and characterizing suitable instrumentation to measure clusters and ions in the sub 3 nm range, where aerosol formation starts, is necessary to clarify the processes that lead to aerosol formation. This thesis presents the results of a series of experimental studies of sub 3 nm aerosol particles and ions. It also shows the results of the technical characterization and instrument development that were made in the process. Specifically, we describe three scientific results achieved from chamber experiments. Firstly the relative contributions of sulfuric acid, ammonia and ions in nucleation processes was quantified experimentally, supporting that sulfuric acid alone cannot explain atmospheric observation of nucleation rates. Secondly, the chemical composition of cluster ions was directly measured for a ternary system, where sulfuric acid, ammonia and water were the condensable vapors. In these measurements we observed a decreasing acidity of the clusters with increasing concentration of gas phase ammonia, with the ratio of sulfuric acid/ammonia staying closer to that of ammonium bisulfate than to that of ammonium sulfate. Finally, in a series of chamber experiments the ion ion recombination coefficient was quantified at different conditions. The ion ion recombination coefficient is a basic physical quantity for modeling ion induced and ion mediated nucleation. We observed a steep increase in the ion ion recombination coefficient with decreasing temperatures and with decreasing relative humidity. This thesis also reviews technical results of: 1) laboratory verification, characterization and testing of different aerosol and ion instruments measuring in the sub 3 nm range; 2) the development of new inlets for such instruments to improve the detection of sub-3 nm particles and ions.
  • Tsona Tchinda, Narcisse (Helsingin yliopisto, 2016)
    Sulfur oxidation products are involved in the formation of acid rain and atmospheric aerosol particles. The formation mechanism of these sulfur-containing species is often complex, especially when ions are involved. The work of this thesis uses computational methods to explore reactions of sulfur dioxide with some atmospheric ions, and to examine the effect of humidity on the stability and electrical mobilities of sulfuric acid-based clusters formed in the first steps of atmospheric particle formation. Quantum chemical calculations are performed to provide insights into the mechanism of the reaction between sulfur dioxide (SO2) and the superoxide ions (O2-) in the gas phase. This reaction was investigated in various experimental studies based on mass spectrometry, but discrepancies on the structure of the product remained disputed. The performed calculations indicate that the peroxy SO2O2- molecular complex is formed upon collision of SO2 and O2-. Due to a high energy barrier, SO2O2- is unable to isomerize to the sulfate radical ion (SO4-), the most stable form of the singly charged tetraoxysulfurous ion. It is suggested that SO2O2- is the major product of SO2 and O2- collision. The gas-phase reaction between SO2 and SO4- is further explored. From quantum chemical calculations and transition state theory, it is found that SO2 and SO4- cluster effectively to form SO2SO4-, which reacts fast at low relative humidity to form SO3SO3-. This species has never been observed in the atmosphere and its decomposition upon collision with other atmospheric species is most likely. First-principles molecular dynamics simulations are used to probe the decomposition by collisions with ozone (O3). The most frequent reactive collisions lead to the formation of SO4-, SO3, and O2. This implies that SO4- acts as a good catalyst in the SO2 oxidation by O3 to SO3. The best structures and the thermochemistry of the stepwise hydration of bisulfate ion, sulfuric acid, base (ammonia or dimethylamine) clusters are determined using quantum chemical calculations. The results indicate that ammonia-containing clusters are more hydrated than dimethylamine-containing ones. The effect of humidity on the mobilities of different clusters is further examined and it is finally found that the effect of humidity is negligible on the electrical mobilities of bisulfate ion, sulfuric acid, ammonia or dimethylamine clusters.
  • Isavnin, Alexey (Helsingin yliopisto, 2014)
    A lot of modern ground-based and space systems, such as navigation satellites, electric power grids, and telecommunication frameworks, can be affected by the changes in the near-Earth space environment, i.e., space weather. The main driver of the space weather is the Sun, which provides a supersonic flow of plasma, known as the solar wind. Coronal mass ejections (CMEs) are the most prominent feature of solar activity. They result from the eruptions on the Sun and propagate almost radially from it embedded into the solar wind. CMEs drive the strongest disturbances of the near-Earth space environment and cause the strongest geomagnetic storms when they encounter the magnetosphere of the Earth. A significant fraction of CMEs exhibit a specific configuration of twisted magnetic field lines, i.e., the flux rope configuration. The geoffectiveness of flux rope CMEs depends on their internal magnetic structure, morphological properties, speed, and the geometry of their propagation through the interplanetary space. In this thesis, the internal structure of flux rope CMEs and their three-dimensional evolution in the interplanetary space were investigated using the combination of white-light and extreme ultraviolet observations and in-situ measurements and modeling. The results of the analysis show that a typical flux rope CME consists of regions of physically different plasma with the flux rope occupying one of them. The methodology for studying the evolution of the individual flux rope in three-dimensional space is described. The presented technique is used to show that solar flux ropes experience significant deflections and rotations during their propagation from the Sun to the Earth's orbit that have to be taken into account for reliable space weather forecasting. These structures deflect predominantly towards the solar equatorial plane and their rotations are affected by the solar wind streams. It is discovered that 40% of the flux rope evolution happens after 30 solar radii. Flux-rope-like structures can also form in the magnetosphere during the periods of geomagnetic disturbances. They are generated in the magnetotail configurations with multiple reconnection sites and travel towards the Earth or away from it. Both types of these helical magnetic structures are addressed in this thesis as well. It is demonstrated that the properties of these structures help to get insight into the dynamics of the magnetosphere. The model of evolution of earthward-traveling flux ropes is presented, according to which they deteriorate and degrade into dipolarization fronts, another magnetic field configuration that is characteristic for geomagnetic disturbances. This thesis contributes both to the improvement of the flux rope analysis techniques as well as conducts a comprehensive analysis of solar and magnetospheric flux ropes and their evolution. The results of the research advance our understanding of the Sun-Earth coupling in one dynamical process and can be used for improving the space weather forecasting tools.
  • Prank, Marje (Helsingin yliopisto, 2017)
    Atmospheric composition has strong influence on human health, ecosystems and also Earth's climate. Among the atmospheric constituents, particulate matter has been recognized as both a strong climate forcer and a significant risk factor for human health, although the health relevance of the specific aerosol characteristics, such as its chemical composition, is still debated. Clouds and aerosols also contribute the largest uncertainty to the radiative budget estimates for climate projections. Thus, reliable estimates of emissions and distributions of pollutants are necessary for assessing the future climate and air-quality related health effects. Chemistry-transport models (CTMs) are valuable tools for understanding the processes influencing the atmospheric composition. This thesis consists of a collection of developments and applications of the chemistry-transport model SILAM. SILAM's ability to reproduce the observed aerosol composition was evaluated and compared with three other commonly used CTM-s in Europe. Compared to the measurements, all models systematically underestimated dry PM10 and PM2.5 by 10-60%, depending on the model and the season of the year. For majority of the PM chemical components the relative underestimation was smaller than that, exceptions being the carbonaceous particles and mineral dust - species that suffer from relatively small amount of available observational data. The study stressed the necessity for high-quality emissions from wild-land fires and wind-suspended dust, as well as the need for an explicit consideration of aerosol water content in model-measurement comparison. The average water content at laboratory conditions was estimated between 5 and 20% for PM2.5 and between 10 and 25% for PM10. SILAM predictions were used to assess the annual mortality attributable to short-term exposures to vegetation-fire originated PM2.5 in different regions in Europe. PM2.5 emitted from vegetation fires was found to be a relevant risk factor for public health in Europe, more than 1000 premature deaths per year were attributed to vegetation-fire released PM2.5. CTM predictions critically depend on emission data quality. An error was found in the EMEP anthropogenic emission inventory regarding the SOx and PM emissions of metallurgy plants on the Kola Peninsula and SILAM was applied to estimate the accuracy of the proposed correction. Allergenic pollen is arguably the type of aerosol with most widely recognised effect to health. SILAM's ability to predict allergenic pollen was extended to include Ambrosia Artemisiifolia - an invasive weed spreading in Southern Europe, with extremely allergenic pollen capable of inducing rhinoconjuctivitis and asthma in the sensitive individuals even in very low concentrations. The model compares well with the pollen observations and predicts occasional exceedances of allergy relevant thresholds even in areas far from the plants' habitat. The variations of allergenicity in grass pollen were studied and mapped to the source areas by adjoint computations with SILAM. Due to the high year-to-year variability of the observed pollen potency between the studied years and the sparse observational network, no clear geographic pattern of pollen allergenicity was detected.
  • Mogensen, Ditte (Helsingin yliopisto, 2015)
    Forests emit biogenic volatile organic compounds (BVOCs) that, together with e.g. sulfuric acid, can operate as aerosol precursor compounds when oxidised. Aerosol particles affect both air visibility, human health and the Earth s radiative budget, thus making the emission inputs and oxidation mechanisms of VOCs absolutely crucial to understand. This thesis discusses the life cycle of compounds in the atmosphere. Specifically, we studied the representations of emission of BVOCs, the atmosphere s oxidation ability along with the sources and sinks of sulfuric acid. The main tool to achieve this was numerical modelling, often compared to field observations. Additionally, we performed computational chemistry simulations in order to calculate transitions in sulfuric acid. The main findings of this thesis can be summarised into the following: (1) Biological understanding of VOC emission processes needs to be enhanced in order to predict VOC concentrations with a high precision. (2) The unexplained fraction of the total OH reactivity in the boreal forest is larger than the known fraction and known secondary organic oxidation products of primary emitted terpenes cannot explain the missing reactivity. (3) OH is the main oxidation agent of organic compounds in the boreal atmosphere. (4) Criegee Intermediates, produced from unsaturated hydrocarbons, can oxidise SO2 effectively in order to provide as an essential source of sulfuric acid in areas with high VOC concentrations. (5) Two-photon electronic excitation did not turn out to be a significant sink of gaseous sulfuric acid in the stratosphere. This thesis closes a large part of the sulfuric acid concentration gap in VOC rich environments. Further, this thesis raises awareness of the fact that we still do not fully comprehend the mechanisms leading to BVOC emissions nor the organic atmospheric chemistry in the boreal forest. Finally, this work encourage to study alternative BVOC emission sources as well as alternative atmospheric oxidants.
  • Majidi, Sharareh (Helsingin yliopisto, 2013)
    Physics has been always one of the most challenging subjects to learn for university and school students. It is also considered a demanding topic for teachers who aim to teach it efficiently. Therefore, one of the most important notions in physics is to find suitable ways to maximize productive learning and teaching outcomes. One of the most important factors that influence physics learning and teaching is the organization of physics knowledge and the ability to arrange its concepts properly. In physics education, the organization of knowledge and meaningful structural patterns is a vital component of teachers subject matter knowledge. Correspondingly, physics textbooks, teachers, and lecturers are expected to translate their subject matter knowledge with the most pedagogically effective approaches. So another central component in physics education is teachers pedagogical content knowledge, which is about a) the most essential representation forms (e.g. analogies, models, examples, simulation) that teachers employ in the classroom, and b) students misconceptions and difficulties as well as the best approaches to diminish those complications. This thesis examines the organization of knowledge and representation forms used by teachers and found in university and upper secondary school textbooks. Magnetostatics is recognized as one of the most challenging topics in physics, which the society of physics education has largely disregarded. In this study, we concentrate on two important magnetic laws of Biot-Savart/magnetic flux density and Ampère. These laws as well as their applications and examples are employed broadly in both upper secondary schools and universities. These topics provide us sufficient space to investigate the organization of physics knowledge as well as the most appropriate representation forms. In these studies, we utilized a variety of qualitative and quantitative methods to collect data. Different samples are selected from standard university textbooks, teachers at the University of Helsinki, Department of Physics, and some teachers from highly reputed upper secondary schools in Helsinki, Finland. To study the organization of knowledge of teachers and textbooks, their structures are first portrayed by means of concept maps. Second, structural measures are applied to evaluate any meaningful patterns detected in these concept maps. Structural measures contain complex network observables such as density of links, hierarchy, clustering, cycles, and loops. In other cases, the structural measures are confined to the number of dead-ended concepts, core concepts, incoming and outgoing links. Results reveal certain similarities and differences between the ways knowledge is organized and arranged within the subject matter of teachers or textbooks. The results report the shared concepts and structural patterns between university teachers and the textbooks they use for their teaching purposes and identify differences between the structural properties of two laws of Biot-Savart and Ampère. The rest of the results inform us about a variety of forms that could be employed to represent these laws. Recognized representation forms include experiments and demonstrations, stating facts in physics, inductive and deductive reasoning, examples, and explanations, and models such as analogies, mathematical models, and visual models are ultimately drawn from the data analysis through this research. The novelty of this thesis is its briefly examination and discussion of the possible link between the organization of knowledge, which functions as teachers subject matter, and their representation forms, which serve as their pedagogical content knowledge. This thesis also discusses the implications for teaching and learning as well as practical applications of it.
  • Paramonov, Mikhail (Helsingin yliopisto, 2015)
    The research conducted and presented herein concentrates primarily on the life cycle of a cloud condensation nucleus CCN. The primary motivation of the work is the importance of CCN in the global aerosol-cloud-climate system, and focus is placed on the production of CCN, their behaviour in the atmosphere and their properties with respect to CCN activation, as well as the removal pathways. The work presented in this thesis covers measurements performed at 16 locations around the world. The results further corroborated the notion that atmospheric new particle formation NPF is an important and widespread source of CCN in the atmosphere. The number of newly formed CCN from NPF depends on many factors, including, but not limited to, biogenic and anthropogenic emissions, frequency of NPF events, nucleation and growth rates and pre-existing CCN concentrations; method of calculation also affects the estimate of NPF contribution to CCN budgets. Highest relative increase in CCN as a result of NPF was observed at a clean remote location in Northern Finland, where in the summer the number concentration of particles above 50 nm in diameter N50 can increase by as much as 800%. Highest absolute increases in NCCN as a result of NPF (up to 3500 particles cm 3 for N50) were found at a dry savannah location of Botsalano in South Africa. In Hyytiälä Type I nucleation events were found to always, at the very least, double NCCN concentrations. It was found that in many environments around the world a rather similar fraction of aerosols activated into cloud droplets at any given level of supersaturation S, and a simple linear parameterisation is provided for an easy calculation of annual mean CCN concentration NCCN based only on total number concentration NCN and the desired S. At the majority of studied locations hygroscopicity was found to increase with size, with accumulation mode hygroscopicity parameter κ values being significantly larger than Aitken mode κ at some locations. Depending on the focus and desired accuracy, the use of κ values as a function of particle dry size rather than the assumption of a size-independent κ should preferably be considered. The photochemistry, aging processes, atmospheric NPF and other atmospheric processes occurring on a diurnal scale were found to affect the CCN activation and hygroscopic properties of Aitken mode aerosol only. The hygroscopicity of the accumulation mode aerosol is more affected by processes occurring on a time scale of a few days to months, e.g. long range transport and seasonal variation in vegetation activity. Below-cloud scavenging by snow was found to be an inefficient mechanism of CCN removal from the atmosphere compared to, e.g., in-cloud scavenging. Additionally, nucleation and Aitken mode particles are scavenged by snow more efficiently that CCN-sized aerosol. No apparent difference in the scavenging efficiency of snow was observed between a rural and an urban site in Southern Finland. Ambient relative humidity was found to correlate positively with the scavenging efficiency of snow, and a new parameterisation for calculating snow scavenging coefficients based on both particle dry size and relative humidity is presented. A reconsideration of the purpose, the focus and the motivation for the cloud condensation nuclei counter CCNC measurements at the SMEAR II is needed if they are to be continued with reasonable, interesting and exciting output.
  • Heikkilä, Anu (Helsingin yliopisto, 2014)
    UV radiation is one of the key factors ageing materials and thus limiting their service life time. Damages induced by UV radiation may be observed as changes in the properties of the material. Various types of weathering experiments are used to study the chemical and physical mechanisms behind the degradation process. Material samples are exposed to UV radiation and other environmental stress agents in natural outdoor and artificial conditions in laboratory. Quantification of the material s response to the exposure yields knowledge on the ageing behaviour of the material. This knowledge is essential when selecting materials to comply with the service requirements, as well as developing materials with extended durability. The focus of this study has been on certain methods used in investigation of the deleterious effects of UV radiation on materials. Various aspects of weathering and analysis of material property changes are considered. As regards weathering, three main topics unfold: sampling of data from outdoor weathering experiments; construction of continuous time series of UV radiation exposures over the weathering periods; and comparability of artificial UV radiation conditions to those encountered under the Sun. Within the scope of the property change analysis, the study has focused on the wavelength dependencies found in damage induction. A new method is proposed for planning the timetable of a natural outdoor weathering experiment in a way that the seasonal and geographical variations are accounted for. The method simplifies the post-exposure analysis of the material properties and improves the comparability of test results obtained at different weathering sites. A new approach is adopted to reconstruct the solar spectral UV irradiance at the Earth s surface through radiative transfer modelling. The method provides a means for deriving UV irradiance in cases where no direct measurements of the quantity are available. As a result, a continuous UV radiation time series is obtained, as required for the determination of the UV radiation exposure accumulated on the materials. To improve the comparability of accelerated ageing to that naturally occurring outdoors, a method for characterizing the UV radiation conditions used in a UV chamber is presented. Knowledge on the exposure is further used to establish a correlation between the artificial and natural ageing environments, facilitating predictions on how the materials would survive in their real service environments. The study aiming at improvements in the analysis of material property changes is two-fold. A novel facility designed and constructed for exposing materials with spectrally resolved UV radiation is reported. The instrument opens new pathways for the investigations on wavelength specific damages in materials. The facility has been exploited in a study on the wavelength sensitivity of newspaper to photoyellowing. A new approach in deriving the action spectrum of photoyellowing of a material is proposed. The resulting action spectrum may be used in optimizing the chemical and/or physical protection of the material against UV radiation.
  • Zhou, Luxi (Helsingin yliopisto, 2015)
    Aerosols are common air pollutants that pose serious threats to health. They also impact the Earth’s climate by interacting with solar radiation and altering cloud properties. One important phenomenon associated with the atmospheric aerosol system is new particle formation. This thesis is devoted to study the processes related to new particle formation as well as aerosol precursor gases in the boundary layer, where exchanges between the surface and the atmosphere take place. Model SOSAA, a one-dimensional transport model with detailed chemistry and aerosol microphysics, was constructed, tested and used as the main tool. Variations in the main biogenic volatile organic compounds (BVOCs), OH and H2SO4 across different environment were analysed. The simulated concentrations of main emitted organic compounds in two rural sites agree well with the observations. Due to missing OH reactivity in rural environment, the concentration of OH at Hyytiälä is overestimated. In contrast, the concentration of OH is likely underestimated in an urban environment due to missing production from HONO. H2SO4 production is underestimated at every site studied. New particle formation was studied in two phases: nucleation and growth. Large uncertainties are introduced to nucleation rates when calculating dependencies based on sulphuric acid alone. The oxidation products of monoterpenes by OH are shown not to be a suitable proxy for nucleation at either the urban site in Beijing or the rural site at Hyytiälä in summer. While they mainly contribute to the growth of particles at the rural site in Hyytiälä, the oxidation products of monoterpenes might only have major role in particle growth at Manitou during the evening. The rapid particle growth at Manitou in the daytime is possibly related to the oxidation products of 2-Methyl-3 Buten-2-Ol (MBO) by OH or any compound that has similar concentration and diurnal variation to the oxidation product of MBO by OH. The last model study assessed the climatic impact of aerosols in a hypothetic scenario with high atmospheric methane loading. The impact associated with aerosols and the change in atmospheric composition is shown to be as large as the impact due to methane itself. A NOx mitigation strategy was evaluated by SOSAA as effective to remove methane but the side product of hazardous tropospheric ozone makes the method unfavourable in real life. The process-based model SOSAA developed during this doctoral study is a useful tool for atmospheric research since it provides flexibility in time and space. The results call for further research into the ultimate mechanisms controlling nucleation and particle growth. This thesis also identifies knowledge gaps in the related precursor gases and encourages further studies in fields such as emission inventory of volatile organic compounds, sulphuric acid production, and atmospheric oxidation cycle.