Browsing by Subject "Aerosolifysiikka"

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  • Clusius, Petri (Helsingin yliopisto, 2020)
    This thesis presents the Atmospherically Relevant Chemistry and Aerosol Box Model (ARCA box), which is used for simulating atmospheric chemistry and the time evolution of aerosol particles and the formation of stable molecular clusters. The model can be used for example in solving of the concentrations of atmospheric trace gases formed from some predefined precursors, simulation and design of smog chamber experiments or indoor air quality estimation. The backbone of ARCAs chemical library comes from Master Chemical Mechanism (MCM), extended with Peroxy Radical Autoxidation Mechanism (PRAM), and is further extendable with any new reactions. Molecular clustering is simulated with the Atmospheric Cluster Dynamics Code (ACDC). The particle size distribution is represented with two alternative methods whose size and grid density are fully configurable. The evolution of the particle size distribution due to the condensation of low volatile organic vapours and the Brownian coagulation is simulated using established kinetic and thermodynamic theories. The user interface of ARCA differs considerably from the previous comparable models. The model has a graphical user interface which improves its usability and repeatability of the simulations. The user interface increases the potential of ARCA being used also outside the modelling community, for example in the experimental atmospheric sciences or by authorities.
  • Li, Xiaoyu (Helsingin yliopisto, 2020)
    Urban areas account for 70% of worldwide energy-related CO2 emissions and play a significant role in the global carbon budget. With the enhanced consumption of fossil fuel and the dramatic change in land use related to urbanization, control and mitigation of CO2 emissions in the urban area is becoming a major concern for urban dwellers and city managers. It is of great importance and demand to estimate the local CO2 emissions in urban areas to assess the effectiveness of mitigation regulation. Surface Urban Energy and Water Balance Scheme (SUEWS) incorporated with a CO2 exchange module provides an advanced method to model total urban CO2 flux and quantify the different local-scale emission sectors involving transportation, human metabolism, buildings and vegetation. Using appropriate input data such as detailed site information and meteorological condition, it can simulate the local or neighbourhood scale CO2 emissions in a specific period, or even under a future scenario. In this study, the SUEWS model is implemented in an urban region, Jätkäsaari, which is an extension of Helsinki city centre, to simulate anthropogenic and biogenic CO2 emissions in the past and future. The construction of this district started in 2009 and was planned to be completed in 2030. Therefore, this region is a good case to investigate the impacts of urban planning on urban CO2 emissions. Based on the urban surface information, meteorological data, and abundant emission parameters, a simulation in this 1650 × 1400 m area with the spatial resolution of 50 × 50 m and the time resolution of an hour was conducted with the aim to get information on the total annual CO2 emissions, and the temporal and spatial variability of CO2 fluxes from different sources and sink in 2008 and 2030. The positive CO2 fluxes indicate the CO2 sources, while the negative indicate the CO2 sinks. In both of the previous and future case, the spatial variation of net CO2 fluxes in Jätkäsaari is dominated by the distribution of traffic and human activities. From April to September, the vegetation acts as the CO2 sink with negative net ecosystem exchange. In 2008, the modelled cumulative CO2 flux is 3.0 kt CO2 year-1, consisting of 1.9 kt CO2 year-1 from metabolism, 1.9 kt CO2 year-1 from traffic, 0.5 kt CO2 year-1 from soil and vegetation respiration, as well as -1.3 kt CO2 year-1 from photosynthesis. In 2030, the total annual CO2 emissions increase to 11.1 kt CO2 year-1 because of the rising traffic volume and amount of inhabitants. Road traffic became the dominant CO2 sources, accounting for 53% of the total emissions. For the diurnal variation, in 2008, the study area remains the CO2 sources with the exception of summertime morning when the net CO2 flux is negative, while in 2030, the net CO2 flux is positive in the whole day.
  • Graeffe, Frans (Helsingin yliopisto, 2019)
    Atmospheric aerosols affect the Earth's radiative balance, visibility and human health. Therefore the formation processes and growth of these particles are important and should be studied to understand how human and natural processes affects these processes. One poorly understood and relatively little studied part of aerosols is particulate organic nitrates (pONs). These pONs are mostly formed during nighttime when NOx, mainly emitted from fossil fuel combustion and industrial processes, and volatile organic compounds (VOCs), from both natural and anthropogenic sources, reacts in the atmosphere. The quantification of these pONs is still hard due to instrumental restrictions, although much improvement has happened during recent years. One main reason for these challenges is the difficulty to separate inorganic nitrates from organic nitrates with real-time instruments. During this work, we generated pure pON in well controlled laboratory conditions and sampled it with an Aerosol Mass Spectrometer (AMS), an instrument widely used for measuring the chemical composition of atmospheric aerosols. We used four different pON precursors to generate pON. I investigated the fragmentation patterns of pON detected by the AMS, utilizing the high resolution of the newest model of the AMS. As older versions of the AMS has difficulties to separate nitrate-containing organic fragments due to lower resolution than the AMS I used, I was able to study pON mass spectrum with better resolution than anyone before me. I found mass spectral differences for the different pON precursors, and was able to find unique fragments for some of the pON precursors that possibly can be used as marker fragments.
  • Besel, Vitus (Helsingin yliopisto, 2020)
    We investigated the impact of various parameters on new particle formation rates predicted for the sulfuric acid - ammonia system using cluster distribution dynamics simulations, in our case ACDC (Atmospheric Cluster Dynamics Code). The predicted particle formation rates increase significantly if rotational symmetry number of monomers (sulfuric acid and ammonia molecules, and bisulfate and ammonium ions) are considered in the simulation. On the other hand, inclusion of the rotational symmetry number of the clusters only changes the results slightly, and only in conditions where charged clusters dominate the particle formation rate because most of the clusters stable enough to participate in new particle formation display no symmetry, therefore have a rotational symmetry number of one, and the few exceptions to this rule are positively charged. Further, we tested the influence of the application of a quasi-harmonic correction for low-frequency vibrational modes. Generally, this decreases predicted new particle formation rates, and significantly alters the shape of the formation rate curve plotted against the sulfuric acid concentration. We found that the impact of the maximum size of the clusters explicitly included in the simulations depends on the simulated conditions and the errors due to the limited set of clusters simulated generally increase with temperature, and decrease with vapor concentrations. The boundary conditions for clusters that are counted as formed particles (outgrowing clusters) have only a small influence on the results, provided that the definition is chemically reasonable and the set of simulated clusters is sufficiently large. We compared predicted particle formation rates with experimental data measured at the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber. A cluster distribution dynamics model shows improved agreement with experiments when using our new input data and the proposed combination of symmetry and quasi-harmonic corrections., compared to an earlier study based on older quantum chemical data.
  • Merikanto, Joonas (Finnish Assosiation for Aerosol Research, 2007)
    Report series in aerosol science
    A better understanding of the limiting step in a first order phase transition, the nucleation process, is of major importance to a variety of scientific fields ranging from atmospheric sciences to nanotechnology and even to cosmology. This is due to the fact that in most phase transitions the new phase is separated from the mother phase by a free energy barrier. This barrier is crossed in a process called nucleation. Nowadays it is considered that a significant fraction of all atmospheric particles is produced by vapor-to liquid nucleation. In atmospheric sciences, as well as in other scientific fields, the theoretical treatment of nucleation is mostly based on a theory known as the Classical Nucleation Theory. However, the Classical Nucleation Theory is known to have only a limited success in predicting the rate at which vapor-to-liquid nucleation takes place at given conditions. This thesis studies the unary homogeneous vapor-to-liquid nucleation from a statistical mechanics viewpoint. We apply Monte Carlo simulations of molecular clusters to calculate the free energy barrier separating the vapor and liquid phases and compare our results against the laboratory measurements and Classical Nucleation Theory predictions. According to our results, the work of adding a monomer to a cluster in equilibrium vapour is accurately described by the liquid drop model applied by the Classical Nucleation Theory, once the clusters are larger than some threshold size. The threshold cluster sizes contain only a few or some tens of molecules depending on the interaction potential and temperature. However, the error made in modeling the smallest of clusters as liquid drops results in an erroneous absolute value for the cluster work of formation throughout the size range, as predicted by the McGraw-Laaksonen scaling law. By calculating correction factors to Classical Nucleation Theory predictions for the nucleation barriers of argon and water, we show that the corrected predictions produce nucleation rates that are in good comparison with experiments. For the smallest clusters, the deviation between the simulation results and the liquid drop values are accurately modelled by the low order virial coefficients at modest temperatures and vapour densities, or in other words, in the validity range of the non-interacting cluster theory by Frenkel, Band and Bilj. Our results do not indicate a need for a size dependent replacement free energy correction. The results also indicate that Classical Nucleation Theory predicts the size of the critical cluster correctly. We also presents a new method for the calculation of the equilibrium vapour density, surface tension size dependence and planar surface tension directly from cluster simulations. We also show how the size dependence of the cluster surface tension in equimolar surface is a function of virial coefficients, a result confirmed by our cluster simulations.
  • Li, Xinyang (Helsingin yliopisto, 2020)
    The impacts of dust aerosols on human health and climate change are increasing as the particulate matter (PM) mass concentrations and frequency of Sand and Dust Storm (SDS) episodes have shown an increasing trend in recent studies, especially for the Middle East and North Africa (MENA). In this thesis, particulate matter (PM10 and PM2.5) concentrations were measured during May 2018–March 2019 in the urban atmosphere of Amman, Jordan. The PM sampling was 24-hours every 6 days. The overall mean PM10 mass concentration was 64±39 μg/m3 with the median (+interquartile range) value of 49.2+53.5 μg/m3, the PM2.5 mass concentration varied between 15 μg/m3 and 190 μg/m3 with an annual average 47±32 μg/m3 and with the median (+interquartile range) value of 35.8+26.3 μg/m3. The PM2.5 / PM10 ratio was 0.8±0.2. According to the Jordanian Air Quality standards, the annual mean PM10 needs to be below a limit value of 120 μg/m3, which was true in this work. However, the PM2.5 mass concentration was three times higher the corresponding limit value (65 μg/m3). However, both exceeded the World Health Organization (WHO) air quality annual guideline of 20 μg/m3 for PM10 and 10 μg/m3 for PM2.5. The results show that the observed PM10 mass concentrations in Jordan were lower than what was reported in other cities in the Middle East but were higher when compared to other Mediterranean cities. During the measurement period, Jordan was affected by Sand and Dust Storms (SDS), which were observed on 14 sampling days. The source origins of these SDS were traced back to North Africa, the Arabian Peninsula, and the Levant. The 24-hour PM10 concentrations during these SDS episodes ranged between 108.1 μg/m3 and 187.3 μg/m3. In the future, measurements with a higher time resolution (one sample per day) are recommended for a more precise seasonal trend interpretation.
  • Al Dulaimi, Qusay (Helsingin yliopisto, 2020)
    Sand and dust storms are one of the major regional environmental problems that affect human health. Many environmental studies have focused on airborne dust concentrations observed at different regions and have tried to connect the observations to specific dust source regions. This thesis aims to provide a new dust classifications scheme for the Eastern Mediterranean region, specifically observed in Amman, Jordan. I utilized a combination of a long-term data-base consisting of aerosol particle number concentration in coarse mode (1–10 µm) during November 2013 – July 2018 and air mass back trajectories analysis to visually identify the Sand and Dust Storm (SDS) episodes. The classification included three main source regions of for the submicron dust, namely Sahara, Arabia, and Levant. I also classified the data according to the, episode intensity according to their corresponding number concentrations as no-dust, mild, intermediate, and strong intensities and further classified the range of back trajectories as short, intermediate, long, and very long, which indicates the distance between the observation site and the source region.. The results showed that majority of the dust events and an elevated number of dust days are influenced by a source in Levant and Sahara source region. These events which dominated during 70 days in 2016. The Levant source governed during 60 days during the same period. Other dust sources contributed less to the dusty days, and the lowest dusty days number was due to emissions from Levant & Arabia (19 days). The episode intensity varied censurably and underlined variability from the different source areas. The maximum intensity in the dust episode concentration was linked to Levant & Sahara with a max number concentration of 95 /cm3. The classification method was successful and it was able to establish a dust source database in the Eastern Mediterranean region based on the long-term observations performed in Amman with variable dust concentration and dust periods in different seasons and different meteorological circumstances.
  • Ovaska, Aino (Helsingin yliopisto, 2021)
    Cloud condensation nuclei (CCN) participate in controlling the climate, and a better understading of their number concentrations is needed to constrain the current uncertainties in Earth’s energy budget. However, estimating the global CCN concentrations is difficult using only localised in-situ measurements. To overcome this, different proxies and parametrisations for CCN have been developed. In this thesis, accumulation mode particles were used as a substitute for CCN, and continental proxy for number concentration of N100 was developed with CO and temperature as tracers for anthropogenic and biogenic emissions. The data utilised in the analysis contained N100 measurements from 22 sites from 5 different continents as well as CO and temperature from CAMS reanalysis dataset. The thesis aimed to construct a global continental proxy. In addition to this, individual proxies for each site (the site proxy) and proxies trained with other sites’ data (the site excluded proxy) were developed. The performance of these proxies was evaluated using a modified version of K-fold cross-validation, which allowed estimating the effect of dataset selection on the results. Additionally, time series, seasonal variation, and parameter distributions for developed proxies were analysed and findings compared against known characteristics of the sites. Global proxy was developed, but no single set of parameters, that would achieve the best performance at all sites, was found. Therefore, two versions of global proxy were selected and their results analysed. For most of the sites, the site proxy performed better than the global proxies. Additionally, based on the analysis from the site excluded proxy, extrapolating the global proxy to new locations produced results with varying accuracy. Best results came from sites with low concentrations and occasional anthropogenic transport episodes. Additionally, some European rural sites performed well, whereas in mountainous sites the proxy struggled. Comparing the proxy to literature, it performed generally less well or similarly as proxies from other studies. Longer datasets and additional measurement sites could improve the proxy performance.
  • Tuovinen, Saana (Helsingin yliopisto, 2019)
    Observations of frequent new particle formation events have been made in severely polluted environ- ments in China. In theory this should not be possible because of the large condensation sink caused by large concentrations of particles. This thesis tries to shed light on reasons why this happens by investigating heterogeneous nucleation in different conditions, for different vapours and seed particles. Especially of interest are those situations where heterogeneous nucleation is considered to be ineffective which would affect the condensation sink of vapours. Theoretical modelling was used to investigate heterogeneous nucleation and measured data was analyzed to complement theoretical results. In this thesis, special focus is on contact angle θ of heterogeneous nucleation, a variable that depends on surface tensions of the vapour and the seed particle the vapour condenses on. θ has a strong effect on the heterogeneous nucleation probability and the larger it is the less likely nucleation is to occur. Many situations where there was at least in theory little heterogeneous nucleation were found. Conditions similar to real atmospheric conditions were investigated and contact angles needed for heterogeneous nucleation to be ineffective for a vapour were determined. Because θ is related to chemical properties of the seed particle, aerosol chemical composition was also investigated alongside with the corresponding condensation sink and particle formation rates using data measured in Beijing, China. This was done in hopes of finding indications of if and how effective condensation sink and aerosol chemical composition are related. However, no clear connection was yet found. Influence of ineffective heterogeneous nucleation on effective condensation sink was considered. It was found that if ineffectiveness of heterogeneous nucleation affects the condensation sink, effective sink can in theory be significantly smaller than condensation sink. Thus, ineffective heterogeneous nucleation due to multiple factors explored in this thesis could in part explain why new particle formation events are observed even in heavily polluted areas.
  • Häkkinen, Ella (Helsingin yliopisto, 2020)
    Atmospheric aerosol particles affect Earth’s radiation balance, human health and visibility. Secondary organic aerosol (SOA) contributes a significant fraction to the total atmospheric organic aerosol, and thus plays an important role in climate change. SOA is formed through oxidation of volatile organic compounds (VOCs) and it consists of many individual organic compounds with varying properties. The oxidation products of VOCs include highly oxygenated organic molecules (HOM) that are estimated to explain a large fraction of SOA formation. To estimate the climate impacts of SOA it is essential to understand its properties in the atmosphere. In this thesis, a method to investigate thermally induced evaporation of organic aerosol was developed. SOA particles were generated in a flow tube from alpha-pinene ozonolysis and then directed into a heated tube to initiate particle evaporation. The size distribution of the particles was measured with parallel identification of the evaporated HOM. This method was capable of providing information of SOA evaporation behaviour and the particle-phase composition at different temperatures. Mass spectra of the evaporated HOM and particle size distribution data were analyzed. The obtained results suggest that SOA contains compounds with a wide range of volatilities, including HOM monomers, dimers and trimers. The volatility behaviour of the particulate HOM and their contribution to SOA particle mass was studied. Furthermore, indications of particle-phase reactions occurring in SOA were found.