On the sources of submicron aerosol particles in savannah : implications for climate and air quality

Aerosol is defined as solid or liquid particles suspended in a gas lighter than the particles, which means that the atmosphere we live in is an aerosol in itself. Although aerosol particles are only a trace component of the atmosphere they affect our lives in several ways. The aerosol particles can cause adverse health effects and deteriorate visibility, but they affect also the Earth s climate directly by scattering and absorbing solar radiation and indirectly by modulating the properties of the clouds. Anthropogenic aerosol particles have a net cooling effect on the climate, but the uncertainty in the amount of cooling is presently as large as the heating effect of carbon dioxide. To reduce the uncertainty in the aerosol climate effects, spatially representative reference data of high quality are needed for the global climate models. To be able to capture the diurnal and seasonal variability the data have to be collected continuously over time periods that cover at least one full seasonal cycle. Until recently such data have been nearly non-existing for continental Africa and hence one aim of this work was to establish a permanent measurement station measuring the key aerosol particle properties in a continental location in southern Africa. In close collaboration with the North-West University in South Africa this aim has now been achieved at the Welgegund measurement station. The other aims of this work were to determine the aerosol particle concentrations including their seasonal and diurnal variation and to study the most important aerosol particle sources in continental southern Africa. In this thesis the aerosol size distribution and its seasonal and diurnal variation is reported for different environments ranging from a clean rural background to an anthropogenically heavily influenced mining region in continental southern Africa. Atmospheric regional scale new particle formation has been observed at a world record high frequency and it dominates the diurnal variation except in the vicinity of low-income residential areas, where domestic heating and cooking are a stronger source. The concentration of aerosol particles in sizes that can act as cloud condensation nuclei was found to increase during the dry season because of reduced wet removal and increased aerosol production from incomplete combustion, which can be either domestic heating or savannah and grassland fires depending on location. During the wet season new particle formation was shown to be an important source of particles in the size range of cloud condensation nuclei.Aerosoli on määritelmän mukaan kiinteitä tai nestemäisiä hiukkasia, jotka leijuvat itseään kevyemmässä kaasussa. Näin ollen ilmakehä, jossa elämme, on jo itsessään aerosoli. Vaikka aerosolihiukkaset muodostavat vain hyvin pienen osan ilmakehästä, ne silti vaikuttavat ihmisiin monin tavoin. Aerosolihiukkasilla voi olla haitallisia terveysvaikutuksia ja ne voivat heikentää näkyvyyttä, mutta ne myös viilentävät maapallon ilmastoa heijastamalla auringon säteilyä ja toisaalta muokkaamalla pilvien ominaisuuksia. Kun tarkastellaan ilmastonmuutokseen vaikuttavia ihmisperäisiä tekijöitä, aerosolihiukkaset lukeutuvat ilmakehää eniten viilentäviin tekijöihin, mutta epävarmuus niiden viilentävässä vaikutuksessa on tällä hetkellä samaa luokkaa kuin hiilidioksidin lämmittävä vaikutus. Yksi edellytys aerosolihiukkasten ilmastovaikutuksiin liittyvän epävarmuuden pienentämiselle on että maailmanlaajuisille ilmastomalleille on käytettävissä spatiaalisesti edustavaa, korkealuokkaista vertailuaineistoa. Tällaisen vertailuaineiston tulisi lisäksi mahdollistaa vuoden- ja vuorokaudenaikaisvaihteluiden tarkastelu, mikä edellyttää vähintään vuoden kestoisia yhtäjaksoisia mittauksia. Koska tällaisia aineistoja ei ole ollut saatavilla Afrikasta etenkään mantereisista ympäristöistä, yksi tämän väitöskirjatyön tavoitteista on ollut perustaa mantereiseen Afrikkaan pysyvä mittausasema, joka tuottaa tietoa aerosolihiukkasten ilmastollisesti tärkeimmistä ominaisuuksista. Tämä tavoite on nyt saavutettu Welgegundin mittausasemalla Etelä-Afrikassa läheisessä yhteistyössä North-West yliopiston kanssa. Lisäksi tämän työn tavoitteina on ollut määrittää aerosolihiukkasten pitoisuuksia, niiden vuoden- ja vuorokaudenaikaisia vaihteluja sekä tärkeimpiä lähteitä mantereisissa ympäristöissä eteläisessä Afrikassa. Tässä väitöskirjassa raportoidaan aerosolihiukkasten kokojakauman vuoden- ja vuorokaudenaikaisvaihteluja eteläisessä Afrikassa niin puhtaassa ympäristössä kuin teollistuneella kaivosalueellakin. Alueellisella mittakaavalla ilmenevää uusien aerosolihiukkasten muodostumista havaittiin kaikissa tutkituissa ympäristöissä ennätyksellisen taajaan, lähes päivittäin. Uusien hiukkasten muodostuminen hallitseekin aerosolihiukkasten vuorokaudenaikaisvaihtelua lukuun ottamatta matalan elintason asuinalueiden välitöntä läheisyyttä, missä ruoanlaitto ja asuinrakennusten lämmitys ovat vielä voimakkaampi lähde. Kuivan kauden aikana sateiden vähyys, maastopalojen yleistyminen ja lisääntynyt asuinrakennusten lämmitys nostivat aerosolihiukkasten pitoisuudet korkeimmilleen niissä kokoluokissa, joissa hiukkaset voivat toimia pilvipisaroiden tiivistymisytiminä. Märän kauden aikana taas alueellinen uusien hiukkasten muodostuminen ilmakehässä oli merkittävä aerosolihiukkasten lähde pilvipisaroiden tiivistymisytimien kokoluokissa

Analysis of change in users' assessment of search results over time

We present the first systematic study of the influence of time on user judgements for rankings and relevance grades of web search engine results. The goal of this study is to evaluate the change in user assessment of search results and explore how users' judgements change. To this end, we conducted a large-scale user study with 86 participants who evaluated two different queries and four diverse result sets twice with an interval of two months. To analyse the results we investigate whether two types of patterns of user behaviour from the theory of categorical thinking hold for the case of evaluation of search results: (1) coarseness and (2) locality. To quantify these patterns we devised two new measures of change in user judgements and distinguish between local (when users swap between close ranks and relevance values) and non-local changes. Two types of judgements were considered in this study: 1) relevance on a 4-point scale, and 2) ranking on a 10-point scale without ties. We found that users tend to change their judgements of the results over time in about 50% of cases for relevance and in 85% of cases for ranking. However, the majority of these changes were local

Incorporating user search behaviour into relevance feedback

In this paper we present five user experiments on incorporating behavioural information into the relevance feedback process. In particular we concentrate on ranking terms for query expansion and selecting new terms to add to the user's query. Our experiments are an attempt to widen the evidence used for relevance feedback from simply the relevant documents to include information on how users are searching. We show that this information can lead to more successful relevance feedback techniques. We also show that the presentation of relevance feedback to the user is important in the success of relevance feedback

Categorical relevance judgment

In this study we aim to explore users' behaviour when assessing search results relevance based on the hypothesis of categorical thinking. In order to investigate how users categorise search engine results, we perform several experiments where users are asked to group a list of 20 search results into a number of categories, while attaching a relevance judgment to each formed category. Moreover, to determine how users change their minds over time, each experiment was repeated three times under the same conditions, with a gap of one month between rounds. The results show that on average users form 4-5 categories. Within each round the size of a category decreases with the relevance of a category. To measure the agreement between the search engine’s ranking and the users’ relevance judgments, we defined two novel similarity measures, the average concordance and the MinMax swap ratio. Similarity is shown to be the highest for the third round as the users' opinion stabilises. Qualitative analysis uncovered some interesting points, in particular, that users tended to categorise results by type and reliability of their source, and particularly, found commercial sites less trustworthy, and attached high relevance to Wikipedia when their prior domain knowledge was limited

Characteristics of LIS Research Articles Affecting Their Citation Impact

The paper analyses the citation impact of Library and Information Science, LIS for short, research articles published in 31 leading international LIS journals in 2015. The main research question is: to what degree do authors' disciplinary composition in association with other content characteristics of LIS articles affect their citation impact? The impact is analysed in terms of the number of citations received and their authority, using outlier normalization and subfield normalization. The article characteristics analysed using quantitative content analysis include topic, methodology, type of contribution, and the disciplinary composition of their author teams. The citations received by the articles are traced from 2015 to May 2021. Citing document authority is measured by the citations they had received up to May 2021. The overall finding was that authors' disciplinary composition is significantly associated with citation scores. The differences in citation scores between disciplinary compositions appeared typically within information retrieval and scientific communication. In both topics LIS and computer science jointly received significantly higher citation scores than many disciplines like LIS alone or humanities in information retrieval, or natural sciences, medicine, or social sciences alone in scientific communication. The paper is original in allowing joint analysis of content, authorship composition, and impact.Comment: 30 pages, 4 figure