Browsing by Subject "Atmospheric Sciences"

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  • Saponaro, Giulia (Helsingin yliopisto, 2020)
    Clouds play a vital role in Earth's energy balance by modulating atmospheric processes, thus it is crucial to have accurate information on their spatial and temporal variability. Furthermore, clouds are relevant in those processes involved in aerosol-cloud-radiation interactions. The work conducted and presented herein concentrates on the retrievals of cloud properties, as well as their application for climate studies. While remote sensing observation systems have been used to analyze the atmosphere and observe its changes for the last decades, climate models predict how climate will change in the future. Altogether, these sources of observations are needed to better understand cloud processes and their impact on climate. In this thesis aerosol and cloud properties from the three above mentioned sources are applied to evaluate their potential in representing cloud properties and applicability in climate studies on local, regional and global scales. One aim of this thesis focuses on evaluating cloud parameters from ground-based remote-sensing sensors and from climate models using the MODerate Imaging Spectroradiometer (MODIS) data as a reference dataset. It is found that ground-based measurements of liquid clouds are in good agreement with MODIS cloud droplet size while poor correlation is found in the amount of cloud liquid water due to the management of drizzle. The comparison of the cloud diagnostic from three climate models with MODIS data, enabled through the application of a satellite simulator, helped to understand discrepancies among models as well as discover deficiencies in their simulation processes. These findings are important to further improve the parametrization of atmospheric constituents in climate models, therefore enhancing the accuracy of climate projections. In this thesis it is also assessed the impact of aerosol particles on clouds. Satellite data can be used to derive climatically crucial quantities that are otherwise not directly retrieved (such as aerosol index and cloud droplet number concentration) which can be used to infer the sensitivity of clouds to aerosols changes. Results on the local and regional scales show that contrasting aerosol backgrounds indicate a higher sensitivity of clouds to aerosol changes in cleaner ambient air and a lower sensitivity in polluted areas, further corroborating the notion that anthropogenic emission modify clouds. On the global scale, the estimates of the aerosol-cloud interactions present, overall, a good agreement between the satellite- and model-based values which are in line with the results from other models.
  • Dada, Lubna (2019)
    New particle formation (NPF) is an atmospheric phenomenon, observed in many environments globally, and it contributes to a major fraction of the global aerosol number budget thereby affecting both climate and human health. In this thesis, we investigate the mechanisms behind NPF in the boreal forest environment and analyze the long-term behavior of the variables associated with the occurrence of this phenomenon. In order to improve the classification of atmospheric NPF events, especially when considering the increasing number of measurement campaigns and stations, we developed an automatic framework to classify NPF events based on the 2–4 nm ion and 7–25 nm aerosol particle concentrations in the atmosphere. This approach categorizes days into four defined classes: Regional NPF events, transported NPF events, ion bursts and non-events. For regional NPF events, the approach additionally determined the precise period (start and end-time) during which the event occurred. We show that, in the boreal forest, NPF events tend to occur under clear sky conditions with low condensation sinks and moderate temperatures. Using chamber simulations, we further investigated the mechanisms of new particle formation and growth in the boreal forest environment. While sulfuric acid is known to be the driver of NPF, we found that pure biogenic NPF is possible in the absence of sulfuric acid, and that the nucleation is mediated by dimers of highly oxygenated monoterpene oxidation products. We also found that anthropogenic vapors, such as NOx, attenuate the particle formation and growth by modifying the chemical composition of highly oxygenated molecules (HOMs) necessary for nucleation and growth. In the present-day-time atmosphere, we found that highly oxygenated molecules (HOMs) govern ion-induced new particle formation in the boreal forest when the ratio of biogenic HOMs to H2SO4 is greater than 30. Our results show that non-nitrate HOM dimers mediate ion-induced nucleation not only during daytime but also during night-time. In the absence of H2SO4, we observed pure biogenic ion-induced clustering mediated by non-nitrate HOM dimers and trimers; however, these clusters did not grow past 6 nm due to insufficient photochemistry needed for producing condensable vapors that would ensure cluster survival.
  • Hooda, Rakesh K (Helsingin yliopisto, 2019)
    Atmospheric aerosol particles are linked to visibility reduction and adverse health effects, and radiation balance of the Earth— directly by reflecting and absorbing solar radiation and indirectly by influencing the cloud properties and processes and, possi-bly, by changing the heterogeneous chemistry of reactive gaseous species. Atmospheric aerosols are the most uncertain driver of global climate change. The South‒Asian region has been increasingly recognized as one of the global hotspots of aerosols; and Indo Gangetic Plains (IGP) is one among them with complex geography, heterogeneity in sources and varying atmospheric dynamics. These factors make IGP’s aerosol and pollution very difficult to characterize. So far, long-term regional observations of aerosol properties have been scarce in this region, but argued necessary in order to bring the knowledge of regional and global distribution of aerosols further. In this context, regional studies of aerosol properties their dynamics and atmospheric processes are very important areas of investigation to better estimate the climatic importance of submicron aerosol particles. Moreover regional studies over IGP-Himalayas domain are inevitable to know how trans-Himalayan valleys are acting as conduits for aerosol and pollution transport from the plains to the Himalayas. Therefore, in this thesis we studied these issues by applying basic to state-of-the-art instrumentation in two different envi-ronments, plains—Gual Pahari, and Himalayan foothills— Mukteshwar; to obtain physical and optical properties of submi-cron particles. Additionally, we used meteorological parameters, emissions and process modelling to determine local and region-al scale transport of atmospheric aerosols. The work carried out as part of the thesis infers four main conclusions, 1) Simultaneous long-term measurements at both the environments in Northern India region are useful to establish linkages between sub-urban environment and high altitude sites. One site represents a source region, while another characterize as a receiver site of atmospheric pollutants; 2) A distinct cycle of aerosol properties, both seasonal and diurnal, is present and provides information of driving factors of aerosol variability at both the sites; 3) The contribution of regional sources seem to dominate over the local /sub-urban sources, in the IGP region bounda-ry layer; 4) Aerosol properties and specific humidity “passive tracer” based analysis clearly reveal that the mountainous terrain sites are under the influence of air from the plains due to convective transport processes enhanced by local and mesoscale topography. The results presented in this thesis are particularly useful, first, when examining the linkages of aerosol properties variability between two different environments. The second, in determining for instance local versus regional influences, and pollutants reaching high altitude sites which can be explained by boundary layer dynamics processes, especially in the mountain terrain where the modelled mixing layer depths have uncertainties. This work outlines future direction of multi-points measurements on vertical profile of atmospheric particles and local boundary layer over mountainous terrain where the atmospheric structure becomes much more complicated. Additionally, investigations including isotope-based analysis and modelling work over the Himalayan region are desirable to be able to describe better the transport of atmospheric aerosols from IGP to high altitudes and further up to Himalayan ice-pack and glaciers where aerosol deposition could have serious environmental impacts.