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Browsing by Author "Kurppa, Mona"

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  • Kurppa, Mona (2016)
    This thesis is a case study of the impact of urban planning on local air quality along a planned city boulevard in western Helsinki. The aim of this study is to analyse ventilation and dispersion of traffic-related air pollutants inside street canyons and courtyards in four alternative city block design versions. In particular, whether the format and variation of building height can improve air quality in future planned neighbourhoods and as such, help improve the decision-making process in city planning. The study employs a large-eddy simulation (LES) model PALM with embedded Lagrangian stochastic particle and canopy models to simulate transport of pollutants (air parcels) and the aerodynamic impact of street trees and a surrounding forest on pollutant transport. The embedded models are revised by the author to take into account the horizontal heterogeneity of the particle sources and plant canopy. Furthermore, three-dimensional two-way self-nesting is used for the first time in PALM in this study. High-resolution simulations are conducted over a real urban topography under two contrasting meteorological conditions with neutral and stable stratification and south-western and eastern wind direction, respectively. The comparison of the different boulevard-design versions is based on analysing the temporal mean particle concentrations, the turbulent vertical particle flux densities and the particle dilution rate. Differences in flux densities between the versions show a strong dependence on urban morphology whereas the advection-related dilution rate depends on the volume of unblocked streamwise street canyons. A suggestive ranking of the versions is performed based on the horizontal mean values of the analysis measures (separately for the boulevard, the other street canyons, the courtyards and the surroundings). Considering both meteorological conditions, the design version with variable building height and short canyons along the boulevard outperforms the other design versions based on the ranking. This is especially pronounced in stable conditions. Surprisingly, variability in building shape did not induce clear improvements in ventilation. This is the first high-resolution LES study conducted over a real urban topography applying sophisticated measures to assess pollutant dispersion and ventilation inside street canyons and courtyards.