Browsing by Subject "LES"

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  • Karttunen, Sasu; Kurppa, Mona; Auvinen, Mikko; Hellsten, Antti; Järvi, Leena (2020)
    Street vegetation has been found to have both positive and negative impacts on pedestrian-level air quality, but the net effect has remained unclear. In this study, the effect of street trees on aerosol mass (PM10 and PM2.5) and number in a boulevard-type street canyon with high traffic volumes in Helsinki is examined using the large-eddy simulation model PALM. Including a detailed aerosol module and a canopy module to comprise permeable trees, PALM allows to examine the effect of street trees in depth. The main aim is to understand the relative importance of dry deposition and the aerodynamic impact of street trees on the different aerosol measures at pedestrian-level and to find a suitable street-tree layout that would minimise the pedestrian-level aerosol particle concentrations over the boulevard pavements. The layout scenarios were decided together with urban planners who needed science-based knowledge to support the building of new neighbourhoods with boulevard-type streets in Helsinki. Two wind conditions with wind being parallel and perpendicular to the boulevard under neutral atmospheric stratification are examined. Adding street trees to the boulevard increases aerosol particle concentrations on the pavements up to 123%, 72% and 53% for PM10, PM2.5 and total number, respectively. This shows decreased ventilation to be more important for local aerosol particle concentrations than dry deposition on vegetation. This particularly for PM10 and PM2.5 whereas for aerosol number, dominated by small particles, the importance of dry deposition increases. Therefore the studied aerosol measure is important when the effect of vegetation on pedestrian-level air quality is quantified. Crown volume fraction in the street space is one of the main determining factors for elevated mass concentrations on the pavements. The lowest pedestrian-level mass concentrations are seen with three rows of trees of variable height, whereas the lowest number concentrations with four rows of uniform trees. The tree-height variation allows stronger vertical turbulent transport with parallel wind and largest volumetric flow rates with perpendicular wind. Introducing low (height <1 m) hedges under trees between the traffic lanes and pavements is found to be a less effective mitigation method for particle mass than introducing tree-height variability, and for particle number less effective than maximising the tree volume in the street canyon. The results show how street trees in a boulevard-type street canyon lead to decreased pedestrian-level air quality with the effect being particularly strong for larger aerosol particles. However, with careful planning of the street vegetation, significant reductions in pedestrian-level aerosol particle concentrations can be obtained.
  • Karttunen, Sasu (Helsingin yliopisto, 2020)
    Air pollution is the most severe environmental problem in the world in terms of human health. The World Health Organisation (WHO) estimates that 91% of the world's population is exposed to high air pollutant levels. The risks are particularly high in urban areas, where often high population densities are combined with high air pollutant levels. Urban street canyons are especially prone to high pollutant levels due to the proximity of traffic and reduced exchange of air with the street canyon and air above, referred to as ventilation. As a result, one of the most important topics in city planning is how to avoid designs that impact the air quality negatively. Street trees are often planted in street canyons for aesthetic purposes while they can also improve thermal comfort. The air quality within street canyons is affected by street trees in two ways. They provide leaf surface for air pollutants to deposit on, thus cleaning the air. On the other hand, they block the airflow within the street canyon, thus decreasing the ventilation of air pollutants. In previous studies the latter effect has generally been found stronger. However, due to the various benefits of street trees, leaving them completely out from street canyon designs is rarely an option. The City of Helsinki is planning to develop its current inbound motorways into city boulevards which has raised concerns towards the local air quality levels due to high projected traffic rates. The aim of this study was to find which of five street-tree scenarios, realistic for the city boulevards, is the best in terms of air quality. Pedestrian-level aerosol mass concentrations were used as the measure of air quality. Furthermore the impacts of vegetation and dependency of aerosol mass concentrations on various flow statistics were studied in order to explain the differences between the scenarios. Large-eddy simulation (LES) model PALM was utilised to study the flow field above and within a city boulevard and to model the dispersion of traffic-related aerosols. Aerosol particles of different sizes were represented using a sectional aerosol model SALSA. The suitability of the used LES setup for such intercomparison studies was also investigated. The results showed that the street trees have generally a considerable negative impact (-2% to 54%) on pedestrian-level aerosol mass concentrations. Trees were find to reduce the mean wind speeds within the street canyon, which correlated strongly with the pedestrian-level concentrations. This was particular with a parallel wind direction to the street canyon due to decreased ventilation. Turbulence produced by the street trees was partially able to compensate for the reduced ventilation in some scenarios. The increased turbulence could be observed up to heights exceeding the maximum building height. Based on the results, it is recommended to prefer variable-height street-tree canopies over uniform ones within street canyons similar to the studied one. Uneven canopy increases turbulence and related pollutant transport which partially compensates decreased ventilation due to decreased wind speeds. It is also recommendable to consider minimising the ratio of the total crown volume to the street canyon volume, as ventilation decreases sharply as the ratio increases.
  • Strömberg, Jani (Helsingin yliopisto, 2021)
    Air temperatures are commonly higher in urban environments compared to rural ones. The energy input of solar radiation and its storage in urban surfaces changes the way the surface interacts with the atmosphere through turbulent fluxes and mixing processes. The complexity of radiative properties combined with the effect of urban geometry makes the magnitude of the effect radiation has on the dynamics of boundary layer flow an important area of study. The aim of this study is to understand and quantify how much the radiative processes alter the flow field and turbulence in a real urban street canyon in Helsinki. The model used is the large-eddy simulation (LES) model PALM, which solves for the flow and the most relevant atmospheric scales that describe interactions between the surface and atmosphere. An additional library called RRTMG (Rapid Radiative Transfer Model for Global Models) is used in this study to provide the radiation input impacting the boundary layer flow. Two embedded surface models in PALM, USM (Urban Surface Model) and LSM (Land-Surface Model) are used to solve the local conditions for radiative balance based on the output of RRTMG. Two model runs are made (RRTMG On & RRTMG Off), both identical in terms of the large-scale forcing boundary conditions and land-use data, but with additional radiation input in RRTMG On. The results show that radiation alters the low level stratification of potential temperature, which leads to more unstable conditions. Near-surface air temperatures within the canyon were increased by 3.9 C on average. Horizontal wind speeds increased by 76 % close to the ground compared to RRTMG Off. RRTMG On also showed a change in the structure of the topographically forced canyon vortex, as the low wind conditions enabled the radiative effects to have a stronger effect in its forcing. The center of the vortex changed in location more towards the center of the canyon and the vertical motions on opposing sides of the street were strengthened by 0.15 m/s in both vertical directions. Additionally both mechanical and thermal turbulence production increased with RRTMG On, while the thermal production remained smaller by one magnitude compared to mechanical production within Mäkelänkatu. Higher wind speeds and their variance gave rise to increased mechanical production of turbulence and radiative effects increased the thermal production. More research is however needed to determine thermal turbulence's role in situations with different meteorological conditions or in other cities.
  • Kurppa, Mona; Hellsten, Antti; Auvinen, Mikko; Raasch, Siegfried; Vesala, Timo; Jarvi, Leena (2018)
    Buildings and vegetation alter the wind and pollutant transport in urban environments. This comparative study investigates the role of orientation and shape of perimeter blocks on the dispersion and ventilation of traffic-related air pollutants, and the street-level concentrations along a planned city boulevard. A large-eddy simulation (LES) model PALM is employed over a highly detailed representation of the urban domain including street trees and forested areas. Air pollutants are represented by massless and passive particles (non-reactive gases), which are released with traffic-related emission rates. High-resolution simulations for four different city-block-structures are conducted over a 8.2 km domain under two contrasting inflow conditions with neutral and stable atmospheric stratification corresponding the general and wintry meteorological conditions. Variation in building height together with multiple cross streets along the boulevard improves ventilation, resulting in 7-9% lower mean concentrations at pedestrian level. The impact of smaller scale variability in building shape was negligible. Street trees further complicate the flow and dispersion. Notwithstanding the surface roughness, atmospheric stability controls the concentration levels with higher values under stably stratified inflow. Little traffic emissions are transported to courtyards. The results provide urban planners direct information to reduce air pollution by proper structural layout of perimeter blocks.