Abstract
The increased traffic emissions and reduced ventilation of urban street canyons lead to the formation of high particle concentrations as a function of the related flow field and geometry. In this context, the use of advanced modelling tools, able to evaluate particle concentration under different traffic and meteorological conditions, may be helpful. In this work, a numerical scheme based on the non-commercial fully explicit AC-CBS algorithm, and the one-equation Spalart-Allmaras turbulence model, was developed to perform numerical simulations of fluid flow and ultrafine particle dispersion in different street canyon configurations and under different wind speed and traffic conditions. The proposed non-commercial numerical tool was validated through a comparison with data drawn from the scientific literature. The results obtained from ultrafine particle concentration simulations show that as the building height increases the dispersion of particles in the canyon becomes weaker, due to the restricted interaction between the flow field in the street canyon and the undisturbed flow. Higher values of approaching wind speed facilitate the dispersion of the particles. The traffic effect has been evaluated by imposing different values of particles emission, depending on the vehicles type, with the lowest concentration values obtained for the Euro 6 vehicles, and the highest for High Duty Vehicles. A parametric analysis was also performed concerning the exposure to particles of pedestrians in different positions at the road level as a function of street canyon geometry, traffic mode, and wind speed. The worst exposure (1.25 × 10 6 part./cm 3 ) was found at the leeward side for an aspect ratio H/W = 1, wind speed of 5 m/s when High Duty Vehicles traffic was considered.
Highlights
Ultrafine Particles in Urban Areas Management of urban air quality is a key aspect in reducing the personal exposure to typical traffic-related pollutants and their resulting health effects
Computational Fluid Dynamics (CFD) Model Results: Flow Characteristics The flow regimes in urban street canyons are determined by the interaction between the vortex generated behind the upwind building and the downwind building, and can be categorized in isolated roughness flow, wake interference flow and skimming flow (Oke, 1988; Hunter et al, 1990)
Higher concentrations were detected on leeward or windward sides according to the number and the direction of rotation of mean vortices; i.e. the aspect ratio: in the H/W = 1 configuration, UFPs concentration values results higher on the leeward side; whereas, in the H/W = 2 configuration the concentration is higher on the windward side
Summary
Ultrafine Particles in Urban Areas Management of urban air quality is a key aspect in reducing the personal exposure to typical traffic-related pollutants and their resulting health effects. Vehicular traffic is considered the main contributor to UFPs emission (Kittelson et al, 2004; Gidhagen et al, 2005) even if a threshold limit value for emission from light passenger and commercial vehicles has been stated in terms of particle number (Commission Regulation (EC) No 692/2008); such local source, along with the development of clusters of buildings and the increasing vehicular traffic, are some of the reasons for deterioration of air quality in urban areas This is the case of the so called street canyon, which is a typical urban configuration of a street flanked by buildings on both sides. Urban microenvironments may increase human short-term exposure to high particle concentrations, and significantly contribute to the increase of the daily dose (Buonanno et al, 2011; Buonanno et al, 2012), leading to worsening of existing pulmonary and cardiovascular disease
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